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diff --git a/old/50068-0.txt b/old/50068-0.txt deleted file mode 100644 index cc810d4..0000000 --- a/old/50068-0.txt +++ /dev/null @@ -1,10368 +0,0 @@ -The Project Gutenberg EBook of Hawkins Electrical Guide Vol. 8 (of 10), by -Nehemiah Hawkins - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Hawkins Electrical Guide Vol. 8 (of 10) - A Progressive Course of Study for Engineers, Electricians, - Students, and Those Desiring to Acquire a Working Knowledge - of Electricity and Its Applications - -Author: Nehemiah Hawkins - -Release Date: September 28, 2015 [EBook #50068] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK HAWKINS ELECTRICAL GUIDE, VOL 8 *** - - - - -Produced by Juliet Sutherland, Paul Marshall and the Online -Distributed Proofreading Team at http://www.pgdp.net - - - - - - -Transcriber's Notes: - - Underscores "_" before and after a word or phrase indicate _italics_ - in the original text. - - Equal signs "=" before and after a word or phrase indicate =bold= - in the original text. - - Small capitals have been converted to SOLID capitals. - - Illustrations have been moved so they do not break up paragraphs. - - Misprints in the table SAVING DUE TO HEATING THE FEED WATER, Pg. 1936 - have been corrected, they are: - - Init. Temp. Pressure Old Value New Value - 130 40 .0954 .0934 - 200 40 .0900 .0999 - 210 40 .1000 .1010 - 230 100 .0117 .1012 - - In the original text, there are two Fig. 2769's and two Fig. 2770's. - The second of each has had an "a" suffix added, i.e. 2769a and 2770a. - - On line 7397 the word "impedence" was corrected to "impedance". - - Superscripts are shown as ^{d} and subscripts are shown as _{d}, where - "d" is an integer. - - Inconsistent spelling and hyphenation has been left as in the original. - - - - - THE THOUGHT IS IN THE QUESTION - THE INFORMATION IS IN THE ANSWER - - HAWKINS ELECTRICAL GUIDE NUMBER EIGHT - - QUESTIONS ANSWERS & ILLUSTRATIONS - - A PROGRESSIVE COURSE OF STUDY FOR ENGINEERS, - ELECTRICIANS, STUDENTS AND THOSE DESIRING TO - ACQUIRE A WORKING KNOWLEDGE OF - - ELECTRICITY AND ITS APPLICATIONS - - A PRACTICAL TREATISE by HAWKINS AND STAFF - - THEO AUDEL & CO. 72 FIFTH AVE. NEW YORK. - - COPYRIGHTED, 1915, BY THEO. AUDEL & CO., - - NEW YORK. - Printed in the United States. - - - - - TABLE OF CONTENTS - GUIDE No. 8 - - - =WAVE FORM MEASUREMENT= 1,839 to 1,868 - - Importance of wave form measurement--=methods=: - step by step; constantly recording--=classes of - apparatus=: wave indication; _oscillographs_--=step - by step methods=--Joubert's; four part commutator; - modified four part commutator; ballistic galvanometer; - zero; Hospitalier ondograph--=constantly recording - methods=: cathode ray; glow light; moving iron; moving - coil; hot wire--=oscillographs=--moving coil type; - construction and operation; production of the time scale; - oscillograms--falling plate camera; its use. - - =SWITCHBOARDS= 1,869 to 1,884 - - =General principles=: diagram--small plant a.c. - switchboard--=switchboard panels=; =generator - panel=; diagram of connections--simple method of - determining bus bar capacity--feeder panel--diagrams of - connection for two phase and three phase installations. - - =ALTERNATING CURRENT WIRING= 1,885 to 1,914 - - Effects to be considered in making - calculations--=induction=; self- and mutual; mutual - induction, how caused--=transpositions=--inductance - per mile of three phase circuit, table--=capacity=; - table--=frequency--skin effect=; calculation; - table--=corona effect=; its manifestation; - critical voltage; spacing of wires--=resistance of - wires--impedance--power factor=; apparent current; - usual power factors encountered; example--=wire - calculations--sizes of wire--table of the property of - copper wire--drop=; example--current--example covering - horse power, watts, apparent load, current, size of wire, - drop, voltage at the alternator, and electrical horse power. - - =POWER STATIONS= 1,915 to 1,988 - - Classification--=central stations=; types: a.c., - d.c., and a.c. and d.c.; reciprocating engine vs. - turbine--=location of central stations=; price of - land; trouble after erection; water supply; service - requiring direct current--=size of plant=; nature - of load; peak load; load factor; machinery required; - example; factors of evaporation; grate surface per - horse power--=general arrangement of station=; - belt drive with counter shaft; desirable features of - belt drive; conditions, suitable for counter shaft - drive; location of engine and boilers; the steam pipe; - piping between engine and condenser; number and type - of engine; superheated steam; switchboard location; - individual belt drive; direct drive--=station - construction=--=foundations=--=walls=-- - =roofs=--=floors=--=chimneys=; - cost of chimneys and mechanical draft; high chimneys - ill advised--=steam turbine=; types: impulse - and reaction; why high vacuum is necessary; the - working pressure--=hydro-electric plants=--water - turbines; types: impulse, reaction--=isolated - plants=--=sub-stations=; arrangement; three phase - installations; reactance coils in sub-stations; portable - sub-stations. - - =MANAGEMENT= 1,989 to 2,114 - - The term "management"--=selection=; general - considerations--=selection of generators=; - efficiency of generators; size and number; - regulation--=installation=; precautions; - handling of armatures; assembling a machine; speed - of generators; calculation of pulley sizes; gear - wheels--=belts=; various belt drives; horse - power transmitted by belts; velocity of belt; endless - belts--=switchboards=; essential points of difference - between single phase and three phase switchboard wiring; - assembling a switchboard; usual equipment. - - =Operation of Alternators--alternators in parallel=; - synchronizing; lamp methods; action of amortisseur winding; - synchronizing three phase alternators; disadvantage of - lamp method--=cutting out alternator=; precautions; - hunting--=alternators in series=. - - =Transformers=; selection; efficiency; kind of oil - used; detection of moisture; drying oil; regulation; - transformers in parallel; polarity test--=motor - generators=; various types and conditions requiring - same--=dynamotors=; precautions--=rotary - converters=; objections to single phase type; operation - when driven by direct current, by alternating current; most - troublesome part; efficiency; overload; starting; starting - current. - - =Electrical measuring instruments=; location; - readings; station voltmeters; points relating to - ammeters; attention necessary; usual remedies to correct - voltmeter--=how to test generators=; commercial - efficiency; various tests. - - =Station Testing:= resistance measurement by "drop" - method--methods of connecting ammeter voltmeter and - wattmeter for measurement of power--=motor testing:= - single phase motor--three phase motor, voltmeter and - ammeter method; two wattmeter method; polyphase wattmeter - method; one wattmeter method; one wattmeter and Y box - method--three phase motor with neutral brought out; single - wattmeter method--temperature test, three phase induction - motor--=three phase alternator testing:= excitation - or magnetization curve test--synchronous impedance - test--load test--three phase alternator or synchronous - motor temperature test--=direct current motor= or - =generator testing:= magnetization curve--(shunt) - external characteristic--direct current motor testing; - load and speed tests--temperature test, "loading back" - method--=compound dynamo testing=: external - characteristic, adjustable load--=transformer testing=: - external characteristic, adjustable load--=transformer - testing=: core loss and leakage or exciting current - test--copper loss--copper loss by wattmeter measurement - and impedance--temperature--insulation--internal - insulation--insulation resistance--polarity--winding - or ratio tests. - - - - -CHAPTER LXIII - -WAVE FORM MEASUREMENT - - -The great importance of the wave form in alternating current work -is never denied, though it has sometimes been overlooked. The -application of large gas engines to the driving of alternators -operated in parallel requires an accurate knowledge of the wave form, -and a close conformation to a sine wave if parallel operation is -to be satisfactory. It is also important that the fluctuations in -magnetism of the field poles should be known, especially if solid -steel pole faces be used. - - If an alternator armature winding be connected in delta, the - presence of a third harmonic becomes objectionable, as it gives - rise to circulating currents in the winding itself, which increase - the heating and lowers the efficiency of the machine. - - That the importance of having a good wave form is being - realized, is proved by the increasing prevalence in alternator - specifications of a clause specifying the maximum divergence - allowable from a true sine wave. It is however perhaps not always - realized that an alternator which gives a good pressure wave on - no load may give a very bad one under certain loads, and the - ability of the machine to maintain a good wave form under severe - conditions of load is a better criterion of its good design than - is the shape of its wave at no load. - - The question of wave form is of special interest to the power - station engineer. Upon it depends the answer to the questions: - whether he may ground his neutral wires without getting large - circulating currents; whether he may safely run any combination - of his alternators in parallel; whether the constants of his - distributing circuit are of an order liable to cause dangerous - voltage surges due to resonance with the harmonics of his - pressure wave; what stresses he is getting in his insulation due - to voltage surges when switching on or off, etc. It has been - shown by Rossler and Welding that the luminous efficiency of the - alternating current arc may be 44 per cent. higher with a flat - topped than with a peaked pressure wave, while on the other hand - it is well known that transformers are more efficient on a peaked - wave. Also the accuracy of many alternating current instruments - depends upon the wave shape. - - In making insulation breakdown tests on cables, insulators, or - machinery, large errors may be introduced unless the wave form at - the time of the test be known. It is not sufficient even to know - that the testing alternator gives a close approximation to a sine - wave at no load; since if the capacity current of the apparatus - under test be moderately large compared with the full load current - of the testing alternator, the charging current taken may be - sufficient to distort the wave form considerably, thus giving - wrong results to the disadvantage of either the manufacturer or - purchaser. - -[Illustration: FIG. 2,583.--General Electric simultaneous record of -three waves with common zero.] - -The desirability of a complete knowledge of the manner in which the -pressure and current varies during the cycle, has resulted in various -methods and apparatus being devised for obtaining this knowledge. -The apparatus in use for such purpose may be divided into two general -classes, - - 1. Wave indicators; - 2. Oscillographs. - -and the methods employed with these two species of apparatus may be -described respectively as, - - 1. Step by step; - 2. Constantly recording. - -that is to say, in the first instance, a number of instantaneous -values are obtained at various points of the cycle, which are plotted -and a curve traced through the several points thus obtained. A -constantly recording method is one in which an infinite number of -values are determined and recorded by the machine, thus giving a -complete record of the cycle, leaving no portion of the wave to be -filled in. - -[Illustration: FIG. 2,584.--General Electric simultaneous record of -three waves with separate zeros.] - -[Illustration: Figs. 2,585 and 2,586.--Oscillograms (from paper by -Morris and Catterson-Smith, Proc. I. E. E., Vol. XXXIII, page 1,023), -showing _how the current varies_ =in one of the armature coils of -a direct current motor=. Fig. 2,585 was obtained with the brushes -in the neutral position, and fig. 2,586 with the brushes shifted -forward.] - - -The various methods of determining the wave form may be further -classified as: - - { Joubert's method; - { Four part commutator method; - { Modified four part commutator method; - 1. Step by step { Ballistic galvanometer method; - { Zero method; - { By Hospitalier ondograph. - -[Illustration: FIG. 2,587.--=Oscillogram= by Bailey and Cleghorne -(Proc. I.E.E., Vol. XXXVIII), =showing= _the sparking pressure or -pressure between the brush and the commutator segment at the moment -of separation_. The waves fall into groups of three owing to the fact -that there were three armature coils in each slot.] - - { cathode ray; - { by use of various types { glow light; - 2. constantly recording { of =oscillograph=, { moving iron; - { such as { moving coil; - { hot wire. - - -[Illustration: FIG. 2,588.--Various wave forms. The sine wave -represents a current or pressure which varies according to the sine -law. A distorted wave is due to the properties of the circuit, for -instance, the effect of hysteresis in an iron core introduced into a -coil is to distort the current wave by adding harmonics so that the -ascending and descending portions may not be symmetrical. A peaked -wave has a large maximum as compared with its virtual value. A peaked -wave is produced by a machine with concentrated winding.] - -=Joubert's Method.=--The apparatus required for determining the -wave form by this step by step method, consists of a galvanometer, -condenser, two, two way switches, resistance and adjustable contact -maker, as shown in fig. =2,589=. - - The contact maker is attached to the alternator shaft so that - it will rotate synchronously with the latter. By means of the - adjustable contact, the instant of "making" that is, of "closing" - the testing circuit may be varied, and the angular position of the - armature, at which the testing circuit is closed, determined from - the scale, which is divided into degrees. - - A resistance is placed in series with one of the alternator - leads, such that the drop across it, gives sufficient pressure for - testing. - -=Ques. Describe the method of making the test.= - -[Illustration: FIG. 2,589.--Diagram illustrating Joubert's =step by -step method= of wave form measurement.] - -Ans. For current wave measurement switch No. 1 is placed on contact -F, and for pressure wave measurement, on contact G, switch No. 2 is -now turned to M and the drop across the resistance (assuming switch -No. 1 to be turned to contact F) measured by charging the condenser, -and then discharging it through the galvanometer by turning the -switch to S. This is repeated for a number of positions of the -contact maker, noting each time the galvanometer reading and position -of the contact maker. By plotting the positions of contact maker as -abscissæ, and the galvanometer readings as ordinates, the curve drawn -through them will represent the wave form. - -[Illustration: FIG. 2,590.--=Four part commutator method= of wave -form measurement. The contact device consists of two slip rings and -a four part commutator. One slip ring is connected to one terminal -of the source, the other to the voltmeter, and the commutator to the -condenser. By adjusting R when a known direct current pressure is -impressed across the terminals, the voltmeter can be rendered direct -reading.] - -[Illustration: FIG. 2,591.--=Modified four part commutator method= -of wave form measurement (Duncan's modification). By this method -one contact maker can be used for any number of waves having the -same frequency. Electro-dynamometers are used and the connections -are made as here shown. The moving coils are connected in series to -the contact maker, and the fixed coils are connected to the various -sources to be investigated, then the deflection will be steady and -by calibration with direct current can be made to read directly in -volts.] - - - The apparatus is calibrated by passing a known constant current - through the resistance. - -[Illustration: FIG. 2,592.--Diagram illustrating the =ballistic -galvanometer method= of wave form measurement. =The test may be made= -as described in the accompanying text, or in case the contact breaker -is belted instead of attached rigidly to the shaft, it could be -arranged to run slightly out of synchronism, then by taking readings -at regular intervals, points will be obtained along the curve without -moving the contact breaker. If this method be used, a non-adjustable -contact breaker suffices. =In arranging the belt drive= so as to run -slightly out of synchronism, if the pulleys be of the same size, the -desired result is obtained by pasting a thin strip of paper around -the face of one of the pulleys thus altering the velocity ratio of -the drive slightly from unity.] - -=Ballistic Galvanometer Method.=--This method, which is due to -Kubber, employs a _contact breaker_ instead of a _contact maker_. -The distinction between these two devices should be noted: A contact -maker keeps the circuit _closed_ during each revolution for a short -interval only, whereas, a contact breaker keeps the circuit _open_ -for a short interval only. - - Fig. 2,592, shows the necessary apparatus and connections for - applying the ballistic galvanometer method. The contact breaker - consists of a commutator having an ebonite or insulating segment - and two brushes. - - _In operation_ the contact breaker keeps the circuit closed - during all of each revolution, except the brief interval in which - the brushes pass over the ebonite segment. - - The contact breaker is adjustable and has a scale enabling its - various positions of adjustment to be noted. - -=Ques. Describe the test.= - -[Illustration: FIGS.. 2,593 and 2,594.--=Two curves= _representing -pressure and current respectively of a rotary converter._ Fig. 2,593, -pressure wave V, fig. 2,594 current wave C. These waves were obtained -from a converter which was being driven by an alternator by means of -an independent motor. The rotary converter was supplying idle current -to some unloaded transformers and the ripples clearly visible in the -pressure wave V, correspond to the number of teeth in the armature of -the rotary converter.] - -Ans. The contact breaker is placed in successive positions and -galvanometer readings taken, the switch being turned to F, fig. -2,592, in measuring the current wave, and to G in measuring the -pressure wave. The results thus obtained are plotted giving -respectively current and pressure waves. - -=Ques. How is the apparatus calibrated?= - -Ans. By sending a constant current of known value through the -resistance R. - -=Zero Method.=--In electrical measurements, a zero method is one _in -which the arrangement of the testing devices is such that the value -of the quantity being measured is shown when the galvanometer needle -points to_ =zero=. - -In the zero method either a contact maker or contact breaker may be -used in connection with a galvanometer and slide wire bridge, as -shown in figs. 2,595 and 2,596. - -[Illustration: FIG. 2,595.--Diagram illustrating zero method of wave -measurement with _contact_ =maker=. The voltage of the battery must -be at least as great as the maximum pressure to be measured and must -be kept constant.] - -=Ques. What capacity of battery should be used?= - -Ans. Its voltage should be as great as the maximum pressure to be -measured. - -=Ques. What necessary condition must be maintained in the battery?= - -Ans. Its pressure must be kept constant. - -=Ques. How are instantaneous values measured?= - -Ans. The bridge contact A is adjusted till the galvanometer shows no -deflection, then the length AS is a measure of the pressure. - - The drop between these points can be directly measured with a - voltmeter if desired. - -=Ques. How did Mershon modify the test?= - -Ans. He used a telephone instead of the galvanometer to determine the -correct placement of the bridge contact A. - -[Illustration: FIG. 2,596.--Diagram illustrating zero method of wave -measurement with _contact_ =breaker=. The voltage of the battery must -be at least as great as the maximum pressure to be measured and must -be kept constant.] - -=Ques. How can the instantaneous values be recorded?= - -Ans. By attaching to the contact A, a pencil controlled by an -electro-magnet arranged to strike a revolving paper card at the -instant of no deflection, the paper being carried on a drum. - -=Hospitalier Ondograph.=--The device known by this name is a -development of the Joubert step by step method of wave form -measurement, that is to say, the principle on which its =action is -based=, consists in _automatically charging a condenser from each -100th wave, and discharging it through a recording galvanometer, each -successive charge of the condenser being automatically taken from a -point a little farther along the wave._ - -[Illustration: FIG. 2,597.--Diagram of Hospitalier ondograph showing -mechanism and connections. It represents a development of Joubert's -step by step method of wave form measurement.] - - As shown in the diagram, fig. 2,597, the ondograph consists of - a synchronous motor A, operated from the source of the wave form - to be measured, connected by gears B to a commutator D, in such a - manner that while the motor makes a certain number of revolutions, - the commutator makes a like number diminished by unity; that is to - say, if the speed of the motor be 900 revolutions per minute, the - commutator will have a speed of 899. - - The commutator has three contacts, arranged to automatically - charge the condenser _cc'_ from the line, and discharge it through - the galvanometer E, the deflection of which will be proportional - to the pressure at any particular instant when contact is made. - - In fig. 2,597, GG' are the motor terminals, HH' are connected to - the condenser _cc'_ through a resistance (to prevent sparking at - the commutator) and I, I' are the connections to the service to be - measured. - - A permanent magnet type of recording galvanometer is employed. - Its moving coil E receives the discharges of the condenser in - rapid succession and turns slowly from one side to the other. - -[Illustration: FIG. 2,598.--View of Hospitalier ondograph. =In -operation=, a long pivoted pointer carrying a pen and actuated by -electro-magnets, records on a revolving drum a wave form representing -the alternating current, pressure or current wave.] - - The movable part operates a long needle (separately mounted) - carrying a pen F, which traces the curve on the rotating cylinder - C. This cylinder is geared to the synchronous motor to run at such - a speed as to register three complete waves upon its circumference. - - By substituting an electromagnetic galvanometer for the - permanent magnet galvanometer, and by using the magnet coils as - current coils and the moving coil as the volt coil, the instrument - can be made to draw watt curves. Fig. 2,598 shows the general - appearance of the ondograph. - -=Cathode Ray Oscillograph.=--This type of apparatus for measuring -wave form was devised by Braun, and consists of a cathode ray tube -having a fluorescent screen at one end, a small diaphragm with a -hole in it at its middle, and two coils of a few turns each, placed -outside it at right angles to one another. These coils carry currents -_proportional to the_ =pressure= _and_ =current= _respectively_ of -the circuit under observation. - -[Illustration: FIG. 2,599.--General Electric =moving coil -oscillograph= complete =with tracing table=. The tracing table is -employed for observing the waves, and by using a piece of transparent -paper, the waves under observation appear as a continuous band of -light which can be traced, thus making a permanent record. This is -not, however, to be regarded as a recording attachment, and can not -be used where instantaneous phenomena are being investigated. =The -synchronous motor= for operating the synchronous mirror in connection -with tracing and viewing attachment is wound for 100 to 115 volts, 25 -to 125 cycles, and should, of course, be run from the same machine -which furnishes power to the circuit under observation. A rheostat -for steadying and adjusting the current should be connected in series -with the motor. =The beam from the vibrator mirrors= _striking this -synchronous mirror moves back and forth over the curved glass, and -gives the length of the wave; the movement of the vibrator mirror -gives the amplitude, and the combination gives the wave complete_. An -arc lamp or projection lantern produces the image reflected by the -mirrors upon the film, tracing table or screen. For the rotation of -the photographic film, a small direct current shunt wound motor is -ordinarily used.] - -The ray then moves so as to produce an energy diagram on the -fluorescent screen. - -[Illustration: FIG. 2,600.--General Electric moving coil -oscillograph. =The moving elements= _consist of single loops of flat -wire carrying a small mirror and held in tension by small spiral -springs_. The current passing down one side and up the other, forces -one side forward and the other backward, thus causing the mirror to -vibrate on a vertical axis. The vibrator elements fit into chambers -between the poles of electro-magnets, and are adjustable, so as to -move the beam from the mirror, both vertically and horizontally. A -sensitized photographic film is wrapped around a drum and held by -spring clamps. The drum, with film, is placed in a case and a cap -then placed over the end, making the case light, when the index is -either up or down. The loading is done in a dark room. A driving dog -is screwed into the drum shaft, and which, when the drum and case are -in place, revolves the film past a slot. =When an exposure is to be -made=, the index is moved from the closed position, thus opening the -slot in the case and exposing the film to the beam of light from the -vibrating mirrors when the electrically operated shutter is open. The -slot is then closed by moving the index to "=Exposed=." A slide with -ground glass can be inserted in place of the film case or roll holder -to arrange the optical system when making adjustments. The shutter -operating mechanism is arranged so as to hold the shutter open during -exactly one revolution of the film drum. There are two devices -connected to the shutter operating mechanism; one opens the shutter -at the instant the end of the film passes the slot; the other opens -immediately, at any part of the film, and both give exposure during -one revolution. The first is useful when making investigations in -which the events are either recurring, or their beginnings known or -under control, and the second when the time of the event is not under -control, such as the blowing of fuses or opening of circuit breakers.] - -The instrument is much used in wireless telegraphy, as it is capable -of showing the characteristics of currents of very high frequency. - -[Illustration: FIG. 2,601.--General Electric =moving coil -oscillograph= _with case removed_, =showing= _interior construction -and arrangement of parts_. The oscillograph is furnished complete -with a three element electro-magnet galvanometer, optical system, -shutter and shutter operating mechanism, film driving motor and cone -pulleys, photographic and tracing attachments, 6 film holders, and -the following repair parts, for vibrators: 6 extra suspension strips; -6 vibrator mirrors; 1 box gold leaf fuses; 1 bottle mirror cement; 1 -bottle damping liquid.] - -[Illustration: FIG. 2,602.--Oscillogram showing the direct current -pressure of a 25 cycle rotary converter (below), and (above) the -pressure wave taken between one collector ring and one commutator -brush. The 12 ripples per cycles in the direct current voltage are -due to a 13th harmonic in the alternating current supply.] - -=Glow Light Oscillograph.=--This device consists of two aluminum rods -in a partially evacuated tube, their ends being about two millimeters -apart. When an alternating current of any frequency passes between -them a sheath of violet light forms on one of the electrodes, passing -over to the other when the current reverses during each cycle. The -phenomenon may be observed or photographed by means of a revolving -mirror. - -[Illustration: FIG. 2,603.--Curves by Morris, _illustrating the_ -=dangerous rush of current which may occur when switching on a -transformer=. The circuit was broken at F and made again at G. The -current was so great as to carry the spot of light right off the -photographic plate due to the fact that a residual field was left in -the core after switching off, and on closing the switch again the -direction of the current was such as to tend to build up the full -flux in the same direction as this residual flux. =The dotted lines= -have been drawn in _to show how the actual waves were distorted from -the normal_.] - -=Moving Iron Oscillograph.=--This type is due to Blondel, to whom -belongs the credit of working out and describing in considerable -detail the principles underlying the construction of oscillographs. - - The moving iron type of oscillograph consists of a very thin - vane of iron suspended in a powerful magnetic field, thus forming - a polarized magnet. Near this strip are placed two small coils - which carry the current whose wave form is to be measured. - - The moving iron vane has a very short period of vibration and - can therefore follow every variation in the current. - -[Illustration: FIG. 2,604.--Siemens-Blondel =moving coil type= -oscillograph. The coil is in the shape of a loop of thin wire, which -is suspended in the field of an electro-magnet excited by continuous -current. The current to be investigated is sent through this loop, -which in consequence of the interaction of current and magnetic -field, begins to vibrate. The oscillations are rendered visible by -directing a beam of light from a continuous current arc lamp onto a -small mirror fixed to the loop. The light reflected by the mirror is -in the form of a light strip, but by suitable means this is drawn -out in respect of time, so that a curve truly representing the -current is obtained. The loop of fine wire is stretched between two -supports and is kept in tension by a spring. As the spring tension -is considerable, the directive force of the vibrating system is -large, and its natural periodicity very high. The mirror is fixed in -the center of the loop, and has an area of 1 square mm. In order to -protect the loops from mechanical injury they are built into special -frames. The mirrors are of various sizes, the loop for demonstration -purposes (projection device) being provided with the largest -mirror and the most sensitive loop with a mirror of the smallest -dimensions.] - - Attached to the vane is a small mirror which reflects a beam of - light upon some type of receiving device. - - The Siemens-Blondel oscillograph shown in fig. 2,604, is of - the _moving coil_ type, being a development of the moving iron - principle. - -=Moving Coil Oscillograph.=--The operation of this form of -oscillograph is based _on the behaviour of a movable coil in a -magnetic field_. - -[Illustration: FIGS. 2,605 and 2,606.--=Oscillograms= reproduced from -a paper by M. B. Field on "A Study of the =Phenomena of Resonance= -by the Aid of Oscillograms" (_Journal_ of _E. E._, Vol. XXXII). =The -effect of resonance= on the wave forms of alternators has been the -subject of much investigation and discussion; it is a matter of vital -importance to the engineer in charge of a large alternating current -power distribution system. Fig. 2,605 shows the pressure curve of an -alternator running on a length of unloaded cable, the 11th harmonic -being very prominent. Fig. 2,606 shows the striking alteration -produced by reducing the length of cable in the circuit and thus -causing resonance with the 13th harmonic.] - - It consists essentially of a modified moving coil galvanometer - combined with a rotating or vibrating mirror, a moving - photographic film, or a falling photographic plate. The - galvanometer portion of the outfit is usually referred to as the - oscillograph as illustrated in figs. 2,608 to 2,612, representing - diagrammatically the moving system. - - In the narrow gap between the poles S, S of a powerful magnet - are stretched two parallel conductors formed by bending a thin - strip of phosphor bronze back on itself over an ivory pulley P. - A spiral spring attached to this pulley serves to keep a uniform - tension on the strips, and a guide piece L limits the length of - the vibrating portion to the part actually in the magnetic field. - - A small mirror M bridges across the two strips as shown. The - effect of passing a current through such a "vibrator" is to cause - one of the strips to advance while the other recedes, and the - mirror is thus turned about a vertical axis. - -[Illustration: FIG. 2,607.--General view of electro-magnet -form of Duddell moving coil oscillograph, showing oil bath and -electro-magnet. This instrument is specially designed to have a very -high natural period of vibration (about 1/10,000 of a second) so as -to be suitable for accurate research work. It is quite accurate for -frequencies up to 300 per second. In the figure, A is the brass oil -bath in which two vibrators are fixed; B, core of electro-magnet -which is excited by two coils, one of which, C, is seen. The ends of -these two coils are brought out to four terminals at D, so that the -coils may be connected in series for 200 volt, or in parallel for 100 -volt circuits. The bolts, E,E, hold the oil bath in position between -the poles of the magnet. F,F,F (one not seen), are levelling screws; -G,G, terminals of one vibrator; H, fuse; K, thermometer with bulb in -center of oil bath.] - -[Illustration: FIGS. 2,608 to 2,612.--=Vibrator= of Duddell moving -coil oscillograph and =section through oil bath= of electro-magnet -oscillograph. =The vibrator consists of= a brass frame W, which -supports two soft iron pole pieces P,P. Between these, a long narrow -groove is divided into two parts by a thin soft iron partition, -which runs up the center. The current being led in by the brass wire -U, passes from an insulated brass plate to the strip, which is led -over an ivory guide block, down one of the narrow grooves and over -another guide block, the loops round the ivory pulley O, which puts -tension on the strip by the spring N, back to the guide block again, -up the other narrow groove, and out by way of the insulated brass -plate and lead U. Halfway up the grooves the center iron partition R -is partially cut away to permit of a small mirror M, bridging across -from one strip to the other, being stuck to the strips by a dot of -shellac at each corner. The figure illustrates one type of vibrator -in which P is removable from W for ease in repairing. In type 1, -these pole pieces P,P are not removable. =The vibrators= are placed -side by side in the gap between the poles S,S of the electro-magnet, -see fig. 2,610. Each vibrator is pivoted about vertical centers, the -bottom center fitting in the base of the oil bath, and the one at -the top being formed by a screw in the cock piece Y. It can thus be -easily turned in azimuth, its position being fixed by the adjusting -screw L, a spiral spring serving to keep the vibrator always in -contact with this screw. Since each cock piece can be independently -moved forward or backward, each vibrator can be tipped slightly in -either of these directions so that complete control over the mirrors -is obtained and reflected spots of light may be made to coincide with -that reflected from the fixed zero mirror, which latter is fixed to a -brass tongue in between the two vibrators. =A plano-convex lens= of -50 cm. focal length is fixed on the oil bath in front of the vibrator -mirrors to converge the reflected beams of light. It will be noticed -that this lens is slightly inclined so that no trouble will be given -by reflections from its own surface. The normal distance from the -vibrator mirrors to the scale of photographic plate is 50 cm., and -at this distance, a convenient working deflection on each side of -the zero line is 3 to 4 cm. This is obtained with a R.M.S. current -through the strips of from .05 to .1 of an ampere according to wave -form, etc. =The maximum deflection= on each side of the zero line -should not exceed 5 cm. while the maximum R.M.S. current through the -strips should in no case exceed .1 ampere.] - - Each strip of the loop passes through a separate gap (not shown - in the figure). The whole of the "vibrator," as this part of the - instrument is called, is immersed in an oil bath, the object - of the oil being to damp the movement of the strips, and make - the instrument dead beat. It also has the additional advantage - of increasing by refraction the movement of the spot of light - reflected from the vibrating mirrors. - - The beam of light reflected from the mirror M is received on - a screen or photographic plate, the instantaneous value of the - current being proportional to the linear displacement of the spot - of light so formed. - - With alternating currents, the spot of light oscillates to and - fro as the current varies and would thus trace a straight line. - - To obtain an image of the wave form, it is necessary to traverse - the photographic plate or film in a direction at right angles to - the direction of the movement of the spot of light. - -[Illustration: FIG. 2,613.--Duddell =moving coil oscillograph= _with -projection and tracing desk outfit_. The outfit is designed for -teaching and lecture purposes. =In operation=, _after the beam of -light from the arc lamp has been reflected from the oscillograph -mirrors, it falls on a vibrating mirror which gives it a deflection -proportional to time in a direction at right angles to the deflection -it already has and which is proportional to the current passing -through the oscillograph_. It is therefore only necessary to place a -screen in the path of the reflected beam of light to obtain a trace -of the wave form. Since the vibrating mirror is vibrated by means -of a cam on the shaft of a synchronous motor, which motor is driven -from, or synchronously with, the source of supply whose wave form is -being investigated, the wave form is repeated time after time in the -same place on the screen, and owing to the "persistence" of vision, -the whole wave appears stationary on the screen. The synchronous -motor with its vibrating mirror, mentioned above, is located -underneath the "tracing desk." When used in this position a wave a -few centimeters in amplitude is seen through a sheet of tracing paper -which is bent round a curved sheet of glass. A permanent record of -the wave form can thus easily be traced on the paper. A _dark box_ -which is designed to hold a sheet of sensitized paper in place of -the tracing paper, can be fitted in place of the tracing desk. Thus -an actual photographic record of the wave form is obtained. If the -synchronous motor be transferred from its position underneath the -tracing desk to the space reserved for it close to the oscillograph, -the beam of light is then received on a large mirror which is placed -at an angle of about 45 degrees to the horizontal and so projects the -wave form onto a large vertical screen which should be fixed about -two and a half meters distant. Under these conditions a wave form of -amplitude 50 cm. each side the zero line may be obtained which is -therefore visible to a large audience.] - -=Ques. How are the oscillograms obtained in the Duddell moving coil -oscillograph?= - -Ans. In all cases the oscillograms are obtained by a spot of light -tracing out the curve connecting current or voltage with time. The -source of light is an arc lamp, the light from which passes first -through a lens, and then, excepting when projecting on a screen, -through a rectangular slit about 10 mm. long by 1 mm. wide. The -position of the lamp from the lens is adjusted till an image of the -arc is obtained covering the three (two moving, one fixed) small -oscillograph mirrors. The light is reflected back from these mirrors -and, being condensed by a lens which is immediately in front of them, -it converges till an image of the slit is formed on the surface where -the record is desired. All that is necessary now to obtain a bright -spot of light instead of this line image is to introduce in the path -of the beam of light a cylindrical lens of short focal length. - -[Illustration: FIGS. 2,614 and 2,615.--Sectional view of =permanent -magnet form= of Duddell =moving coil oscillograph.= This instrument -has a lower natural period of vibration (1/3000 second) than the -type shown in fig. 2,612, and therefore is not capable of accurately -following wave forms of such high frequency, but it is sufficiently -quick acting to follow wave forms of all ordinary frequencies with -perfect accuracy. It is easier to repair, and more portable owing to -the fact that the magnetic field is produced by a permanent magnet -instead of an electro-magnet. This also renders the instrument -suitable for use on high tension circuits without earth connection, -as, owing to the fact that no direct current excitation is required, -the instrument is more easily insulated than other types.] - -=Ques. What is the function of the mirrors on the vibrating vane?= - -[Illustration: FIG. 2,616.--Diagram of connections of Duddell -oscillograph =to high pressure circuit.= The modification necessary -for high pressure circuit only applies to the vibrator which gives -the pressure wave and consists in adding two more resistances, R_{4} -and R_{5}. Referring to fig. 2,617, it will be seen that in case fuse -f_{2} blows, or the vibrator be accidentally broken, the full supply -voltage is immediately thrown on the instrument itself. This is not -permissible in high voltage work and therefore the resistance R_{5} -is introduced as a permanent shunt to the oscillograph vibrator. -The resistance R_{4} is an exact duplicate of R_{2} being a 21 ohm -plug resistance box for adjusting the sensitivity of the vibrator -to an even figure. =In practice= R_{5} is usually a part of R_{1}, -and in most of the high voltage resistances, two taps are brought -out near one end to serve as R_{5}. One of these taps is usually 50 -ohms distant from the end terminal and the other only 5 ohms from -the end. =The use of these taps is as follows:= The large resistance -consisting of R_{1} + R_{5} is so chosen with respect to the voltage -of the circuit under investigation that the current through R_{1} is -about .1 ampere. _It should never be more than this continuously._ -Then R_{4} is connected to the 50 ohm tap, and since the resistance -of the oscillograph vibrator circuit is variable from about 5 to -26 ohms by means of R_{4}, the current can be controlled through -the oscillograph from about .066 to .091 of an ampere, enabling an -open wave form to a convenient scale to be obtained. =If it now -be desired to record large rises of pressure,= such as may occur -in cases of resonance, _the height of the wave must be reduced in -order to keep these rises on the plate_. This is accomplished by -disconnecting R_{4} from the 50 ohm tap and connecting it to the 5 -ohm tap, when the current through the vibrator will be from .05 to -.016 of an ampere according to whether the resistance R_{4} is in -or out of circuit. When, instead of using the _falling plate_, the -_cinematograph_ camera is being used, it becomes necessary always -to work on the 5 ohm tap since the width of the film is much less -than that of the plate, and the current must therefore be less. -=In experiments where sudden rises of voltage are expected= _it is -often advisable to keep_ R_{1} _as great as possible._ That end of -the resistance R_{1} referred to as R_{5} in the diagram should be -securely connected to the supply main and no switch or fuse used. A -switch may, if desired, be used in series with R_{1}, provided it be -inserted at the point where R_{1} joins the supply main remote from -R_{5}. It will be seen that fuses f_{1} and f_{2} are shown. Provided -that the connections are always made in accordance with the diagram, -and the vibrators are always shunted by R_{5} or R_{3} respectively, -there is not much objection to the use of these fuses, but on general -principles it is wise to avoid fuses in high tension work and -accordingly with each permanent magnet oscillograph, dummy fuses are -supplied, which can be inserted in place of the ordinary fuses when -desired. _The remark previously made about keeping both vibrators -and the frame of the instrument at approximately the same pressure -applies with additional emphasis in high pressure work._] - -Ans. They simply control the direction of a beam of light in a -horizontal plane in such a manner that its deflection from a zero -position depends on the current passing through the instrument, and -it is therefore evident that the oscillograph is not complete without -means of producing a time scale. - -[Illustration: FIG. 2,617.--Diagram of connections of Duddell -oscillograph =to low pressure circuit=, R_{1} is a high non-inductive -resistance connected across the mains in series with one of -the vibrators. S_{2} is a switch, and f_{2}, the fuse (on the -oscillograph in this circuit). The resistance of R_{1} in ohms -should be rather more than ten times the voltage of the circuit, -so that a current of a little less than .1 of an ampere will pass -through it. The vibrator will then give the curve of the circuit -on an open scale. (For the projection oscillograph, the resistance -R_{1} should be only twice the supply voltage, since .5 of an ampere -is required to give full scale deflection on a large screen.) =To -obtain the current wave form=, _the shunt_ R_{3} _is connected in -series with the circuit under investigation and the second vibrator -is connected across this shunt_. Here also f_{1} is a fuse, S_{1} a -switch, and R_{2} an adjustable resistance box. The switch S_{1} is -however unnecessary if the plug resistance box supplied for R_{2} -be used, since an infinity plug is included in this box. The shunt -R_{3} should have a drop of about 1 volt across it in order to give -a suitable working current through the vibrator. The resistance -R_{2} is not absolutely essential, but it is a great convenience in -adjusting the current through the vibrator. It is a plug resistance -box, the smallest coil being .04 of an ohm and the total 21 ohms. -Being designed to carry .5 ampere continuously it can be used -with any other type of Duddell oscillograph, and by its use the -sensitiveness of the vibrator can be adjusted so that a round number -of amperes in the shunt gives 1 mm. deflection. This adjustment is -best made with direct current. =It should be noted= in connecting -the oscillograph in circuit, that _the two vibrators should be so -connected to the circuit that it is impossible that a higher pressure -difference than_ 50 _volts should exist between one vibrator and the -other, or between either vibrator and the frame_. To ensure attention -to this important point, a brass strap is provided which connects -the two vibrators together and to the frame of the instrument. This -does not mean that this point must necessarily be earthed since the -frame of the instrument is insulated from the earth. It is advisable, -however, to earth it when possible.] - -[Illustration: FIGS. 2,618 and 2,619.--=Two curves= _obtained with -the_ =falling plate camera= and illustrating _the discharge of a -condenser through an inductive circuit_. =When taking curve A= the -resistance in the circuit was very small compared to the inductance, -while =before taking curve B= an additional non-inductive resistance -was inserted in the circuit so that the oscillations were damped out -much more rapidly although the periodic time remained approximately -constant.] - -=Ques. How is the time scale produced?= - -Ans. Either the surface on which the beam of light falls may be -caused to move in a vertical plane with a certain velocity, so that -the intersection of the beam and the plane surface traces out a curve -connecting current with time (a curve which becomes a permanent -record if a sensitized surface be used); or, the surface may remain -stationary and in the path of the horizontally vibrating beam may be -introduced a mirror which rotates or vibrates about a horizontal -axis, thus superposing a vertical motion proportional to time on the -horizontal vibration which is proportional to current, and causing -the beam of light to trace out a curve connecting current and time on -the stationary surface. - -=Ques. What kind of recording apparatus is used with the Duddell -oscillograph?= - -Ans. A falling plate camera, or a cinematograph film camera. - -[Illustration: FIG. 2,620.--Synchronous motor with vibrating mirror -as used with Duddell moving coil oscillograph. =Since the motor must -run synchronously= with the wave form it is required to investigate, -_it should be supplied with current from the same source_. The motor -can be used over a wide range of frequencies (from 20 to 120). When -working at frequencies below 40, it is advisable to increase the -moment of inertia of the armature, and for this purpose a suitable -brass disc is used. =The armature carries a sector=, _which cuts off -the light from the arc lamp during a fraction of each revolution, and -a cam which rocks the vibrating mirror_. =It makes one revolution -during two complete periods=, and the cam and sector are so arranged -that during 1½ periods, the mirror is turning with uniform angular -velocity, while during the remaining half period, the mirror is -brought back quickly to its angular position, the light being cut off -by the sector during this half period.] - -=Ques. Explain the operation of the falling plate camera.= - -Ans. In this arrangement a photographic plate is allowed to fall -freely by the force of gravity down a dark slide. At a certain point -in its fall it passes a horizontal slit through which the beams of -light from the oscillograph pass, tracing out the curves on the plate -as it falls. - -[Illustration: FIGS. 2,621 to 2,623.--Interior of cinematograph -camera as used on Duddell moving coil oscillograph =for obtaining -long records=. The loose side of case is shown removed and one of -the reels which carry the film lying in front. =The spool of film= -which is placed on the loose reel A, passes over the guide pulley -B, then vertically downward between the brass gate D (shown open in -the figure), and the brass plate C. =The exposure aperture= is in -the plate C and can be opened or closed by a shutter controlled by -the lever M. The groove in the plate C, and the springs which press -the gate D flat on the plate C, prevent the film having any but a -vertical motion as it passes the exposure slit. E is the sprocket -driving pulley which engages with the perforations on the film and -unwinds it from the reel A to reel H. Outside the case on the far -side of it is secured to the axle G a three speed cone pulley. This -is driven by a motor of about 1/7 horse power, which also drives, -through the gears shown, the sprocket pulley E. Close to the grooved -cone pulley is a lever carrying a jockey pulley L, and a brake, which -latter is normally held onto the cone pulley by a spring and so -causes the loose belt to slip. By pressing a lever which is attached -to the falling plate camera case, the brake can be suddenly released -and at the same time the jockey pulley caused to tighten the belt -onto the grooved cone pulley, so that the starting and stopping of -the film is controlled independently of the driving motor, and being -quickly accomplished avoids waste of film. =Both reels= are alike and -each is made in two pieces. =The upper reel= is loose on its axle and -its motion is retarded slightly by a friction brake. =The lower reel= -is also loose on its axle, but it is driven by means of a friction -clutch, the clutch always rotating faster than the reel so that the -used film delivered by the sprocket pulley E is wound up as fast as -delivered. K is the front face of one reel, the boss on it pushes -into the tube on the other half H, which serves not only to unite the -two halves, but also to secure the end of the film which is doubled -through J.] - - The mean speed of the plate at the moment of exposure is about - 13 feet per second. This speed is very suitable for use with - frequencies of from 40 to 60 periods per second. A cloth bag is - used to introduce the plate to the slide. - - A catch holds the plate until it is desired to let it fall. - Inside the case, is a small motor, 100 or 200 volts direct - current, driving four mirrors which are fixed about a common axis - with their planes parallel to it. - -[Illustration: FIG. 2,624.--Portion of oscillograph record taken with -cinematograph film camera, =showing the rush of current= and =sudden -rise of voltage= _at the moment of switching on a high pressure -feeder_.] - - By looking through a small slot in the end of the camera into - these rotating mirrors, the observer sees the wave form which the - oscillograph is tracing out and is thus able to make sure that - he is obtaining the particular wave form or other curve desired - before exposing the plate. - -[Illustration: FIG. 2,625.--Portion of oscillograph record taken with -a cinematograph film camera =showing the effect of switching off -a high pressure feeder= and illustrating the violent fluctuations -produced by sparking at the switch contacts.] - - The plate falls into a second red cloth bag which is placed on - the bottom of the slide. The plates used are "stereoscopic size", - 6¾" × 3¼" (17.1 × 8.3 cm.). - -=Ques. For what use is the cinematograph camera adapted?= - -Ans. For long records. - - For instance, in investigations, such as observation on the - paralleling of alternators, the running up to speed of motors, - and the surges which may occur in switching on and off cable, - etc. The cinematograph camera fits on to the falling plate case - and by means of which a roll of cinematograph film can be driven - at a uniform speed past the exposure aperture, enabling records - up to 50 metres in length to be obtained. An interior view of the - cinematograph camera is shown in fig. 2,621. - -[Illustration: FIG. 2,626.--Curves reproduced from an article by J. -T. Morris in the _Electrician_. "On recording transitory phenomena by -the oscillograph."] - -[Illustration: FIG. 2,627.--First rush of current from an alternator -when short circuited, showing unsymmetrical initial wave of current, -becoming symmetrical after a few cycles. 25 cycles.] - -[Illustration: FIG. 2,628.--Pressure wave obtained from narrow -exploring coil on alternator armature, indicating distribution of -field flux. The terminal voltage of the alternator is very nearly a -sine wave, 60 cycles; about 17 volts.] - -SOME OSCILLOGRAPH RECORDS - -[Illustration: FIG. 2,629.--The waves of voltage and current of an -alternating arc. A, voltage wave; B, current wave showing low power -factor of the arc without apparent phase displacement. 60 cycles.] - -[Illustration: FIG. 2,630.--Rupturing 650 volt circuit. A, current -wave; B, 25 cycle wave to mark time scale.] - -[Illustration: FIG. 2,631.--First rush of current from alternator -when short circuited, showing unsymmetrical current wave, also wave -of field current caused by short circuit current in armature. Upper -curve, armature current; lower curve, field current.] - -[Illustration: FIG. 2,632.--Mazda (tungsten) lamp, showing rapid -decrease to normal current as filament heats up. 25 cycles.] - -[Illustration: FIG. 2,633.--Current wave in telephone line -corresponding to sustained vowel sound "_i_," as in machine; voice -pitched at A 110.] - -[Illustration: FIG. 2,634.--Carbon lamp, showing rapid increase to -normal current as filament heats up. 25 cycles.] - -[Illustration: FIG. 2,635.--Short circuit current on direct current -end of rotary converter, 21,500 amperes maximum. Upper curve, direct -current voltage; lower curve, direct current amperage. Duration of -short circuit about .1 second.] - - - - -CHAPTER LXIV - -SWITCHBOARDS - - -=General Principles of Switchboard Connections.=--The interconnection -of generators, transformers, lines, bus bars, and switches with -their relays, in modern switchboard practice is shown by the -diagrams, figs. 2,636 to 2,645. The figures being lettered A to J -for simplicity, the generators are indicated by black discs, and the -switches by open circles, while each heavy line represents a set of -bus bars consisting of two or more bus bars according to the system -of distribution. It will be understood, also, in this connection, -that the number of pole of the switches and the type of switch will -depend upon the particular system of distribution employed. - - Diagram A, shows the simplest system, or one in which a single - generator feeds directly into the line. There are no transformers - or bus bars and only one switch is sufficient. - - In B, a single generator supplies two or more feeders through a - single set of bus bars, requiring a switch for each feeder, and a - single generator switch. - - In C, two generators are employed and required and the addition - of a bus section switch. - - D, represents a number of generators supplying two independent - circuits. The additional set of bus bars employed for this purpose - necessitates an additional bus section switch, and also additional - selector switches for both feeders and generators. - - E, shows a standard system of connection for a city street - railway system having a large number of feeders. - -[Illustration: FIGS. 2,645 and 2,646.--Diagrams illustrating general -principles of switchboard connections.] - - This arrangement allows any group of feeders to be supplied from - any group of generators. - -[Illustration: FIG. 2,646.--Fort Wayne switchboard panel for one -alternator and one transfer circuit. Diagram giving dimensions, -arrangement of instruments of board, and method of wiring. The -different forms of standard alternating current switchboard panels -for single phase circuits made by the Fort Wayne Electric Works are -designed to fulfill all the usual requirements of switchboards for -this class of work. The line includes panels equipped for a single -generator; for one generator and two circuits; one generator and one -transfer circuit; one generator, an incandescent and an arc lighting -circuit; and also feeder panels of different kinds.] - - It also permits the addition of a generator switch for each - generator. - - F, represents the simplest system with transformers. - - It requires a single generator transformer bank, switch and - line. The arrangement as show at F is used where a number of - plants supply the same system. - - G, represents a system having more than one line. - - In this case a bus bar and transformer switch is used on the - high tension side. - - H, shows a number of generators connected to a set of low - tension bus bars through generator switches, and employing a low - tension transformer switch. - - I, shows the connections of a system having a large number of - feeders supplied by several small generators. In this case, the - plant is divided into two parts, each of which may be operated - independently. - -[Illustration: FIG. 2,647.--General Electric =small plant alternating -current switchboard=, _designed for use in small central stations and -isolated plants_. They are for use with one set of bus bars, to which -all generators and feeders are connected by means of single throw -lever switches or circuit breakers, suitable provision being made for -the parallel operation of the generators.] - - J, represents the arrangement usually employed in modern plants - where the generator capacity is large enough to permit of a - generator transformer unit combination with two outgoing lines. By - operating in parallel on the high tension side only, any generator - can be run with any transformer. The whole plant can be run in - parallel, or the two parts can be run separately. - -[Illustration: FIG. 2,648.--Crouse-Hinds =voltmeter and ground -detector radial switch=, arranged for mounting on the switchboard. -The switch proper is placed on the rear of the board with hand wheel, -dial, and indicator only on the front side. The current carrying -parts are of hard brass, with contact surfaces machined after -assembling. The contact parts are of the plunger spring type, and the -cross bar has fuse connections. Ground detector circuits are marked -G+ and G- for two wire system, and G+, G-, GN+ and GN- for three wire -system. When the voltmeter switch is to be used as a ground detector, -two circuits are required for a two wire system, and four circuits -for a three wire system, that is, a six circuit voltmeter and ground -detector switch for use on a two wire system has two circuits for -ground detector and four circuits for voltmeter readings. A six -circuit voltmeter and ground detector switch, for use on a three wire -system, has four circuits for ground detector and two circuits for -voltmeter readings.] - -=Switchboard Panels.=--The term "panel" means the slab of marble or -slate upon which is mounted the switches, and the indicating and -controlling devices. There are usually several panels comprising -switchboards of moderate or large size, these panels being classified -according to the division of the system that they control, as for -instance: - - 1. Generator panel; - 2. Feeder panel; - 3. Regulator panel, etc. - - In construction, the marble or slate should be free from - metallic veins, and for pressures above, say, 600 volts, live - connections, terminals, etc., should preferably be insulated from - the panels by ebonite, mica, or removed from them altogether, as - is generally the case with the alternating gear where the switches - are of the oil type. - -[Illustration: FIGS. 2,649 and 2,650,--Wiring diagrams of -Crouse-Hinds voltmeter and ground detector switches. Fig. 2,649 -voltmeter switch; fig. 2,650 voltmeter and ground detector switch. A -view of the switch is shown in fig. 2,648; it is designed for use on -two or three wire systems up to 300 volts.] - - The bus bars and connections should be supported by the - framework at the back of the board, or in separate cells, and the - instruments should be operated at low pressure through instrument - transformers. - - The panels are generally held in position by bolting them to an - angle iron, or a strip iron framework behind them. - -=Generator Panel.=--This section of a switchboard carries the -instruments and apparatus for measuring and electrically controlling -the generators. On a well designed switchboard each generator has, as -a rule, its own panel. - -[Illustration: FIGS. 2,651 to 2,653.--Diagrams of connections for -generator panels. =Key to symbols=: =A=, ammeter; =A.S.=, ammeter -switch; =C.T.=, current transformer; =F.=, fuse; =F.A.=, direct -current field ammeter; =F.S.=, field switch; =G.C.S.=, governor -control switch; =L.S.=, limit switch (included with governor motor); -=O.S.=, oil switch; =P.I.W.=, polyphase indicating wattmeter; -=P.W.M.=, polyphase watthour meter; =P.R.=, pressure receptacle; -=P.P.=, pressure plug; =Rheo.=, rheostat; =S.=, shunt; =S.R.=, -synchronizing receptacle; =S.P.=, synchronizing plugs; =T.B.=, -terminal board for instrument leads; =V=, alternating current -voltmeter.] - -[Illustration: FIGS. 2,654 and 2,655.--Diagrams illustrating =a -simple method of determining bus capacity= as suggested by the -General Electric Co. Fig. 2,654 relates to any panel; the method is -as follows: =1.= Make a rough plan of the _entire board_, regardless -of the number of panels to be ordered. _The order of panels_ -shown is recommended, it being most economical of copper and best -adapted to future extensions. =2.= To avoid confusion keep on one -side of board everything pertaining to exciter buses, and on other -side everything pertaining to A. C. buses. =3.= With single lines -represent the exciter and A. C. buses across such panels as they -actually extend and by means of arrows indicate that portion of each -bus which is connected to feeders and that portion which is connected -to generators. _Remember that "Generator" and "Feeder" arrows must -always point toward each other_, otherwise the rules given below do -not hold. Note also that the field circuits of alternator panels are -treated as D. C. feeders for the exciter bus. =4.= On each panel -mark its ampere rating, that is, the maximum current it supplies -to or takes from the bus. For A. C. alternator panels the D. C. -rating is the excitation of the machines. =5.= Apply the following -rules _consecutively_, and note their application in fig. 2,654. -(For the sake of clearness ampere ratings are shown in light face -type and bus capacities in large type.) =A.= _Always begin with the -tail of the arrow and treat "generator" and "feeder" sections of -the bus separately._ =B.= _Bus capacity for first panel = ampere -rating of panel._ =C.= _Bus capacity for each succeeding panel = -ampere rating of panel plus bus capacity for preceding panel._ (See -sums marked above the buses in fig. 2,654.) =D.= _For a panel not -connected to a bus extending across it, use the smaller value of -the bus capacities already obtained for the two adjoining panels._ -(See exciter bus for panel C.) =E.= _The bus capacity for any feeder -panel need not exceed the maximum for the generator panels_ (see A. -C. bus for panel G) _and vice versa_ (see exciter bus for panel B). -Hence the corrections made in values obtained by applying rules =B= -and =C=. The arrangement of panels shown in fig. 2,654 is the one -which is mostly used. The above method may, however, be applied to -other arrangements, one of which is shown in fig. 2,655. Here the -generators must feed both ways to the feeders at either end of the -board so that in determining A. C. bus capacities it is necessary to -first consider the generators with the feeders at one end, and then -with the feeders at the other end as shown by the dotted A. C. buses. -The required bus capacities are then obtained by taking the maximum -values for the two cases.] - -[Illustration: FIG. 2,656.--End view showing =general arrangement of -switchboards= for 240, 480, and 600 volt alternating current. The cut -shows a single throw oil switch mounted on the panel.] - -In the case of a dynamo, a good representative panel would have -mounted upon it a reverse current circuit breaker, an ammeter, a -double pole main switch (or perhaps a single pole switch, since -the circuit breaker could also be used as a switch) a double pole -socket into which a plug could be inserted to make connection with -a voltmeter mounted on a swinging bracket at the end of the board; -a rheostat handle, the spindle of which operates the shunt rheostat -of the machine, the rheostat being placed either directly behind -the spindle, if of small size, or lower down with chain drive from -the hand wheel spindle, if of larger size, a field discharge switch -and resistance, a lamp near the top of the panel for illuminating -purposes, a fuse for the voltmeter socket, and, if desired, a -watthour meter. If the dynamo be compound wound, the equalizing -switch will generally be mounted on the frame of the machine, and in -some cases the field rheostat will be operated from a pillar mounted -in front of the switchboard gallery. If the generator be for traction -purposes, the circuit breaker is more often of the maximum current -type, and a lightning arrester is often added, without a choke coil, -the latter as well as further lightning arresters being mounted on -the feeder panels. - -[Illustration: FIGS. 2,657 and 2,658.--Two views of a =feeder panel=, -showing general arrangement of the devices assembled thereon. A, -circuit breaker; B, ammeter; C, voltmeter; D, switches.] - -In the case of a high pressure alternating current plant of -considerable size, the bus bars oil switches, and the current and -pressure transformers are generally mounted either in stoneware -cells, or built on a framework in a space guarded by expanded metal -walls, and no high pressure apparatus of any sort is brought on to -the panels themselves. - -[Illustration: FIGS. 2,659 to 2,666.--Diagram of connections for -three phase feeder panels. =Key to symbols=: A, ammeter; A.S., -three way ammeter switch; B.A.S., bell alarm switch; C.T., current -transformer; F, fuse; O.S., oil switch; P.I.W., polyphase indicating -wattmeter; P.W.M., polyphase watthour meter; T.B., terminal board; -T.C., trip coils for oil switch.] - -=Feeder Panel.=--The indicating and control apparatus for a feeder -circuit is assembled on a panel called the feeder panel. - -The most common equipment in the case of a direct current feeder -panel comprises an ammeter, a double pole switch, and double pole -fuses or instead of the fuses, a circuit breaker on one or both -poles; in the case of a traction feeder a choke coil and a lightning -arrester are often added. - -[Illustration: FIGS. 2,667 and 2,668.--Diagrams of connections for -two phase and three phase installations: A and A1, ammeter; C.C., -constant current transformer; C.T., current transformer; D.R., -discharge resistance; F, fuse; F.S., field switch; L.A., lightning -arrester; O.S., oil switch; P.P., pressure plug; P.R., pressure -receptacle; P.T., pressure transformer; S and S1, plug switches; -T.C., oil switch trip coil; V, voltmeter.] - -The equipment of a typical high pressure three phase feeder panel is -an ammeter (sometimes three ammeters, one in each phase) operated -by a current transformer, and oil break switch with two overload -release coils, or three if the neutral of the circuit be earthed, the -releases being operated by current transformers. - -[Illustration: FIG. 2,669.--Crouse-Hinds radial ammeter switch, -arranged for mounting directly on the switchboard. It is designed -for use with external shunt ammeters of any make or capacity, and in -connection with the required number of shunts, makes possible the -taking of current readings of a corresponding number of circuits by -means of one ammeter. The wiring diagram is shown in fig. 2,670.] - -The switch when on a large system is often in a cell some distance -behind the panel, and is then controlled by a system of levers, or by -a small motor which is started and stopped by a throw over switch on -the panel, in which case there is generally a lamp or lamps on the -panel to show whether the switch is open or closed. - - Air brake switches or links are placed between the bus bars - and the oil switch to allow of the latter being isolated for - inspection purposes, and as a general rule no apparatus carrying - high pressure current is allowed on the front of the panel. With - both direct and alternating current feeders, a watthour meter is - often added to show the total consumption of the circuit. - -[Illustration: FIG. 2,670.--Wiring diagram for Crouse-Hinds radial -ammeter switch as illustrated in fig. 2,669. The switch proper is on -the rear of the switchboard, and the hand wheel dial and indicator on -the front.] - - A typical three phase generator panel is provided with three - ammeters, one in each phase, operated from three current - transformers, one to each ammeter, a volt meter, a power factor - indicator, and an indicating watthour meter, all operated from - one or more pressure transformers, and the necessary current - transformers, the operating handle of the oil switch, which is - connected to the switch itself by means of rods, two maximum - releases operated by current transformers, or a reverse relay - for automatically tripping the switch, lamps for indicating when - the switch is tripped, a socket for taking the plug which makes - connection between the secondary of a pressure transformer and - the synchronizer on the synchronizing panel, and a lamp for - illuminating purposes, while on the base of the panel or on a - pillar at the front of the gallery is mounted the gear for the - field circuit. This consists of a double pole field switch and a - discharge resistance, an ammeter, a handle for the rheostat in - the generator field, and (if each alternator have its own direct - coupled exciter) possibly also a small rheostat for the exciter - field. - - NOTE.--In some cases where the capacity of - the plant is not very great, the oil switch - is mounted on the back of the panel, and the - bus bars, current transformers, &c., on the - framework, also just at the back of the panel, - but under no circumstances, in good modern - practice, is high pressure apparatus permitted - on the front of the board. Where the capacity of - the plant is very large, the oil switches are - operated electrically by means of small motors, - and in this case the small switch gear for - starting and stopping this motor is mounted on - the generator panel, also the lamp or lamps to - indicate when the switch is open, and when closed. - - - - -CHAPTER LXV - -=ALTERNATING CURRENT WIRING= - - -In the case of alternating current, because of its peculiar -behaviour, there are several effects which must be considered in -making wiring calculations, which do not enter into the problem with -direct current. - -Accordingly, in determining the size of wires, allowance must be made -for - - 1. Self-induction; - 2. Mutual-induction; - 3. Power factor; - 4. Skin effect; - 5. Corona effect; - 6. Frequency; - 7. Resistance. - - Most of these items have already been explained at such length, - that only a brief summary of facts need be added, to point out - their connection and importance with alternating current wiring. - -=Induction.=--The effect of induction, whether self-induction -or mutual induction, is to set up a back pressure of _spurious -resistance_, which must be considered, as it sometimes materially -affects the calculation of circuits even in interior wiring. - - _Self-induction is the effect produced by the action of the - electric current upon itself during variations in strength._ - -=Ques. What conditions besides variations of current strength governs -the amount of self-induction in a circuit?= - -Ans. The shape of the circuit, and the character of the surrounding -medium. - - If the circuit be straight, there will be little self-induction, - but if coiled, the effect will become pronounced. If the - surrounding medium be air, the self-induction is small, but if it - be iron, the self-induction is considerable. - -[Illustration: FIGS. 2,671 to 2,676.--=The effect of self-induction.= -In a non-inductive circuit, as in fig. 2,672, the whole of the -virtual pressure is available to cause current to flow through the -lamp filament, hence it will glow with maximum brilliancy. If an -inductive coil be inserted in the circuit as in fig. 2,674, the -reverse pressure due to self-induction will oppose the virtual -pressure, hence the effective pressure (which is the difference -between the virtual and reverse pressures), will be reduced and -the current flow through the lamp diminished, thus reducing the -brilliancy of the illumination. The effect may be intensified to such -degree by interposing an iron core in the coil as in fig. 2,676, as -to extinguish the lamp.] - -=Ques. With respect to self-induction, what method should be followed -in wiring?= - -Ans. When iron conduits are used, the wires of each circuit should -not be installed in separate conduits, because such arrangement will -cause excessive self-induction. - - The importance of this may be seen from the experience of one - contractor, who installed feeders and mains in separate iron - pipes. When the current was turned on, it was found that the - self-induction was so great as to reduce the pressure to such an - extent that the lamps, instead of giving full candle power, were - barely red. This necessitated the removal of the feeders and main - and re-installing them, so that those of the same circuit were in - the same pipe. - -=Ques. What is mutual induction?= - -Ans. Mutual induction is the effect of one alternating current -circuit upon another. - -[Illustration: FIG. 2,677.--Measurement of self induction when -the frequency is known. The apparatus required consists of a high -resistance or electrostatic a.c. voltmeter, d.c. ammeter, and a -non-inductive resistance. Connect the inductive resistance to be -measured as shown, and close switch M, short circuiting the ammeter. -Connect alternator in circuit and measure drop across R and across -X_{_i_}. Disconnect alternator and connect battery in circuit, then -open switch M and vary the continuous current until the drop across R -is the same as with the alternating current, both measurements being -made with the same voltmeter; read ammeter, and measure drop across -X_{_i_}. Call the drop across X_{_i_} with alternating current E, and -with direct current E_{_i_}, and the reading of the ammeter J. Then - ____________________ -L = √E^{2} + E_{_i_}^{2} ÷ 2π _f_ I. If the resistance X_{_i_} -be known, and the ammeter be suitable for use with alternating -current, the switch and R may be dispensed with. - - ______________________________ -Then L = √E^{2} - X_{_i_}^{2} I_{_i_}^{2} ÷ 2π _f_ I, -where I_{_i_} is the value of the alternating current. The resistance -of the voltmeter should be high enough to render its current -negligible as compared with that through X_{_i_}.] - -=Ques. How is it caused?= - -Ans. It is due to the magnetic field surrounding a conductor cutting -adjacent conductors and inducing back pressures therein. - - This effect as a rule in ordinary installations is negligible. - -=Transpositions.=--The effect of mutual induction between two -circuits is proportional to the inter-linkage of the magnetic fluxes -of the two lines. This in turn depends upon the proximity of the -lines and upon the general relative arrangement of the conductors. - -[Illustration: FIG. 2,678.--Transposition diagram for two parallel -lines consisting of two wires each.] - -[Illustration: FIG. 2,679.--Transposition diagram for three phase, -three wire line, transposing at the vertices of an equilateral -triangle. The line is originally balanced and becomes unbalanced on -transposing, a procedure which should be resorted to only to prevent -_mutual induction_.] - -[Illustration: FIG. 2,680.--Transposition diagram of three phase, -three wire line, with center arranged in a straight line.] - -The inductive effect of one line upon another is equal to the -algebraic sum of the fluxes due to the different conductors of the -first line, considered separately, which link the secondary line. - -The effect of mutual induction is to induce surges in the line where -a difference of frequency exists between the two currents, and to -induce high electrostatic charges in lines carrying little or no -current, such as telephone lines. - - INDUCTANCE PER MILE OF THREE PHASE CIRCUIT - - ---------+-------+----------+------------ - | Diam. | Distance | Self - Size | in | _d_ in | Inductance - B. & S. | inch. | inches. | L henrys. - ---------+-------+----------+------------ - 0000 | .46 | 12 | .00234 - | | 18 | .00256 - | | 24 | .00270 - | | 48 | .00312 - | | | - 000 | .41 | 12 | .00241 - | | 18 | .00262 - | | 24 | .00277 - | | 48 | .00318 - | | | - 00 | .365 | 12 | .00248 - | | 18 | .00269 - | | 24 | .00285 - | | 48 | .00330 - | | | - 0 | .325 | 12 | .00254 - | | 18 | .00276 - | | 24 | .00293 - | | 48 | .00331 - | | | - 1 | .289 | 12 | .00260 - | | 18 | .00281 - | | 24 | .00308 - | | 48 | .00338 - | | | - 2 | .258 | 12 | .00267 - | | 18 | .00288 - | | 24 | .00304 - | | 48 | .00314 - | | | - 3 | .229 | 12 | .00274 - | | 18 | .00294 - | | 24 | .00310 - | | 48 | .00351 - | | | - 4 | .204 | 12 | .00280 - | | 18 | .00300 - | | 24 | .00315 - | | 48 | .00358 - | | | - 5 | .182 | 12 | .00286 - | | 18 | .00307 - | | 24 | .00323 - | | 48 | .00356 - | | | - 6 | .162 | 12 | .00291 - | | 18 | .00313 - | | 24 | .00329 - | | 48 | .00369 - | | | - 7 | .144 | 12 | .00298 - | | 18 | .00310 - | | 24 | .00336 - | | 48 | .00377 - | | | - 8 | .128 | 12 | .00303 - | | 18 | .00325 - | | 24 | .00341 - | | 48 | .00384 - | | | - 9 | .114 | 12 | .00310 - | | 18 | .00332 - | | 24 | .00348 - | | 48 | .00389 - | | | - 10 | .102 | 12 | .00318 - | | 18 | .00340 - | | 24 | .00355 - | | 48 | .00396 - ---------+-------+----------+------------ - -This effect may be nullified by separating the lines and by -transposing the wires of one of the lines so that the effect produced -in one section is opposed by that in another. Of two parallel lines -consisting of two wires each, one may be transposed to neutralize the -mutual inductance. - - Fig. 2,678 shows this method. The length L' should be an even - factor of L so that to every section of the line transposed there - corresponds an opposing section. - -[Illustration: FIG. 2,681.--Capacity effect in single phase -transmission line. The effect is the same as would be produced by -shunting across the line at each point an infinitesimal condenser -having a capacity equal to that of an infinitesimal length of -circuit. For the purpose of calculating the charging current, a very -simple and sufficiently accurate method is to determine the current -taken by a condenser having a capacity equal to that of the entire -line when charged to the pressure on the line at the generating end. -The effect of capacity of the line is to reduce the pressure drop, -that is, improve the regulation, and to decrease or increase the -power loss depending on the load and power factor of the receiver.] - -[Illustration: FIG. 2,682.--Capacity effect in a three phase -transmission line. It is the same as would be produced by shunting -the line at each point by three infinitesimal condensers connected in -star with the neutral point grounded, the capacity of each condenser -being twice that of a condenser of infinitesimal length formed by -any two of the wires. The effect of capacity on the regulation and -efficiency of the line can be determined with sufficient accuracy -in most cases by considering the line shunted at each end by three -condensers connected in star, the capacity of each condenser being -equal to that formed by any two wires of the line. An approximate -value for the charging current per wire is the current required to -charge a condenser, equal in capacity to that of any two of the -wires, to the pressure at the generating end of the line between any -one wire and the neutral point.] - - - The self inductance of lines is readily calculated from the - following formula: - - L = .000558 {2.303 log (2A ÷ _d_) + .25} per mile of circuit - -where - - L = inductance of a loop of a three phase circuit in henrys. - _Note._--The inductance of a complete single phase - circuit = L × 2 ÷ √3. - A = distance between wires; - _d_ = diameter of wire. - - - CAPACITY IN MICRO-FARADS PER MILE OF CIRCUIT - FOR THREE PHASE SYSTEM - - ---------+-------+----------+-------------- - | Diam. | Distance | Capacity - Size | in | A in | C in - B. & S. | inch. | inches. | micro-farads - ---------+-------+----------+-------------- - 0000 | .46 | 12 | .0226 - | | 18 | .0204 - | | 24 | .01922 - | | 48 | .01474 - | | | - 000 | .41 | 12 | .0218 - | | 18 | .01992 - | | 24 | .01876 - | | 48 | .01638 - | | | - 00 | .365 | 12 | .0124 - | | 18 | .01946 - | | 24 | .01832 - | | 48 | .01604 - | | | - 0 | .325 | 12 | .02078 - | | 18 | .01898 - | | 24 | .01642 - | | 48 | .01570 - | | | - 1 | .289 | 12 | .02022 - | | 18 | .01952 - | | 24 | .01748 - | | 48 | .0154 - | | | - 2 | .258 | 12 | .01972 - | | 18 | .01818 - | | 24 | .01710 - | | 48 | .01510 - | | | - 3 | .229 | 12 | .01938 - | | 18 | .01766 - | | 24 | .01672 - | | 48 | .01480 - | | | - 4 | .204 | 12 | .01874 - | | 18 | .01726 - | | 24 | .01636 - | | 48 | .01452 - | | | - 5 | .182 | 12 | .01830 - | | 18 | .01690 - | | 24 | .01602 - | | 48 | .01426 - | | | - 6 | .162 | 12 | .01788 - | | 18 | .01654 - | | 24 | .01560 - | | 48 | .0140 - | | | - 7 | .144 | 12 | .01746 - | | 18 | .01618 - | | 24 | .01538 - | | 48 | .01374 - | | | - 8 | .128 | 12 | .01708 - | | 18 | .01586 - | | 24 | .01508 - | | 48 | .01350 - | | | - 9 | .114 | 12 | .01660 - | | 18 | .01552 - | | 24 | .01478 - | | 48 | .01326 - | | | - 10 | .102 | 12 | .01636 - | | 18 | .01522 - | | 24 | .01452 - | | 48 | .01304 - ---------+-------+----------+-------------- - -=Capacity.=--In any given system of electrical conductors, a -pressure difference between two of them corresponds to the presence -of a quantity of electricity on each. With the same charges, the -difference of pressure may be varied by varying the geometrical -arrangement and magnitudes and also by introducing various -dielectrics. The constant connecting the charge and the resulting -pressure is called the capacity of the system. - - All circuits have a certain capacity, because each conductor - acts like the plate of a condenser, and the insulating medium, - acts as the dielectric. The capacity depends upon the insulation. - - For a given grade of insulation, the capacity is proportional - to the surface of the conductors, and universally to the distance - between them. - - A three phase three wire transmission line spaced at the corners - of an equilateral triangle as regards capacity acts precisely - as though the neutral line were situated at the center of the - triangle. - - The capacity of circuits is readily calculated by applying the - following formulae: - - C = 38.83 sc 10^{-3} / log (D ÷ d) per mile, - insulated cable with lead sheath; - C = 38.83 × 10^{-3} / log (4h ÷ d) per mile, - single conductor with earth return; - C = 19.42 × 10^{-3} / log (2A ÷ d) per mile of - parallel conductors forming metallic circuit; - - in which - - C = Capacity in micro-farads; for a metallic circuit, C = - capacity between wires; - - sc = Specific inductive capacity of insulating material; = 1 for - air, and 2.25 to 3.7 for rubber; - - D = Inside diameter of lead sheath; - d = Diameter of conductor; - h = Distance of conductors above ground; - A = Distance between wires. - -=Frequency.=--The number of cycles per second, or the frequency, -has a direct effect upon the inductance reactance in an alternating -current circuit, as is plainly seen from the formula. - - X_{i} = 2π_f_L - - In the case of a transmission line alone; the lower frequencies - are the more desirable, in that they tend to reduce the inductance - drop and charging current. The inductance drop is proportional to - the frequency. - - The natural period of a line, with distributed inductance and - capacity, is approximately given by - __ - P = 7,900 / √LC - - where L is the total inductance in millihenrys, and C, the total - capacity in micro-farads. Accordingly some lower odd harmonic of - the impressed frequency may be present which corresponds with the - natural period of the line. When this obtains, oscillations of - dangerous magnitude may occur. Such coincidences are less likely - with the lower harmonics than with the higher. - -=Skin Effect.=--The tendency of alternating current to confine itself -to the _outer_ portions of a conductor, instead of passing uniformly -through the cross section, is called _skin effect_. The effect is -proportional to the size of the conductor and the frequency. - -=Ques. What effect has "skin effect" on the current?= - -Ans. It is equivalent to an increase of ohmic resistance and -therefore opposes the current. - -[Illustration: FIGS. 2,683 to 2,687.--Skin effect and shield effect. -Fig. 2,683, section of conductor illustrating skin effect or tendency -of the alternating current to distribute itself unequally through the -cross section of a conductor as shown by the varied shading, which -represents the current flowing most strongly in the outer portions -of the conductor. For this reason it has been proposed to use hollow -or flat instead of solid round conductors; however, with frequency -not exceeding 100, the skin effect is negligibly small in copper -conductors of the sizes usually employed. In figs. 2,684 and 2,685, -or 2,686 and 2,687, if two adjacent conductors be carrying current -in the same direction, concentration will occur on those parts of -the two conductors remote from one another, and the nearer parts -will have less current, that is to say, they will be =shielded=. -In this case, the induction due to one conductor will exert its -opposing effect to the greatest extent on those parts of the other -conductor nearest to it; this effect decreasing the deeper the latter -is penetrated. After crossing the current axis, the induction will -still decrease in magnitude, but will now aid the current in the -conductor. Hence, the effect of these two conductors on one another -will make the current density more uniform than is the case where -the two conductors adjacent to one another are carrying current in -opposite directions, as in figs. 2,685 and 2,686, therefore, the -resistance and the heating for a given current will be smaller. If -the two return conductors be situated on the line passing through -the center of the conductors just considered, the effect will be to -still further concentrate the current; the distribution symmetry will -be further disturbed, and the resistance of the conductor system -increased. It is therefore difficult to say which of the two cases -considered holds the advantage so far as increasing the resistance is -concerned. The case, however, in which the phases are mixed has much -the smaller reactive drop.] - - If the conductor be large, or the frequency high, the central - portion of the conductor carries little if any current, hence the - resistance is therefore greater for alternating current than for - direct current. - -=Ques. For what condition may "skin effect" be neglected?= - -Ans. For frequencies of 60 or less, with conductors having a diameter -not greater than 0000 B. & S. gauge. - -=Ques. How is the "skin effect" calculated for a given wire?= - -Ans. Its area in circular mils multiplied by the frequency, gives the -ratio of the wire's ohmic resistance to its combined resistance. - - That is to say, the factor thus obtained multiplied by the - resistance of the wire to direct current will give its combined - resistance or resistance to alternating current. - - The following table gives these ratio factors for large - conductors. - - RATIO FACTOR FOR COMBINED RESISTANCE - - +---------------+---------+---------------+---------+ - | Cir. mils. | Ratio | Cir. mils. | Ratio | - | × frequency | factor | × frequency | factor | - +---------------+---------+---------------+---------+ - | 10,000,000 | 1.00 | 70,000,000 | 1.13 | - | 20,000,000 | 1.01 | 80,000,000 | 1.17 | - | 30,000,000 | 1.03 | 90,000,000 | 1.20 | - | 40,000,000 | 1.05 | 100,000,000 | 1.25 | - | 50,000,000 | 1.08 | 125,000,000 | 1.34 | - | 60,000,000 | 1.10 | 150,000,000 | 1.43 | - +---------------+---------+---------------+---------+ - -=Corona Effect.=--When two wires, having a great difference of -pressure are placed near each other, a certain phenomenon occurs, -which is called _corona effect_. When the spacing or distance -between the wires is small and the difference of pressure in the -wires very great, a continuous passage of energy takes place through -the dielectric or atmosphere, the amount of this energy may be an -appreciable percentage of the power transmitted. Therefore in laying -out high pressure transmission lines, this effect must be considered -in the spacing of the wires. - -=Ques. How does the corona effect manifest itself?= - -Ans. It is visible at night as a bluish luminous envelope and audible -as a hissing sound. - -=Ques. What is the critical voltage?= - -Ans. The voltage at which the corona effect loss takes place. - -=Ques. Upon what does the critical voltage depend?= - -Ans. Upon the radius of the wires, the spacing, and the atmospheric -conditions. - -[Illustration: FIG. 2,688.--Electromagnetic and electrostatic -fields surrounding the conductors of a transmission line. The -electromagnetic field is represented by lines of magnetic force -that surround the conductors in circles, (the solid lines), and -the electrostatic field by (dotted) circles passing from conductor -to conductor across at right angles to the magnetic circles. For -any given size of wire and distance apart of wires there is a -certain voltage at which the critical density or critical gradient -is reached, where the air breaks down and luminosity begins--the -critical voltage where corona manifests itself. At still higher -voltages corona spreads to further distances from the conductor -and a greater volume of air becomes luminous. Incidentally, it -produces noise. Now to produce light requires power and to produce -noise requires power. Air is broken down and is heated in breaking -down, and to heat also requires power; therefore, as soon as corona -forms, power is consumed or dissipated in its formation. When this -phenomenon occurs on the conductors of an alternating current circuit -a change takes place in relation to current and voltage. On the -wires of an alternating current transmission line, at a voltage -below that where corona forms--at a voltage where wires are not -luminous--considerable current, more or less depending on voltage -and length of wire, flows into the circuit as capacity current or -charging current.] - - The critical voltage increases with both the diameter of the - wires, and the spacing. - - The losses due to corona effect increase very rapidly with - increasing pressure beyond the critical voltage. - - The magnitude of the losses as well as the critical voltage is - affected, by atmospheric conditions, hence they probably vary with - the particular locality, and the season of the year. Therefore, - for a given locality, a voltage which is normally below the - critical point, may at times be above it, depending upon changes - in the weather. - - Such elements as smoke, fog, moisture, or other particles (dust, - snow, etc.) floating in the air, increase the losses; rain, - however, apparently has no appreciable effect upon the losses. - It follows then that in the design of a transmission line, the - atmospheric conditions of the particular locality through which - the line passes should be considered. - -=Ques. How should live wires be spaced?= - -Ans. They should be so spaced as to lessen the tendency to leakage -and to prevent the wires swinging together or against towers. The -spacing should be only sufficient for safety, since increased spacing -increases the self-induction of the line, and while it lessens the -capacity, it does so only in a slight degree. - - The following spacing is in accordance with average practice. - - SPACING FOR VARIOUS VOLTAGES - - +---------+----------+ - | Volts | Spacing | - +---------+----------+ - | 5,000 | 28 ins. | - | 15,000 | 40 ins. | - | 30,000 | 48 ins. | - | 45,000 | 60 ins. | - | 60,000 | 60 ins. | - | 75,000 | 84 ins. | - | 90,000 | 96 ins. | - | 105,000 | 108 ins. | - | 120,000 | 120 ins. | - +---------+----------+ - -=Resistance of Wires.=--For quick calculation the following method -of obtaining the resistance (approximately) of wires will be found -convenient: - -1,000 feet No. 10 B. & S. wire, which is about .1 inch in diameter -(.1019), has a resistance of one ohm, at a temperature of 68° F. and -weighs 31.4 pounds. A wire three sizes larger, that is No. 7, has -twice the cross section and therefore one-half the resistance. A wire -three sizes smaller than No. 10, that is No. 13, has one-half the -cross section and therefore twice the resistance. - -Thus, starting with No. 10, any number three sizes larger will double -the cross sectional area and any wire three sizes smaller will halve -the cross sectional area of the preceding wire. This is true to the -extreme limits of the table, so that the area, weight and resistance -of any wire may be at once calculated to a close approximation from -this rule, intermediate sizes being obtained by interpolation. - -For alternating current, the combined resistance, that is, the total -resistance, including skin effect, is obtained by multiplying the -resistance, as found above by the "ratio factor" (see table page -1,894). - -[Illustration: FIGS. 2,689 to 2,692.--Triangles for obtaining -graphically, impedance, impressed pressure, etc., in alternating -current circuits. For a full explanation of this method the reader is -referred to Guide 5, Chapter XLVII on Alternating Current Diagrams. A -thorough study of this chapter is recommended.] - -=Impedance.=--_The total opposition to the flow of electricity in an -alternating current circuit_, or the impedance may be resolved into -two components representing the ohmic resistance and the spurious -resistance; these components have a phase difference of 90°, and -they may be represented graphically by the two legs of a right angle -triangle, of which the hypothenuse represents the impedance. - -Similarly, the volts lost or "drop" in an alternating circuit may be -resolved into two components representing respectively - -1. The loss due to resistance. - -2. The loss due to reactance. - -These components have a phase difference of 90° and are represented -graphically similar to the impedance components. This has been -explained at considerable length in Chapter XLVII (Guide V). - -[Illustration: FIG. 2,693.--Mechanical analogy of power factor, as -exemplified by a locomotive "poling" a car off a siding. The car and -locomotive are shown moving in parallel directions, and the pole -AB, inclined at an angle ϕ. Now, if the length of AB be taken to -represent the pressure exerted on the pole by the locomotive, then -the imaginary lines AC and BC, drawn respectively parallel and at -right angles to the direction of motion will represent respectively -the useful and no energy (wattless) components; that is to say, if -the pressure AB be applied to the car at an angle ϕ, only part of it, -AC, is useful in propelling the car, the other component, BC, being -wasted in tending to push the car off the track at right angles to -the rails, being resisted by the flanges of the outer wheels.] - -=Power Factor.=--When the current falls out of step with the -pressure, as on inductive loads, the power factor becomes less than -unity, and the effect is to increase the current required for a -given load. Accordingly, this must be considered in calculating the -size of the wires. As has been explained, the current flowing in -an alternating current circuit, as measured by an ammeter, can be -resolved into two components, representing respectively the _active -component_ and the _wattless component_ or idle current. These are -graphically represented by the two legs of a right triangle, of which -the hypothenuse represents the current measured by the ammeter. - -This _apparent_ current, as is evident from the triangle, exceeds -the _active_ current and lags behind the pressure by an amount -represented by the angle ϕ between the hypothenuse and leg -representing the energy current as shown in fig. 2,694. - -[Illustration: FIG. 2,694.--Diagram showing that the apparent current -is more than the active current, the excess depending upon the angle -of phase difference.] - -[Illustration: FIG. 2,695.--Diagram showing components of impedance -volts. Compare this diagram with figs. 2,689 and 2,671, and note that -the term "reactance" is the difference between the inductance drop -and the capacity drop if the circuit contain capacity, for instance, -if inductance drop be 10 volts and capacity drop be 7 volts then -reactance 10-7 = 3 volts.] - -=Ques. What determines the heating of the wires on alternating -current circuits with inductive loads?= - -Ans. The apparent current, as represented by the hypothenuse of the -triangle in fig. 2,694. - -=Ques. How is the apparent current obtained?= - -Ans. Divide the true watts by the product of the power factor -multiplied by the voltage. - - Example.--A certain circuit supplies 20 kw. to motors at 220 - volts and .8 power factor. What is the apparent current? - - true watts 20,000 - Apparent Current = -------------------- = -------- = 113.6 amperes - power factor × volts .8 × 220 - -=Ques. What else, besides power factor, should be considered in -making wire calculations for motor circuits?= - -Ans. The efficiency of the motor, and the heavy starting current. - - The product of the efficiency of the motor multiplied by the - power factor gives the _apparent efficiency_, which governs the - size of the wires, apparatus, etc., necessary to feed the motors. - - Allowance should be made for the heavy starting current required - for some motors to avoid undue drop. - - TABLE OF APPROXIMATE AMPERES PER TERMINAL - FOR INDUCTION MOTORS - - Column headings: A-110 volts B-220 volts C-440 volts D-550 volts - - +-----+----------------+------------------+------------------------+ - | | | Two phase | Three phase | - |Horse| Single phase | four wire | three wire | - |power+----+-----+-----+-----+-----+------+-----+-----+------+-----+ - | | A | B | C | A | B | C | A | B | C | D | - +-----+----+-----+-----+-----+-----+------+-----+-----+------+-----+ - | .5| 6.6| 3.4 | 1.8| 3.3 | 1.7 | .9 | 3.7 | 1.8 | 1 | | - | 1 | 14 | 7 | 3.5| 6.4 | 3.2 | 1.6 | 7.4 | 3.7 | 1.9 | | - | 2 | 24 | 12 | 6 | 11 | 5.7 | 2.9 | 13 | 6.6 | 3.3 | 2.5 | - | 3 | 34 | 17 | 8.5| 16 | 8.1 | 4.1 | 19 | 9.3 | 4.7 | 3.5 | - | 4 | 52 | 26 | 13 | 26 | 13 | 6.5 | 30 | 15 | 7.5 | 6 | - | 5 | 74 | 37 | 18.5| 38 | 19 | 9.5 | 44 | 22 | 11 | 9 | - | 10 | 94 | 47 | 23.5| 44 | 22 | 11 | 50 | 25 | 12.5 | 11 | - | 15 | | | | 66 | 33 | 16.5 | 76 | 38 | 19 | 16 | - | 20 | | | | 88 | 44 | 22 | 102 | 51 | 25.5 | 22 | - | 25 | | | | 111 | 55 | 28 | 129 | 64 | 32 | 25 | - | 30 | | | | 134 | 67 | 33.5 | 154 | 77 | 38.5 | 32 | - | 40 | | | | 178 | 89 | 44.5 | 204 | 107 | 53.5 | 44 | - | 50 | | | | 204 | 102 | 51 | 236 | 118 | 59 | 52 | - | 75 | | | | 308 | 154 | 77 | 356 | 178 | 89 | 77 | - | 100 | | | | 408 | 204 | 102 | 472 | 236 | 118 | 100 | - +-----+----+-----+-----+-----+-----+------+-----+-----+------+-----+ - -=Ques. What are the usual power factors encountered on commercial -circuits?= - -Ans. A mixed load of incandescent lamps and induction motors will -have a power factor of from .8 to .85; induction motors above .8 to -.85; incandescent and Nernst lamps .98; arc lamps, .85. - -=Wire Calculations.=--In the calculation of alternating current -circuits, the two chief factors which make the computation different -from that for direct current circuits, is _induction_ and _power -factor_. The first depends on the frequency, and physical condition -of the circuit, and the second upon the character of the load. - -=Ques. Under what conditions may inductance be neglected?= - -[Illustration: FIGS. 2,696 to 2,698.--Example of wiring showing where -inductance is negligible, and where it must be considered in wire -calculations.] - -Ans. In cases where the wires of a circuit are not spaced over an -inch apart, or in conduit work, where both wires are in the same -conduit. - - Under these conditions the calculation is the same as for direct - current after making proper allowance for power factor. - -=Ques. Under what conditions must induction be considered?= - -Ans. On exposed circuits with wires separated several inches, -particularly in the case of large wires. - -=Sizes of Wire.=--The size of wire for any alternating circuit may be -determined by slightly modifying the formula used in direct current -work, and which, as derived in Guide No. 4, page 748, is - - amperes × feet × 21.6 - circular mils = --------------------- (1) - drop - - The quantity 21.6, is twice the resistance (10.8) of a foot of - copper wire one mil in diameter (_mil foot_). This resistance - (10.8) is multiplied by 2, giving the quantity 21.6, because the - length of a circuit, or feet in the formula, is given as the "run" - or distance one way, that is, one-half the total length of wire in - the circuit, must be multiplied by 2 to get the total drop, viz.: - - amperes × feet × 10.8 × 2 amperes × feet × 21.6 - circular mils = ------------------------- = --------------------- - drop drop - -It is sometimes however convenient to make the calculation in terms -of watts. Formula (1) may be modified for such calculation. - -In modifying the formula, the "drop" should be expressed in -percentage instead of actual volts lost, that is, instead of the -difference in pressure between the beginning and the end of the -circuit. - -In any circuit the loss in percentage, or - - drop - % loss = ------------------ × 100 - impressed pressure - -from which - - % loss × impressed pressure - drop = --------------------------- (2) - 100 - -Substituting equation (2) in equation (1) - - amperes × feet × 21.6 - circular mils = ----------------------------- - % loss × imp. pressure - ------------------------ - 100 - - amperes × feet × 2,160 - = -------------------------- (3) - % loss × imp. pressure - -Equation (3) is modified for calculation in terms of watts as -follows: The power in watts is equal to the _applied voltage_ -multiplied by the current, that is to say, the power is equal to -the _volts at the consumer's end of the circuit_ multiplied by the -current, or simply - - watts = volts × amperes - -from which - - watts - amperes = ----- (4) - volts - -[Illustration: FIGS. 2,699 to 2,703.--Stranded copper cables. For -conductors of large areas and in the smaller sizes where extra -flexibility is required it becomes necessary to employ stranded -cables made by grouping a number of wires together in either -concentric or rope form. The concentric cable as here illustrated is -formed by grouping six wires around a central wire thereby forming -a seven wire cable. The next step is the application in a reverse -direction of another layer of 12 wires and a nineteen wire cable is -produced. This is again increased by a third layer of eighteen wires -for a 37 wire cable and a fourth layer of 24 wires for a 61 wire -cable. Successive layers, each containing 6 more wires than that -preceding, may be applied until the desired capacity is obtained. The -cuts show sectional views of concentric cables each formed from No. -10 B. & S. gauge wires.] - -Substituting this value for the current in equation (3) and -remembering that the pressure taken is the volts at the consumer's -end of the line - - watts - ----- × feet × 2,160 - volts - circular mils = --------------------- - % loss × volts - - watts × feet × 2,160 - = -------------------- (5) - % loss × volts^{2} - -This formula (5) applies to a direct current two wire circuit, and to -adapt it to any alternating current circuit it is only necessary to -use the letter M instead of the number 2,160, thus - - watts × feet × M - circular mils = ------------------ (6) - % loss × volts^{2} - -in which M is a coefficient which has various values according to the -kind of circuit and value of the power factor. These values are given -in the following table: - - =VALUES OF M= - - --------+----------------------------------------------------------- - | POWER FACTOR - SYSTEM +-----+-----+-----+-----+-----+-----+-----+-----+-----+----- - | 1.00| .98| .95| .90| .85| .80| .75| .70| .65| .60 - --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- - Single |2,160|2,249|2,400|2,660|3,000|3,380|3,840|4,400|5,112|6,000 - phase | | | | | | | | | | - --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- - Two |1,080|1,125|1,200|1,330|1,500|1,690|1,920|2,200|2,556|3,000 - phase | | | | | | | | | | - (4 wire)| | | | | | | | | | - --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- - Three |1,080|1,125|1,200|1,330|1,500|1,690|1,920|2,200|2,556|3,000 - phase | | | | | | | | | | - (3 wire)| | | | | | | | | | - --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- - - NOTE.--The above table is calculated as - follows: For =single phase= M = 2,160 ÷ power - factor^{2} × 100; for =two phase= four wire, or - three phase three wire, M = ½ (2,160 ÷ power - factor^{2})× 100. Thus the value of M for a - single phase line with power factor .95 = 2,160 ÷ - .95^{2} × 100 = 2,400. - -It must be evident that when 2,160 is taken as the value of M, -formula (6) applies to a two wire direct current circuit and also to -a single phase alternating current circuit when the power factor is -unity. - -In the table the value of M for any particular power factor is found -by dividing 2,160 by the square of that power factor for single phase -and twice the square of the power factor for two phase and three -phase. - -=Ques. For a given load and voltage how do the wires of a single and -two phase system compare in size and weight, the power factor being -the same in each case?= - -Ans. Since the two phase system is virtually two single phase -systems, the four wires of the two phase systems are half the size of -the two wires of the single phase system, and accordingly, the weight -is the same for either system. - - =VALUES OF T= - - +-------------------+--------------------------------------------+ - | | POWER FACTOR | - | SYSTEM +--------+--------+--------+--------+--------+ - | | 1.00 | .98 | .90 | .80 | .70 | - +-------------------+--------+--------+--------+--------+--------+ - | Single phase | 1.00 | .98 | .90 | .80 | .70 | - +-------------------+--------+--------+--------+--------+--------+ - | Two phase, 4 wire | 2.00 | 1.96 | 1.80 | 1.60 | 1.40 | - +-------------------+--------+--------+--------+--------+--------+ - |Three phase, 3 wire| 1.73 | 1.70 | 1.55 | 1.38 | 1.21 | - +-------------------+--------+--------+--------+--------+--------+ - - NOTE.--This table is for finding the value - of the current in line, using the formula I = - W ÷ (E × T), in which I = current in line; E = - voltage between main conductors at receiving or - consumers' end; W = watts. For instance, what - is the current in a two phase line transmitting - 1,000 watts at 550 volts, power factor .80? I = - 1,000 ÷ (550 × 1.60) = 1.13. - -=Ques. Since there is no saving in copper in using two phases, what -advantage has the two phase system over the one phase system?= - -Ans. It is more desirable on power circuits, because two phase motors -are self-starting. - - That is to say, the rotating magnetic field that can be - produced by a two phase current, permits an induction motor to - start without being equipped with any special phase splitting - devices which are necessary on single phase motors, because the - oscillating field produced by a single phase current does not - produce any torque on a squirrel cage armature at rest. - -=Ques. For equal working conditions, what is the comparison between -the single, two and three phase system as to size and weight of -wires?= - -Ans. Each wire of the three phase system is half the size of one of -the wires of the single phase system, hence the weight of copper -required for the three phase system is 75% of that required for the -single phase system. Since in the two phase system half of the load -is carried by each phase, each wire of the three phase system is the -same size as one of the wires of the two phase system, hence, the -copper required by the three phase system is 75% of that required by -the two phase system. - - =MISCELLANEOUS FORMULÆ FOR COPPER WIRES= - - Diameter squared = circular mils - Circular mils × .7854 = square mils - .000003027 × circular mils = pounds per foot - .003027 × circular mils = pounds per 1,000 feet - .0159847 × circular mils = pounds per mile - .003879 × square mils = pounds per 1,000 feet - .33033 ÷ circular mils = feet per pound - .0000002924 × circular mils = pounds per ohm - .342 ÷ circular mils = ohms per pound - .096585 × circular mils = feet per ohm - 10.353568 ÷ circular mils = ohms per foot - -Breaking weight of wire ÷ area = breaking weight per square inch. - -Breaking weight per square inch × area = breaking weight of wire. - -The weight of copper wire is 1-1/7 times the weight of iron wire of -same diameter. - - EXAMPLE.--What size wires must be used on a single phase circuit - 2,000 feet in length to supply 30 kw. at 220 volts with loss of - 4%, the power factor being .9? - - The formula for circular mils is - - watts × feet × M - circular mils = ------------------ (1) - % loss × volts^{2} - - Substituting the given values and the proper value of M from the - table, in (1) - - 30,000 × 2,000 × 2,660 - circular mils = ---------------------- = 82,438 - 4 × 220^{2} - - Referring to the accompanying table of the properties of copper - wire, the nearest _larger_ size wire is No. 1 B. & S. gauge having - an area of 83,690 circular mils. - - - =TABLE OF THE PROPERTIES OF COPPER WIRE= - - Giving weights, length and resistances of wires of Matthiessen's - Standard Conductivity for both B. & S. G. (Brown & Sharpe Gauge) - and B. W. G. (Birmingham Wire Gauge) from Transactions October - 1903, of the American Institute of Electrical Engineers. - - __________________________________________________________________ - | | | - Gauges. To the nearest fourth | | Length.|Resistance. - significant digit. | | | - _____________________________________| |________|_________ - | | | | Weight. | | - | | Diameter. | Area. | Lbs. | Feet | Ohms per - | | | | per | per lb.|1,000 ft. - ______|_______|___________|__________| 1,000 |________|__________ - | | | | feet. | | - | | | Circular | | | - B.& S.| B.W.G.| Inches. | mils. | | | @ 68° F. - ______|______ |___________|__________|_________|________|_________ - | | | | | | - 0000 | | 0.460 | 211,600 | 640.5 | 1.561 | .04893 - | 0000 | 0.454 | 206,100 | 623.9 | 1.603 | .05023 - | 000 | 0.425 | 180,600 | 546.8 | 1.829 | .05732 - | | | | | | - 000 | | 0.4096 | 167,800 | 508.0 | 1.969 | .06170 - | 00 | 0.380 | 144,400 | 437.1 | 2.288 | .07170 - 00 | | 0.3648 | 133,100 | 402.8 | 2.482 | .07780 - | | | | | | - | 0 | 0.340 | 115,600 | 349.9 | 2.858 | .08957 - 0 | | 0.3249 | 105,500 | 319.5 | 3.130 | .09811 - | 1 | 0.3000 | 90,000 | 272.4 | 3.671 | .1150 - | | | | | | - 1 | | 0.2893 | 83,690 | 253.3 | 3.947 | .1237 - | 2 | 0.2840 | 80,660 | 244.1 | 4.096 | .1284 - | 3 | 0.2590 | 67,080 | 203.1 | 4.925 | .1543 - | | | | | | - 2 | | 0.2576 | 66,370 | 200.9 | 4.977 | .1560 - | 4 | 0.2380 | 56,640 | 171.5 | 5.832 | .1828 - 3 | | 0.2294 | 52,630 | 159.3 | 6.276 | .1967 - | | | | | | - | 5 | 0.2200 | 48,400 | 146.5 | 6.826 | .2139 - 4 | | 0.2043 | 41,740 | 126.4 | 7.914 | .2480 - | 6 | 0.2030 | 41,210 | 124.7 | 8.017 | .2513 - | | | | | | - 5 | | 0.1819 | 33,100 |100.2 | 9.98 | .3128 - | 7 | 0.1800 | 32,400 | 98.08 | 10.20 | .3196 - | 8 | 0.1650 | 27,230 | 82.41 | 12.13 | .3803 - | | | | | | - 6 | | 0.1620 | 26,250 | 79.46 | 12.58 | .3944 - | 9 | 0.1480 | 21,900 | 66.30 | 15.08 | .4727 - 7 | | 0.1443 | 20,820 | 63.02 | 15.87 | .4973 - | | | | | | - | 10 | 0.1340 | 17,960 | 54.35 | 18.40 | .5766 - 8 | | 0.1285 | 16,510 | 49.98 | 20.01 | .6271 - | 11 | 0.1200 | 14,400 | 43.59 | 22.94 | .7190 - | | | | | | - 9 | | 0.1144 | 13,090 | 39.63 | 25.23 | .7908 - | 12 | 0.1090 | 11,880 | 35.96 | 27.81 | .8715 - 10 | | 0.1019 | 10,380 | 31.43 | 31.82 | .9972 - | | | | | | - | 13 | 0.0950 | 9,025 | 27.32 | 36.60 | 1.147 - 11 | | 0.09074 | 8,234 | 24.93 | 40.12 | 1.257 - | 14 | 0.08300 | 6,889 | 20.85 | 47.95 | 1.503 - | | | | | | - 12 | | 0.08081 | 6,530 | 19.77 | 50.59 | 1.586 - | 15 | 0.07200 | 5,184 | 15.69 | 63.73 | 1.997 - 13 | | 0.07196 | 5,178 | 15.68 | 63.79 | 1.999 - | | | | | | - | 16 | 0.06500 | 4,225 | 12.79 | 78.19 | 2.451 - 14 | | 0.06408 | 4,107 | 12.43 | 80.44 | 2.521 - | 17 | 0.0580 | 3,364 | 10.18 | 98.23 | 3.078 - | | | | | | - 15 | | 0.05707 | 3,257 | 9.858 | 101.4 | 3.179 - 16 | | 0.05082 | 2,583 | 7.818 | 127.9 | 4.009 - | 18 | 0.04900 | 2,401 | 7.268 | 137.6 | 4.312 - | | | | | | - 17 | | 0.045260 | 2,048 | 6.200 | 161.3 | 5.055 - | 19 | 0.042000 | 1,764 | 5.340 | 187.3 | 5.870 - 18 | | 0.040300 | 1,624 | 4.917 | 203.4 | 6.374 - | | | | | | - 19 | | 0.035890 | 1,288 | 3.899 | 256.5 | 8.038 - | 20 | 0.035000 | 1,225 | 3.708 | 269.7 | 8.452 - | 21 | 0.032000 | 1,024 | 3.100 | 322.6 | 10.11 - | | | | | | - 20 | | 0.031960 | 1,022 | 3.092 | 323.4 | 10.14 - 21 | | 0.028460 | 810.1 | 2.452 | 407.8 | 12.78 - | 22 | 0.028000 | 784.0 | 2.373 | 421.4 | 13.21 - 22 | | 0.025350 | 642.4 | 1.945 | 514.2 | 16.12 - | 23 | 0.025000 | 625.0 | 1.892 | 528.6 | 16.57 - 23 | | 0.022570 | 509.5 | 1.542 | 648.4 | 20.32 - | | | | | | - | 24 | 0.022000 | 484.0 | 1.465 | 682.6 | 21.39 - 24 | | 0.020100 | 404.0 | 1.223 | 817.6 | 25.63 - | 25 | 0.020000 | 400.0 | 1.211 | 825.9 | 25.88 - | | | | | | - | 26 | 0.018000 | 324.0 | .9808 | 1,020 | 31.96 - 25 | | 0.017900 | 320.4 | .9699 | 1,031 | 32.31 - | 27 | 0.016000 | 256.0 | .7749 | 1,290 | 40.45 - | | | | | | - 26 | | 0.015940 | 254.1 | .7692 | 1,300 | 40.75 - 27 | | 0.014200 | 201.5 | .6100 | 1,639 | 51.38 - | 28 | 0.014000 | 196.0 | .5933 | 1,685 | 52.83 - | | | | | | - | 29 | 0.013000 | 169.0 | .5116 | 1,955 | 61.27 - 28 | | 0.012640 | 159.8 | .4837 | 2,067 | 64.79 - | 30 | 0.012000 | 144.0 | .4359 | 2,294 | 71.90 - | | | | | | - 29 | | 0.011260 | 126.7 | .3836 | 2,607 | 81.70 - 30 | | 0.010030 | 100.5 | .3042 | 3,287 | 103.0 - | 31 | 0.010000 | 100.0 | .3027 | 3,304 | 103.5 - | | | | | | - | 32 | 0.009000 | 81.0 | .2452 | 4,078 | 127.8 - 31 | | 0.008928 | 79.70 | .2413 | 4,145 | 129.9 - | 33 | 0.008000 | 64.0 | .1937 | 5,162 | 161.8 - | | | | | | - 32 | | 0.007950 | 63.21 | .1913 | 5,227 | 163.8 - 33 | | 0.007080 | 50.13 | .1517 | 6,591 | 206.6 - | 34 | 0.007000 | 49.0 | .1483 | 6,742 | 211.3 - | | | | | | - 34 | | 0.006305 | 39.75 | .1203 | 8,311 | 260.5 - 35 | | 0.005615 | 31.52 | .09543 |10,480 | 328.4 - 36 | 35 | 0.005000 | 25.0 | .07568 |13,210 | 414.2 - | | | | | | - 37 | | 0.004453 | 19.83 | .06001 |16,660 | 522.2 - | 36 | 0.004000 | 16. | .04843 |20,650 | 647.1 - 38 | | 0.003965 | 15.72 | .04759 |21,010 | 658.5 - | | | | | | - 39 | | 0.003531 | 12.47 | .03774 |26,500 | 830.4 - 40 | | 0.003145 | 9.888 | .02993 |33,410 |1047. - -------------------------------------------------------------------- - -=Drop.=--In order to determine the drop or volts lost in the line, -the following formula may be used - -drop = ((% loss × volts) / 100) × S (1) - -in which the % loss is a percentage of the applied power, that is, -the power delivered to the consumer and not a percentage of the power -at the alternator. "Volts" is the pressure at the consumer's end of -the circuit. - - =VALUE OF "S" FOR 60 CYCLES= - - -----------------+------------------------+------------------------+ - | .98 power factor | .90 power factor | - --------+--------+------------------------+------------------------+ - Size of | Area | Spacing of | Spacing of | - wire | in | conductors | conductors | - B.&S. |circular| | | - gauge | mils. | | | - | +----+----+----+----+----+----+----+----+----+----+ - | | 1" | 3" | 6" | 12"| 24"| 1" | 3" | 6" | 12"| 24"| - --------+--------+----+----+----+----+----+----+----+----+----+----+ - 500,000 |500,000 |1.21|1.45|1.61|1.77|1.92|1.32|1.80|2.11|2.44|2.75| - 300,000 |300,000 |1.15|1.29|1.38|1.48|1.57|1.19|1.47|1.66|1.84|2.02| - 0,000 |211,600 |1.12|1.22|1.28|1.34|1.41|1.13|1.33|1.45|1.58|1.63| - 000 |167,800 |1.09|1.18|1.22|1.28|1.29|1.08|1.23|1.33|1.44|1.53| - 00 |133,100 |1.07|1.14|1.18|1.21|1.25|1.03|1.16|1.24|1.32|1.40| - 0 |105,500 |1.05|1.10|1.14|1.17|1.20|1.00|1.09|1.16|1.22|1.28| - 1 | 83,690 |1.04|1.08|1.10|1.13|1.15|1.00|1.05|1.09|1.14|1.19| - 2 | 66,370 |1.02|1.05|1.08|1.10|1.12|1.00|1.00|1.04|1.08|1.12| - 3 | 52,630 |1.02|1.04|1.06|1.07|1.09|1.00|1.00|1.00|1.03|1.06| - | | | | | | | | | | | | - 4 | 41,740}|1.00|1.02|1.03|1.04|1.07|1.00|1.00|1.00|1.00|1.00| - 5 | 33,100}| | | | | | | | | | | - | | | | | | | | | | | | - 6 | 26,250}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - 7 | 20,820}| | | | | | | | | | | - | | | | | | | | | | | | - 8 | 16,510}| | | | | | | | | | | - 9 | 13,090}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - 10 | 10,380}| | | | | | | | | | | - --------+--------+----+----+----+----+----+----+----+----+----+----+ - --------+--------+----+----+----+----+----+----+----+----+----+----+ - Size | | .80 power factor | .70 power factor | - of | Area +------------------------+------------------------+ - wire | in | Spacing of | Spacing of | - B.&S. |circular| conductors | conductors | - gauge | mils. +----+----+----+----+----+----+----+----+----+----+ - | | 1" | 3" | 6" | 12"| 24"| 1" | 3" | 6" | 12"| 24"| - --------+--------+----+----+----+----+----+----+----+----+----+----+ - 500,000 | 500,000|1.27|1.89|2.25|2.64|3.03|1.14|1.72|2.12|2.53|2.92| - 300,000 | 300,000|1.11|1.46|1.68|1.90|2.12|1.00|1.33|1.56|1.78|2.01| - 0,000 | 211,600|1.03|1.27|1.43|1.58|1.75|1.00|1.14|1.29|1.45|1.69| - 000 | 167,800|1.00|1.16|1.28|1.41|1.53|1.00|1.02|1.15|1.28|1.50| - 00 | 133,100|1.00|1.07|1.15|1.22|1.00|1.00|1.00|1.03|1.13|1.21| - 0 | 105,500|1.00|1.00|1.07|1.15|1.00|1.00|1.00|1.00|1.01|1.09| - 1 | 83,690}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - 2 | 66,370}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - 3 | 52,630}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - | | | | | | | | | | | | - 4 | 41,740}| | | | | | | | | | | - 5 | 33,100}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - | | | | | | | | | | | | - 6 | 26,250}| | | | | | | | | | | - 7 | 20,820}| | | | | | | | | | | - | | | | | | | | | | | | - 8 | 16,510}| | | | | | | | | | | - 9 | 13,090}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - 10 | 10,380}| | | | | | | | | | | - --------+--------+----+----+----+----+----+----+----+----+----+----+ -The coefficient S has various values as given in the accompanying -tables. As will be seen from the table, the value of S to be used -depends upon the size of wire, spacing, power factor and frequency. - -These values are accurate enough for all practical purposes, and may -be used for distances of 20 miles or less and for voltages up to -25,000. - -The capacity effect on very long high voltage lines, makes this -method of determining the drop somewhat inaccurate beyond the limits -above mentioned. - - =VALUE OF "S" FOR 25 CYCLES= - - ---------+--------+------------------------+------------------------+ - Size | | .98 power factor | .90 power factor | - of | Area +------------------------+------------------------+ - wire | in | Spacing of | Spacing of | - B.&S. |circular| conductors | conductors | - gauge | mils. +----+----+----+----+----+----+----+----+----+----+ - | | 1" | 2" | 6" | 12"| 24"| 1" | 3" | 6" | 12"| 24"| - ---------+--------+----+----+----+----+----+----+----+----+----+----+ - 500,000 |500,000 |1.01|1.17|1.23|1.29|1.36|1.02|1.22|1.35|1.43|1.61| - 300,000 |300,000 |1.04|1.10|1.13|1.18|1.21|1.00|1.08|1.16|1.25|1.31| - 0,000 |211,600 |1.03|1.07|1.09|1.11|1.14|1.00|1.02|1.07|1.13|1.15| - 000 |167,800 |1.00|1.05|1.06|1.09|1.10|1.00|1.00|1.02|1.07|1.11| - 00 |133,100 |1.00|1.03|1.05|1.06|1.08|1.00|1.00|1.00|1.02|1.05| - 0 |105,500 |1.00|1.01|1.02|1.03|1.04|1.00|1.00|1.00|1.00|1.00| - | | | | | | | | | | | | - 1 | 83,690}| | | | | | | | | | | - 2 | 66,370}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - 3 | 52,630}| | | | | | | | | | | - | | | | | | | | | | | | - 4 | 41,740}| | | | | | | | | | | - 5 | 33,100}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - 6 | 26,250}| | | | | | | | | | | - | | | | | | | | | | | | - 7 | 20,820}| | | | | | | | | | | - 8 | 16,510}| | | | | | | | | | | - 9 | 13,090}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - 10 | 10,380}| | | | | | | | | | | - ---------+--------+----+----+----+----+----+----+----+----+----+----+ - Size | | .80 power factor | .70 power factor | - of | Area +------------------------+------------------------+ - wire | in | Spacing of | Spacing of | - B.&S. |circular| conductors | conductors | - gauge | mils. +----+----+----+----+----+----+----+----+----+----+ - | | 1" | 3" | 6" | 12"| 24"| 1" | 3" | 6" | 12"| 24"| - ---------+--------+----+----+----+----+----+----+----+----+----+----+ - 500,000 | 500,000|1.00|1.15|1.30|1.47|1.62|1.00|1.00|1.16|1.33|1.49| - 300,000 | 300,000|1.00|1.00|1.09|1.16|1.25|1.00|1.00|1.00|1.02|1.12| - 0,000 | 211,600|1.00|1.00|1.00|1.03|1.10|1.00|1.00|1.00|1.00|1.00| - 000 | 167,800|1.00|1.00|1.00|1.00|1.01|1.00|1.00|1.00|1.00|1.00| - 00 | 133,100|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - 0 | 105,500|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - | | | | | | | | | | | | - 1 | 83,690}| | | | | | | | | | | - 2 | 66,370}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - 3 | 52,630}| | | | | | | | | | | - | | | | | | | | | | | | - 4 | 41,740}| | | | | | | | | | | - 5 | 33,100}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - 6 | 26,250}| | | | | | | | | | | - | | | | | | | | | | | | - 7 | 20,820}| | | | | | | | | | | - 8 | 16,510}| | | | | | | | | | | - 9 | 13,090}|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00|1.00| - 10 | 10,380}| | | | | | | | | | | - ---------+--------+----+----+----+----+----+----+----+----+----+----+ - - EXAMPLE.--A circuit supplying current at 440 volts, 60 - frequency, with 5% loss and .8 power factor is composed of No. 2 - B. & S. gauge wires spaced one foot apart. What is the drop in the - line? - - According to the formula - - % loss × volts - drop = -------------- × S - 100 - - Substituting the given values, and value of S as obtained from - the table for frequency 60 - - 5 × 440 - drop = ------- × 1 = 22 volts - 100 - - -=Current=.--As has been stated, the effect of power factor less than -unity, is to increase the current; hence, in inductive circuit -calculations, the first step is to determine the current flowing in a -circuit. This is done as follows: - - apparent load - current = ------------- (1) - volts - - and - - watts - apparent load = ------------ (2) - power factor - - Substituting (2) in (1) - - watts - ------------ - power factor watts - current = ------------ = --------------------- (3) - volts power factor × volts - - -[Illustration: FIG. 2,704.--Rope type of stranded copper cable -which is used when a high degree of flexibility is required. The -construction of this cable is the stranding together of seven groups, -each containing seven wires and producing a total of 49 wires. -In cases when a greater carrying capacity is desired than can be -obtained through the use of the 7 × 7 or 49 wire cable, the number of -groups is increased to nineteen thereby making a total of 133 wires -(19 × 7).] - - EXAMPLE.--A 50 horse power 440 volt motor has a full load - efficiency of .9 and power factor of .8. How much current is - required? - - Since the brake horse power of the motor is given, it is - necessary to obtain the electrical horse power, thus - - brake horse power 50 - E.H.P. = ----------------- = -- = 55.5 - efficiency .9 - - which in watts is - - 55.5 × 746 = 41,403 - - which is the actual load, and from which - - actual load 41,403 - apparent load = ------------ = ------ = 51,754 - power factor .8 - - The current therefore at 440 volts is - - apparent load 51,754 - ------------- = ------ = 117.6 amperes - volts 440 - - EXAMPLE.--A 50 horse power single phase 440 volt motor, having - a full load efficiency of .92 and power factor of .8, is to be - operated at a distance of 1,000 feet from the alternator. The - wires are to be spaced 6 inches apart and the frequency is 60, and - % loss 5. Determine: =A=, _electrical horse power_; =B=, _watts_; - =C=, _apparent load_; =D=, _current_; =E=, _size of wires_; =F=, - _drop_; =G=, _voltage at the alternator_. - - =A=. _Electrical horse power_ - - brake horse power 50 - E. H. P. = ----------------- × --- = 54.3 - efficiency .92 - - or, - - 54.3 × 746 = 40,508 watts - - =TABLE OF WIRE EQUIVALENTS= - (Brown and Sharpe gauge) - ------+--------+--------+--------+---------+---------+--------- - 0000 | 2 # 0 | 4 # 3 | 8 # 6 | 16 # 9 | 32 # 12 | 64 # 15 - 000 | 2 " 1 | 4 " 4 | 8 " 7 | 16 " 10 | 32 " 13 | 64 " 16 - 00 | 2 " 2 | 4 " 5 | 8 " 8 | 16 " 11 | 32 " 14 | 64 " 17 - 0 | 2 " 3 | 4 " 6 | 8 " 9 | 16 " 12 | 32 " 15 | 64 " 18 - 1 | 2 " 4 | 4 " 7 | 8 " 10 | 16 " 13 | 32 " 16 | 64 " 19 - 2 | 2 " 5 | 4 " 8 | 8 " 11 | 16 " 14 | 32 " 17 | 64 " 20 - 3 | 2 " 6 | 4 " 9 | 8 " 12 | 16 " 15 | 32 " 18 | 64 " 21 - 4 | 2 " 7 | 4 " 10 | 8 " 13 | 16 " 16 | 32 " 19 | 64 " 22 - 5 | 2 " 8 | 4 " 11 | 8 " 14 | 16 " 17 | 32 " 20 | 64 " 23 - 6 | 2 " 9 | 4 " 12 | 8 " 15 | 16 " 18 | 32 " 21 | 64 " 24 - 7 | 2 " 10 | 4 " 13 | 8 " 16 | 16 " 19 | 32 " 22 | 64 " 25 - 8 | 2 " 11 | 4 " 14 | 8 " 17 | 16 " 20 | 32 " 23 | 64 " 26 - 9 | 2 " 12 | 4 " 15 | 8 " 18 | 16 " 21 | 32 " 24 | 64 " 27 - 10 | 2 " 13 | 4 " 16 | 8 " 19 | 16 " 22 | 32 " 25 | 64 " 28 - 11 | 2 " 14 | 4 " 17 | 8 " 20 | 16 " 23 | 32 " 26 | 64 " 29 - 12 | 2 " 15 | 4 " 18 | 8 " 21 | 16 " 24 | 32 " 27 | 64 " 30 - 13 | 2 " 16 | 4 " 19 | 8 " 22 | 16 " 25 | 32 " 28 | - 14 | 2 " 17 | 4 " 20 | 8 " 23 | 16 " 26 | 32 " 29 | - 15 | 2 " 18 | 4 " 21 | 8 " 24 | 16 " 27 | 32 " 30 | - 16 | 2 " 19 | 4 " 22 | 8 " 25 | 16 " 28 | | - 17 | 2 " 20 | 4 " 23 | 8 " 26 | 16 " 29 | | - 18 | 2 " 21 | 4 " 24 | 8 " 27 | 16 " 30 | | - 19 | 2 " 22 | 4 " 25 | 8 " 28 | | | - 20 | 2 " 23 | 4 " 26 | 8 " 29 | | | - 21 | 2 " 24 | 4 " 27 | 8 " 30 | | | - ------+--------+--------+--------+---------+---------+--------- - -=B.= _Watts_ - - watts = E.H.P. × 746 = 54.3 × 746 = 40,508 - -=C.= _Apparent load_ - - apparent load or kva = (actual load or watts ÷ power factor) - - = 40,508 ÷ .8 = 50,635 - -=D.= _Current_ - - current = (apparent load or kva ÷ volts) - = 50,635 ÷ 440 - = 115 amperes - -=E.= _Size of wires_ - - cir. mils = (watts × feet × M) ÷ (% loss × volts^{2}) - = (40,508 × 1,000 × 3,380) ÷ (5 × 440^{2}) - = 141,443 - -From table page 1,907, nearest size _larger_ wire is No. 00 B.&S. -gauge. - -=F.= _Drop_ - - drop = ((% loss × volts) ÷ 100) × S - = ((5 × 440) ÷ 100) × 1.17 - = 25.74 volts - -NOTE.--Values of S are given on page 1910. - -=G.= _Voltage at alternator_ - - alternator pressure = (volts at motor + drop) - = 440 + 25.74 - = 465.7 volts. - - - - -CHAPTER LXVI - -POWER STATIONS - - -The term _power station_ is usually applied to any building -containing an installation of machinery for the conversion of energy -from one form into another form. There are three general classes of -station: - - 1. Central stations; - 2. Sub-stations; - 3. Isolated plants. - -These may also be classified with respect to their function as - - 1. Generating stations; - 2. Distributing stations; - 3. Converting stations. - -and with respect to the form of power used in generating the electric -current, generating stations may be classed as - - 1. Steam electric; - 2. Hydro-electric; - 3. Gas electric, etc. - -=Central Stations.=--It must be evident that the general type of -central station to be adapted to a given case, that is to say, the -general character of the machinery to be installed depends upon the -kind of natural energy available for conversion into electrical -energy, and the character of the electrical energy required by the -consumers. - -This gives rise to a further classification, as - - 1. Alternating current stations; - 2. Direct current stations; - 3. Alternating and direct current stations. - -The alternators or dynamos may be driven by steam or water turbines, -reciprocating engines, or gas engines, according to the character of -the natural energy available. - -[Illustration: FIG. 2,705.--Elevation of small station with direct -drive, showing arrangement of the boiler and engine, piping, etc.] - -=Ques. Why is the reciprocating engine being largely replaced by the -steam turbine, especially for large units?= - -Ans. Because of its higher rotative speed, and absence of a -multiplicity of bearings which in the case of a high speed, -reciprocating engine must be maintained in close adjustment for the -proper operation of the engine. - - The higher speed of rotation results in a more compact unit, - desirable for driving high frequency alternators. - -=Ques. Is the steam turbine more economical than a high duty -reciprocating engine?= - -Ans. No. - -=Location of Central Stations.=--As a rule, central stations should -be so located that the average loss of voltage in overcoming the -resistance of the lines is a minimum, and this point is located -at the center of gravity of the system. In fig. 2,706 is shown a -graphical method of locating this important spot. - -[Illustration: Fig. 2,706.--Diagram illustrating graphical method -of determining the _center of gravity_ of a system in locating the -central station.] - - Suppose a rough canvass of prospective consumers in a district - to be supplied with electric light or power shows the principal - loads to be located at A, B, C, D, E, etc., and for simplicity - assume that these loads will be approximately equal, so that each - may be denoted by 1 for example: - -The relative locations of A, B, C, D, E, etc., should be drawn to -scale (say 1 inch to the 1,000 feet) after which the problem resolves -itself into finding the location of the station with respect to this -scale. - -[Illustration: FIG. 2,707.--Exterior of central station at Lewis, -Ia.; example of very small station located in the principal business -section of a town. It also illustrates the use of a direct connected -gasoline electric set. The central station is located on Main Street, -which is the principal thoroughfare, and is installed in a low one -story building for which a mere nominal rental charge is paid, the -company having the option to buy the property later at the value of -the land plus the cost of the improvements and simple interest on -the same. To the front of an old frame building about 16 feet by -28 feet has been built a neat, well lighted concrete block room, -about 16 feet by 16 feet, carrying the building to the lot line and -affording ample space for the generating set and switchboards, and -such desk room as is needed for the ordinary office business of -the company. In this room, which is finished in natural pine with -plastered walls, has been installed a standard General Electric 25 -kw. gasoline electric generating set consisting of a four cylinder, -four cycle, vertical water cooled, 43-54 H.P. gasoline engine, direct -connected to a three phase, 2,300 volt, 600 R.P.M. alternator with -a 125 volt exciter mounted on the same shaft and in the same frame. -With the generating set is a slate switchboard panel equipped with -three ammeters, one voltmeter, an instrument plug switch for voltage -indication, one single pole carbon break switch, one automatic oil -circuit breaker line switch and rheostats. Instrument transformers -are mounted above and back of the board. For street lighting service -a 4 kw. constant current transformer has been installed, and with -it a gray marble switchboard panel mounted on iron frames and -carrying an ammeter and a four point plug switch. On a board near -the generator set are mounted in convenient reach suitable wrenches, -spanners, and repair parts and tools. To cool the engine cylinders -five 6 × 8 steel tanks have been installed in the old building, a -pump on engine giving forced circulation.] - - The solution consists in first finding the center of gravity - of any two of the loads, such as those at A and B. Since each of - these is 1, they will together have the same effect on the system - as the resultant load of 1 and 1, or 2, located at their center of - gravity, this point being so chosen that the product of the loads - by their respective distances from this point will in both cases - be equal. - - The loads being equal in this case the distances must be equal - in order that the products be the same, so that the center of - gravity of A + B is at G, which point is midway between A and B. - - Considering, next, the resultant load of 2 at G and the load of - 1 at C, the resultant load at the center or gravity of these will - be 3, and this must be situated at a distance of two units from C - and one unit from G so that the distance 2 times the load 1 at C - equals the distance 1 times the load 2 at G. Having thus located - the load 3 at H, the same method is followed in finding the load 4 - at I. Then in like manner the resultant load 4 and the load 1 at E - gives a load 5 at S. - - The point S being the last to be determined represents, - therefore, the position of the center of gravity of the entire - system, and consequently the proper position of the plant in order - to give the minimum loss of voltage on the lines. - -=Ques. Is the center of gravity of the system, as obtained in fig. -2,706, the proper location for the central station?= - -Ans. It is very rarely the best location. - -=Ques. Why?= - -Ans. Other conditions, such as the price of land, difficulty of -obtaining water, facilities for delivery of coal and removal of -ashes, etc., may more than offset the minimum line losses and copper -cost due to locating the station at the center of gravity of the -system. - -[Illustration: FIG. 2,708.--Map of Cia Docas de Santos hydro-electric -system; an example of station location remote from the center of -distribution. In the figure A is the intake; B, flume; C, forebay; -D, penstocks; E, power house; F, narrow gauge railway; G, general -store; H, point of debarkation; I, transmission line; J, dead ends; -K, sub-station. Santos, in the republic of Brazil, is one of the -great coffee shipping ports of the world, and for the development of -its water front has required an elaborate system of quays. These have -been developed by the Santos Dock Company, which holds a concession -for the whole water front. The company, needing electric power for -its own use, has developed a system deriving its power from a point -about thirty miles from the city, where a small stream plunges -down the sea coast from the mountain range that runs along it. The -engineers have estimated that 100,000 horse power can be obtained -from this source.] - -=Ques. How then should the station be located?= - -Ans. The more practical experience the designer has had, and the -more common sense he possesses, the better is he equipped to handle -the problem, as the solution is generally such that it cannot be -worked out by any rule of thumb method. - -[Illustration: FIG. 2,709.--Station location. The figure shows -two distribution centers as a town A and suburb B supplied with -electricity from one station. For minimum cost of copper the location -of the station would be at G, the center of gravity. However, it is -very rarely that this is the best location. For instance at C, land -is cheaper than at G, and there is room for future extension to the -station, as shown by the dotted lines, whereas at G, only enough -land is available for present requirements. Moreover C is near the -railroad where coal may be obtained without the expense of cartage, -and being located at the river, the plant may be run condensing thus -effecting considerable economy. The conditions may sometimes be such -that any one of the advantages to be secured by locating the station -at C may more than offset the additional cost of copper.] - -=Ques. What are the general considerations with respect to the price -of land?= - -Ans. The cost for the station site may be so high as to necessitate -building or renting room at a considerable distance from the district -to be supplied. - - If the price of land selected for the station be high, the - running expenses will be similarly affected, inasmuch as more - interest must then be paid on the capital invested. - - The price or rent of real estate might also in certain - instances alter the proposed interior arrangement of the station, - particularly so in the case of a company with small capital - operating in a city where high prices prevail. In general, - however, it may be stated that whatever effect the price of real - estate would have upon the arrangement, operation and location - of a central station it can quite readily and accurately be - determined in advance. - -=Ques. With respect to the cost of the land what should be especially -considered?= - -Ans. Room for the future extension of the plant. - - Although such additional space need not be purchased at the time - of the original installation it is well, if possible, to make - provision whereby it can be obtained at a reasonable figure when - desired. The preliminary canvass of consumers will aid in deciding - the amount of space advisable to allow for future extensions; as a - rule, however, it is wise to count on the plant enlarging to not - less than twice its original size, as often the dimensions have to - be increased four and even six times those found sufficient at the - beginning. - -[Illustration: FIG. 2,710.--Section of the central station or -"electricity works" at Derby, showing boiler and engine room and -arrangement of bunkers, conveyor, ash pit, grates, boilers (drum, -heating surface and superheater), economizer, flue, turbines, -condenser pumps, etc.; also location of switchboard gallery and -system of piping.] - -=Ques. What trouble is likely to be encountered with an illy located -plant after it is in operation?= - -Ans. It may be considered a nuisance by those residing in the -vicinity, occasioning many complaints. - -[Illustration: FIG. 2,711.--View of old and new Waterside stations. -The new station at the right has an all turbine equipment of ten -units, some Curtis and some Parsons machines, two have a capacity of -14,000 kw., and the remaining eight are of 12,000 kw. each. The old -Riverside station, seen at the left is described on page 1940.] - - Thus, if the plant be placed in a residential section of the - community the smoke, noise and vibration of the machines may - become a nuisance to the surrounding inhabitants, and eventually - end in suits for damage against the company responsible for the - same. For these and the other reasons just given a company is - sometimes forced to disregard entirely the location of a central - station near the center of gravity of the system, and build at a - considerable distance; such a proceeding would, if the distance be - great, necessitate the installation of a high pressure system. - - There might, however, be certain local laws in force restricting - the use of high pressure currents on account of the danger - resulting to life, that would prevent this solution of the - problem. In such cases there could undoubtedly be found some site - where the objections previously noted would be tolerated; thus, - there would naturally be little objection to locating next to a - stable, a brewery, or a factory of any description. - -=Ques. Why is the matter of water supply important for a central -station?= - -Ans. Because, in a steam driven plant, water is used in the boilers -for the production of steam by boiling, and if the engines be of the -condensing type it is also used in them for creating a vacuum into -which the exhaust steam passes so as to increase the efficiency of -the engine above what it would be if the exhaust steam were obliged -to discharge into the comparatively high pressure of the atmosphere. - - The force of this will be apparent by considering that the - water consumption of the engine ordinarily is from 15 to 25 lbs. - of "feed water" per horse power per hour, and the amount of - "circulating water" required to maintain the vacuum is about 25 to - 30 times the feed water, and in the case of turbines with their 28 - or 29 inch vacuum, much more. For instance, a 1,000 horse power - plant running on 15 lbs. of feed water and 30 to 1 circulating - water would require (1,000 × 15) × (30 + 1) = 465,000 lbs. or - 55,822 gals. per hour at full capacity. - -=Ques. Besides price what other considerations are important with -respect to water?= - -Ans. Its quality and the possibility of a scarcity of supply. - -It is quite necessary that the water used in the boilers should be -as free as possible from impurities, so as to prevent the deposition -within them of any scale or sediments. The quality of the water used -for condensing purposes, however, is not quite so important, although -the purer it is the better. - -If the plant is to be located in a city, the matter of water supply -need not generally be considered, because, as a rule, it can be -obtained from the waterworks; there will then, of course, be a water -tax to consider and this, if large, may warrant an effort being -made to obtain the water in some other way. In any event, however, -the possibility of a scarcity in the supply should be reduced to a -minimum. - -If the plant be located in the country, some natural source of water -would be utilized unless the place be supplied with waterworks, which -is not generally the case. It is usual, however, to find a stream, -lake or pond in the vicinity, but if none such be conveniently near, -an artesian or other form of well must be sunk. - -If abundance of water exist in the vicinity of the proposed -installation, not only would the location of the plant be governed -thereby, but the kind of power to be used for its operation would -depend thereon. Thus, if the quantity of the water were sufficient -throughout the entire year to supply the necessary power, water -wheels might be installed and used in place of boilers and steam -engines for driving the generators. The station would then, of -course, be situated close to the waterfall, regardless of the center -of gravity of the system. - -[Illustration: FIG. 2,712.--View illustrating the location of a -station as governed by the presence of a water falls. In such cases -the natural water power may be at a considerable distance from the -center of gravity of the distribution system because of the saving -in generation. In the case of long distance transmission very -high pressure may be used and a transformer step down sub-station -be located at or near the center of gravity of the system, thus -considerably reducing the cost of copper for the transmission line.] - -=Ques. What should be noted with respect to the coal supply?= - -Ans. The facility for transporting the coal from the supply point to -the boiler room. - - In this connection, an admirable location, other conditions - permitting, is adjacent to a railway line or water front so that - coal delivered by car or boat may be unloaded directly into the - bins supplying the boilers. - - If the coal be brought by train, a side or branch track will - usually be found convenient, and this will usually render any - carting of the fuel entirely unnecessary. - - In whatever way the coal is to be supplied, the liability of a - shortage due to traffic or navigation being closed at any time of - the year should be well looked into, as should also the facility - for the removal of ashes, before deciding upon the final location - for the plant. - -[Illustration: FIG. 2,713.--View of a station admirably located with -respect to transportation of the coal supply. As shown, the coal may -be obtained either by boat or rail, and with modern machinery for -conveying the coal to the interior of the station, the transportation -cost is reduced to a minimum.] - -[Illustration: FIG. 2,714.--Floor plan of part of the turbine central -station erected by the Boston Edison Co., showing two 5,000 kw. -Curtis steam turbines in place. The complete installation contains -twelve 5,000 kw. Curtis steam turbines, a sectional elevation being -shown in fig. 2,758, page 1,971.] - -=Choice of System.=--The chief considerations in the design of a -central station are economy and capacity. When the current has to be -transmitted long distances for either lighting or power purposes, -economy is attainable only by reducing the weight of the copper -conductors. This can be accomplished only by the use of the high -voltage currents obtainable from alternators. - -Again, where the consumers are located within a radius of two miles -from the central station, thereby requiring a transmission voltage of -550 volts or less, dynamos may be employed with greater economy. - -Alternating current possesses serious disadvantages for certain -important applications. - -For instance, in operating electric railways and for lighting it is -often necessary to transmit direct current at 500 volts a distance -of five or ten miles. In such cases, the excessive drop cannot be -economically reduced by increasing the sizes of the line wire, while -a sufficient increase of the voltage would cause serious variations -under changes of load. Hence, it is common practice to employ some -form of auxiliary generator or booster, which when connected in -series with the feeder, automatically maintains the required pressure -in the most remote districts so long as the main generators continue -to furnish the normal or working voltage. - -The advantage of a direct current installation in such cases over a -similar plant supplying alternating current line is the fact that a -storage battery may be used in connection with the former for taking -up the fluctuations of the current, thereby permitting the dynamo to -run with a less variable load, and consequently at higher efficiency. - -=Ques. Name some services requiring direct current.= - -Ans. Direct current is required for certain kinds of electrolytic -work, such as electro-plating, the electrical separation of -metals, etc., also the charging of storage batteries for electric -automobiles. - -[Illustration: FIG. 2,715.--Example of central station located -remote from the distributing center and furnishing alternating -current at high pressure to a sub-station where the current is passed -through step down transformers and supplied at moderate pressure -to the distribution system. In some cases the sub-station contains -also converters supplying direct current for battery charging, -electro-plating, etc.] - -=Ques. How is direct current supplied?= - -Ans. Sometimes the central station is equipped with suitable -apparatus for supplying both direct and alternating current. This may -be accomplished in several different ways: By installing both direct -and alternating current generators in the central station; by the use -of double current generators or dynamotors, from which direct current -may be taken from one side and alternating current from the other -side; or by installing, in the sub-station of an alternating current -central station, in addition to the transformers usually placed -therein, a rotary converter for changing or converting alternating -current into direct current. - - Thus, it is evident that the character of a central station - will be governed to a great extent by the class of services to be - supplied. - - An exception to this is where the entire output has to be - transmitted a long distance to the point of utilization. - - In such cases a copper economy demands the use of high tension - alternating current, and its distribution to consumers may be made - directly by means of step down transformers mounted near by or - within the consumers' premises, or it may be transformed into low - voltage alternating current by a conveniently located sub-station. - - Where the current is to be used chiefly for lighting and there - are only a few or no motors to be supplied, the choice between - direct current and alternating current will depend greatly upon - the size of the installation, direct current being preferable - for small installations and alternating current for large - installations. - - If the current is to be used primarily for operating machinery, - such as elevators, travelling cranes, machine tools and other - devices of a similar character, which have to be operated - intermittently and at varying speeds and loads, direct current is - the more suitable; but if the motors performing such work can be - operated continuously for many hours at a time under practically - constant loads, as, for instance in the general work of a pumping - station, alternating current may be employed with advantage. - -[Illustration: FIG. 2,716.--Diagram illustrating diversity factor. -By definition _diversity factor = combined actual maximum demand of -a group of customers divided by the sum of their individual maximum -demands_. Example, a customer has fifty (50) watt lamps and, of -course, the sum of the individual maximum demands of the lamps is 2.5 -kw. watts ("connected load"). The customer's maximum demand, however, -is 1.5 kw. Hence, the diversity factor[A] of the customer's group of -lamps is 1.5 ÷ 2.5 = .6. In the diagram the ordinates of the curves -show the ratio _maximum demand_ to _connected load_ for various kinds -of electric lighting service in Chicago.] - -[A] NOTE.--The diversity factor of a customer's group of lamps, -namely, the ratio of maximum demand to connected load is usually -called the _demand factor_ of the customer. - -=Size of Plant.=--Before any definite calculation can be made, or -plans drawn, the engineer must determine the probable load. This is -usually ascertained in terms of the number and distances of lamps -that will be required, by making a thorough canvass of the city or -town, or that portion for which electrical energy is to be supplied. -The probable load that the station is to carry when it begins -operation, the nature of this load, and the probable rate of increase -are matters upon which the design and construction chiefly depend. - -[Illustration: FIG. 2,717.--Load curve for one day.] - -=Ques. What is the nature of the load carried by a central station?= - -Ans. It fluctuates with the time of day and also with the time of -year. - -=Ques. How is a fluctuating load best represented?= - -Ans. Graphically, that is to say by means of a curve plotted on -coordinate paper of which ordinates represent load values and the -corresponding abscissæ time values, as in the accompanying curves. - -=What is the nature of a power load?= - -Ans. Where electricity is supplied for power purposes to a number of -factories, the load is fairly steady, dropping, of course, during -meal hours. In the case of traction, the average value of the load is -fairly steady but there are momentarily violent fluctuations due to -starting cars or trains. - -[Illustration: FIG. 2,718.--Load curve for one year.] - -=Ques. What is the peak load?= - -Ans. The maximum load which has to be carried by the station at any -time of day or night as shown by the highest point of the load curve. - -=Ques. Define the load factor.= - -Ans. The machinery of the station evidently must be large enough to -carry the peak load, and therefore considerably in excess of that -required for the average demand. The ratio of the average to the -maximum load is called the load factor. - - There are two kinds of load factor: the annual, and the daily. - - The annual load factor is obtained as a percentage by - multiplying the number of units sold (per year) by 100, and - dividing by the product of the maximum load and the number of - hours in the year. The daily load factor is obtained by taking the - figures for 24 hours instead of a year. - -[Illustration: FIG. 2,719.--Load curve of plant supplying power for -the operation of motors in a manufacturing district. The horizontal -dotted lines show suitable power ratings. A properly designed steam -plant has a large overload capacity, a hydraulic plant has a small -overload capacity, and a gasoline engine plant has no overload -capacity. Accordingly, the peak of the load (maximum load) may be 25 -or 30 per cent. in excess of the rated capacity of a steam plant, -not more than 5 or 10 per cent. in excess of the rated capacity of a -hydraulic plant, not at all in excess of the rated capacity of a gas -engine plant.] - -=Ques. What must be provided in addition to the machinery required to -supply the peak load?= - -Ans. Additional units must be installed for use in case of repairs or -break down of some of the other units. - - EXAMPLE.--What would be the boiler horse power required to - generate 5,000 kw. under the following conditions: Efficiency of - generators 85%; efficiency of engines 90%; feed water of engines - and auxiliaries 15 lbs. per I. H. P.; boiler pressure 175 lbs.; - temperature of feed water 150° Fahr? With a rate of combustion of - 15 lbs. of coal per sq. foot of grate per hour and an evaporation - (from and at 212°) of 8 lbs. of water per lb. of coal, what area - of grate would be required and how much heating surface? - - 5,000 kw. = 5,000 ÷ .746 = 6,702= electrical horse power - - To obtain this electrical horse power with alternators whose - efficiency is 85% requires - - 6,702 ÷ .85 = 7,885 brake horse power at the engine - - This, with mechanical efficiency of 90% is equivalent to - - 7,885 ÷ .9 = 8,761 indicated horse power - - Since 15 lbs. of feed water are required for the engines and - auxiliaries per indicated horse power per hour, the total feed - water or evaporation required to generate 5,000 kw. is - - 15 × 8,761 = 131,415 lbs. per hour. - - that is to say, the boilers must be of sufficient capacity - to generate 131,415 lbs. of steam per hour from water at a - temperature of 150° Fahr. This must be multiplied by the _factor - of evaporation_ for steam at 175 lbs. pressure from feed water at - a temperature of 150°, in order to get the equivalent evaporation - "_from and at 212_°." - - The formula for the factor of evaporation is - - H - _h_ - factor of evaporation = ------- (1) - 965.7 - - in which - - H = total heat of steam at the observed pressure; - _h_ = total heat of feed water of the observed temperature; - 965.7 = latent heat, of steam at atmospheric pressure. - - Substituting in (1) values for the observed pressure and - temperature as obtained from the steam table - - 1,197 - 118 - factor of evaporation = ------------ = 1.117 - 965.7 - - for which the equivalent evaporation "_from and at 212_°" is - - 131,415 × 1.117 = 146,791 lbs.= per hour - - - =FACTORS OF EVAPORATION= - - -------------+-----------------------------------------------------+ - Temp of | STEAM PRESSURE BY GAUGE | - feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+ - Deg. Fahr. | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 | - -------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+ - 32 |1.214|1.216|1.220|1.222|1.225|1.227|1.229|1.231|1.232| - 40 |1.206|1.209|1.212|1.214|1.216|1.219|1.220|1.222|1.224| - 50 |1.195|1.197|1.201|1.204|1.206|1.208|1.210|1.212|1.214| - 60 |1.185|1.188|1.191|1.193|1.196|1.198|1.200|1.202|1.203| - 70 |1.175|1.178|1.180|1.183|1.185|1.187|1.189|1.191|1.193| - 80 |1.164|1.167|1.170|1.173|1.175|1.177|1.179|1.181|1.183| - 90 |1.154|1.157|1.160|1.162|1.165|1.167|1.169|1.170|1.172| - 100 |1.144|1.147|1.150|1.152|1.154|1.156|1.158|1.160|1.162| - 110 |1.133|1.136|1.139|1.142|1.144|1.146|1.148|1.150|1.152| - 120 |1.123|1.126|1.129|1.131|1.133|1.136|1.138|1.140|1.141| - 130 |1.113|1.116|1.118|1.121|1.123|1.125|1.127|1.129|1.130| - 140 |1.102|1.105|1.108|1.110|1.113|1.115|1.117|1.119|1.120| - 150 |1.091|1.095|1.098|1.100|1.102|1.104|1.106|1.108|1.110| - 160 |1.081|1.084|1.087|1.090|1.092|1.094|1.096|1.098|1.100| - 170 |1.070|1.074|1.077|1.079|1.081|1.083|1.085|1.087|1.089| - 180 |1.060|1.063|1.066|1.069|1.071|1.073|1.075|1.077|1.079| - 190 |1.050|1.053|1.056|1.058|1.060|1.063|1.065|1.066|1.068| - 200 |1.039|1.043|1.045|1.048|1.050|1.052|1.054|1.056|1.058| - 210 |1.029|1.032|1.035|1.037|1.040|1.042|1.044|1.046|1.047| - -------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+ - Temp of | STEAM PRESSURE BY GAUGE | - feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+ - Deg. Fahr. | 140 | 150 | 160 | 170 | 180 | 190 | 200 | 210 | 220 | - -------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+ - 32 |1.234|1.236|1.237|1.239|1.240|1.241|1.243|1.244|1.245| - 40 |1.226|1.227|1.229|1.230|1.232|1.233|1.234|1.236|1.237| - 50 |1.215|1.217|1.218|1.220|1.221|1.223|1.224|1.225|1.226| - 60 |1.205|1.207|1.208|1.210|1.211|1.212|1.214|1.215|1.216| - 70 |1.194|1.196|1.197|1.199|1.200|1.202|1.203|1.205|1.206| - 80 |1.184|1.186|1.187|1.189|1.190|1.192|1.193|1.194|1.195| - 90 |1.174|1.176|1.177|1.179|1.180|1.181|1.183|1.184|1.185| - 100 |1.164|1.165|1.167|1.168|1.170|1.171|1.172|1.174|1.175| - 110 |1.153|1.155|1.156|1.158|1.159|1.160|1.162|1.163|1.164| - 120 |1.143|1.145|1.146|1.147|1.149|1.150|1.151|1.153|1.154| - 130 |1.132|1.134|1.136|1.137|1.138|1.140|1.141|1.142|1.144| - 140 |1.122|1.124|1.125|1.127|1.128|1.129|1.131|1.132|1.133| - 150 |1.111|1.113|1.115|1.116|1.118|1.119|1.120|1.121|1.123| - 160 |1.101|1.103|1.104|1.106|1.107|1.108|1.110|1.111|1.112| - 170 |1.091|1.092|1.094|1.095|1.097|1.098|1.099|1.101|1.102| - 180 |1.080|1.082|1.083|1.085|1.086|1.088|1.089|1.090|1.091| - 190 |1.070|1.071|1.073|1.074|1.076|1.077|1.078|1.080|1.081| - 200 |1.059|1.061|1.063|1.064|1.065|1.067|1.068|1.069|1.071| - 210 |1.049|1.051|1.052|1.053|1.055|1.056|1.057|1.059|1.060| - -------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+ - Temp. of | STEAM PRESSURE BY GAUGE | - feed water. +-----+-----+-----+-----+-----+-----+-----+-----+-----+ - Deg. Fahr. | 230 | 240 | 250 | 260 | 270 | 280 | 290 | 300 | | - -------------+-----+-----+-----+-----+-----+-----+-----+----+------+ - 32 |1.246|1.247|1.248|1.250|1.251|1.252|1.253|1.254| - 40 |1.238|1.239|1.240|1.241|1.242|1.243|1.244|1.245| - 50 |1.228|1.229|1.230|1.231|1.232|1.233|1.234|1.235| - 60 |1.217|1.218|1.219|1.220|1.221|1.222|1.223|1.224| - 70 |1.207|1.208|1.209|1.210|1.211|1.212|1.213|1.214| - 80 |1.196|1.198|1.199|1.200|1.201|1.202|1.203|1.204| - 90 |1.186|1.187|1.188|1.189|1.190|1.191|1.192|1.193| - 100 |1.176|1.177|1.178|1.179|1.180|1.181|1.182|1.183| - 110 |1.166|1.167|1.168|1.169|1.170|1.171|1.172|1.173| - 120 |1.155|1.156|1.157|1.158|1.159|1.160|1.161|1.162| - 130 |1.145|1.146|1.147|1.148|1.149|1.150|1.151|1.152| - 140 |1.134|1.135|1.136|1.137|1.138|1.139|1.140|1.141| - 150 |1.124|1.125|1.126|1.127|1.128|1.129|1.130|1.131| - 160 |1.113|1.115|1.116|1.117|1.118|1.119|1.120|1.121| - 170 |1.103|1.104|1.105|1.106|1.107|1.108|1.109|1.110| - 180 |1.093|1.094|1.095|1.096|1.097|1.098|1.099|1.100| - 190 |1.082|1.083|1.084|1.085|1.086|1.087|1.088|1.089| - 200 |1.072|1.073|1.074|1.075|1.076|1.077|1.078|1.079| - 210 |1.061|1.062|1.063|1.064|1.065|1.066|1.067|1.068| - -------------+-----+-----+-----+-----+-----+-----+-----+-----+ - - One boiler horse power being equal to _an evaporation of_ 34½ - _lbs. of water from a feed water temperature of 212° Fahr., into - steam at the same temperature_, the boiler capacity is accordingly - - 148,105 ÷ 34.5 = 4,293 boiler horse power. - - The rate of evaporation is given at 8 lbs. of water (from and at - 212° Fahr.), for which the fuel required is - - 148,105 ÷ 8 = 18,513 lbs. of coal per hour. - - For a rate of combustion of 15 lbs. of coal per hour per square - foot of grate, - - grate area = 18,513 ÷ 15 = 1,234 sq. ft. - - For stationary boilers the usual ratio of heating surface to - grate area is 35:1, accordingly the heating surface corresponding - to this ratio is - - 1,234 × 35 = 43,190 sq.ft. - - The above calculation is based on a rate of evaporation of 8 - lbs. of water per lb. of coal and a rate of combustion of 15 lbs. - of coal per sq. ft. of grate. For other rates the required grate - area may be obtained from the following table: - - ---------------------------------------------------------------------- - GRATE SURFACE PER HORSE POWER (KENT) - ------------+------+-----+-------------------------------------------- - |Pounds| | - | of | Lbs.| Pounds of coal burned per square foot of - |water | of | grate per hour - | from | coal+----+----+----+----+----+----+----+----+---- - |and at| per | | | | | | | | | - | 212° | h.p.| 8 | 10 | 12 | 15 | 20 | 25 | 30 | 35 | 40 - | per | per | | | | | | | | | - |pound | hour+----+----+----+----+----+----+----+----+---- - | of | | Square feet grate per horse power - | coal | | - ------------+------+-----+----+----+----+----+----+----+----+----+---- - Good coal | }10 | 3.45| .43|.35 | .28| .23| .17| .14| .11| .10| .09 - and boiler | } 9 | 3.83| .48| .38| .32| .25| .19| .15| .13| .11| .10 - | | | | | | | | | | | - Fair coal |{8.61 | 4. | .50| .40| .33| .26| .20| .16| .13| .12| .10 - or boiler |{8 | 4.31| .54| .43| .36| .29| .22| .17| .14| .13| .11 - |{7 | 4.93| .62| .49| .41| .33| .24| .20| .17| .14| .12 - | | | | | | | | | | | - Poor coal |{6.9 | 5. | .63| .50| .42| .34| .25| .20| .17| .15| .13 - or boiler |{6 | 5.75| .72| .58| .48| .38| .29| .23| .19| .17| .14 - |{5 | 6.9 | .86| .69| .58| .46| .35| .28| .23| .22| .17 - | | | | | | | | | | | - Lignite and|}3.45 |10. |1.25|1.00| .83| .67| .50| .40| .33| .29| .25 - poor boiler|} | | | | | | | | | | - ------------+------+-----+----+----+----+----+----+----+----+----+---- - -=General Arrangement of Station.=--In designing an electrical -station, it is preferable that whatever rooms or divisions of the -interior space are desired should determine the total outside -dimensions of the plant in the original plans of the building than -that these latter dimensions be fixed and the rooms, etc., be fitted -in afterward. - - =SAVING DUE TO HEATING THE FEED WATER= - - Table showing the percentage of saving for each degree of increase in - temperature of feed water heated by waste steam. - - ---------------------------------------------------------------------- - Init| Pressure of steam in boiler, lbs. per sq. inch above atmosphere - temp|----------------------------------------------------------------- - of | - feed| 0 | 20 | 40 | 60 | 80 | 100 | 120 | 140 | 160 | 180 | 200 - ----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- - 32°|.0872|.0861|.0855|.0851|.0847|.0844|.0841|.0839|.0837|.0835|.0833 - 40 |.0878|.0867|.0861|.0856|.0853|.0850|.0847|.0845|.0843|.0841|.0839 - 50 |.0886|.0875|.0868|.0864|.0860|.0857|.0854|.0852|.0850|.0848|.0846 - 60 |.0894|.0883|.0876|.0872|.0867|.0864|.0862|.0859|.0856|.0855|.0853 - 70 |.0902|.0890|.0884|.0879|.0875|.0872|.0869|.0867|.0864|.0862|.0860 - 80 |.0910|.0898|.0891|.0887|.0883|.0879|.0877|.0874|.0872|.0870|.0868 - 90 |.0919|.0907|.0900|.0895|.0888|.0887|.0884|.0883|.0879|.0877|.0875 - 100 |.0927|.0915|.0908|.0903|.0899|.0895|.0892|.0890|.0887|.0885|.0883 - 110 |.0936|.0923|.0916|.0911|.0907|.0903|.0900|.0898|.0895|.0893|.0891 - 120 |.0945|.0932|.0925|.0919|.0915|.0911|.0908|.0906|.0903|.0901|.0899 - 130 |.0954|.0941|.0934|.0928|.0924|.0920|.0917|.0914|.0912|.0909|.0907 - 140 |.0963|.0950|.0943|.0937|.0932|.0929|.0925|.0923|.0920|.0918|.0916 - 150 |.0973|.0959|.0951|.0946|.0941|.0937|.0934|.0931|.0929|.0926|.0924 - 160 |.0982|.0968|.0961|.0955|.0950|.0946|.0943|.0940|.0937|.0935|.0933 - 170 |.0992|.0978|.0970|.0964|.0959|.0955|.0952|.0949|.0946|.0944|.0941 - 180 |.1002|.0988|.0981|.0973|.0969|.0965|.0961|.0958|.0955|.0953|.0951 - 190 |.1012|.0998|.0989|.0983|.0978|.0974|.0971|.0968|.0964|.0062|.0960 - 200 |.1022|.1008|.0999|.0993|.0988|.0984|.0980|.0977|.0974|.0972|.0969 - 210 |.1033|.1018|.1010|.1003|.0998|.0994|.0990|.0987|.0984|.0981|.0979 - 220 | -- |.1029|.1019|.1013|.1008|.1004|.1000|.0997|.0994|.0991|.0989 - 230 | -- |.1039|.1031|.1024|.1018|.1012|.1010|.1007|.1003|.1001|.0999 - 240 | -- |.1050|.1041|.1034|.1029|.1024|.1020|.1017|.1014|.1011|.1009 - 250 | -- |.1062|.1052|.1045|.1040|.1035|.1031|.1027|.1025|.1022|.1019 - ----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- - - NOTE.--An approximate rule for the conditions of ordinary - practice is a saving of 1 per cent. made by each increase of - 11° in the temperature of the feed water. This corresponds to - .0909 per cent. per degree. The calculation of saving is made as - follows: Boiler pressure, 100 lbs. gauge; total heat in steam - above 32° = 1,185 B.T.U. feed water, original temperature 60°, - final temperature 209°F. Increase in heat units, 150. Heat units - above 32° in feed water of original temperature = 28. Heat units - in steam above that in cold feed water, 1,185-28 = 1,157. Saving - by the feed water heater = 150 ÷ 1,157 = 12.96 per cent. The - same result is obtained by the use of the table. Increase in - temperature 150° × tabular figure .0864 = 12.96 per cent. Let - total heat of 1 lb. of steam at the boiler pressure = H; total - heat of 1 lb. of feed water before entering the heater = _h'_, and - after passing through the heater = _h''_; then the saving made by - the heater is (_h''_-_h'_) ÷ (H-_h'_). - -Under usual conditions the plans of an electrical station are readily -drawn, as they are generally of a simple nature. The engines and -generators will occupy the majority of the space, and these are -usually placed in one large room; in some stations, however, they are -located respectively in two adjacent rooms. The boilers are generally -located in a room apart from the engines and dynamos, and in some -cases a separate building is provided for them; the pumps, etc., must -be installed not far from the boilers, and space must also be allowed -near the boilers for coal and ashes. - -[Illustration: FIG. 2,720.--Floor plan of an electrical station -having a belted drive with counter shaft.] - - Fig. 2,720 shows the floor plan of an electrical station, in - which a countershaft and belted connections are used between - the engines and generators. Referring first to the plan of the - building itself, A represents the engine and dynamo room, B - denotes the boiler room, C the office, D the store room, and E the - chimney connected with the boilers by means of the uptake _w_. - Referring next to the apparatus installed, S, S, S, S represents - a battery of four boilers; these are connected by steam piping - VV to the two steam engines, M and M, which are belted to the - countershaft O. Belted to the countershaft are the generators, - T, T, T, T, the circuits from which are controlled on the - switchboard, H. - -=Ques. What are the objections to the arrangement shown in -fig. 2,720.=? - -Ans. The large space required by the belt drive especially in -locations where land is expensive. Another objection is the -frictional loss due to the belt drive with its countershaft, etc. - -[Illustration: FIG. 2,721.--Elevation of station having a belted -drive with countershaft, as shown in plan in fig. 2,720.] - -=Ques. What are the desirable features of the belt drive?= - -Ans. High speed generators may be used, thus reducing the first -cost, and the multiplicity of speeds and flexibility of the system -resulting from the use of a friction clutch. - - Thus in fig. 2,720, each pulley may be mounted on the counter - shaft O with a friction clutch. A jaw clutch may also be provided - at Z, thus permitting the shaft O to be divided into two sections. - It is therefore possible by this arrangement to cause either of - the engines to drive any one of the generators, or all of them, or - both of the engines to drive all of the generators simultaneously. - -=Ques. Under what condition is the counter shaft belt drive -particularly valuable?= - -Ans. In case of a break down of any one of the engines or generators, -and also when it becomes necessary to clean them without interrupting -the service. - -[Illustration: FIG. 2,722.--Plan of station arranged for extension. -The space required for a central station depends upon the number and -kind of lights to be supplied, and upon the character and arrangement -of the machinery. In calculating the size of building required, two -things must be carefully considered: first, the building must be -adapted to the plant to be installed in the beginning; and second, it -must be arranged so that enlargement can be made without disarranging -or interfering with the plant already in existence. This is usually -best secured by providing for expansion in one or two definite -directions, the building being made large enough to accommodate -additional units that will be necessary at some future time because -of the growth of the community and consequent increased demand for -electric current.] - -=Ques. How may the design in fig. 2,720 be modified for the -installation of a storage battery?= - -Ans. If a storage battery be necessary, a partition may be -constructed across the room A, as indicated by the dotted lines, and -the battery installed in the room thus formed. - -[Illustration: FIG. 2,723.--Interior of old Riverside station -showing at the right, seven 6,000 horse power alternators driven by -reciprocating engines, and at the left, a number of turbine units -aggregating 90,000 horse power.] - -=Ques. Mention a few details in the general arrangement of the -building fig. 2,720.= - -Ans. Two doors to the room A may conveniently be provided at K and L, -the former connecting with the boiler room B, and the latter serving -as the main entrance to the station. There is little that need be -added to what has already been stated regarding the boiler room B. -The door at F provides for the entrance of coal and the removal of -ashes, while at P, the pump and heaters may conveniently be located. -In the office C, visitors may be received, the station reports made -out, bulletins issued from time to time, and whatever engineering -problems arise may here be solved on paper by the engineer in -charge of the plant. The store room D will be found convenient for -various supplies, tools and appliances needed in the operation of -the station. These may here be kept under lock and key and the daily -waste and loss resulting from carelessness avoided. - -=Ques. What important point should be noted in locating the engines -and boilers?= - -Ans. They should be so placed that the piping between them will be as -short and direct as possible. - -=Ques. Why?= - -Ans. The steam pipe should be short to reduce the loss of heat -between engine and boiler to a minimum, and both short and direct to -avoid undue friction and consequent drop in pressure of the steam in -passing through the pipe to the engine. - - Entirely too little attention is given to this matter on the - part of designers and it cannot be too strongly emphasized that, - for economy, the steam pipe between an engine and boiler should - be as short and direct as possible, having regard of course, for - proper piping methods. - -=Ques. What should be provided for the steam pipe?= - -Ans. A heavy covering of approved material should be placed around -the pipe to reduce the loss of heat by radiation. For this purpose -hair felt, mineral wool and asbestos are used. - -[Illustration: FIG. 2,724.--View of engine and condenser, showing how -to arrange the piping to secure good vacuum. _Locate the condenser as -near the engine as possible_; =use easy bends= _instead of elbows; -place the pump_ =below= _bottom of condenser so the water will -drain to pump_. At A is a relief valve, for protection in case the -condenser become flooded through failure of the pump, and at B is -a gate valve to shut off condenser in case atmospheric exhaust is -desired to permit repairs to be made to condenser during operation. -=A water seal= should be maintained on the relief valve and =special -attention= _should be given to the stuffing box_ of the gate valve -=to prevent air leakage=. _The discharge valve of the pump should be -water sealed._] - -=Ques. How should the piping be arranged between the engine and -condenser, and why?= - -Ans. It should be as short and direct as possible; especially should -elbows be avoided so that the back pressure on the engine piston will -be reduced as near as can be to that of the condenser. - - That is to say, in order to get nearly the full effect of the - vacuum in the condenser the frictional resistance of the piping - should be reduced to a minimum. - - Where 90° turns are necessary, easy bends should be used instead - of sharp elbows. The force of this argument must be apparent by - noting the practice of steam turbine builders of placing the - turbine right up against the condenser, and remembering that a - high vacuum is necessary to the economical working of a turbine. - See fig. 1,445, page 1,182. - -=Ques. What are the considerations respecting the number and type of -engine to be used?= - -Ans. In the illustration fig. 2,720, two engines M and M' are -employed, one belted to each end of the countershaft O. These engines -should be of similar or identical pattern; for a small output -they may be either simple or compound, as the conditions of fuel -expenditure may dictate, but if the output be large, triple expansion -engines or turbines are advisable. - -[Illustration: FIG. 2,725.--"Dry pipe" for horizontal boiler: it is -connected to the main outlet and its upper surface is perforated with -small holes, the far end being closed. With this arrangement steam is -taken from the boiler over a large area, so that it will contain very -little moisture. _All horizontal boilers without a dome should be -fitted with a dry pipe;_ most engineers do not realize the importance -of obtaining dry steam for engine operation.] - - Corliss or similar slow speed engines may advantageously be used - in either case. In all cases the engine should be run condensing - unless the cost for circulating water is prohibitive; even in such - cases cooling towers may be installed and effect a saving. - - In operation, during the greater part of the day, one engine - running two or perhaps three of the generators, will carry the - load, but when the load is particularly heavy, as in the morning - and evening, both engines and all the generators may be required - to meet the demands. - -[Illustration: FIG. 2,726.--Method of connecting a header to a -battery of boilers. Where two or more boilers are connected to a -single header, the use of a reliable non-return boiler stop valve is -necessary, and in some countries their installation is compulsory. A -non-return boiler stop valve will instantly close should the pressure -in the boiler to which it is attached suddenly decrease below that -in the header, and thereby prevent the entrance of steam from the -other boilers of the battery. This sudden decrease in pressure may be -caused by a ruptured fitting or the blowing out of a tube, in which -event an ordinary stop valve taking the place of a non-return boiler -stop valve would be inadequate, as the loss of steam from the other -boilers of the battery would be tremendous before an ordinary valve -could be reached and closed, assuming that it would be possible to do -so, which in the majority of cases it is not. Should it be desired -to cut out a boiler for cleaning or repairs, the non-return boiler -stop valve will not permit steam to enter the boiler from the header, -even should the handwheel be operated for this purpose, as it cannot -be opened by hand, but can, however, be closed. A non-return boiler -stop valve should be attached to each boiler and connected to an -angle valve on the header. A pipe bend should be used for connecting -the valves, as this will allow for expansion and contraction. The -pipe should slope a trifle downward toward the header and a suitable -drain provided. This drain should be opened and all water permitted -to escape before the angle valve is opened, thereby preventing any -damage due to water hammer.] - - By exercising a little ingenuity in shifting the load on - different machines at different times, both engines and dynamos, - may readily be cleaned and repaired without interrupting the - service. - -=Ques. For economy what kind of steam should be used?= - -Ans. Super-heated steam. - - The saving due to the use of superheated steam is about 1% for - every ten degrees Fahr. of super-heat. It should be used in all - cases. - -=Ques. How should the machines be located?= - -Ans. Sufficient space should be allowed between them that cleaning -and repairing may be done easily, quickly and effectually. - -[Illustration: FIGS. 2,727 and 2,728.--Method of preventing vibration -and of supporting pipes. The figures show top and side views of -a main header carried in suitable frames fitted with adjustable -roller. While the pipe is illustrated as resting on the adjustable -rollers, nevertheless the rollers may also be placed at the sides -or on top of the pipe to prevent vibration, or in cases where the -thrust from a horizontal or vertical branch has to be provided for. -This arrangement will take care of the vibration without in any way -preventing the free expansion and contraction of the pipe.] - -=Ques. How should the switchboard be located?= - -Ans. In fig. 2,720, the switchboard H is mounted against the wall -dividing the room A from the room B, and is in line with the machines. - - The advantages arising from a switchboard thus installed are, - that the switchboard attendant working thereon can obtain at any - time an unobstructed view of the performance of each individual - machine, and he has in consequence a much better control of them; - then, too, while he is engaged at the engines or generators he - can also see the measuring instruments on the switchboard, and - ascertain approximately the readings upon them. - - In cases of emergency it is sometimes necessary for the - engineer in charge of a plant to be in several places at the same - time in order to prevent an accident, and that this seemingly - impossibility may be approximated as nearly as possible, it is - essential that the controlling devices be located as closely - together as is consistent, and that no moving belt or pulley - intervene between them. - - These conditions are well satisfied in fig. 2,720, and owing to - the short distances between the generators and the switchboard the - drop of voltage in each of the conducting wires between them will - be low. - - This latter advantage is worthy of notice in a station - generating large currents at a low pressure. To offset the - advantages just mentioned, the location of the switchboard in - line with the machines introduces an element of danger to the - switchboard, its apparatus, and the attendant, on account of the - possible bursting of a flywheel or other parts of the machines - from centrifugal force. - -[Illustration: FIGS. 2,729 and 2,730.--Points on placing stop valves. -The first and most important feature is to ascertain whether the -valve will act as a water trap for condensed steam. Fig. 2,729 -illustrates a common error in the placing of valves, as this -arrangement permits of an accumulation of condensed steam above the -valve when closed, and should the engineer be careless and open the -valve suddenly, serious results might follow owing to water-hammer. -Fig. 2,730 illustrates the correct method of placing the valve. It -sometimes occurs, however, that it is not convenient to place the -valve as shown in fig. 2,730 and that fig. 2,729 is the only manner -in which the valve can be placed. In such cases, the valve should -have a drain, and this drain should always be opened before the large -valve is opened.] - - If the switchboard be placed in the dotted position at H', or, - in fact, at the opposite end of the room A, the damage to life - and property that might result from the effects of centrifugal - force would be eliminated, but in place thereof would be the - disadvantages of an obstructed view of the machines from the - switchboard, an obstructed view of the switchboard from the - machines, inaccessibility between these two, and a greater drop - of voltage in the majority of the conducting wires between the - generators and the switchboard. - -=Ques. Describe a second arrangement of station with belt drive and -compare it with the design shown in fig. 2,720.= - -[Illustration: FIG. 2,731.--Plan of electrical station with belt -drive without counter shaft. The installation here represented -consists of two boilers, S, etc., and three sets of engines and -generators, T, M, etc. Sufficient allowance has been made in the -plans, however, for future increase of business, as additional space -has been provided for an extra engine and generator set, as indicated -by the dotted lines.] - -Ans. A floor plan somewhat different from that presented in fig. -2,720 is shown in fig. 2,731. Here a belt drive is employed, but no -countershaft is used. Each generator, therefore, is dependent upon -its respective engine, and in consequence the flexibility obtained by -the use of a countershaft is lost. On the other hand, there is less -loss of mechanical power between the engines and generators in the -driving of the latter, and less floor space is necessary in the room -A. If, however, the floor area of this room be made the same as in -the previous arrangement and the same number of machines are to be -installed, they may be spaced further apart, affording in consequence -considerably more room for cleaning and repairing them. - - In operation, the normal conditions should be such that any two - of the engine and generator sets may readily carry the average - load, the third set to be used only as a reserve either to aid the - other two when the load is unusually heavy or to replace one of - the other sets when it becomes necessary to clean or repair the - latter. - - The switchboard may perhaps be best located at H, as a similar - position on the opposite side of the room A would bring it beneath - one or more of the steam pipes and thus endanger it should a - possible leakage occur from these pipes. If located at H, however, - it will be in line with the machines, and therefore will be - subject to the disadvantages previously mentioned for such cases; - consequently it might be as well to place it at the further end - of the room, either against the partition (shown dotted) of the - storage battery room if this be built, or else (if no storage - battery is to be installed), against the end wall itself. The - nearer end of the room A would not be very desirable for the - switchboard installation on account of being so far removed from - the machines, and therefore more or less inaccessible from them. - Outside of what has now been mentioned, the division of the floor - plan and the arrangement therein is practically the same as in - fig. 2,720, accordingly what has already been stated regarding the - former installation applies, therefore, with equal force to the - present installation. - -=Ques. Describe a plant with direct drive.= - -Ans. This type of drive is shown in fig. 2,732. Each engine is -directly connected to a generator, that is, the main shafts of both -are joined together in line so that the generator is driven without -the aid of a belt. - -=Ques. What is the advantage of direct drive?= - -Ans. The great saving in floor space, which is plainly shown in fig. -2,732, the portion A' representing the saving which results over the -installations previously illustrated in figs. 2,720 and 2,731. - -=Ques. How could the floor space be further reduced?= - -Ans. By employing vertical instead of horizontal engines. - -=Ques. What should be done before drawing the plans for the station?= - -Ans. The types of the various machines and apparatus to be installed -should, as nearly as possible, be selected in advance so that their -approximate dimensions may serve as a guide in drawing up the plans -of the building. - -[Illustration: FIG. 2,732.--Plan of electrical station containing -direct connected units. As shown, space is provided for an extra -boiler and engine and generator set, as indicated by the dotted -lines. Space also exists for a storage battery room if necessary, and -the partition dividing this room from the engine and dynamo room is -shown by a dotted line as in previous cases.] - - Owing to the great difference in these dimensions for the - various types, and in fact for the same types as manufactured by - different concerns, no definite rules regarding the necessary - space required can here be given. In a general way, however, the - author has endeavoured to indicate by the drawings the relative - amounts of space that ordinarily would be considered sufficient. - -=Ques. What is the disadvantage of direct drive?= - -Ans. A more expensive generator is required because it must run at -the same speed as the engine, which is relatively low as compared -with that of a belted generator. - -=Station Construction.=--The construction or rearrangement of the -building intended for the plant is a problem that under ordinary -conditions would be solved by an architect, or at least by an -architect with the assistance of an electrical or mechanical -engineer, still there are many installations where the electrical -engineer has been compelled to design the building. - -In such instances he should be equipped with a general knowledge of -the construction of buildings. - -=Foundations.=--The foundation may be either natural or artificial; -that is, it may be composed of rock or soil sufficiently solid -to serve the purpose unaided, or it may be such as to require -strengthening by means of wood or iron beams, etc. In either case any -tendency toward a considerable settling or shifting of the foundation -due to the action of water, frost, etc., after the station has been -completed must be well guarded against. To this end special attention -should be given to the matter of drainage. - -=Ques. How should the foundation be constructed for the machines?= - -Ans. The foundations constructed for the machines should be entirely -separate from that built for the walls of the building, so that the -vibrations of the former will not affect the latter. - - If there be several engines and dynamos to be installed, it is - best to construct two foundations, one for the engines and one for - the dynamos. If, however, there be considerable distance between - the units, it may be advisable to build a separate foundation - for each engine and for each dynamo. The material of which these - foundations are composed should if the machines be of 20 horse - power or over, possess considerable strength and be impervious to - moisture. Brick, stone and concrete are desirable for the purpose, - and only the best quality of cement mortar should be employed. - Care must be taken that lime mortar is not used in place of - cement mortar, as the former is not well adapted to withstand the - vibrations of the machines without crumbling. - -[Illustration: FIG. 2,733.--Angle for foundation footing. In ordinary -practice the footing courses upon which the walls of the building -proper rest, consist of blocks or slabs of stone as large as are -available and convenient to handle. Footings of brick or concrete are -also used in very soft soils; footings consisting of timber grillage -are often employed. A grillage of iron or steel beams has also -been used successfully. The inclination of the angle φ, of footing -should be about as follows: for metal footings 75°; for stone, 60°; -for concrete, 45°; for brick, 30°. Damp proof courses of slate, or -layer of asphalt are laid in or on the foundations or lower walls to -prevent moisture arising or penetrating by capillary attraction.] - -=Ques. Describe a method of constructing foundations.= - -Ans. An excavation is made to the desired depth and a form inserted -corresponding to the desired dimensions for the foundation. A -template is placed on top locating all the centers, with iron -pipes suspended from these centers, two or three sizes larger than -the anchor bolts. At the lower end of the pipes are core boxes. -Concrete is poured into the mould thus formed, and when hard, the -forms are removed thus leaving the solid foundation. The anchor -bolts are inserted through the pipes and passed through iron plates -at the lower end as shown in fig. 2,734, being secured by nuts. By -using pipe of two or three bolt diameters a margin is provided for -adjustment so the bolts will pass through the holes in the frame of -the machine thus allowing for any slight errors in laying out the -centers on the template. - -[Illustration: FIG. 2,734.--Concrete foundation showing method of -installing the anchor bolts.] - -=Ques. What is the object of the openings in the bottom of the -foundation?= - -Ans. In case of a defective bolt, it may be replaced by a new one -without injury to the foundation. - -=Walls.=--Regarding the material for the walls of the station iron, -stone, brick and wood may be considered. Of these, iron in the form -of sheets or plates would be entirely fireproof, but being itself -a conductor would introduce difficulties in maintaining a high -insulation resistance of the current carrying circuits; it would -also make the building difficult to heat in winter and to keep cool -in summer. Stone in the form of limestone, granite or sandstone, as -a building material is desirable for solidity and attractiveness; -it is also fireproof and an insulator, but the high cost of such a -structure for an electrical station usually prohibits its use except -in private plants or in electrical stations located in large cities. - -[Illustration: FIG. 2,735.--View showing part of template for -locating anchor bolt centers, pipes through which the bolts pass -and bolt boxes at lower end of bolts. The completed foundation is -shown in fig. 2,734, with template removed. The template is made of -plain boards upon which the center lines are drawn, and bolt center -located. Holes are bored at the bolt centers to permit insertion of -the pipes as shown.] - -Brick is a good material and is readily obtained in nearly all parts -of the country; it is comparatively cheap, and is also an insulating -and fireproof material. The bricks selected for this purpose should -possess true sharp edges, and be hard burned. - -=Ques. What are the features of wood?= - -Ans. Wood forms the cheapest material that can be used for the walls -of electrical stations, and it usually affords satisfaction, but has -the disadvantage of high fire risk. - -=Roofs=.--In fig. 2,736 is shown one form of construction for the -roof of an electrical station. The end view here presented shows -the upper portion of the walls at B and D; these support the iron -trusses C, and the roof proper MN. In many stations there is provided -throughout the length of the building, a monitor or raised structure -on the peak of the roof for ventilation and light. The end view of -the monitor is shown at S in the figure; its sides should be fitted -with windows adjustable from the floor. - -[Illustration: FIG. 2,736.--One form of roof construction.] - -=Floors.=--The floor of the station should be so designed that -it will be capable of supporting a reasonable weight, but as the -weights of the machines are borne entirely by their respective -foundations the normal weight upon the floor will not be great; for -short periods, however, it may be called upon to support one or two -machines while they are being placed in position or interchanged, -and due allowance must be made for such occurrences. - -Station floors for engine and dynamo rooms are, as a rule, -constructed of wood. Where very high currents are generated, however, -insulated floors of special construction mounted on glass are -necessary as a protection from injurious shocks. Brick, concrete, -cement, and other substances of a similar nature are objectionable -as a floor material for engine and dynamo rooms on account of the -grit from them, caused by constant wear, being liable to get into the -bearings of the machines. - -Where there are no moving parts, however, as in the boiler room, the -materials just mentioned possess no disadvantages and are preferable -to wood on account of being fireproof. - - =THEORETICAL DRAFT PRESSURE IN INCHES OF WATER IN= - =A CHIMNEY 100 FEET HIGH= - - (For other heights the draft varies directly as the height) - - Temp. in TEMP. OF EXTERNAL AIR. (BAROMETER 30 INCHES) - Chimney, °F. 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100° - 200° .453 .419 .384 .353 .321 .292 .263 .234 .209 .182 .157 - 220 .488 .453 .419 .388 .355 .326 .298 .269 .244 .217 .192 - 240 .520 .488 .451 .421 .388 .359 .330 .301 .276 .250 .225 - 260 .555 .528 .484 .453 .420 .392 .363 .334 .309 .282 .257 - 280 .584 .549 .515 .482 .451 .422 .394 .365 .340 .313 .288 - 300 .611 .576 .541 .511 .478 .449 .420 .392 .367 .340 .315 - 320 .637 .603 .568 .538 .505 .476 .447 .419 .394 .367 .342 - 340 .662 .638 .593 .563 .530 .501 .472 .443 .419 .392 .367 - 360 .687 .653 .618 .588 .555 .526 .497 .468 .444 .417 .392 - 380 .710 .676 .641 .611 .578 .549 .520 .492 .467 .440 .415 - 400 .732 .697 .662 .632 .598 .570 .541 .513 .488 .461 .436 - 420 .753 .718 .684 .653 .620 .591 .563 .534 .509 .482 .457 - 440 .774 .739 .705 .674 .641 .612 .584 .555 .530 .503 .478 - 460 .793 .758 .724 .694 .660 .632 .603 .574 .549 .522 .497 - 480 .810 .776 .741 .710 .678 .649 .620 .591 .566 .540 .515 - 500 .829 .791 .760 .730 .697 .669 .639 .610 .586 .559 .534 - -=Chimneys.=--These are generally constructed of brick and iron, -sometimes of concrete. Iron chimneys cost less than brick chimneys, -necessitate less substantial foundations, and are free from the -liability of cracking. They must be painted to prevent corrosion, are -less substantial, and lose considerably more heat by radiation than -do brick chimneys. - -[Illustration: FIG. 2,737.--An example of direct connected unit with -gas engine power. The view shows a Westinghouse 200 kva., 4,000 volt, -three phase, 60 cycle alternator direct connected to a gas engine.] - -[Illustration: FIG. 2,738.--Curves showing comparative costs of -chimney and mechanical draft. In certain of these, the cost of the -existing chimney is known, and that of the complete mechanical draft -plant is estimated, while in others, the cost of mechanical draft -installation is determined from the contract price, and the expense -of a chimney to produce equivalent results is calculated. Costs are -shown for both single, forced and induced engine driven fans and -for duplex engine driven plants, in which either fan may serve as a -relay. An apparatus of the latter type is the most expensive, and -finds its greatest use where economizers are employed.] - -Both brick and iron chimneys, require an inner wall or lining of -brick, which forms the flue proper, and in order that this wall be -not cracked by sudden cooling an air space is left between it and -the outer wall. In a brick chimney the inner wall need not extend -much beyond half the height of the chimney, but when iron is used it -should reach to the top. - -=Ques. Upon what does the force of natural draught in a chimney -depend?= - -Ans. It depends upon the difference between the weight of the column -of hot gases inside the chimney and the weight of a like column of -the cold external air. - -[Illustration: FIGS. 2,739 and 2,740.--Substituting mechanical -draught in place of chimney. The relative proportions of a brick -chimney, and of the smoke pipe required when mechanical draft is -introduced are forcibly shown in the illustrations, which show the -works of the B.F. Sturtevant Co., at Jamaica Plain, Mass. The removal -of the boilers to a position too far distant from the existing -chimney to permit of its longer fulfilling its office, led to the -substitution of an induced draft fan and the subsequent removal of -the chimney. The present stack or smoke pipe, barely visible in fig. -2,740, extends only 31 feet above the ground, and no trouble is -experienced from smoke.] - -=Ques. How is the intensity of the draught expressed?= - -Ans. In terms of the number of inches of a water column sustained by -the pressure produced. - -=Ques. Are high chimneys necessary?= - -Ans. No. - - _Chimneys above 150 feet in height are very costly, and their - increased cost is not justified by increased efficiency._ - -[Illustrations: FIGS. 2,741 to 2,744.--Installation of forced draft -system to old boiler plant. The figures illustrate the simplest -method. The fan which is of steel plate with direct connected double -cylinder engine, is placed immediately over the end of a brick duct -into which the air is discharged. This duct is carried under ground -across the front of the boilers, to the ash pits of each of which -connection is made through branch ducts. Each branch duct opening -is provided with special ash pit damper, operated by notched handle -bar, as illustrated in the detail. This method of introduction -serves to distribute the air within the ash pit, and to secure even -flow through the fuel upon the grate above. Of course, the ash pit -doors must remain closed in order to bring about this result. A -chimney of sufficient height to merely discharge the gases above -objectionable level is all that is absolutely necessary with this -arrangement. Although the introduction of a fan in an old plant is -usually evidence of the insufficiency of the existing chimney to meet -the requirements, such a chimney, will, however, usually serve as a -discharge pipe for the gases when the fan is employed. The fan thus -becomes more than a mere auxiliary to the chimney; it practically -supplants it so far as the method of draught production is concerned.] - - The latest chimney practice is to build two or more small - chimneys instead of one large one. A notable example is the - Spreckels Sugar Refinery in Philadelphia, where three separate - chimneys are used for one boiler plant of 7,500 horse power. The - three chimneys are said to have cost several thousand dollars less - than an equivalent single chimney. - - =Very tall chimneys= have been characterized by one writer as - "_monuments to the folly of their builders._" - -[Illustration: FIGS. 2,745 and 2,746.--Comparison of chimney draft -and mechanical draft. The illustrations show a plant of 2,400 H.P. of -modern water tube boilers, 12 in number, set in pairs and equipped -with economizers. Fig. 2,745 indicates the location of a chimney, 9 -feet in internal diameter by 180 feet high, designed to furnish the -necessary draft; fig. 2,746 represents the same plant with a complete -duplex induced draught apparatus substituted for the chimney, and -placed above the economizer connections. Each of the two fans is -driven by a special engine, direct connected to the fan shaft, and -each is capable of producing draft for the entire plant. A short -steel plate stack unites the two fan outlets and discharges the gases -just above the boiler house roof. All of the room necessary for the -chimney is saved, and no valuable space is required for the fans.] - -=Ques. How is mechanical draft secured?= - -Ans. In two ways, known respectively as _induced draught_ and _forced -draught_. - -=Ques. Describe the method of induced draft.= - -Ans. A fan is located in the smoke flue, and which in operation -draws the gases through the furnace and discharges them into a -_short_ chimney. - -=Ques. Describe the method of forced draft.= - -Ans. In this method, air is forced into the furnace underneath the -grate bars by means of a fan or a steam jet blower. - -[Illustration: FIG. 2,747.--Forced draft plant with hollow bridge -wall at the Crystal Water Co., Buffalo, N. Y. The air is delivered to -the ash pit via the hollow bridge wall, being supplied under pressure -by the blower seen at the side of the boiler setting. As shown, the -blower is operated by a small reciprocating engine; however, compact -blowing units with steam turbine drive can be had and which are -designed to be placed in the boiler setting.] - -=Ques. What is the application of the two systems?= - -Ans. Induced draft is installed mostly in new plants, while forced -draft is better adapted to old plants. - -=Steam Turbines=.--It is not the author's intention to discuss at -length the steam end of the electric plant, because too much space -would be required, and also because the subject belongs properly -to the field of mechanical engineering rather than electrical -engineering. However, because of the recent introduction of the steam -turbine for the direct driving of large generators, and the fact -that it is now almost universally used in large central stations, a -detailed explanation of its principles and construction may not be -out of place. - -[Illustration: FIG. 2,748.--Longitudinal section of elementary -Parsons type steam turbine. The turbine consists essentially of a -fixed casing, or cylinder, and a revolving spindle or drum. The ends -of the spindle are extended in the form of a shaft, carried in two -bearings A and B, and, excepting the small parts of the governing -mechanism and the oil pump, these bearings are the only rubbing -parts in the entire turbine. Steam enters from the steam pipe at C -and passes through the main throttle or regulating valve D, which, -as actually constructed, is a balanced valve. This valve is operated -by the governor through suitable controlling mechanism. The steam -enters the cylinder through the passage E and, turning to the left -passes through alternate stationary and revolving rows of blades, -finally emerging from them at F and flowing through the connection -G to the condenser or to the atmosphere, depending upon whether the -turbine is condensing or non-condensing. Each row of blades, both -stationary and revolving, extends completely around the turbine and -the steam flows through the full annulus between the spindle and -the cylinder. In an ideal turbine the lengths of the blades and the -diameter of the spindle which carries them would continuously and -gradually increase from the steam inlet to the exhaust. Practically, -however, the desired effect is produced by making the spindle in -steps, there being generally three such steps or stages, H, J and K. -The blades in each step are arranged in groups of increasing length. -At the beginning of each of the larger steps, the blades are usually -shorter than at the end of the preceding smaller step, the change -being made in such a way that the correct relation of blade length -to spindle diameter is secured. The steam, acting as previously -described, produces a thrust tending to force the spindle toward -the left, as seen in the cut. This thrust, however, is counteracted -by the "balance pistons," L, M and N, which are of the necessary -diameter to neutralize the thrust on the spindle steps, H, J and K, -respectively. These elements are called "pistons" for convenience, -although they do not come in contact with the cylinder, but both -the pistons and the cylinder are provided with alternate rings -which form a labyrinth packing to retard the leakage of steam. In -order that each balance piston may have the proper pressure on both -sides, equalizing passages O, P and Q are provided connecting the -balance pistons with the corresponding stages of the blading. The end -thrust being thus practically neutralized by means of the balance -pistons, the spindle "floats" so that it can be easily moved in one -direction or the other. In order to definitely fix the position of -the spindle, a small adjustable collar bearing is provided at R, -inside the housing of the main bearing B. This collar bearing is -adjustable so as to locate and hold the spindle in such position so -that there will be such a clearance between the rings of the balance -piston and those of the cylinder, that the leakage of steam will be -reduced to a minimum and, at the same time, prevent actual contact -under varying conditions of temperature. Where the shaft passes out -of the cylinder, at S and T, it is necessary to provide against -in-leakage of air or out-leakage of steam by means of glands. These -glands are made tight by water packing without metallic contact. The -shaft of the turbine is extended at U and coupled to the shaft of -the alternator by means of a flexible coupling. The high pressure -turbines are so proportioned that, when using steam as previously -described, they have enough capacity to take care of the ordinary -fluctuations of load when controlled by the governor through the -valve D, thus insuring maximum economy of steam consumption at -approximately the rated load. To provide for overloads, the valve V -is supplied to admit steam to an intermediate stage of the turbine. -This valve shown diagrammatically in the illustration, is arranged -to be operated by the governor and is, according to circumstances, -located either as shown by the illustration, or at another stage of -the turbine.] - -[Illustration: FIG. 2,749.--Arrangement of blading in Parsons type -turbine, consisting of alternate moving and stationary blades. The -path taken by the steam is indicated by the arrows.] - -A turbine is a machine in which a rotary motion is obtained by -transference of the _momentum_ of a fluid or gas. In general the -fluid is guided by fixed blades, attached to a casing, and, impinging -on other blades mounted on a drum or shaft, causing the latter to -revolve. - -Turbines are classed in various ways as: 1, _radial flow_, when the -steam enters near the center and escapes toward the circumference; -and 2, _parallel flow_, when the steam travels _axially_ or parallel -to the length of the turning body. - -Turbines are commonly, yet erroneously classed as: - - 1. Impulse; - 2. Reaction. - -=Ques. What is the distinction between these two types?= - -Ans. In the so called impulse type, _steam enters and leaves the -passages between the vanes at the same pressure_. In the so called -reaction type, _the pressure is less on the exit side of the vanes -than on the entrance side_. - - Fig. 2,750 is a sectional view of the Parsons-Westinghouse - parallel flow turbine. Steam from the boiler enters first a - receiver in which are the governor controlled admission valves. - These valves are actuated by a centrifugal governor. - -[Illustration: FIG. 2,750.--Sectional view of Parsons-Westinghouse -turbine, showing rotor and governor.] - - _Steam does not enter the turbine in a continuous blast, but - intermittently, or in puffs._ The speed regulation is therefore - accomplished by proportioning the duration of these puffs to the - load of the engine, this being effected by the governor, fig. - 2,752. - - The governor of the turbine has only to move a small pilot - valve, or slide, E, which admits steam under the piston F, and - lifts the throttle valve proper off its seat. - - As soon as the pilot valve closes, the spring shifts the main - throttle valve. Thus, at light loads, the main throttle or - admission valve is continually opening and shutting at uniform - intervals, the length of time during which it remains open - depending upon the load. - - As the load increases, the duration of the valve opening also - increases, until at full load the valve does not reach its seat at - all and the steam flows steadily through the turbine. The steam - thus admitted flows into the annular passage A, fig. 2,750, by the - opening S, and then past the blades, revolving the rotor. - - When the load increases above the normal rated amount a - secondary pilot valve is moved by the same means, this in turn - admitting steam to a piston, similar to F, which lifts another - throttle valve. This admits steam into the annular space I, so - that it acts upon the larger diameter of the drum or rotor, giving - largely increased power for the time being. - - The levers or arms of the governor are mounted upon knife edges - instead of pins, making it extremely sensitive. The tension spring - may be adjusted by hand while the turbine is running. - -[Illustration: FIG. 2,751.--Sectional view of a combination impulse -and reaction single flow turbine. This is a modification of the -single flow type, in which the smallest barrel of reaction blading is -replaced by an impulse wheel. Steam is admitted to the nozzle block -A, is expanded in the nozzles and discharged against a portion of the -periphery of the impulse wheel. The intermediate and low pressure -stages are identical with the corresponding stages in the single flow -type. The substitution of the impulse element for the high pressure -section of reaction blading has no influence one way or another on -the efficiency. That is to say the efficiency of an impulse wheel is -about the same at the least efficient section of reaction blading. -This design is attractive, however, in that it shortens the machine -materially, and gives a stiffer design of rotor. The entering steam -is confined in the nozzle chamber until its pressure and temperature -have been materially reduced by expanding through the nozzles. -As the nozzle chamber is cast separately from the main cylinder, -the temperature and pressure differences to which the cylinder is -subjected are correspondingly lessened. However, probably on account -of its small diameter at the high pressure section, the straight -Parsons type has always shown itself to be adequate for all of the -steam pressures and temperatures encountered in ordinary practice.] - - The governor does not actually move the pilot valve, but shifts - the point L in fig. 2,752. A reciprocating motion is given to the - rod I by a small eccentric on the governor shaft; this is driven - by worm gearing shown near O in fig. 2,750, so that the eccentric - makes one revolution to about eight of the turbine. Thus, with a - turbine running 1,200 revolutions, the rod I would be moved up and - down 150 times per minute. As the points A and H are fixed, the - motion is conveyed to the small pilot valve E, thus giving 150 - puffs a minute. The governor in shifting the point L brings the - edge of the pilot valve nearer the port and so cuts off the steam - earlier. - - The annular diameter or space between the rotor and the stator - is gradually increased from inlet to exhaust, the blades being - made longer in each ring. When the mechanical limit is reached, - the diameter of the rotor is increased as at I and D so as to - keep the length of blade within bound. - - Balance pistons as at B, C, F are attached to the rotor, - their office being to oppose end thrust upon those blades in - corresponding diameter of the rotor. Communication is established - through the passage V and pipe M between the eduction pipe and the - back of these pistons, thus increasing the efficiency of their - balancing and also taking care of any leakage past them. - - A small thrust bearing T prevents end play of the rotor, and is - adjustable to maintain the proper clearance between the rings of - blades; this varies from ⅛ inch at the admission to 1 inch at the - exhaust. This bearing also takes up any extra unbalanced thrust. A - turbine should operate with a high vacuum, because without this it - does not compare favorably with an ordinary reciprocating engine - from the point of economy. - -[Illustration: FIG. 2,752.--Sectional view of governor of the -Parsons-Westinghouse turbine.] - - _Separate air pumps are provided to create the vacuum._ - - Where the ordinary type of vertical air pump is employed, a - booster or _vacuum increaser_ is added, as nothing below 26 inches - is advisable, 28 and 29 inches being always striven for. It is - also preferable to use a certain amount of _super-heat_ with steam - turbines. - - To assist in producing the high vacuum, exhaust passages are - made large, the eduction passage E in fig. 2,750 being nearly - twenty-three times the area of the steam pipe. - - Among other details, a noteworthy feature is a small oil pump K, - which circulates oil through bearings of the machinery, the oil - being drawn from the tank under the governor shaft and gravitating - there after use. No pressure of oil is employed. Stuffing rings - prevent leakage; these consist of alternate grooves and collars in - shaft and bearing, like the grooves in an indicator piston. - -=Ques. Why is a high vacuum desirable?= - -Ans. Because the turbine is capable of expanding the steam to a very -low terminal pressure, and this is necessary for economy. - -=Ques. What may be said of the working pressures for turbines?= - -Ans. To meet the varied conditions of service, turbines are designed -to operate with: 1, high pressure, 2, low pressure, or 3, mixed -pressure. - -[Illustration: FIG. 2,753.--Sectional view of a double flow turbine. -The maximum economical capacity of a single flow turbine is limited -by the rotative speed. The economical velocity at which the steam may -pass through the blades of the turbine depends on the velocity of -the moving blades. The capacity of the turbine depends on the weight -of the steam passed per unit of time, which in turn depends on the -mean velocity and the height of the blades. For a given rotative -speed, the mean diameter of blade ring practicable is limited by -the allowable stresses due to centrifugal force, and there is a -practical limit for the height of the blades. Now if the rotative -speed be taken only half as great, the maximum diameter of the rotor -may be doubled and, without increasing the height of the blades, the -capacity of the turbine will be doubled. So with the single flow -steam turbine as well as with the single crank reciprocating engine, -there is a practical limiting economical capacity for any given -speed. If this limit be reached with a single crank reciprocating -engine, a unit of double the power may be produced at the same speed -by coupling two single crank engines to one shaft. Similar results -are secured making a double flow turbine which is in effect, as will -be seen from the figure, two single flow turbines made up in a single -rotor in a single casing with a common inlet and two exhausts. Steam -enters the nozzle block, acts on the impulse element, and then the -current divides, one-half of the steam going through the reaction -blading at the left of the impulse wheel; the remainder passes over -the top of the impulse wheel and through the impulse blading at the -right.] - - High pressure turbines operate at about the same initial - pressure as triple expansion engines. - - Low pressure, as here applied, means the exhaust pressure of the - reciprocating engine from which the exhaust steam passes through - the turbine before entering the condenser. - - Mixed pressure implies that the exhaust steam is supplemented, - for heavy loads, by the admission of live steam. - -=Ques. What determines the working pressure?= - -Ans. When all the power is furnished by the turbine, it is designed -for high pressure; when operated in combination with a reciprocating -engine, low pressure is used for constant load, and mixed pressure -for variable load. - -[Illustration: FIG. 2,754.--Sectional view of a semi-double flow -turbine. This is a modification in which the intermediate section -of reaction blading is single flow, and the low pressure section -only is double flow. This would be analogous to a four cylinder -triple expansion engine, that is, one with one high pressure, one -intermediate pressure and two low pressure cylinders--a design not at -all uncommon in very large engines in which the required dimensions -of a single low pressure cylinder would be prohibitive. Such turbines -are useful for capacities greater than is desirable for a single -flow turbine, and which are still below the maximum possibilities of -a double flow turbine of the same speed. In such machines the best -efficiency is secured by making the intermediate blading in a single -section large enough to pass the entire quantity of steam. A "dummy" -similar to those used on the single flow Parsons type, shown at the -right of the impulse wheel, compels all of the steam to pass through -the single intermediate section of the reaction blading, and balances -the end thrust due to this section. When the steam issues from the -intermediate section, the current is divided, one-half passing -directly to the adjacent low pressure section, while the other half -passes through the holes shown in the periphery of the hollow rotor -and through the rotor itself, beyond the dummy ring, into the other -low pressure section at the left hand end of the turbine.] - - NOTE.--There are logical engineering reasons for the existence - of the several types of turbine, viz., single flow, double flow, - and semi-double flow. The double flow turbine is not inherently - superior to the single flow design, but is used under conditions - for which the single flow machine is unsuitable. Similarly, the - semi-double flow is recommended only for conditions which it can - meet more satisfactorily than either of the other types. - - NOTE.--Low pressure turbines use exhaust steam from - non-condensing engines and are valuable as an adjunct to existing - plants for the purpose of increasing economy and capacity with a - minimum outlay for new equipment. - - NOTE.--Bleeder turbines are for use in plants which are required - to furnish, not only power, but also considerable and varying - quantities of low pressure steam for heating purposes. In these - turbines a part of the steam after it has done work in the high - pressure stages may be diverted to the heating system, and the - remainder expanded through the low pressure blading and exhausted - into the condenser. In this way none of the energy of the heating - steam, due to the difference of pressure between the boiler and - the heating system is wasted. On the other hand if no steam is - required for heating purposes, the turbine operates just as - efficiently as though the bleeder feature were absent. - -[Illustration: FIG. 2,755.--Westinghouse valve gear with steam relay. -In the smaller turbines, the governor acts directly on the steam -admission valves, opening first the primary valve, and then, if -necessary, the secondary valve, after the primary is fully open. In -turbines of the single flow Parsons type, the governor actuates two -small valves controlling ports leading to steam relay cylinders which -operate the admission valves. The little valve controlling the relay -cylinder for the secondary valve has more lap than the other and -consequently does not come into action until the primary valve has -attained its maximum effective opening. The figure shows the general -design of this type of valve gear.] - - _The De Laval steam turbine_ is termed by its builders a high - speed rotary steam engine. It has but a single wheel, fitted with - vanes or buckets of such curvature as has been found to be best - adapted for receiving the impulse of the steam jet. There are no - stationary or guide blades, the angular position of the nozzles - giving direction to the jet. The nozzles are placed at an angle - of 20 degrees to the plane of motion of the buckets. The best - energy in the steam is practically devoted to the production of - velocity in the expanding or divergent nozzle, and the velocity - thus attained by the issuing jet of steam is about 4,000 feet per - second. To attain the maximum efficiency, the buckets attached to - the periphery of the wheel against which this jet impinges should - have a speed of about 1,900 feet per second, but, owing to the - difficulty of producing a material for the wheel strong enough to - withstand the strains induced by such a high speed, it has been - found necessary to limit the peripheral speed to 1,200 or 1,300 - feet per second. - - It is well known that in a correctly designed nozzle the - adiabatic expansion of the steam from maximum to minimum pressure - will convert the entire static energy of the steam into kinetic - energy. Theoretically this is what occurs in the De Laval nozzle. - The expanding steam acquires great velocity, and the energy - of the jet of steam issuing from the nozzle is equal to the - amount of energy that would be developed if an equal volume of - steam were allowed to adiabatically expand behind the piston - of a reciprocating engine, a condition, however, which for - obvious reasons has never yet been attained in practice with the - reciprocating engine. But with the divergent nozzle the conditions - are different. - - _The Curtis turbine_ is built by the General Electric Company at - their works in Schenectady, N. Y., and Lynn, Mass. They are of the - horizontal and vertical types. In the vertical type the revolving - parts are set upon a vertical shaft, the diameter of the shaft - corresponding to the size of the machine. - - The shaft is supported by and runs upon a step bearing at the - bottom. This step bearing consists of two cylindrical cast iron - plates bearing upon each other and having a central recess between - them into which lubricating oil is forced under pressure by a - steam or electrically driven pump, the oil passing up from beneath. - -[Illustration: FIGS. 2,756 and 2,757.--Westinghouse valve gear with -oil relay. Governors for the larger turbines, particularly those of -the combination impulse and reaction double, or single double flow -type, employ an oil relay mechanism, as shown in the figure, for -operating the steam valves. In these turbines the lubricating oil -circulating pump, maintains a higher pressure than is required for -the lubricating system. The governor controls a small relay valve -A which admits pressure oil to, or exhausts it from the operating -cylinder. When oil is admitted to the operating cylinder raising the -piston, the lever C lifts the primary valve E. The lever D moves -simultaneously with C, but on account of the slotted connection -with the stem of the secondary valve F, the latter does not begin -to lift until the primary valve is raised to the point at which its -effective opening ceases to be increased by further upward travel. -In the Westinghouse designs, the operating valve, A is connected not -only to the governor, but also to a vibrator, which gives it a slight -but continuous reciprocating motion, while the governor controls its -mean position. The effect of this is manifested in a slight pulsation -throughout the entire relay system, which, so to speak, keeps it -"alive" and ready to respond instantly, to the smallest change in -the position of the governor. The oil relay can be made sufficiently -powerful to operate valves of any size, and it is also in effect -a safety device in that any failure of the lubricating oil supply -will automatically and immediately shut off the steam and stop the -turbine.] - - A weighted accumulator is sometimes installed in connection with - the oil pipe as a convenient device for governing the step bearing - pumps, and also as a safety device in case the pumps should fail, - but it is seldom required for the latter purpose, as the step - bearing pumps have proven after a long service in a number of - cases, to be reliable. The vertical shaft is also held in place - and kept steady by three sleeve bearings one just above the step, - one between the turbine and generator, and the other near the top. - -[Illustration: FIG. 2,758.--Elevation of new turbine central station -erected by the Boston Edison Co. The turbine room is 68 feet, 4 -inches wide and 650 feet long from outside to outside of the walls. -The boiler room is 149 feet, 6 inches by 640 feet and equipped with -twelve groups of boiler, one group consisting of eight 512 H.P. -boilers for each turbine. The switching arrangements are located in -a separate building as shown in the elevation. The total floor space -covered by boiler room, turbine room and switchboard room is 2.64 -square feet per kw. The boilers are all on the ground floor. See fig. -2,714 for plan.] - - These guide bearings are lubricated by a standard gravity feed - system. It is apparent that the amount of friction in the machine - is very small, and as there is no end thrust caused by the action - of the steam, the relation between the revolving and stationary - blades may be maintained accurately. As a consequence, therefore, - the clearances are reduced to the minimum. - - The Curtis turbine is divided into two or more stages, and each - stage has one, two or more sets of revolving blades bolted upon - the peripheries of wheels keyed to the shaft. There are also the - corresponding sets of stationary blades bolted to the inner walls - of the cylinder or casing. - - The governing of speed is accomplished in the first set of - nozzles and the control of the admission valves here is effected - by means of a centrifugal governor attached to the top end of the - shaft. This governor, by a very slight movement, imparts motion - to levers, which in turn work the valve mechanism. - - The admission of steam to the nozzles is controlled by piston - valves which are actuated by steam from small pilot valves which - are in turn under the control of the governor. - -[Illustration: FIG. 2,759.--Illustration of a weir. To make a weir, -place a board across the stream at some point which will allow a -pond to form above. The board should have a notch cut in it with -both side edges and the bottom sharply beveled toward the intake, -as shown in the above cut. The bottom of the notch, which is called -the "crest" of the weir, should be perfectly level and the sides -vertical. In the pond back of the weir, at a distance not less than -the length of the notch, drive a stake near the bank, with its top -precisely level with the crest. By means of a rule, or a graduated -stake as shown, measure the depth of water over the top of stake, -making allowance for capillary attraction of the water against the -sides of the weir. For extreme accuracy this depth may be measured -to thousandths of a foot by means of a "hook gauge," familiar to all -engineers. Having ascertained the depth of water over the stake, -refer to the accompanying table, from which may be calculated the -amount of water flowing over the weir. There are certain proportions -which must be observed in the dimensions of this notch. Its length, -or width, should be between four and eight times the depth of water -flowing over the crest of the weir. The pond back of the weir should -be at least fifty per cent. wider than the notch and of sufficient -width and depth that the velocity of flow or approach be not over one -foot per second. In order to obtain these results it is advisable to -experiment to some extent.] - - Speed regulation is effected by varying the number of nozzles - in flow, that is, for light loads fewer nozzles are open and a - smaller volume of steam is admitted to the turbine wheel, but the - steam that is admitted impinges against the moving blades with the - same velocity always, no matter whether the volume be large or - small. With a full load and all the nozzle sections in flow, the - steam passes to the wheel in a broad belt and steady flow. - - WEIR TABLE - giving cubic feet of water per minute that will flow over a weir - one inch wide and from ⅛ to 20⅞ inches deep. - - --------+------+------+------+------+------+------+------+------ - Depth | | | | | | | | - inches | | ⅛ | ¼ | ⅜ | ½ | ⅝ | ¾ | ⅞ - --------+------+------+------+------+------+------+------+------ - =0= | .00 | .01 | .05 | .09 | .14 | .19 | .26 | .32 - =1= | .40 | .47 | .55 | .64 | .73 | .82 | .92 | 1.02 - =2= | 1.13 | 1.23 | 1.35 | 1.36 | 1.58 | 1.70 | 1.82 | 1.95 - =3= | 2.07 | 2.21 | 2.34 | 2.48 | 2.61 | 2.76 | 2.90 | 3.05 - =4= | 3.20 | 3.35 | 3.50 | 3.66 | 3.81 | 3.97 | 4.14 | 4.30 - =5= | 4.47 | 4.64 | 4.81 | 4.98 | 5.15 | 5.33 | 5.51 | 5.69 - =6= | 5.87 | 6.06 | 6.25 | 6.44 | 6.62 | 6.82 | 7.01 | 7.21 - =7= | 7.40 | 7.60 | 7.80 | 8.01 | 8.21 | 8.42 | 8.63 | 8.83 - =8= | 9.05 | 9.26 | 9.47 | 9.69 | 9.91 |10.13 |10.35 |10.57 - =9= |10.80 |11.02 |11.25 |11.48 |11.71 |11.94 |12.17 |12.41 - =10= |12.64 |12.88 |13.12 |13.36 |13.60 |13.85 |14.09 |14.34 - =11= |14.59 |14.84 |15.09 |15.34 |15.59 |15.85 |16.11 |16.36 - =12= |16.62 |16.88 |17.15 |17.41 |17.67 |17.94 |18.21 |18.47 - =13= |18.74 |19.01 |19.29 |19.56 |19.84 |20.11 |20.39 |20.67 - =14= |20.95 |21.23 |21.51 |21.80 |22.08 |22.37 |22.65 |22.94 - =15= |23.23 |23.52 |23.82 |24.11 |24.40 |24.70 |25.00 |25.30 - =16= |25.60 |25.90 |26.20 |26.50 |26.80 |27.11 |27.42 |27.72 - =17= |28.03 |28.34 |28.65 |28.97 |29.28 |29.59 |29.91 |30.22 - =18= |30.54 |30.86 |31.18 |31.50 |31.82 |32.15 |32.47 |32.80 - =19= |33.12 |33.45 |33.78 |34.11 |34.44 |34.77 |35.10 |35.44 - =20= |35.77 |36.11 |36.45 |36.78 |37.12 |37.46 |37.80 |38.15 - --------+------+------+------+------+------+------+------+------ - - NOTE.--The weir table on this page contains - figures 1, 2, 3, etc., in the first vertical - column which indicates the inches depth of water - running over weir board notches. Frequently the - depths measured represent also fractional inches, - between 1 and 2, 2 and 3, etc. The horizontal - line of fraction at the top represents these - fractional parts, and can be applied between any - of the numbers of inches depth, from 1 to 25. The - body of the table shows the cubic feet, and the - fractional parts of a cubic foot, which will pass - each minute for each inch in depth, and for each - fractional part of an inch by eighths for all - depths from 1 to 25 inches. Each of these results - is for only one inch width of weir. To estimate - for any width of weir the result obtained for one - inch width must be multiplied by the number of - inches constituting the whole horizontal length - of weir. - -[Illustration: FIGS. 2,760 and 2,761.--Samson vertical runner and -shaft, and complete Samson vertical turbine. The runner is composed -of two separate and distinct types of wheel, having thereby also -two diameters. Each wheel or set of buckets receives its separate -quantity of water from one and the same set of guides, but each set -acts only once and singly upon the water used, and the water does not -act twice upon the combined wheel, as some suppose. In construction=, -the lower or main set of buckets is made of flanged plate steel, and -cast solidly into a heavy ring surrounding the outer and lower edges, -and into a heavy diaphragm, separating the two sets of buckets.] - -[Illustration: FIG. 2,762.--Water discharging from a needle nozzle -due to a pressure of 169 lbs. per sq. in.] - -=Hydro-Electric Plants.=--The economy with which electricity can be -transmitted long distances by high tension alternating currents, has -led to the development of a large number of water powers in more or -less remote regions. - -[Illustration: FIG. 2,763.--Photograph of an operating tangential -water wheel equipped with Pelton buckets.] - - This economy is possible by the facility with which alternating - current can be transformed up and down. Thus at the hydro-electro - plant, the current generated by the water wheel driven alternator - is transformed to very high pressure and transmitted with economy - a long distance to the distributing point where it is transformed - down to the proper pressure for distribution. - - A water wheel or turbine is a machine in which a rotary motion - is obtained by transference of the momentum of water; broadly - speaking, the fluid is guided by fixed blades, attached with a - casing, and impinging on other blades mounted on a drum or shaft, - causing the latter to revolve. - - There are two general classes of turbine: - 1. Impulse turbines; - 2. Reaction turbines. - - -[Illustration: FIG. 2,764.--Sectional elevation of one of the 5,000 -horse power vertical Pelton-Francis turbines directly connected to -generator, as installed for the Schenectady Power Co.] - -=Ques. What is an impulse turbine?= - -Ans. One in which the fluid is directed by means of a series of -nozzles against vanes which it drives. - -=Ques. What is a reaction turbine?= - -Ans. One in which the pressure or head of the water is employed -rather than its velocity. The current is deflected upon the wheel by -the action of suitably disposed guide blades, the passages being full -of water. Rotary motion is obtained by the change in the direction -and momentum of the fluid. - -[Illustration: Figs. 2,765 to 2,768.--Cross sections of Lowel dam -power house, and wheel pits containing sixteen Samson turbines: The -section C-D gives an end view of the generator room showing the -locations of the generators below the head level water. They are -secure against flood water, or leakage, by well constructed stuffing -boxes in the iron bulkheads, through which the turbine wheel shafts -pass and connect to the generators. Section E-F gives an end view of -one of these wheel rooms or penstocks, and shows the extension of the -draft tube from wheel case into tail water. The section A-B shows the -sub-structure of gravel and macadam under the controlling gates, this -forming also a portion or extension of the dam proper. These gates -turn on an axis made of two 15 inch I beams securely riveted together -with plates and angle irons to which the wooden frame is attached. -The radius of the gates is 14 feet. They are designed to allow the -water to pass underneath the gate, thus controlling any height of -head water. They are intended to take care of an excess of water at -unusual stages of the river. The whole affair has been well designed -and executed. This plant furnishes a good example of a secure, and -level foundation, since the wheel houses and generator room are -immediately on the rock. It is necessary in all tandem plants to -provide a very secure, substantial super-structure so that the long -line of turbines and shaft will always remain straight and in proper -alignment with the generator and the turbine cases. Users cannot be -reminded of this too often.] - -=Ques. Name three classes of reaction turbines.= - -Ans. Parallel flow, inward flow, and outward flow. - - Parallel flow turbines have an efficiency of about 70% and are - suited for low falls not over 30 feet. Inward and outward flow - turbines have an efficiency of about 85%. Impulse turbines are - suitable for high heads. - -[Illustration: FIGS. 2,769 and 2,770.--Exterior and interior of -hydro-electric plant at Harrisburg, Va. It is located on the south -fork of the Shenandoah River, twelve and one-half miles distant. A -dam 720 feet long and 15 feet high was built on a limestone ledge -running across the river; which with a fall of 5 feet from the dam to -the power house, a quarter of a mile distant, secured an effective -head pressure of 20 feet. The power house, comprising the generator -room and the wheel room, also the machinery room, are here shown. The -wheel room, which is 20 × 40 feet, extends across the head race, and -rests upon solid concrete walls, forming the sides and ends of the -wheel pits. The end wall is 6 feet thick at the bottom, and 4½ feet -at the top. It has three arched openings, each 8 feet wide and 9 feet -high, through which the water escapes after leaving the turbines. -The intake is protected by a wrought iron rack 40 feet long. The -power is obtained by three 50 inch vertical shaft Samson turbines, -with a 20 inch Samson for an exciter. The three large turbines -have a rating of 1,350 horse power; and are connected to the main -horizontal line shaft by bevel mortise gears 7 feet diameter and 15 -inches face. The couplings on the main shaft have 48 inch friction -clutch hubs, permitting either or each turbine being operated, or -shut down independently of the others. The main shaft is 85 feet long -and 6 inches diameter; making 280 revolutions. This shaft carries -two pulleys 70 inches diameter and 38 inches face for driving the -generators. The accompanying illustration shows the harness work, -gears, pulleys, etc., furnished with the turbines. The 20 inch -horizontal shaft Samson turbine of 72 horse power is direct connected -to an exciter generator of 20 kw., running 700 rev. per min. The -two large generators are driven 450 revolutions per minute by belts -producing a three phase current of 60 cycles of 11,500 volts for the -twelve and one-half miles transmission. The line consists of three -strands of No. 4 bare copper wire. This current is used for lighting -and power purposes, and the plant is of the latest improved design -and construction.] - -=Isolated Plants.=--When electric power transmission from central -stations first came into commercial use, the distance from the -station at which current could be obtained at a reasonable cost was -exceedingly limited. - -[Illustration: FIG. 2,769a.--Triumph direct current generator set with -upright slide valve engine.] - -[Illustration: FIG. 2,770a.--Murray alternating current direct -connected unit with high speed Corliss engine and belt driven -exciter, 50, 75 and 100 kva. alternator and 150 R.P.M. engine.] - -[Illustration: FIG. 2,771.--Direct connected direct current unit with -Ridgway high speed four valve engine.] - -[Illustration: FIG. 2,772.--Buckeye mobile, or self contained unit -consisting of compound condensing engine, boiler, superheater, -reheater, feed and air pumps; it produces one horse power on 1½ lbs. -of coal, built in sizes from 75 to 600 horse power.] - -[Illustration: FIG. 2,773.--Westinghouse three cylinder gas engine, -direct connected to dynamo, showing application of gas engine drive -for small direct connected units.] - - Consequently, persons desiring electrical power were in the - majority of cases forced to install their own apparatus for - producing it, this being the origin of isolated plants. - - From the nature of the case it is evident that an isolated - plant is as a rule smaller and more simple in construction than - a central station, and in consequence much more readily operated - and managed. It is generally owned by a private individual or a - corporation and operated in conjunction with other affairs of a - similar character. A basement or other portion of a building is - usually set aside in which the necessary apparatus is installed. - -[Illustration: FIG. 2,774.--General Electric 25 kw., gasoline -electric generating set for lighting and power. The engine has -four cylinders 7¼ × 7½, and runs at a speed of 560 revolutions per -minute. The total candle power capacity in Mazda lamps is 20,000. The -ignition is by low tension magneto, coil and battery. Carburetter is -of the constant level type to which gasoline is delivered by a pump -driven by the engine. Forced lubrication; five crank shaft bearings -babbitted; valves in side; overall dimensions 96 × 34 × 60 high; -weight 5,000.] - - Although electricity is now transmitted economically to great - distances from central stations, there is still a field for the - isolated plant. - - The average type of isolated plant has enlarged from a small - dynamo driven by a little slide valve engine located in an out - of the way corner to direct connected generators and engines of - hundreds and even thousands of horse power assembled in a large - room specially adapted to the purpose. - - In the more modern of these, the electrical outputs are each - frequently equal to that of a town central station of respectable - size, and the auxiliary equipments are similar in every - particular. As a matter of fact, in certain modern isolated plants - the only feature that distinguishes them from central stations - is that in the former case the owner of the plant represents the - sole consumer and conducts other business in connection with it, - whereas in the latter case there are a large number of consumers - uninterested financially in the enterprise, which is itself - generally owned and operated by a company conducting no other - business. - -[Illustration: FIG. 2,775.--Plan of sub-station with air blast -transformers and motor operated oil switches and underground 11,000 -or 13,200 volt high tension lines.] - -=Sub-Stations.=--According to the usual meaning of the term, a -sub-station is a building provided with apparatus for changing high -pressure alternating current received from the central station into -direct current of the requisite pressure, which in the case of -railways is 550 to 600 volts. - -Where traffic is heavy and the railway system of considerable -distance, sub-stations are provided at intervals along the line, -each receiving high pressure current from one large central station -and converting it into moderate pressure direct current for their -districts. - -=Ques. Upon what does the arrangement of the sub-station depend?= - -Ans. Upon the character of the work and the type of apparatus -employed for converting the high pressure alternating current into -direct current. - -[Illustration: FIG. 2,776.--Plan of small sub-station with single -phase oil insulated self-cooling transformers and hand operated oil -switches 11,000 or 13,200 volts, overhead high tension lines.] - - In general it should be substantial, convenient to install or - replace the heavy machines, and the layout arranged so that the - apparatus can be readily operated by those in attendance. - - An overhead traveling crane is the most convenient method of - handling the heavy machinery, and is frequently used in large - sub-stations. - - Fig. 2,776 shows a sectional view, and fig. 2,777, a plan for - a small sub-station containing two rotary converters and two - banks of three single phase static transformers operating on a - three phase system at 11,000 or 13,200 volts, together with the - auxiliary apparatus. - -[Illustration: FIG. 2,777.--Elevation of small sub-station, as shown -in plan in Fig. 2,776.] - -=Ques. For three phase installations, what are the merits of -separate and combined transformers?= - -Ans. With separate transformer for each phase, repairs are more -readily made in case of accident or burnouts in the coils. The three -phase units have the advantage of low first cost. - - Sub-station transformers produce considerable heat, due to the - hysteresis and eddy currents, and it is necessary to get rid of it. - - Small transformers radiate the heat from the shell and the - medium sizes have corrugated shells which increase the surface and - provide more rapid radiation. - - Large transformers are cooled by an air blast supplied by motor - driven blowers or by water pumped through a coil of pipe which is - immersed in the insulating oil of the transformer. The large size - oil insulated, water cooled transformers are used on circuits of - 33,000 volts or more. In water turbine plants, the water may be - piped to the transformer under pressure and the pump omitted which - cuts down the cost of operating. Air blast transformers usually - have a damper or shutter for air control. - -[Illustration: FIG. 2,778.--Marine portable transformer station on -Los Angeles Aqueduct. The view shows three 20 kva. Westinghouse out -door transformers installed on a float, 33,000 volts high pressure; -440 volts low pressure; 50 cycles.] - -=Ques. Explain the use of reactance coils in sub-stations.= - -Ans. In order that the direct current voltage of the ordinary rotary -may be regulated by a field rheostat, which calls for a corresponding -change in the alternating current voltage, a reactance coil is -provided between the low tension winding and the converter. - - Without such a reactance the maintenance of the same voltage at - full load as at no load involves excessive leading and lagging - currents and consequently excessive heating in the armature - inductors, unless the resistance drop from the source of constant - pressure is small, or the natural reactance of the circuit high. - -=Ques. What is the effect of weakening the converter field?= - -Ans. A lagging current is set up which causes a drop in the reactance -coil. - -[Illustration: FIG. 2,779.--Sectional elevation of portable outdoor -transformer type sub-station. The high voltage switching and -protective apparatus is mounted, out of the way, on the roof of the -car, but is operated from the switchboard with a standard remote -control handle. The transformer is carried directly over the truck at -the uncovered end of the car and the low-tension leads from it run in -conduit beneath the floor and up into the cab, (which contains the -converter and switchboard) to the converter. The positive lead runs -through a conduit and ends in a terminal on the roof. The energy thus -makes a complete circuit of the car leaving at a point close to that -at which it entered. The low pressure alternating current as well as -the direct current positive leads are carried below the car floor -in iron conduit supported from the channel frame. The field wires -are carried through this conduit to the rheostat. Wiring for the -lights is arranged to supply two, 5 light clusters. One is fed with -the 600 volt direct current and the other with 420 volt alternating -current. All lighting conductors are carried in metal moulding -carried between the flanges of the channel iron ribs. High wiring -is carried entirely on the roof of the car where it is entirely out -of the way and where the operator cannot come in contact with it. -The switchboard should be of the utmost simplicity. Usually the -negative and equalizer switches, and the field break-up switch are -mounted on the frame of the converter. The double throw switch for -starting and running the converter can be mounted under the floor -of the car and operated by handle at the switchboard. The rheostat -can be mounted back of the switchboard on brackets bolted to the car -super-structure. The switchboard need only carry the positive knife -switch and circuit breaker, and the alternating current ammeter, -voltmeter and power factor meter. Sometimes a watthour meter is -added. The positive lead is brought out through a conduit on the -roof of the car and is arranged for bolting to the positive feeder. -The negative and equalizer terminals are located at the cab end of -the car and are arranged so that connection can be easily made from -them to the ground and, if necessary, to an equalizer circuit. There -is usually a sliding door at each end of the cab and two windows on -each side. Above the doors, transoms, extending the width of the cab, -are arranged to drop so that a current of air will circulate through -the cab under the roof, carrying out the heated air. There are also -several ventilating holes beneath the converter in the floor of the -car. These provisions insure a constant circulation of air through -the car which carries away all heated air.] - -=Ques. State the effect of strengthening converter field.= - -Ans. A leading current is set up which gives a rise of voltage in the -reactance coil. - - Hence when a heavy current passes through the series coil of a - compound wound converter and tends to produce a leading current, - the reactance coil will balance it, and improve the power factor - of the whole line. - -[Illustration: FIG. 2,780.--Westinghouse 300 kw. converter in -portable sub-station.] - -=Portable Sub-Stations.=--A portable sub-station constitutes a spare -equipment for practically any number of permanent sub-stations and -renders unnecessary the installation of spare equipment in each. - - It can be used to increase the capacity of a permanent - sub-station when the load is unusually heavy, or to provide - service while a permanent sub-station is being overhauled or - rebuilt. - - The transformer can be used for emergency lighting, the primary - being connected to a high pressure line and the secondary to the - load, if special provision be made at the time the transformer is - built to adapt it for these applications. - -[Illustration: FIG. 2,781.--Switchboard end of Westinghouse portable -sub-station.] - -When an electric railway has a portable sub-station, direct current -can be provided at any point on the system where there is track at -the high pressure line. The direct current can be made available -very quickly as its production involves only the transferring of the -sub-station, and its connection to the high pressure line. - - Portable sub-stations range in capacity from 200 to 500 kw., and - for all alternating current voltages up to 66,000, and frequencies - of 25 and 60 cycles. - - Although portable sub-stations usually must be of more or - less special design to adapt them to the conditions under which - they must operate, there are certain general features that are - common to all. All members are readily accessible and there are - no unnecessary parts. The weight and dimensions are a minimum - insuring ease of transportation. Live parts are so protected that - the danger of accidental contact with them is minimized. - -[Illustration: FIGS. 2,782 and 2,783.--Views of levelling device for -Westinghouse converter.] - -=Ques. What are the advantages of using outdoor transformers on -portable sub-stations?= - -Ans. All high pressure wiring is kept out of the car. The transformer -is more effectively cooled and the heat dissipated by the transformer -does not warm the interior of the cab. The transformer is much more -accessible. The car can be run under a crane and the transformer -coils pulled out with a hoist. - - Taps for different high and low pressure voltages can be readily - provided at the time the transformer is being built. - - - - -CHAPTER LXVII - -MANAGEMENT - - -The term "management," broadly speaking, includes not only the -actual skilled attention necessary for the proper operation of the -machines, after the plant is built, but also other duties which must -be performed from its inception to completion, and which may be -classified as - - 1. Selection; - 2. Location; - 3. Erection; - 4. Testing; - 5. Running; - 6. Care; - 7. Repair. - -That is to say, someone must select the machinery, determine where -each machine is to be located, install them, and then attend to the -running of the machines and make any necessary repairs due to the -ordinary mishaps likely to occur in operation. - -These various duties are usually entrusted to more than one -individual; thus, the selection and location of the machinery is done -by the designer of the plant, and requires for its proper execution -the services of an electrical engineer, or one possessing more than -simply a practical knowledge of power plants. - -The erection of the machines is best accomplished by those making a -specialty of this line of work, who by the nature of the undertaking -acquire proficiency in methods of precision and an appreciation of -the value of accuracy which is so essential in the work of aligning -the machines, and which if poorly done will prove a constant source -of annoyance afterward. - -The attention required for the operation of the machines, embracing -the running care and repair, is left to the "man in charge," who -in most cases of small and medium size plants is the chief steam -engineer. He must therefore, not only understand the steam apparatus, -but possess sufficient knowledge of electrical machinery to operate -and maintain it in proper working order. - -The present chapter deals chiefly with alternating current machinery, -the management of direct current machines having been fully explained -in Guide No. 3, however, some of the matter here presented is common -to both classes of apparatus. - - * * * * * - -=Selection.=--In order to intelligently select a machine so that it -will properly harmonize with the conditions under which it is to -operate, there are several things to be considered. - - 1. Type; - 2. Capacity; - 3. Efficiency; - 4. Construction. - -The general type of machine to be used is, of course, dependent on -the system employed, that is, whether it be direct or alternating, -single or polyphase. - - Thus, the voltage in most cases is fixed except on transformer - systems where a choice of voltage may be had by selecting a - transformer to suit. - -In alternating current constant pressure transmission circuits, an -average voltage of 2,200 volts with step down transformer ratios of -1/10 and 1/20 is in general use, and is recommended. - - For long distance, the following average voltages are - recommended 6,000; 11,000; 22,000; 33,000; 44,000; 66,000; 88,000; - and higher, depending on the length of the line and degree of - economy desired. - -In alternating circuits the standard frequencies are 25, and 60 -cycles. These frequencies are already in extensive use and it is -recommended to adhere to them as closely as possible. - -[Illustration: FIG. 2,784.--Diagram of connections for testing to -obtain the saturation curve of an alternator. The saturation curve -shows the relation between the volts generated in the armature and -the amperes of field current (or ampere turns of the field) for a -constant armature current. The armature current may be zero, in which -case the curve is called _no load saturation curve_, or sometimes the -_open circuit characteristic curve_. A saturation curve may be taken -with full load current in the armature; but this is rarely done, -except in alternators of comparatively small output. If a full load -saturation curve be desired, it can be approximately calculated from -the no load saturation curve. The figure shows the connections. If -the voltage generated is greater than the capacity of the voltmeter, -a multiplying coil or a step down pressure transformer may be used, -as shown. A series of observations of the voltage between the -terminals of one of the phases, is made for different values of the -field current. Eight or nine points along the curve are usually -sufficient, the series extending from zero to about fifty per cent. -above normal rated voltage. The points should be taken more closely -together in the vicinity of normal voltage than at other portions of -the curve. Care must be taken that the alternator is run at its rated -speed, and this speed must be kept constant. Deviations from constant -speed may be most easily detected by the use of a tachometer. If the -machine be two phase or three phase, the voltmeter may be connected -to any one phase throughout a complete series of observations. The -voltage of all the phases should be observed for normal full load -excitation by connecting the voltmeter to each phase successively, -keeping the field current constant at normal voltage. This is done in -order to see how closely the voltage of the different phases agree.] - - In fixing the capacity of a machine, _careful consideration - should be given to the conditions of operation both_ =present= - _and_ =future= in order that the resultant efficiency may be - maximum. - - Most machines show the best efficiency at or near full load. - If the load be always constant, as for instance, a pump forcing - water to a given head, it would be a simple matter to specify the - proper size of machine, but in nearly all cases, and especially - in electrical plants, the load varies widely, not only the daily - and hourly fluctuations, but the varying demands depending on - the season of the year and growth of the plant's business. All - of these conditions tend to complicate the matter, so that - intelligent selection of capacity of a machine requires not only - calculation but mature judgment, which is only obtained by long - experience. - -[Illustration: FIG. 2,785.--Saturation curve taken from a 2,000 kw., -three phase alternator of the revolving field type, having 16 poles, -and generating 2,000 volts, and 576 amperes per phase when run at 300 -R.P.M.] - -In selecting a machine, or in fact any item connected with the plant -_its construction should be carefully considered_. - - Standard construction should be insisted upon so that in the - event of damage a new part can be obtained with the least possible - delay. - - The parts of most machines are _interchangeable_, that is to - say, with the refined methods of machinery a duplicate part - (usually carried in stock) may be obtained at once to replace a - defective or broken part, and made with such precision that little - or no fitting will be required. - -The importance of standard construction cannot be better illustrated -than in the matter of steam piping, that is, the kind of fittings -selected for a given installation. - -With the exception of the exhaust line from engine to condenser, -where other than standard construction may sometimes be used to -reduce the frictional resistance to the steam, the author would -adhere to standard construction except in very exceptional cases. -Those who have had practical experience in pipe fitting will -appreciate the wisdom of this. - -For installations in places remote from large supply houses, the more -usual forms of standard fittings should be employed, such as ordinary -T's, 45° and 90° elbows, etc. - -In such locations, where designers specify the less usual forms of -standard fittings such as union fittings, offset reducers, etc., -or special fittings made to sketch, it simply means, in the first -instance that they usually cannot be obtained of the local dealer, -making it necessary to order from some large supply house and -resulting in vexatious delays. - -As a rule, those who specify special fittings have found that their -making requires an unreasonable length of time, and the cost to be -several times that of the equivalent in standard fittings. - -An examination of a few installations will usually show numerous -special and odd shape fittings, which are entirely unnecessary. - -Moreover, a standard design, in general, is better than a special -design, because the former has been tried out, and any imperfection -or weakness remedied, and where thousands of castings of a kind are -turned out, a better article is usually the result as compared with a -special casting. - -In the matter of construction, in addition to the items just -mentioned, it should be considered with respect to - - 1. Quality; - 2. Range; - 3. Accessibility; - 4. Proportion; - 5. Lubrication; - 6. Adjustment. - -It is poor policy, excepting in very rare instances, to buy a "cheap" -article, as, especially in these days of commercial greed, the best -is none too good. - -[Illustration: FIGS. 2,786 and 2,787.--Wheel and roller pipe cutters -illustrating =range=. The illustrations show the comparative -movements necessary with the two types of cutter to perform their -function. The wheel cutter requiring only a small arc of movement -will cut a pipe in an inaccessible place as shown, which with a -roller cutter would be impossible. Accordingly, the wheel cutter is -said to have a greater _range_ than the roller cutter.] - -Perhaps next in importance to quality, at least in most cases, -is _range_. This may be defined as _scope of operation_, -_effectiveness_, or _adaptability_. The importance of range is -perhaps most pronounced in the selection of tools, especially for -plants remote from repair shops. - - For instance, in selecting a pipe cutter, there are two general - classes: wheel cutters, and roller cutters. A wheel cutter has - three wheels and a roller cutter one wheel and two rollers, the - object of the rollers being to keep the wheel perpendicular to - the pipe in starting the cut and to reduce burning. It must be - evident that in operation, a roller cutter requires sufficient - room around the pipe to permit making a complete revolution of - the cutter, whereas, with a wheel cutter, the work may be done - by moving the cutter back and forth through a small arc, as - illustrated in figs. 2,786 and 2,787. Thus a wheel cutter has a - _greater range_ than a roll cutter. - - Range relates not only to ability to operate in inaccessible - places but to the various operations that may be performed by one - tool. - - PROPERTIES OF STANDARD WROUGHT IRON PIPE - - --------------------------+------+-----------------+ - | | | - | | | - | | | - | | | - Diameter |Thick-| Circumference. | - | ness.| | - --------+--------+--------+ +--------+--------+ - Nominal| Actual | Actual | |External|Internal| - internal|external|internal| | | | - --------+--------+--------+------+--------+--------+ - Inches | Inches | Inches |Inches| Inches | Inches | - --------+--------+--------+------+--------+--------+ - ⅛ | .405 | .27 | .068 | 1.272 | .848 | - ¼ | .54 | .364 | .088 | 1.696 | 1.144 | - ⅜ | .675 | .494 | .91 | 2.121 | 1.552 | - ½ | .84 | .623 | .109 | 2.639 | 1.957 | - ¾ | 1.05 | .824 | .113 | 3.299 | 2.589 | - 1 | 1.315 | 1.048 | .134 | 4.131 | 3.292 | - 1¼ | 1.66 | 1.38 | .14 | 5.215 | 4.335 | - 1½ | 1.9 | 1.611 | .145 | 5.969 | 5.061 | - 2 | 2.375 | 2.067 | .154 | 7.461 | 6.494 | - 2½ | 2.875 | 2.468 | .204 | 9.032 | 7.753 | - 3 | 3.5 | 3.067 | .217 | 10.996 | 9.636 | - 3½ | 4. | 3.548 | .226 | 12.566 | 11.146 | - 4 | 4.5 | 4.026 | .237 | 14.137 | 12.648 | - 4½ | 5. | 4.508 | .246 | 15.708 | 14.162 | - 5 | 5.563 | 5.045 | .259 | 17.477 | 15.849 | - 6 | 6.625 | 6.065 | .28 | 20.813 | 19.054 | - 7 | 7.625 | 7.023 | .301 | 23.955 | 22.063 | - 8 | 8.625 | 7.982 | .322 | 27.096 | 25.076 | - 9 | 9.625 | 8.937 | .344 | 30.238 | 28.076 | - 10 | 10.75 | 10.019 | .366 | 33.772 | 31.477 | - 11 | 12. | 11.25 | .375 | 37.699 | 35.343 | - 12 | 12.75 | 12. | .375 | 40.055 | 37.7 | - --------+--------+--------+------+--------+--------+ -------+-------------------------+---------------+--------+-------+------ - | | | | | - | | | | | - | | | Length | | - | |Length of pipe |of pipe |Nominal| - Diam.| Transverse areas. | per square |contain-|weight |Number - | | foot of |ing one | per | of -------+--------+--------+-------+-------+-------+ cubic | foot. |thread - Nom. |External|Internal| Metal |Ext'nal|Int'nal| foot. | | per -intern| | | |surface|surface| | | inch -------+--------+--------+-------+-------+-------+--------+-------+ of -Inches|Sq. ins.|Sq. ins.|Sq.ins.| Feet | Feet | Feet |Pounds |screw -------+--------+--------+-------+-------+-------+--------+-------+----- - ⅛ | .129 | .0573| .0717| 9.44 |14.15 |2513. | .241 | 27 - ¼ | .229 | .1041| .1249| 7.075 |10.49 |1383.3 | .42 | 18 - ⅜ | .358 | .1917| .1663| 5.657 | 7.73 | 751.2 | .559 | 18 - ½ | .554 | .3048| .2492| 4.547 | 6.13 | 472.4 | .837 | 14 - ¾ | .866 | .5333| .3327| 3.637 | 4.635 | 270. | 1.115 | 14 - 1 | 1.358 | .8626| .4954| 2.904 | 3.645 | 166.9 | 1.668 | 11½ - 1¼ | 2.164 | 1.496 | .668 | 2.301 | 2.768 | 96.25 | 2.244 | 11½ - 1½ | 2.835 | 2.038 | .797 | 2.01 | 2.371 | 70.66 | 2.678 | 11½ - 2 | 4.43 | 3.356 | 1.074 | 1.608 | 1.848 | 42.91 | 3.609 | 11½ - 2½ | 6.492 | 4.784 | 1.708 | 1.328 | 1.547 | 30.1 | 5.739 | 8 - 3 | 9.621 | 7.388 | 2.243 | 1.091 | 1.245 | 19.5 | 7.536 | 8 - 3½ | 12.566 | 9.887 | 2.679 | .955 | 1.077 | 14.57 | 9.001 | 8 - 4 | 15.904 | 12.73 | 3.174 | .849 | .949 | 11.31 |10.665 | 8 - 4½ | 19.635 | 15.961 | 3.674 | .764 | .848 | 9.02 |12.34 | 8 - 5 | 24.306 | 19.99 | 4.316 | .687 | .757 | 7.2 |14.502 | 8 - 6 | 34.472 | 28.888 | 5.584 | .577 | .63 | 4.98 |18.762 | 8 - 7 | 45.664 | 38.738 | 6.926 | .501 | .544 | 3.72 |23.271 | 8 - 8 | 58.426 | 50.04 | 8.386 | .443 | .478 | 2.88 |28.177 | 8 - 9 | 72.76 | 62.73 |10.03 | .397 | .427 | 2.29 |33.701 | 8 - 10 | 90.763 | 78.839 |11.924 | .355 | .382 | 1.82 |40.065 | 8 - 11 |113.098 | 99.402 |13.696 | .318 | .339 | 1.450|45.95 | 8 - 12 |127.677 |113.098 |14.579 | .299 | .319 | 1.27 |48.985 | 8 -------+--------+--------+-------+-------+-------+--------+-------+---- - -Open construction should be employed, wherever possible, so that -all parts of a machine that require attention, or that may become -deranged in operation, may be accessible for adjustment or repair. - - The design should be such that there is ample strength, and the - bearings for moving parts should be of liberal proportions to - avoid heating with minimum attention. - - A comparison of the proportions used by different manufacturers - for a machine of given size might profitably be made before a - selection is made. - -The matter of lubrication is important. - - Fast running machines, such as generators and motors, should be - provided with ring oilers and oil reservoirs of ample capacity, as - shown in figs. 2,788 to 2,794. - -[Illustration: FIG. 2,788.--Sectional view showing a ring oiler or -self oiling bearing. As shown the pedestal or bearing standard is -cored out to form a reservoir for the oil. The rings are in rolling -contact with the shaft, and dip at their lower part into the oil. -In operation, oil is brought up by the rings which revolve because -of the frictional contacts with the shaft. The oil is in this way -brought up to the top of the bearing and distributed along the shaft -gradually descending by gravity to the reservoir, being thus used -over and over. A drain cock, is provided in the base so that the oil -may be periodically removed from the reservoir and strained to remove -the accumulation of foreign matter. This should be frequently done to -minimize the wear of the bearing.] - -All bearings subject to appreciable wear should be made adjustable so -that lost motion may be taken up from time to time and thus keep the -vibration and noise of operation within proper limits. - -=Selection of Generators.=--This is governed by the class of work -to be done and by certain local conditions which are liable to vary -considerably for different stations. - -These variable factors determine whether the generators must be of -the direct or alternating current type, whether they must be wound -to develop a high or a low voltage, and whether their outputs in -amperes must be large or small. Sufficient information has already -been given to cover these various cases; there are, however, certain -general rules that may advantageously be observed in the selection of -generators designed to fill any of the aforementioned conditions, and -it is well to possess certain facts regarding their construction. - -[Illustration: FIGS. 2,789 to 2,794.--Self oiling self aligning -bearing open. Views showing oil grooves, rings, bolts etc.] - -=Ques. Name an important point to be considered in selecting a -generator.= - -Ans. Its efficiency. - -=Ques. What are the important points with respect to efficiency?= - -Ans. A generator possessing a high efficiency at the average load is -more desirable than a generator showing a high efficiency at full -load. - -=Ques. Why?= - -Ans. The reason is that in station practice the full load limit is -seldom reached, the usual load carried by a generator ordinarily -lying between the one-half and three-quarter load points. - -=Ques. How do the efficiencies of large and small generators compare?= - -Ans. There is little difference. - -[Illustration: FIG. 2,795.--Rotor of Westinghouse type T turbine -dynamo set. The dynamo is of the commutating pole type either shunt -or compound wound. The turbine is of the single wheel impulse type. -The wheel is mounted directly on the end of the shaft as shown. Steam -is used two or more times on the wheel to secure efficiency. A fly -ball governor is provided with weights hung on hardened steel knife -edges. In case of over speeding, an automatic safety stop throttle -valve is tapped shutting off the steam supply. This type of turbine -dynamo set is especially applicable for exciter service in modern, -superheated steam generating stations where the steam pressure -exceeds 125 pounds. Westinghouse Type T turbines operate directly -(that is, without a reducing valve) on pressures up to 200 pounds per -square inch with steam superheated to 150 degrees Fahrenheit.] - -=Ques. How are the sizes and number of generator determined?= - -Ans. The sizes and number of generator to be installed should be -such as to permit the engines operating them being worked at nearly -full load, because the efficiencies of the latter machines decrease -rapidly when carrying less than this amount. - -=Ques. What is understood by regulation?= - -Ans. The accuracy and reliability with which the pressure or current -developed in a machine may be controlled. - - It is generally possible if purchasing of a reputable concern, - to obtain access to record sheets on which may be found results - of tests conducted on the generator in question, and as these are - really the only means of ascertaining the values of efficiency and - regulation, the purchaser has a right to inspect them. If, for - some reason or other, he has not been afforded this privilege, he - should order the machine installed in the station on approval, and - test its efficiency and regulation before making the purchase. - -[Illustration: FIG. 2,796.--Cross section of electrical station -showing small traveling crane.] - -=Installation.=--The installation of machines and apparatus in an -electrical station is a task which increases in difficulty with -the size of the plant. When the parts are small and comparatively -light they may readily be placed in position, either by hand, by -erecting temporary supports which may be moved from place to place -as desired, or by rolling the parts along on the floor upon pieces -of iron pipe. If, however, the parts be large and heavy, a traveling -crane such as shown in fig. 2,797, becomes necessary. - -=Ques. What precaution should be taken in moving the parts of -machines?= - -Ans. Care should be taken not to injure the bearings and shafts, the -joints in magnetic circuits such as those between frame and pole -pieces, and the windings on the field and armature. - -[Illustration: FIG. 2,797.--Cross section of electrical station -showing a traveling crane for the installation or removal of large -and heavy machine parts. A traveling crane consists of an iron beam -which, being supplied with wheels at the ends, can be made to move -either mechanically or electrically upon a track running the entire -length of the station. This track is not supported by the walls -of the building, but rests upon beams specially provided for the -purpose. In addition to the horizontal motion thus obtained, another -horizontal motion at right angles to the former is afforded by means -of the carriage which, being also mounted on wheels, runs upon a -track on the top of the beam. Electrical power is generally used to -move the carriage and also the revolving drums contained thereon, -the latter of which give a vertical motion to the main hoist or the -auxiliary hoist, these hoists being used respectively for raising -or lowering heavy or light loads. In the larger sizes of electric -traveling crane, a cage is attached to the beam for the operator, -who, by means of three controllers mounted in the cage, can move a -load on either the main or auxiliary hoist in any direction.] - - The insulations of the windings are perhaps the most vital parts - of a generator, and the most readily injured. The prick of a pin - or tack, a bruise, or a bending of the wires by resting their - weight upon them or by their coming in contact with some hard - substance, will often render a field coil or an armature useless. - - Owing to its costly construction, it is advisable when - transporting armatures by means of cranes to use a wooden - spreader, as shown in fig. 2,798 to prevent the supporting rope - bruising the winding. - -[Illustration: FIG. 2,798.--View of armature in transit showing use -of a wooden spreader as a protection. If a chain be used in place of -the rope, a padding of cloth should be placed around the armature -shaft and special care taken that the chain does not scratch the -commutator.] - -=Ques. If an armature cannot be placed at once in its final position -what should be done?= - -Ans. It may be laid temporarily upon the floor, if a sheet of -cardboard or cloth be placed underneath the armature as a protection -for the windings; in case the armature is not to be used for some -time, it is better practice to place it in a horizontal position on -two wooden supports near the shaft ends. - -=Ques. What kind of base should be used with a belt driven generator -or motor?= - -Ans. The base should be provided with V ways and adjusting screws -for moving the machine horizontally to take up slack in the belt, as -shown in fig. 2,799. - - Owing to the normal tension on the belt, there is a moment - exerted equal in amount to the distance from the center of gravity - of the machine to the center of the belt, multiplied by the - effective pull on the belt. This force tends to turn the machine - about its center of gravity. By placing the screws as shown, any - turning moment, as just mentioned, is prevented. - -[Illustration: FIG. 2,799.--Plan of belt drive machine showing V ways -and adjusting screws for moving the machine forward from the engine -or counter shaft to take up slack in the belt.] - -=Ques. How should a machine be assembled?= - -Ans. The assembling should progress by the aid of a blue print, or by -the information obtained from a photograph of the complete machine -as it appears when ready for service. Each part should be perfectly -clean when placed in position, especially those parts between which -there is friction when the machine is in operation, or across which -pass lines of magnetic force; in both cases the surfaces in contact -must be true and slightly oiled before placing in position. - - Contact surfaces forming part of electrical circuits must also - be clean and tightly screwed together. An important point to bear - in mind when assembling a machine is, to so place the parts that - it will not be necessary to remove any one of them in order to get - some other part in its proper position. By remembering this simple - rule much time will be saved, and in the majority of instances the - parts will finally be better fitted together than if the task has - to be repeated a number of times. - - When there are two or more parts of the machine similarly - shaped, it is often difficult to properly locate them, but in such - cases notice should be taken of the factory marks usually stamped - upon such pieces and their proper places determined from the - instructions sent with the machine. - -[Illustration: FIGS. 2,800 to 2,802.--Starrett's improved speed -indicator. In construction, the working parts are enclosed like a -watch. The graduations show every revolution, and with two rows of -figures read both right and left as the shaft may run. While looking -at the watch, each hundred revolutions may be counted by allowing -the oval headed pin on the revolving disc to pass under the thumb as -the instrument is pressed to its work. A late improvement in this -indicator consists in the rotating disc, which, being carried by -friction may be moved to the starting point where the raised knobs -coincide. When the spindle is placed in connection with the revolving -shaft, pressing the raised knob with the thumb will prevent the disc -rotating, while the hand of the watch gets to the right position to -take the time. By releasing the pressure the disc is liberated for -counting the revolutions of the shaft when every 100 may be noted by -feeling the knob pass under the thumb lightly pressed against it, -thus relieving the eye, which has only to look on the watch to note -the time.] - -=Ques. What should be noted with respect to speed of generator?= - -Ans. Each generator is designed to be run at a certain speed in -order to develop the voltage at which the machine is rated. The -speed, in revolutions per minute, the pressure in volts, and the -capacity or output in watts (volts × amperes) or in kilowatts -(thousands of watts) are generally stamped on a nameplate screwed to -the machine. - - This requirement frequently requires calculations to be made by - the erectors to determine the proper size pulleys to employ to - obtain the desired speed. - -[Illustration: FIG. 2,803.--Home made belt clamp. It is made with -four pieces of oak of ample size to firmly grip the belt ends where -the bolts are tightened. The figure shows the clamp complete and -in position on the belt and clearly illustrates the details of -construction. In making the long bolts the thread should be cut about -three-quarter length of bolt and deep enough so that the nuts will -easily screw on.] - - =Example.=--What diameter of engine pulley is required to run - a dynamo at a speed of 1,450 revolutions per minute the dynamo - pulley being 10 inches in diameter and the speed of engine, 275 - revolutions per minute? - -The diameter of pulley required on engine is 10 × (1,450 ÷ 275) = 53 -inches, nearly. - - =Rule.=--To find the diameter of the driving pulley, _multiply - the speed of the driven pulley by its diameter, divide the product - by the speed of the driver and the answer will be the size of the - driver required_. - - _Example._--If the speed of an engine be 325 revolutions per - minute, diameter of engine pulley 42 inches, and the speed of - the dynamo 1,400 revolutions per minute, how large a pulley is - required on dynamo? - - The size of the dynamo pulley is - -42 × (325 ÷ 1,400) = 9¾ inches. - - =Rule.=--To find the size of dynamo pulley, _multiply the speed - of engine by the diameter of engine wheel and divide the product - by the speed of the dynamo_. - -[Illustration: FIGS. 2,804 and 2,805.--A good method of lacing a -belt. The view at the left shows outer side of belt, and at the -right, inner or pulley side.] - - _Example._--If a steam engine, running 300 revolutions per - minute, have a belt wheel 48 inches in diameter, and be belted - to a dynamo having a pulley 12 inches in diameter, how many - revolutions per minute will the dynamo make? - - The speed of dynamo will be 300 × (48 ÷ 12) = 1,200 rev. per min. - - =Rule.=--When the speed of the driving pulley and its diameter - are known, and the diameter of the driven pulley is known, the - speed of the driven pulley is found by _multiplying the speed of - the driver by its diameter in inches and dividing the product by - the diameter of the driven pulley_. - - =Example.=--What will be the required speed of an engine - having a belt wheel 46 inches in diameter to run a dynamo 1,500 - revolutions per minute, the dynamo pulley being 11 inches in - diameter? - - The speed of the engine is 1,500 × (11 ÷ 46) = 359 rev. per min. - nearly. - -[Illustration: FIG. 2,806.--Wiring diagram and directions for -operating Holzer-Cabot single phase self-starting motor. =Location:= -The motor should be placed in as clear and dry a location as -possible, away from acid or other fumes which would attack the -metal parts or insulation, and should be located where it is easily -accessible for cleaning and oiling. =Erection:= The motor should be -set so that the shaft is level and parallel with the shaft it is -to drive so that the belt will run in the middle of the pulleys. -Do not use a belt which is too heavy or too tight for the work it -has to do, as it will materially reduce the output of the motor. -The belt should be from one-half to one inch narrower than the -pulley. =Rotation:= In order to reverse the direction of rotation, -interchange leads A and B. =Suspended Motors:= Motors with ring oil -bearings may be used on the wall or ceiling by taking off end caps -and revolving 90 or 180 degrees until the oil wells come directly -below the bearings. =Starting:= Motors are provided with link across -two terminals on the upper right hand bracket at the front of the -motor and with this connection should start considerable overloads. -If the starting current be too great with this connection, it may be -reduced by removing the link. =Temperatures:= At full load the motor -will feel hot to the hand, but this is far below the danger point. If -too hot for touch, measure temperature with a thermometer by placing -bulb against field winding for 10 minutes, covering thermometer -with cloth or waste. The temperature should not exceed 75 degrees -Fahr. above the surrounding air. =Oiling:= Fill the oil wells to the -overflow before starting and keep them full. See that the oil rings -turn freely with shaft. =Care:= The motor must be kept clean. Smooth -collector rings with sandpaper and see that the brushes make good -contact. When brushes become worn they may be reversed. When fitting -new brushes or changing them always sandpaper them down until they -make good contact with the collector rings, by passing a strip of -sandpaper beneath the brush.] - - =Rule.=--To find the speed of engine when diameter of both - pulleys, and speed of dynamo are given, _multiply the dynamo speed - by the diameter of its pulley and divide by the diameter of engine - pulley_. - -=Ques. How are the diameters and speeds of gear wheels figured?= - -Ans. The same as belted wheels, using either the pitch circle -diameters or number of teeth in each gear wheel. - -[Illustration: FIGS. 2,807 to 2,809.--Wiring diagrams and directions -for operating Holzer-Cabot slow speed alternating current motors. -=Erecting:= In installing the motor, be sure the transformer and -wiring to the motor are large enough to permit the proper voltage -at the terminals. If too small, the voltage will drop and reduce -the capacity of the motor. =Oiling:= Maintain oil in wells to the -overflow. =Starting: Single phase= motors are started by first -throwing the starting switch down into the starting position, and -when the motor is up to speed, throwing it up into the running -position. _Do not hold the switch in starting position over 10 -seconds._ Starter for single phase motors above ½ H.P. are arranged -with an adjusting link at the bottom of the panel. The link is shown -in the position of least starting torque and current. Connect from -W to 2 or W to 3 for starting heavier loads. _Two or three phase_ -motors are started simply by closing the switch. These motors start -full load without starters. The motor should start promptly on -closing the switch. It should be started the first time without -being coupled to the line shaft. If the motor start free, but will -not start loaded, it shows either that the load upon the motor is -too great, the line voltage too low, or the frequency too high. The -voltage and frequency with the motor running should be within 5% of -the name plate rating and the voltage with 10 to 15% while starting. -If the motor do not start free, either it is getting no current or -something is wrong with the motor. In either case an electrician -should be consulted. =Solution:= To reverse the direction of rotation -interchange the leads marked "XX" in the diagrams. =Temperature:= At -full load the motor should not heat over 75 degrees Fahr. above the -temperature of the surrounding air; if run in a small enclosed space -with no ventilation, the temperature will be somewhat higher.] - -=Ques. What should be noted with respect to generator pulleys?= - -Ans. A pulley of certain size is usually supplied with each generator -by its manufacturer, and it is not generally advisable to depart much -from the dimensions of this pulley. Accordingly, the solution of the -pulley problem usually consists in finding the necessary diameter of -the driving pulley relative to that of the pulley on the generator in -order to furnish the required speed. - -=Ques. What is the chief objection to belt drive?= - -Ans. The large amount of floor space required. - -[Illustration: FIG. 2,810.--Tandem drive for economizing floor space -with belt transmission. Belts of different lengths are used, as -shown, each of which passes over the driving wheel _d_ of the engine, -and then over the pulley wheel of one of the generators. In such -an arrangement the belts would be run lengthwise through the room -in which the machines are placed, and it is obvious that since the -width of the room would be governed by the width of the machines thus -installed, this method is a very efficient one for accomplishing the -end in view.] - -=Ques. How may the amount of space that would ordinarily be required -for belt drive, be reduced?= - -Ans. By driving machines in tandem as in fig. 2,810, or by the double -pulley drive as in fig. 2,811. - -=Ques. What is the objection to the tandem method?= - -Ans. The most economical distance between centers cannot be employed -for all machines. - -=Ques. What is the objectionable tendency in resorting to floor -economy methods with belt transmission?= - -Ans. The tendency to place the machines too closely together. This is -poor economy as it makes the cleaning of the machines a difficult and -dangerous task; it is therefore advisable to allow sufficient room -for this purpose regardless of the method of belting employed. - -[Illustration: FIG. 2,811.--Double pulley drive for economizing floor -space with belt transmission. Where a center crank engine is used -both pulleys may be employed by belting a machine to each as shown. -Although considerable floor space would be saved by the use of this -scheme if the generators thus belted were placed at M and G yet still -more floor space would be saved by having them occupy the positions -indicated at M and S.] - -=Ques. What is the approved location for an alternator exciter?= - -Ans. To economize floor space the exciter may be placed between the -alternator and engine at S in fig. 2,811. - -=Belts.=--In the selection of a belt, the quality of the leather -should be first under consideration. The leather must be firm, yet -pliable, free from wrinkles on the grain or hair side, and of an even -thickness throughout. - -[Illustration: FIG. 2,812.--Separately excited belt driven alternator -showing approved location of exciter. In an electrical station where -alternating current is generated, the alternators for producing -the current generally require separate excitation for their field -windings; that is, it is usually necessary to install in conjunction -with an alternator a small dynamo for supplying current to the -alternator field. The exciter is a comparatively small machine; in -fact, it requires only about 1 per cent. of the capacity of the -alternator which it excites, and so being small is often belted to -an auxiliary pulley mounted on the alternator shaft. Considerable -floor space would be occupied by an installation of this nature if -the exciter be placed at M, and belted to the alternator as indicated -by the dotted lines. By locating the exciter at S, between the -alternator and the engine, much floor space will be saved and the -general appearance of the installation improved.] - -If the belt be well selected and properly handled, it should do -service for twenty years, and even then if the worn part be cut off, -the remaining portion may be remade and used again as a narrower and -shorter belt. - - Besides leather belts, there are those made of rubber which - withstand moisture much better than leather belts, and which also - possess an excellent grip on the pulley; they are, however, more - costly and much less durable under normal conditions. - - In addition to leather and rubber belts, there are belts - composed of cotton, of a combination of cotton and leather, and - of rope. The leather belt, however, is the standard and is to be - recommended. - -Equally important with the quality of a belt is its size in order to -transmit the necessary power. - - The average strain under which leather will break has been found - by many experiments to be 3,200 pounds per square inch of cross - section. A good quality of leather will sustain a somewhat greater - strain. In use on the pulleys, belts should not be subjected to a - greater strain than one eleventh their tensile strength, or about - 290 pounds to the square inch or cross section. This will be about - 55 pounds average strain for every inch in width of single belt - three-sixteenths inch thick. The strain allowed for all widths - of belting--single, light double, and heavy double--is in direct - proportion to the thickness of the belt. - -=Ques. How much horse power will a belt transmit?= - -Ans. The capacity of a belt depends on, its width, speed, and -thickness. _A single belt one inch wide and travelling 1,000 feet per -minute will transmit one horse power; a double belt under the same -conditions, will transmit two horse power._ - -[Illustration: FIG. 2,813.--One horse power transmitted by belt to -illustrate the rule given above. A pulley is driven by a belt by -means of the friction between the surfaces in contact. Let T be the -tension on the driving side of the belt, and T', the tension on the -loose side; then the driving force = T-T'. In the figure T is taken -at 34 lbs. and T' at 1 lb.; hence driving force = 34-1 = 33 lbs. -Since the belt is travelling at a velocity of 1,000 feet per minute -the power transmitted = 33 lbs. × 1,000 ft. = 33,000 ft. lbs. per -minute = 1 horse power.] - - This corresponds to a working pull of 33 and 66 lbs. per inch of - width respectively. - - =Example.=--What width double belt will be required to transmit - 50 horse power travelling at a speed of 3,000 feet per minute? - - The horse power transmitted by each inch width of double belt - travelling at the stated speed is - - 3,000 - 1 × ----- × 2 = 6, - 1,000 - -hence the width of belt required to transmit 50 horse power is - - 50 ÷ 6 = 8.33, say 8 inches. - -=Ques. At what velocity should a belt be run?= - -Ans. At from 3,000 to 5,000 feet per minute. - -=Ques. How may the greatest amount of power transmitting capacity be -obtained from belts?= - -Ans. By covering the pulleys with leather. - -=Ques. How should belts be run?= - -Ans. With the tight side underneath as in fig. 2,814. - -[Illustration: FIGS. 2,814 and 2,815.--Right and wrong way to run a -belt. The tight side should be underneath so as to increase the arc -of contact and consequently the adhesion, that is to say, a _better -grip_, is in this way obtained.] - -=Ques. What is a good indication of the capacity of a belt in -operation?= - -Ans. Its appearance after a few days' run. - - If the side of the belt coming in contact with the pulley assume - a mottled appearance, it is an indication that the capacity of - the belt is considerably in excess of the power which it is - transmitting, inasmuch as the spotted portions of the belt do not - touch the pulley; and in consequence of this there is liable to be - more or less slipping. - - Small quantities of a mixture of tallow and fish oil which - have previously been melted together in the proportion of two of - the former to one of the latter, will, if applied to the belt - at frequent intervals, do much toward softening it, and thus - by permitting its entire surface to come in contact with the - pulley, prevent any tendency toward slipping. The best results - are obtained when the smooth side of the belt is used next to the - pulley, since tests conducted in the past prove that more power is - thus transmitted, and that the belt lasts longer when used in this - way. - -[Illustration: FIG. 2,816.--The Hill friction clutch pulley for power -control. The clutch mechanism will start a load equivalent to the -double belt capacity of the pulley to which the clutch is attached.] - -=Ques. What is the comparison between the so called endless belts and -laced belts?= - -Ans. With an endless belt there is no uneven or noisy action as with -laced belts, when the laced joint passes over the pulleys, and the -former is free from the liability of breakage at the joint. - -=Ques. How should a belt be placed on the pulleys?= - -Ans. The belt should first be placed on the pulley at rest, and then -run on the other pulley while the latter is in motion. - - The best results are obtained, and the strain on the belt is - less, when the speed at which the moving pulley revolves is - comparatively low. With heavy belts, particular care should be - taken to prevent any portion of the clothing being caught either - by the moving belt or pulleys, as many serious accidents have - resulted in the past from carelessness in regard to this important - detail. The person handling the belt should, therefore, be sure of - a firm footing, and when it is impossible to secure this, it is - advisable to stop the engine and fit the belt around the engine - pulley as well as possible by the aid of a rope looped around the - belt. - -[Illustration: FIG. 2,817--Sectional view of Hill clutch mechanism. -In every case the mechanism hub A, and in a clutch coupling the -ring W, is permanently and rigidly secured to the shaft and need -not be disturbed when removing the wearing parts. When erected, the -adjustment should be verified, and always with the clutch and ring -engaged and at rest. If the jaws do not press equally on the ring, or -if the pressure required on the cone be abnormal, loosen the upper -adjusting nuts T´ on eye bolts and set up the lower adjusting nuts -T´´ until each set of jaws is under the same pressure. Should the -clutch then slip when started it is evident that the jaw pressure is -insufficient and a further adjustment will be necessary. All clutches -are equipped throughout with split lock washers. Vibration or shock -will not loosen the nuts if properly set up. The jaws can be removed -parallel to the shaft as follows: Remove the gibs V, and withdraw the -jaw pins P, then pull out the levers D. Do not disturb the eye bolt -nuts T´ and T´´. The outside jaws B can now be taken out. Remove the -bolt nuts I allowing the fulcrum plates R to be taken off. On the -separable hub pattern the clamping bolts must be taken out before -fulcrum plate is removed. The inside jaws C may now be withdrawn. -Always set the clutch operating lever in the position as shown in -fig. 2,816 to avoid interference with mechanism parts. Oil the moving -parts of the clutch. Keep it clean. Examine at regular intervals.] - -=Ques. Under what conditions does a belt drive give the best results?= - -Ans. When the two pulleys are at the same level. - - If the belt must occupy an inclined position it should not form - a greater angle than 45 degrees with the horizontal. - -=Ques. What is a characteristic feature in the operation of belts, -and why?= - -Ans. Belts in motion will always run to the highest side of a -pulley; this is due partially to the greater speed in feet per minute -developed at that point owing to the greater circumference of the -pulley, and also to the effects of centrifugal force. - - If, therefore, the highest sides of both pulleys be in line with - each other, and the shafts of the respective pulleys be parallel - to each other, there will be no tendency for the belt to leave - the pulleys when once in its proper position. In order that these - conditions be maintained, the belt should be no more than tight - enough to prevent slipping, and the distance between the centers - of the pulleys should be approximately 3.5 times the diameter of - the larger one. - -[Illustration: FIG. 2,818.--Hill clutch mechanism Smith type. The -friction surfaces are wood to iron, the wood shoes being made from -maple. All parts of the toggle gear are of steel and forgings with -the exception of the connection lever which is of cast iron.] - -=Ques. What minor appurtenances should be provided in a station?= - -Ans. Apparatus should be installed as a prevention against accidents, -such as fire, and protection of attendants from danger. - - In every electrical station there should be a pump, pipes - and hose; the pump may be either directly connected to a small - electric motor or belted to a countershaft, while the pipes - and hose should be so placed that no water can accidentally - reach the generators and electrical circuits. A number of fire - bucket filled with water should be placed on brackets around the - station, and with these there should be an equal number of bucket - containing dry sand, the water being used for extinguishing fire - occurring at a distance from the machines and conductors, and the - sand for extinguishing fire in current carrying circuits where - water would cause more harm than benefit. To prevent the sand - being blown about the station, each sand bucket, when not in use, - should be provided with a cover. - - Neat cans and boxes should be mounted in convenient places for - greasy rags, waste, nuts, screws, etc., which are used continually - and which therefore cannot be kept in the storeroom. - - While it is important to guard against fire in the station, - it is equally necessary to provide for personal safety. All - passages and dark pits should therefore be thoroughly lighted - both day and night, and obstacles of any nature that are not - absolutely necessary in the operation of the station, should be - removed. Moving belts, and especially those passing through the - floor, should be enclosed in iron railings. If high voltages be - generated, it is well to place a railing about the switchboard to - prevent accidental contact with current carrying circuits, and in - such cases it is also advisable to construct an insulated platform - on the floor in front of the switchboard. - -[Illustration: FIG. 2,819.--Method of joining adjacent switchboard -panels.] - -=Switchboards.=--The plan of switchboard wiring for alternating -current work depends upon the system in use and this latter may be -either of the single phase, two phase, three phase, or monocyclic -types. The general principles in all these cases, however, are -practically identical. - - Fig. 2,820 shows the switchboard wiring for a single phase - alternator. As an aid in reading the diagram, the conductors - carrying alternating current are represented by solid lines, and - those carrying direct current, by dotted lines. - -[Illustration: FIG. 2,820.--Switchboard wiring for a single phase -separately excited alternator. The direct current circuits are -represented by dotted lines, and the alternating current circuit, by -solid lines.] - - The exciter shown at the right is a shunt wound machine. By - means of the exciter rheostat, the voltage for exciting the field - winding of the alternator is varied; this, in turn, varies the - voltage developed in the alternator since the main leads of the - exciter are connected through a double pole switch G to the field - winding of the alternator. - -[Illustration: FIGS. 2,821 to 2,825.--General Electric diagrams of -connections. A, ammeter; C.B, circuit breaker; C.P, candle power; -C.T, current transformer; D.R, discharge resistance; F, fuse; F.S, -field switch; L, lamp; O.C, overload coil; P.P, pressure plug; -P.R, pressure receptacle; R.C, reactance; rheo, rheostat; R.P, -synchronizing plug, running; R.S, resistance; S, switch; S.I, -synchronism indicator; S.P, synchronizing plug, starting; S.R, -synchronizing receptacle; V, voltmeter.] - - A rheostat is also introduced in the alternator field winding - circuit to adjust the alternator pressure. It may seem unnecessary - to employ a rheostat in each of two separate field circuits to - regulate the voltage of the alternator, but these rheostats are - not both used to produce the same result. When a considerable - variation of pressure is required, the exciter rheostat is - manipulated, whereas for a fine adjustment of voltage the - alternator rheostat is preferably employed. - - Sometimes a direct current ammeter is introduced in the - alternator's field circuit to aid in the adjustment. - - The main circuit of alternator after being protected on both - sides by fuses, runs to the double pole switch K. These fuses - serve as a protection to the alternator in case of a short circuit - at the main switch. It will be noticed the fuses are of the single - pole type and are mounted a considerable distance apart; this is - to prevent any liability of a short circuit between them in case - of action. Enclosed fuses are now used entirely for such work, - since in these there is no danger of heated metal being thrown - about and causing damage when the fuse wire is melted. Enclosed - fuses are also more readily and quickly replaced than open fuses, - the containing tube of each being easy to adjust in circuit, and - when the fuse wire within is once melted the tube is discarded for - a new one. - - The main circuit after passing through the main switch is - further protected on both sides by circuit breakers. Leaving these - protective devices, the left hand side of the circuit includes - the alternating current ammeter, and then connects with one of - the bus bars. The right hand side of the circuit runs from the - circuit breaker to the other bus bar. As many feeder circuits - may be connected to the bus bars and supplied with current by - the alternator as the capacity of this machine will permit. If, - however, there be more than one feeder circuit, each must be wired - through a double pole switch. - - In alternating current work the pressures dealt with are much - greater than those in direct current installations, so that - proportionate care must be taken in the wiring to remove all - possibility of grounds. - - To locate such troubles, however, should they occur, a ground - detector is provided. For this class of work the ground detector - must be an instrument especially designed for high pressure - circuits. Two of its terminals should be connected to the line - wires and the third, to ground; in case of a leak on the line, a - current will then flow through the detector and by the position of - the pointer the location and seriousness of the leak may be judged. - - A step down transformer is also rendered necessary for the - voltmeter and the pilot lamps, owing to the high voltage in use. - The primary winding of the transformer is connected across the - main circuit of the alternator. This connection should never be - made so that it will be cut out of circuit when the main switch is - open, for it is always advisable to consult the voltmeter before - throwing on the load by closing this switch. - -[Illustration: FIGS. 2,826 to 2,829.--General Electric diagrams of -connections. A, ammeter; C.B, circuit breaker; C.P, candle power; -C.T, current transformer; D.R, discharge resistance; F, fuse; F.S, -field switch; L, lamp; O.C, overload coil; P.P, pressure plug; -P.R, pressure receptacle; R.C, reactance; rheo, rheostat; R.P, -synchronizing plug, running; R.S, resistance; S, switch; S.I, -synchronous indicator: S.P, synchronizing plug, starting; S.R, -synchronizing receptacle: V, voltmeter.] - -=Ques. How does the switchboard wiring for a two phase system differ -from the single phase arrangement shown in fig. 2,820?= - -Ans. It is practically the same, except for the introduction of an -extra ammeter and a compensator in each of the outside wires, and in -the use of a four pole switch in place of the two pole main switch. - - The ammeters, of course, are for measuring the alternating - currents in each of the two phases or legs of the system, and the - compensators are two transformers with their primary coils in - series with the outside wires and their secondary coils in series - with each other across the outside wires. The transformers thus - connected are known as compensators or pressure regulators, and - as such compensate for the drop in pressure on either side of the - system. - -=Ques. How is the four pole main switch wired?= - -Ans. Its two central terminals which connect directly with the line -wires, are joined together by a conductor, and from this point one -wire is led off. This wire, together with the two outside wires, form -the feeders of the system. - -=Ques. How many voltmeters are required for the two phase system?= - -Ans. One voltmeter is sufficient on the board if a proper switching -device be employed to shift its connections across either of the two -circuits; otherwise, two voltmeters will be necessary, one bridged -across each of these respective circuits. - - The same reasoning holds true in regard to ground detectors, - so that one or two of these will be required, depending upon the - aforementioned conditions. - -=Ques. What are the essential points of difference between the single -phase switchboard wiring as shown in fig. 2,820, and that required -for a three wire three phase system?= - -Ans. The three phase system requires the use of a three pole switch -in place of the two pole switch; the insertion of an ammeter, a -circuit breaker, and a compensator in each of the three wires of the -system; the presence of two ground detectors instead of one, and the -addition of a voltmeter switch if but one voltmeter be provided, or -else the installation of two voltmeters, connected the one between -the middle wire and outer right hand wire, and the other between the -middle wire and outer left hand wire. - -[Illustration: FIG. 2,830.--Diagram of switchboard connections for -General Electric automatic voltage regulator with two exciters and -two alternators.] - -=Ques. Mention a few points relating to lightning arresters.= - -Ans. In most cases where direct current is used they are mounted -on the walls of the station near the place at which the line wires -enter. If they be mounted outside the station at this point, special -precautions should be taken to keep them free from moisture by -enclosing them in iron cases, but no matter where they are located it -is necessary that they be dry in order to work properly. - -[Illustration: FIGS. 2,831 and 2,832.--Garton-Daniels alternating -current lightning arrester; diagram showing connections. A lightning -discharge takes the path indicated by the dotted line, across the -upper air gap A, through resistance rod B, C, D, across copper strip -R on the base, thence flowing to ground through the movable plunger -M, lower on gap N, and ground binding post L. The discharge path is -practically straight, contains an air gap, distance of but 3/32 inch, -a series resistance averaging but 225 ohms. The lightning discharge -does not flow through the flexible lead connecting band D on the -lower end of the resistance rod with the top of the movable plunger. -These two points are electrically connected by the heavy copper -strip R, and lightning discharges generally, if not always, take the -path across this copper strip in preference to flowing through the -inductance of the one turn of flexible cable. When a discharge occurs -from line to ground through any lightning arrester, the air gaps arc -over, and so there is offered a path from line to ground for the line -current. This flow of line current following the lightning discharge -to ground may vary anywhere from a small capacity current where -the arrester is installed on an ungrounded circuit, a moderately -heavy flow on a partially grounded circuit, to a very heavy flow on -a grounded circuit--either a circuit operated as a dead grounded -circuit, or a circuit which has become accidentally grounded during -a storm. The path taken by this flow of line current from line to -ground may be traced by following the path shown by the dashed line. -It, as seen, crosses upper air gap A, flows through section B of the -resistance rod to band C. Leaving band C it flows through the magnet -winding H, thence to band D on the resistance rod, through flexible -lead to upper end of movable plunger, through movable plunger, across -lower air gap N, to ground binding post L, thence to ground. The -function of the short length of resistance rod CD is as follows: It -has an ohmic resistance of about 30 ohms but is _non-inductive_. -Magnet winding H, connected to bands C and D on the ends of this -short length of rod has an ohmic resistance of 3 ohms, but is _highly -inductive_. Lightning discharges being of _high frequency_ take the -higher resistance _but non-inductive_ path CD in their passage from -line to ground. The flow of normal current from line to ground being -of a very low frequency, 25 or 60 cycles in ordinary alternating -current circuits, zero in direct current circuits--takes the _low -resistance_ path through coil H in its path to ground. Section CD of -the rod is used therefore simply to shunt the inductance of winding -H to high frequency lightning discharges, leaving the lightning -discharge path in the arrester a _non-inductive_ highly efficient -path. In all Garton-Daniels A. C. lightning arresters operating on -non-grounded or partially grounded circuits, the action of the air -gaps and series resistance are together sufficient to extinguish the -flow of normal current to ground at the zero point of the generator -voltage wave. If, however, as frequently happens, the line grounds -accidentally during a storm, then the arrester does not have to -depend for its proper operation on the arc extinguishing properties -of the air gaps and resistance, but the heavier flow of line current -through the arrester energizes the movable plunger, which raises -upward in the coil, opening the circuit between the discharge point M -and the lower end of the plunger. To limit the flow of line current -to ground the resistance rod B is provided, there being approximately -225 ohms between the discharge point A and clamp C in the 2,500 volt -arrester. This feature is particularly effective where the circuit is -temporarily or accidentally grounded. The series resistance prevents -a heavy short circuit through the arrester and limits the current to -a value that is readily broken by the cut out and is not enough to -impede the passage of the discharge.] - - -[Illustration: FIG. 2,833.--Diagram of switchboard connections for -General Electric automatic voltage regulator with three exciters and -three alternators.] - - If possible, one place should be set aside for them and a marble - or slate panel provided on which they may be mounted. - - Wooden supports are undesirable for lightning arresters on - account of the fire risk incurred; this, however, may be reduced - to a minimum by employing skeleton boards and using sheets of - asbestos between the arresters and the wood. - - In parts of the country where lightning is of common occurrence - and where overhead circuits are installed which carry high - pressures, heavy currents, and extend over considerable territory, - it is advisable to have the station well equipped with lightning - arresters of the most improved types. - - In each side of the main circuit, between the lightning arrester - connections and the switchboard apparatus there should be - connected a choke coil or else each of the main conductors at this - point should be tightly coiled up part of its length to answer the - same purpose. - - A quick and effective way of coiling up a wire consists in - wrapping around a cylindrical piece of iron or wood that part - of the conductor in which it is desired to have the coils, the - desired number of times, and then withdrawing the cylindrical - piece. The coils, each of which may contain 50 or 200 turns, - thus inserted in the main circuit introduce a high resistance or - reluctance to a lightning current, and thus prevent it passing - to the generator; there will, however, be an easy path to earth - afforded it through the lightning arrester, and so no damage will - be done. Coils of the nature just mentioned may advantageously be - introduced between the generator and switchboard to take up the - reactive current developed upon the opening of the circuit, and in - the case of suspended conductors, the coils may be used to take up - the slack by the spring-like effect produced by them. - - The safety of the operator should be especially considered in - the design of high pressure alternating current switchboards. - - Such protection may be secured by screening all the exposed - terminals, or preferably by mounting all the switch mechanism on - the back of the board with simply the switch handle projecting - through to the front; by pushing or pulling the switch handle, the - connections can thus be shifted either to one side of the system - or to the other. - -=Ques. Upon what does the work of assembling a switchboard depend?= - -Ans. It depends almost entirely upon the size of the plant, varying -from the simple task of mounting a single panel in the case of an -isolated plant, to the more difficult problem of supporting a large -number of panels in a central station. - -=Ques. When the material chosen for a switchboard must be shipped a -considerable distance, what form of board should be used?= - -Ans. The board units or "slabs" should be of small dimensions, to -avoid the liability of breakage and expense of renewal when a unit -becomes cracked or machine injured. - -[Illustration: FIGS. 2,834 and 2,835.--Front and rear views showing -General Electric automatic voltage regulator mounted on switchboard -panel.] - - Ordinarily, switchboards vary from five to eight feet in - height and the widths of the panels vary from five to six feet. - In some boards the seams between the slabs run vertically, and - in others horizontally. In order to render the assembling of - the switchboard as simple as possible, and its appearance when - finished the most artistic, these seams should run horizontally - rather than vertically. The edges of each of the slabs should also - be chamfered so that there will be less danger of their breaking - out when being mounted on the framework. - -=Ques. In assembling a switchboard, how should the lower slabs be -placed, and why?= - -Ans. They should be suspended a little distance from the floor to -prevent contact with any oil, dirt, water or rubbish that might be on -the floor. - -=Ques. How are the slabs or panels supported?= - -Ans. They are carried on an iron or wooden framework with braces to -give stability. - - The braces should be securely fastened at one end to the wall of - the station, and at the other end to the framework of the board, - as shown in fig. 2,836. - - To fasten the switchboard end of the brace directly to the - slate, marble or other material composing the board is poor - practice and should never be attempted. - - If the station be constructed of iron, these switchboard braces - must be such that they will thoroughly insulate the board and its - contents from the adjoining wall. - -[Illustration: FIG. 2,836.--Method of supporting the framework of a -switchboard.] - -=Ques. What is the usual equipment of a switchboard?= - -Ans. It comprises switching devices, current or pressure limiting -devices, indicating devices, and fuses for protecting the apparatus -and circuits. - -[Illustration: FIG. 2,837.--Diagram showing elementary connections of -General Electric automatic regulator for direct current. It consists -essentially of a main control magnet with two independent windings -and a differentially wound relay magnet. One winding, known as the -pressure winding, of the main control magnet is connected across the -dynamo terminals, the other across a shunt in one of the load mains. -The latter is the "compensating winding" and it opposes the action -of the pressure winding so that as the load increases, a higher -pressure at the dynamo is necessary to "over compound" for line drop. -In ordinary practice, the voltage terminals are connected to the bus -bars, and the compensating shunt inserted in one of the principal -feeders of the system. In operation the shunt circuit across the -dynamo field rheostat is first opened by means of a switch provided -for that purpose on the base of the regulator and the rheostat turned -to a point that will reduce the generator voltage 35 per cent below -normal. The main control magnet is at once weakened and allows the -spring to pull out the movable core until the main contacts are -closed. This closes the second circuit of the differential relay, -thus neutralizing its windings. The relay spring then lifts the -armature and closes the relay contacts. The switch in the shunt -circuit across the dynamo field rheostat is now closed, practically -short circuiting the rheostat, and the dynamo voltage at once rises. -As soon as it reaches the point for which the regulator has been -adjusted, the main control magnet is strengthened, which causes the -main contacts to open, which in turn open the relay contacts across -the rheostat. The rheostat is now in the field circuit, the voltage -at once falls off, the main contacts are closed, and relay armature -released, and shunt circuit across the rheostat again completed. The -voltage then starts to rise and this cycle of operation is continued -at a high rate of vibration, maintaining not a constant but a steady -voltage at the bus bars. When neither the compensating winding nor -pressure wires are used, there will be no "over compounding" effect -due to increase of load and a constant voltage will be maintained at -the bus bars. The compensating winding on the control magnet, which -opposes the pressure winding is connected across an adjustable shunt -in the principal feeder circuit. As the load increases the voltage -drop across the shunt increases and the effect of the compensating -winding becomes greater. This will require a higher voltage on the -pressure winding to open the main contacts and the regulator will -therefore cause the dynamo to compensate for line drop, maintaining -at the bus bars a steady voltage without fluctuations, which rises -and falls with a load on the feeders, giving a constant voltage at -the lamps or center of distribution. The compensating shunt may be -adjusted so as to compensate for any desired line drop up to 15 per -cent; it is preferably placed in the principal lighting feeder, but -may be connected to the bus bars so that the total current will pass -through it. The latter method, however, is sometimes desirable, as -large fluctuating power loads on separate feeders might disturb the -regulation of the lighting feeders. Adjustment is made by sliding -the movable contact at the center of the shunt. This contact may be -clamped at any desired point and determines the pressure across the -compensating winding of the regulator's main control magnet. Where -pressure wires are run back to the central station from the center -of distribution they may be connected directly to the pressure -winding of the main control magnet, and it is unnecessary to use the -compensating shunt. The pressure wires take the place of the leads -from the control magnet to the bus bars and maintain a constant -voltage at the center of distribution.] - - On some switchboards are also mounted small transformers for - raising or lowering the voltages, and lightning arresters as a - protection from lightning. In addition to the apparatus previously - mentioned nearly all switchboards carry at or near their top two - or more incandescent lamps provided with shades or reflectors, for - lighting the board. - -=Ques. What should be done before wiring a switchboard?= - -Ans. The electrical connections between the various apparatus -mounted on the face or front of the board, are made on the back of -the board. It is necessary that these connections be properly made -else considerable electrical power will be wasted at this point. The -wiring on the back of the board should therefore be planned out on -paper before commencing the work. - -[Illustration: FIG. 2,838.--Diagram showing connections of General -Electric automatic voltage regulator for direct current as connected -for maintaining balanced voltage on both sides of a three wire system -using a balancer set. In operation, should the voltage on the upper -bus bars become greater than that on the lower ones, the middle and -upper contacts on the regulator will close, thus opening the relay -contacts to the left and closing those to the right. This inserts all -the resistance in the field of balancer A, and short circuits the -resistance in the field of balancer B. A will then be running as a -motor, and B as a dynamo, thereby equalizing the two voltages until -that on the lower bus bars becomes greater than that of the upper -ones; then the regulator contacts operate in the opposite direction -and balancer A is run as a dynamo, and balancer B as a motor. This -cycle of operation is repeated at the rate of from three to four -hundred times per minute, thus maintaining a balanced voltage on the -system.] - - In laying out the plan of wiring care must be taken to allow - sufficient contact surface at each connection; there should be not - less than one square inch of contact surface allowed for each 160 - amperes of current transmitted. - -[Illustration: FIG. 2,839.--Diagram of connections of General -Electric voltage regulators for one or more alternators using one -exciter.] - - For the bus bars, which, by the way are always of copper, one - square inch per 1,000 amperes is the usual allowance; this is - equal to 1,000 circular mils of cross sectional area per ampere. - - Every effort should be made to give the bus bars the greatest - amount of radiation consistent with other conditions, in order - that their resistances may not become excessive owing to the heat - developed by the large currents they are forced to carry. Suppose, - for instance, the number of amperes to be generated is such as - to require bus bars having each a cross sectional area of one - square inch. If the end dimensions of these bars were each 1 inch - by 1 inch, there would be less radiating surface than if their - dimensions were each 2 inches by ½ inch. - -=Operation of Alternators.=--The operation of an alternator when run -singly differs but little from that for a dynamo. - - As to the preliminaries, the exciter must first be started. - This is done in the same way as for any shunt dynamo. At first - only a small current should be sent through the field winding of - the alternator; then, if the exciter operates satisfactorily and - the field magnetism of the operator show up well, the load may - gradually be thrown on until the normal current is carried, the - same method of procedure being followed as in the similar case of - a dynamo. - -[Illustration: FIGS. 2,840 and 2,841.--General Electric equalizer -regulator designed to equalize the load on two machines, and diagram -of connections.] - -On loading an alternator, a noticeable drop in voltage occurs -across its terminals. This drop in voltage is caused in part by the -demagnetization of the field magnets due to the armature current, -and so depends in a measure upon the position and form of the pole -pieces as well as upon those of the teeth in the armature core. The -resistance of the armature winding also causes a drop in voltage -under an increase of load. - - Another cause which may be mentioned is the inductance of the - armature winding, which is in turn due to the positions of the - armature coils with respect to each other and also with respect to - the field magnets. - -[Illustration: FIG. 2,842.--Connection of General Electric equalizing -regulator for equalizing loads on an engine driven dynamo and rotary -converter running in parallel. Should the load on the dynamo become -greater than that on the rotary converter, the middle and upper -contacts on the regulator close, and thus by means of the relay -switch and control motor, cause the feeder regulator to boost the -voltage on the rotary until the loads again become equal. Should -the load on the rotary converter become greater than that on the -generator, the regulator contacts operate in the reverse direction -and the feeder regulator is caused to buck the rotary voltage.] - -=Alternators in Parallel.=--When the load on a station increases -beyond that which can conveniently be carried by one alternator, it -becomes necessary to connect other alternators in parallel with it. -To properly switch in a new machine in parallel with one already in -operation and carrying load, requires a complete knowledge of the -situation on the part of the attendant, and also some experience. - - The connections for operating alternators in parallel are - shown in fig. 2,843. In the illustration the alternator A is in - operation and is supplying current to the bus bars. The alternator - B is at rest. The main pole switch B' by means of which this - machine can be connected into circuit is therefore open. - -[Illustration: FIG. 2,843.--Method of synchronizing with one lamp; -_dark lamp method_. Assuming A to be in operation, B, may be brought -up to approximately the proper speed, and voltage. Then if B, be -run a little slower or faster than A, the synchronizing lamp will -glow for one moment and be dark the next. At the instant when the -pressures are equal and the machines in phase, the lamp will become -dark, but when the phases are in quadrature, the lamp will glow at -its maximum brilliancy. Since the flickering of the lamp is dependent -upon the difference in frequency, the machines should not be thrown -in parallel while this flickering exists. The nearer alternator -approaches synchronism, in adjusting its speed, the slower the -flickering, and when the flickering becomes very slow, the incoming -machine may be thrown in the moment the lamp is dark by closing the -switch. The machines are then in phase and tend to remain so, since -if one slow down, the other will drive it as a motor.] - - Now, if the load increase to such extent as to require the - service of the second alternator B, it must be switched in - parallel with A. In order that both machines may operate properly - in parallel, three conditions must be satisfied before they are - connected together, or else the one alternator will be short - circuited through the other, and serious results will undoubtedly - follow. - -Accordingly before closing main switch B, it is necessary that - - 1. The frequencies of both machines be the same; - 2. The machines must be in synchronism; - 3. The voltages must be the same. - -=Ques. How are the frequencies made the same?= - -Ans. By speeding up the alternator to be cut in, or change the speed -of both until frequency of both machines is the same. - -[Illustration: FIG. 2,844.--Diagram of connections of General -Electric automatic voltage regulator for several alternators running -in parallel with exciters in parallel.] - -=Ques. How are the alternators synchronized or brought in phase?= - -Ans. The synchronism of the alternators is determined by employing -some form of synchronizer, as by the single lamp method of fig. -2,843, or the two lamp method of fig. 2,845. - -=Ques. In synchronizing by the one lamp method, when should the -incoming machine be thrown in?= - -Ans. It is advisable to close the switch when the machines are -approaching synchronism rather than when they are receding from it, -that is to say, the instant the lamp becomes dark. - -[Illustration: FIG. 2,845.--Method of synchronizing with two lamps; -_dark lamp method_. The two synchronizing lamps are connected as -shown, and each must be designed to supply its rated candle power -at the normal voltage developed by the alternators. Now since the -alternators are both running under normal field excitation the left -hand terminals of each of them will alternately be positive and -negative in polarity, while the right hand terminals are respectively -negative and positive in polarity. If, however, the alternators be -in phase with each other, the left hand terminals of both of them -will be positive while the right hand terminals are negative, and -when the left hand terminals of both machines are negative the right -hand terminals will be positive. Hence, when the machines are in -phase there will be no difference of pressure between the left hand -terminals or between the right hand terminals of the two machines. -Hence, if the synchronizing lamps be connected as shown, both will -be dark. The instant there is a difference of phase, both lamps -will glow attaining full candle power when the difference of phase -has reached a maximum. As the alternators continue to come closer -in step, the red glow will gradually fade away until the lamps -become dark. Then the switch may be closed, thereby throwing the -two machines in parallel. If the intervals between the successive -lighting up of the lamps are of short duration it is advisable to -wait until these become longer even though the other conditions are -satisfied, because where the phases pass each other rapidly there is -a greater possibility of not bringing them together at the proper -instant. An interval of not less than five seconds should therefore -be allowed between the successive lighting up of the lamps, before -closing the switch.] - -[Illustration: FIG. 2,846.--Inductor type synchroscope. This type -is especially applicable where pressure transformers are already -installed for use with other meters. As it requires only about ten -apparent watts it may be used on the same transformers with other -meters. There are three stationary coils, N, M and C, and a moving -system, comprising an iron armature, A, rigidly attached to a shaft -suitably pivoted and mounted in bearings. A pointer is also attached -to the shaft. The moving system is balanced and is not subjected -to any restraining force, such as a spring or gravity control. The -axes of the coils N and M are in the same vertical plane, but 90 -degrees apart, while the axis of C is in a horizontal plane. The -coils N and M are connected in "split phase" relation through an -inductive resistance P and non-inductive resistance Q, and these two -circuits are parallel across the bus bar terminals 3 and 4 of the -synchroscope. Coil C is connected through a non-inductive resistance -across the upper machine terminals 1 and 2 of the synchroscope. =In -operation=, current in the coil C magnetizes the iron core carried -by the shaft and the two projections, marked A and "iron armature." -There is however, no tendency to rotate the shaft. If current be -passed through one of the other coils, say M, a magnetic field will -be produced parallel with its axis. This will act on the projections -of the iron armature, causing it to turn so that the positive and -negative projections assume their appropriate position in the field -of the coil M. A reversal of the direction in both coils will -obviously not affect the position of the armature, hence alternating -current of the same frequency and phase in the coils C and M cause -the same directional effect upon the armature as if direct current -were passed through the coils. If current lagging 90 degrees behind -that in the coils M and C be passed through the coil N, it will cause -no rotative effect upon the armature, because the maximum value of -the field which it produces will occur at the instant when the pole -strength of the armature is zero. The two currents in the coils M and -N produce a shifting magnetic field which rotates about the shaft as -an axis. As all currents are assumed to be of the same frequency, the -rate of rotation of this field is such that its direction corresponds -with that of the armature projections at the instant when the poles -induced in them by the current in the coil C are at maximum value, -and the field shifts through 180 degrees in the same interval as is -required for reversal of the poles. This is the essential feature of -the instrument, namely, that the armature projections take a position -in the rotating magnetic field which corresponds to the direction of -the field at the instant when the projections are magnetized to their -maximum strength by their current in the coil C. If the frequency -of the currents in the coils which produce the shifting field be -less than that in the coil which magnetized the armature, then the -armature must turn in order that it may be parallel with the field -when its poles are at maximum strength.] - -=Ques. What are the objections to the one lamp method?= - -Ans. The filament of the lamp may break, and cause darkness, or the -lamp may be dark with considerable voltage as it takes over 20 volts -to cause a 100 volt lamp to glow. - -=Ques. What capacity of single lamp must be used?= - -Ans. It must be good for twice the voltage of either machine. - -[Illustration: FIG. 2,847.--Brilliant lamp method of synchronizing. -The synchronizing lamps are connected as shown, and must be of the -alternator voltage. When the voltages are equal and the machines in -phase, the difference of pressure between _a_ and a given point is -the same as that between _a'_ and the same point; this obtains for -_b_ and _b'_. Accordingly, a lamp connected across _a b'_ will burn -with the same brilliancy as across _a' b_; the same holds for the -other lamp. When the voltages are the same and the phase difference -is 180° the lamps are dark, and as the phase difference is decreased, -the lamps glow with increasing brightness until at synchronism they -glow with maximum brilliancy. Hence the incoming alternator should be -thrown in at the instant of maximum brilliancy.] - -=Ques. What modification of the synchronizing methods shown in -the accompanying illustrations is necessary when high pressure -alternators are used?= - -Ans. Step down transformers must be used between the alternators and -the lamps to obtain the proper working voltages for the lamps. - -[Illustration: FIG. 2,848.--Synchronizing with high pressure -alternators; dark and brilliant lamp methods. In both methods the -primaries of the transformers are connected in the same way across -the terminals of the alternators as shown. In the dark lamp method, -the connections between the secondary coils of the transformers must -be made so that when each is subjected to the same conditions the -action of the one coil opposes that of the other as in the dark lamp -method; then, if the transformers be both of the same design, there -will be no voltage across the lamps when the alternators are in phase -with each other. If the ratio of each transformer is such as to give, -for example, 100 volts across its secondary terminals, then the two -incandescent lamps since they are joined together in series must -each be designed for 100 volts. One 200 volt lamp could be used in -either method in place of the two 100 volt lamps. When, therefore, -the alternators are directly opposite in phase to each other, both -the lamps will burn brightly; as the alternators come together in -phase the lamps will produce less and less light, until when the -machines are exactly in phase no light will be emitted at all, at -which instant the incoming alternator should be thrown in. It must -be evident, if the transformer secondary connections are arranged as -in the brilliant lamp method, so that they do not oppose each other, -the lamps will be at maximum brilliancy when the alternators are in -phase and dark when the phase difference is 180°, assuming of course -equalized voltage.] - -=Ques. How is the voltage of an incoming machine adjusted so that it -will be the same as the one already in operation?= - -Ans. By varying the field excitation with a rheostat in the -alternator field circuit. - -=Ques. How may two or more alternators be started simultaneously?= - -Ans. After bringing each of them up to its proper speed so as to -obtain equal frequencies, the main switches may be closed, thereby -joining their armature circuits in parallel. As yet, however, their -respective field windings have not been supplied with current, so -that no harm can result in doing this. The exciters of these machines -after being joined in parallel, should then be made to send direct -current simultaneously through the field windings of the alternators, -and from this stage on the directions previously given may be -followed in detail. - -=Ques. What are the conditions when two or more alternators are -directly connected together?= - -Ans. If rigidly connected together, or directly connected to the same -engine, they must necessarily run in the same manner at all times. - - When machines connected in this way are once properly adjusted - so that they are in phase with each other, their operation in - parallel is even a simpler task than when they are all started - together but are not directly connected. - -=Ques. When an alternator is driven by a gas engine, what provision -is sometimes made to insure successful operation in parallel?= - -Ans. An amortisseur winding is provided to counteract the tendency to -"hunting." - -[Illustration: FIG. 2,849.--Diagram of Lincoln Synchronizer. =In -construction=, a stationary coil F, has suspended within it a coil A, -free to move about an axis in the planes of both coils and including -a diameter of each. If an alternating current be passed through both -coils, A, will take a position with its plane parallel to F. If now -the currents in A and F be reversed with respect to each other, coil -A will take up a position 180° from its former position. Reversal -of the relative directions of currents in A and F is equivalent to -changing their phase relation by 180°, and therefore this change -of 180° in phase relation is followed by a corresponding change of -180° in their mechanical relation. Suppose now, instead of reversing -the relative direction of currents in A and F, the change in phase -relation between them be made gradually and without disturbing the -current strength in either coil. It is evident that when the phase -difference between A and F reaches 90°, the force between A and F -will become reduced to zero, and a movable system, of which A may -be made a part, is in condition to take up any position demanded -by any other force. Let a second number of this movable system -consist of coil B, which may be fastened rigidly to coil A, with its -plane 90° from that of coil A, and the axis of A passing through -diameter of B. Further, suppose a current to circulate through B, -whose difference in phase relation to that in A, is always 90°. It -is evident under these conditions that when the difference in phase -between A and F is 90°, the movable system will take up a position, -such that B is parallel to F, because the force between A and F is -zero, and the force between B and F is a maximum; similarly when the -difference in phase between B and F is 90°, A will be parallel to F. -That is, beginning with a phase difference between A and F of zero -a phase change of 90° will be followed by a mechanical change on a -movable system of 90°, and each successive change of 90° in phase -will be followed by a corresponding mechanical change of 90°. For -intermediate phase relation, it can be proved that under certain -conditions the position of equilibrium assumed by the movable element -will exactly represent the phase relations. That is, with proper -design, the mechanical angle between the plane of F and that of A -and also between the plane of F and that of B, is always equal to -the phase angle between the current flowing in F and those in A and -B respectively. =As commercially constructed= coil F consists of a -small laminated iron field magnet with a winding whose terminals -are connected with binding posts. The coils A and B are windings -practically 90° apart on a laminated iron armature pivoted between -the poles of the magnet. These two windings are joined, and a tap -from the junction is brought out through a slip ring to one of two -other binding posts. The two remaining ends are brought out through -two more slip rings, one of which is connected to the remaining -binding post, through a non-inductive resistance, and the other -to the same binding post through an inductive resistance. A light -aluminum hand attached to the armature shaft marks the position -assumed by the armature.] - -=Ques. What is the action of the amortisseur winding?= - -Ans. Any sudden change in the speed of the field, generates a current -in the amortisseur winding which resists the change of velocity that -caused the current. - - The appearance of an amortisseur winding is shown in the cut - below (fig. 2,850) illustrating the field of a synchronous - condenser equipped with amortisseur winding. - -[Illustration: FIG. 2,850.--General Electric field of synchronous -condenser provided with amortisseur winding. Hunting is accompanied -by a shifting of flux across the face of the pole pieces due to the -variation in the effect of armature reaction on the main field flux -as the current varies and the angular displacement between the field -and armature poles is changed. Copper short circuited collars placed -around the pole face have currents induced in them by this shifting -flux, which have such a direction as to exert a torque tending to -oppose any change in the relative position of the field and armature. -This action is similar to that of the running torque of an induction -motor and the damping device has been still further developed until -in its best form it resembles the armature winding of a "squirrel -cage" induction motor. The pole pieces are in ducts, and low -resistance copper bars placed in them with their ends joined by means -of a continuous short circuiting ring extending around the field. -Such a device has proven very effective in damping out oscillations -started from any cause, the same winding doing duty as a damping -device and to assist the starting characteristics.] - -=Ques. How are three phase alternators synchronized?= - -Ans. In a manner similar to the single phase method. - - Thus the synchronizing lamps may be arranged as in fig. 2,581, - which is simply an extension of the single phase method. - -=Ques. Are three lamps necessary?= - -Ans. Only to insure that the connections are properly made, after -which one lamp is all that is required. - -=Ques. How is it known that the connections of fig. 2,851 are -correct?= - -Ans. If, in operation, the three lamps become bright or dark -_simultaneously_, the connections are correct; if this action takes -place _successively_, the connections are wrong. - - If wrong, transpose the leads of one machine until simultaneous - action of the lamps is secured. - -[Illustration: FIG. 2,851.--Method of synchronizing three phase -alternators with, three lamps, being an extension of the single phase -method.] - -=Ques. What is the disadvantage of the lamp method of synchronizing?= - -Ans. Lack of sensitiveness. - -=Ques. Which is the accepted lamp method, dark or brilliant?= - -Ans. In the United States it is usual to make the connections for -a dark lamp at synchronism, while in England the opposite practice -obtains. - - With the dark lamp method, the breaking of a filament might - cause the machines to be connected with a great phase difference, - whereas, with the brilliant lamp it is difficult to determine the - point of maximum brilliancy. This latter method, therefore may be - called the safer. - -=Ques. What may be used in place of lamps for synchronizing?= - -Ans. Some form of synchroscopes, or synchronizers. - -=Ques. How does the Lincoln synchronizer work?= - -Ans. The construction is such that a hand moves around a dial so -that the angle between the hand and the vertical is always the phase -angle between the two sources of electric pressure to which the -synchronizer is connected. - - If the incoming alternator be running too slow, the hand - deflects in one direction, if too fast, in the other direction. - When the hand shows no deflection, that is, when it stands - vertical, the machines are in phase. A complete revolution of the - hand indicates a gain or loss of one cycle in the frequency of the - incoming machine, as referred to the bus bars. - -=Cutting Out Alternator.=--When it is desired to cut out of circuit -an alternator running in parallel with others, the method of -procedure is as follows: - - 1. Reduce driving power until the load has been transferred - to the other alternators, adjusting field rheostat to obtain - minimum current; - 2. Open main switch; - 3. Open field switch. - -=Ques. What precaution should be taken?= - -Ans. _Never_ open field switch before main switch. - -[Illustration: FIG. 2,852.--General Electric 500 kw., horizontal -mixed pressure Curtis turbine connected to a 500 kw. dynamo. In -a Curtis turbine it is not necessary to use the whole periphery -of the first stage for low pressure steam nozzles. A section can -be partitioned off and equipped with special expanding nozzles to -receive steam at high pressure direct from the boilers. Such nozzles -deliver their steam against the same wheel as do the low pressure -nozzles, but occupy only a small portion of its periphery. The steam -is expanded in these nozzles from high pressure all the way down -to the normal pressure of the first stage, and in such expansion -acquires a high velocity and consequently contains a great deal -of energy--much more than does an equal quantity of low pressure -steam. In consequence of this, high pressure steam is used with -a far lower water rate than is obtained with low pressure steam, -or with high pressure steam reduced to low pressure in a reducing -valve. This construction is called "mixed pressure." Its function -is the same as that of the reducing valve, that is, it makes up -for a deficiency of low pressure steam by drawing direct on the -boilers. With this construction, the full power of the turbine can be -developed with: All low-pressure steam, all high pressure steam, or, -any necessary proportion of steam of each pressure. Furthermore, the -transition from all low pressure to all high pressure, through all -the conditions intermediate between these extremes, is provided for -automatically by the turbine governor; a deficiency of low pressure -steam causes the high pressure nozzles to open automatically.] - -=Ques. What is the ordinary method of cutting out an alternator?= - -Ans. The main switch is usually opened without any preliminaries. - -=Ques. What is the objection to this procedure?= - -Ans. It suddenly throws all the load on the other alternators, and -causes "hunting." - -=Ques. What forms of drive are especially desirable for running -alternators in parallel, and why?= - -Ans. Water turbine or steam turbine because of the uniform torque, -thus giving uniform motion of rotation. - - With reciprocating engines, the crank effect is very variable - during the revolution, resulting in pulsations driving the - alternator too fast or too slow, and causing cross current between - the alternators. - -=Ques. Is a sluggish, or a too sensitive governor preferable on an -engine driving alternators in parallel?= - -Ans. A sluggish governor. - -=Alternators in Series.=--Alternators are seldom if ever connected -in series, for the reason that the synchronizing tendency peculiar -to these machines causes them to oppose each other and fall out of -phase when they are joined together in this way. If, however, they -be directly connected to each other, or to an engine, so that they -necessarily keep in phase at all times, and thus add their respective -voltages instead of counteracting them, series operation is possible. - - NOTE.--According to the practice of the General - Electric Co., 2½ degrees of phase difference - from a mean is the limit allowable in ordinary - cases. It will, in certain cases, be possible to - operate satisfactorily in parallel, or to run - synchronous apparatus from machines whose angular - variation exceeds this amount, and in other cases - it will be easy and desirable to obtain a better - speed control. The 2½ degree limit is intended to - imply that the maximum departure from the mean - position during any revolution shall not exceed - 2½ ÷ 360 of an angle corresponding to two poles - of a machine. The angle of circumference which - corresponds to the 2½ degree of phase variation - can be ascertained by dividing 2½ by ½ the - number of pole; thus, in a 20 pole machine, the - allowable angular variation from the mean would - be 2½ ÷ 10 = ¼ of one degree. - -[Illustration: FIG. 2,853.--Diagram of connections for synchronizing -two compound wound three phase alternators. A and A' are the -armatures of the two machines, the fields of which are partly -separately excited, the amount of excitation current being controlled -by the series compounding rheostats B and B', which form a stationary -shunt. It is assumed that the alternator A is connected to the -bus bars 1, 2, and 3, by the switch 1S. If an increase make it -necessary to introduce the alternator A', it is first run up to -speed and excited to standard pressure by its exciter, and then -the double plug switch 3S is closed, connecting the primary of the -station transformer T and T' with the bus bars through the secondary -coil, so that the synchronizing lamps light up when the secondary -circuit is closed through the single pole switch 4S. The primary of -the station transformer T is thus excited through the double pole -switch 5S, connecting it with the outer terminals of the armature -A'. The two alternators will now work in opposition to each other -upon the synchronizing lamps, the transformer T being operated by -the new alternator A' through the switch 2S, and the transformer -T' being operated by the working alternator A, from the bus bars. -If the new alternator be not in step with the working alternator, -the synchronizing lamps will glow, growing brighter and dimmer -alternately with greater or lesser rapidity. In this case, the -armature speed of the new alternator must be controlled in such a -manner that the brightening and dimming will occur more and more -slowly, until the lamps cease to glow or remain extinguished for -a decided interval of time. The extinction of the light is due to -the disappearance of the secondary current, and indicates that -the alternators are in step. The switch 2S should now be thrown, -thus coupling the two machines electrically, and both of them will -continue to operate in step. The double pole equalizer switch 6S -should now be closed, connecting the two field windings in parallel -and equalizing the compounding, so that any variations of load will -affect the two alternators equally. After the alternators have been -connected in parallel, the switches 4S and 5S, may be opened leaving -the switch 3S closed, to operate the switchboard lamps K, K, as pilot -lights from the bus bars.] - -=Transformers.=--These, as a whole, are simple in construction, high -in efficiency, and comparatively inexpensive. Their principles of -operation are also readily understood. - -The efficiency of a transformer, that is, the ratio between full -load primary and full load secondary is greatest when the load on it -is such that the sum of the constant losses equals the sum of the -variable losses. - - In general, transformers designed for high frequencies and - large capacities are more efficient than those designed for - low frequencies and small capacities. As a whole, however, a - transformer leaves but little to be desired as regards efficiency, - a modern 60 cycle transformer of 50 kilowatts capacity or more - possesses an efficiency of approximately 98 per cent. at full load - and an efficiency of about 97 per cent. at half load. - -=Ques. How should a transformer be selected, with respect to -efficiency?= - -Ans. One should be chosen, whose parts are so proportioned that the -point of maximum efficiency occurs at that load which the transformer -usually carries in service. - - In many alternating current installations, comparatively light - loads are carried the greater part of the time, the rated full - load or an overload being occurrences of short durations. For such - purposes special attention should be given to the designing or - selecting of transformers having low core losses rather than low - resistance losses, because the latter are then of relatively small - importance. - -=Ques. What kind of efficiency is the station manager interested in?= - -Ans. The "all day efficiency." - - This expression, as commonly met with in practice, denotes - _the percentage that the amount of energy actually used by the - consumer is of the total energy supplied to his transformer during - 24 hours_. The formula for calculating the all day efficiency of - a transformer is based upon the supposition that the amount of - energy used by the consumer during 24 hours is equivalent to full - load on his transformer during five hours and is as follows: - - 5w - E = ------------- - 24c + 5r + 5w - where - E = the all day efficiency of the transformer, - w = the full load in watts on the primary, - c = the core loss in watts, - r = the resistance loss in watts. - -[Illustration: FIG. 2,854.--Performance curves of Westinghouse air -blast 550 kw, 10,500 volt transformer, 3,000 alternations.] - -=Ques. What are the usual all day efficiencies?= - -Ans. The average is about 85 per cent. for those of 1 kilowatt -capacity, 92 per cent. for those of 5 kilowatts capacity, 94 per -cent. for those of 10 kilowatts capacity, and about 94.5 per cent. -for those of 15 kilowatts capacity. - -=Ques. What becomes of the energy lost by a transformer?= - -Ans. It reappears as heat in the windings and core. - - This heat not only increases the resistances of the windings and - core, producing thereby a further increase of their respective - losses, but in addition causes in time a peculiar effect on the - iron core which is intensified by the reversals of magnetism - constantly going on within it. - - After about two years' service, the iron apparently becomes - fatigued or tired, and this phenomenon is called aging of the - iron. Since the life of the transformer depends to a great extent - upon this factor, the conditions responsible for its existence - should as far as possible be removed. Means must therefore be - provided in the construction to radiate the heat as quickly as it - is generated. - -=Ques. What kind of oil is used in oil cooled transformers?= - -Ans. Mineral oil. - -[Illustration: FIG. 2,855.--General arrangement of air blast -transformers and blowers.] - -=Ques. How is it obtained?= - -Ans. By fractional distillations of petroleum unmixed with any other -substances and without subsequent chemical treatment. - -=Ques. What is the important requirement for transformer oil?= - -Ans. It should be free from moisture, acid, alkali or sulphur -compounds. - -=Ques. How may the presence of moisture be determined?= - -Ans. By thrusting a red hot iron rod in the oil; if it "crackle," -moisture is present. - -=Ques. Describe the Westinghouse method of drying oil.= - -Ans. It is circulated through a tank containing lime, and afterwards, -through a dry sand filter. - -=Ques. What is the objection to heating the oil (raising its -temperature slightly above boiling point of water) to remove the -moisture?= - -Ans. The time consumed (several days) is excessive. - -[Illustration: FIG. 2,856.--Small Curtis turbine generator set as -made by the General Electric Co., in sizes from 5 kw., to 300 kw. -It can be arranged to operate either condensing or non-condensing, -and at any steam pressure above 80 lbs. for the smaller sizes and -100 lbs. for the larger. There are only two main bearings. A thrust -bearing, consisting of roller bearings and running between hardened -steel face washers located at either end of the main bearings is -provided solely for centering the rotor so as to equalize the -clearance. A centrifugal governor is provided (in the smaller sizes) -completely housed, and mounted directly on the main shaft end. It -controls a balanced poppet valve through a bell crank. In the larger -sizes (75 kw. and above) the governor is mounted on a vertical -secondary shaft geared to the main shaft and controls a cam shaft -which opens or closes a series of valves in rotation, admitting the -steam to different sections of the first stage nozzles. In this -way throttling of the steam is avoided. There is also an emergency -governor which closes the throttle valve in the event of the speed -reaching a predetermined limit. The speeds of operation range from -5,000 R.P.M. for the smallest size to 1,500 R.P.M. for the largest. -The lubrication system is enclosed and is automatic. Air leakage -where the shaft passes through the wheel casing is prevented by steam -seal.] - -=Ques. What effect has moisture?= - -Ans. It reduces the insulation value of the oil. .06 per cent. of -moisture has been found to reduce the dielectric strength of oil -about 50 per cent. "dry" oil will withstand a pressure of 25,000 -volts between two 9½ inch knobs separated .15 inch. - -=Ques. What is understood by transformer regulation?= - -Ans. It is the difference between the secondary voltage at no load -and at full load, and is generally expressed as a percentage of the -secondary voltage at no load. - -=Ques. What governs its value?= - -Ans. The resistance and reactance of the windings. - -[Illustration: FIG. 2,857.--Cut off coupling for power transmission -by line shafting. It is used to cut off a driving shaft from a driven -shaft. Its use obviates the use of a _quill_, such as is shown in -fig. 2,858.] - -=Ques. How may the regulation be improved?= - -Ans. By decreasing the resistances of the windings by employing -conductors of greater cross section, or decreasing their reactance by -dividing the coils into sections and closely interspersing those of -the primary between those of the secondary. - - NOTE.--_The term_ ="regulation"= as here used - is synonymous with "drop." The _voltage drop_ - in a transformer denotes the drop of voltage - occurring across the secondary terminals of a - transformer with load. This drop is due to two - causes: 1, the resistance of the windings; and - 2, the reactance or magnetic leakage of the - windings. On non-inductive load, the reactive - drop, being in quadrature, produces but a slight - effect, but on inductive loads it causes the - voltage to drop, and on _leading current loads_ - it causes the voltage to rise. As the voltage - drop of a good transformer is very small even - on inductive load, direct accurate measurement - is difficult. It is best to measure the copper - loss with short circuited secondary by means of - a wattmeter, and at the same time the voltage - required to drive full load current through. From - the watts, the resistance drop can be found, and - from this and the impedance voltage, the reactive - drop may be calculated. From these data a simple - vector diagram will give, near enough for all - practical purposes, the drop for any power - factor, or the following formula may be used - which has been deduced from the vector diagram. - _________________________ - D = √(W + X)^{2} + (R + P)^{2} - 100 where R - = % resistance drop; X = % reactive drop; P = % - power factor of load; W = % wattless factor of - ________ - load (√1 - P^{2}); D = % resultant secondary - drop. For non-inductive loads where P = 100 and W - _____________________ - = 0, D = √X^{2} + (100 + R)^{2} - 100. In the - case of leading currents it should be considered - negative. - - In transformers where there is a great difference in voltage - between the primary and secondary windings, however, this remedy - has its limitations on account of the great amount of insulation - which must necessarily be used between the windings, and which - therefore causes the distances between them to become such as to - cause considerable leakage of the lines of force. - -=Ques. How does the regulation vary for different transformers, and -what should be the limit?= - -Ans. Those of large capacity usually have a better regulation than -those of small capacity, but in no case should its value exceed 2 per -cent. - -[Illustration: FIG. 2,858.--Quill drive. This is the proper -transmission arrangement substitute for heavy service, requiring -large pulleys, sheaves, gears, rotors, etc. It is a hollow shaft -supported by independent bearings. The main driving shaft running -through the quill is thus relieved of all transverse stresses. The -power is transmitted to the quill by means of a friction or jaw -clutch. When the clutch is thrown out the pulley or sheave stands -idle and the driving shaft revolves freely within the quill. As there -is no contact between moving parts there is no wear. Jaw clutches -should be used for drives demanding positive angular displacement. -They can only be thrown in and out of engagement when at rest. All -very large clutch pulleys, sheaves, or gears designed to run loose on -the line shaft are preferably mounted on quills. The letters A, B, C, -etc., indicate the dimensions to be specified in ordering a quill.] - -=Ques. What advantages have shell type transformers over those of the -core type?= - -Ans. They have a larger proportion of core surface exposed for -radiation of heat, and a shorter magnetic circuit which reduces the -tendency for a leakage of the lines of force into the air. - - Both types have advantages and disadvantages as compared with - the other. In the shell type, there is less magnetic leakage, but - also less surface exposed for radiation, and greater difficulty in - providing efficient insulation between the two circuits; in the - core type there is more surface exposed for radiation and less - difficulty in insulating the windings, but there is also a great - leakage of the lines of magnetic force into the outer air. - -=Ques. How are the windings usually arranged?= - -Ans. As a rule, there is only one primary winding but the secondary -winding is generally divided into two equal sections, the four -terminals of which are permanently wired to four connection blocks -which may be connected so as to throw the secondary sections either -in parallel or in series with each other at will. - -=Ques. What is necessary for satisfactory operation of transformers -in parallel?= - -Ans. They must be designed for the same pressures and capacities, -their percentages of regulation should be the same and they must have -the same polarity at a given instant. - - One may satisfy himself as to the first of these conditions by - examining the name plates fastened to the transformers, whereon - are stamped the values of the respective pressures and capacities - of each. - - Although equal values of regulation is given as one of the - conditions to be satisfied, transformers may be operated in - parallel when their percentages of regulation are not the same. - Ideal operation, however, can be attained only under the former - state of affairs. Suppose, for instance, a transformer having a - regulation of two per cent. be operated in parallel with another - of similar size and design but having a regulation of one per - cent. The secondary pressures of these transformers at no load - will of course be the same, but at full load if the secondary - pressure of the one be 98 volts, that of the other will be 99 - volts. There will, therefore, be a difference of pressure of one - volt between them which will tend to force a current backward - through the secondary winding of the transformer delivering 98 - volts. This reversed current, although comparatively small in - value, lowers the efficiency of the installation by causing a - displacement of phase and a decrease in the combined power factor - of the transformers. - -=Ques. Describe the polarity test.= - -Ans. The test for polarity consists in joining together by means of a -fuse wire, a terminal of the secondary winding of each transformer, -and then with the primary windings supplied with normal voltage, -connecting temporarily the remaining terminals of the secondary -windings. The melting of the fuse wire thus connected indicates that -the secondary terminals joined together are of opposite polarities, -and that the connections must therefore be reversed, whereas if the -fuse wire do not melt, it shows that the proper terminals have been -joined and that the connections may be made permanent. - -[Illustration: FIG. 2,859.--Single overhung tangential water wheel -equipped with Doble ellipsoidal buckets. The central position of the -front entering wedge or lip of the bucket is cut away in the form of -a semi-circular notch, which allows a solid circular water jet to -discharge upon the central dividing wedge of the bucket without being -split in a horizontal plane.] - - The object of this test is, obviously, not to determine the - exact polarity of each secondary terminal, but merely to indicate - which of them are of the same polarity. - -[Illustration: FIG. 2,860.--Motor generator exciter set driven by -a Pelton-Doble tangential water wheel. The water wheel runner is -mounted on the shaft overhung and the jet is regulated by either a -hand actuated or governor controlled needle nozzle. The speed of the -water wheel is equivalent to the synchronous speed of the induction -motor, hence, the latter floats on the line, and under certain -conditions may perform the functions of an alternator by feeding into -the circuit, should the water wheel tend to operate above synchronous -speed. Should any interruption to the operation of the wheel occur, -causing a diminution of speed, the induction motor would drop back -to full load speed and take up the exciter load, resulting in no -appreciable drop of exciter voltage. The only variation of speed -possible is dependent upon the "slip" of the motor. Where two or more -exciter sets are employed in the station, an advantageous arrangement -embraces the installation of a water wheel driven motor generator set -and an exciter set, consisting of merely the direct current generator -and water wheel. The induction motor being electrically tied into -the circuit, the possibility of a runaway of the water wheel is -eliminated, since its speed can only slightly exceed the synchronous -speed of the system.] - -=Motor Generators.=--In motor generator sets, either the shunt or -series wound type of motor may be employed at the power producing -end of the set, but the field of the generator is either shunt -or compound wound, depending upon whether or not it is desired -to maintain or to raise the secondary voltage near full load. In -either case a rheostat introduced in the shunt field winding of the -generator will be found very essential. Both generator and motor are -so mounted on the base that their respective commutators are at the -outer ends of the set. By this means ample space surrounds all of the -working parts, and repairs can readily be made. - -Motor generators are frequently used as boosters to raise or boost -the voltage near the extremities of long distance, direct current -transmission lines. Of these, electric railway systems in which it is -desired to extend certain of the longer lines, form a typical example. - -[Illustration: FIG. 2,861.--Automatically governed Pelton-Doble -tangential water wheel driving exciter dynamo. The water wheel is -mounted on one end of the shaft, while the opposite end is extended -to carry a fly wheel of suitable design to compensate for the low fly -wheel effect of the direct current armature. Two bearings support -the shaft which carries the rotating elements of the unit. A needle -nozzle actuated by a direct motion Pelton-Doble governor (designed -for operation by either oil or water pressure) maintains constant -speed.] - -Owing to the great cost of changing such a system over to one -employing alternating current, or storage batteries, or of -constructing an additional power station, these solutions of the -problem are usually at variance with good judgment and the amount -of money at hand. The choice then remains between the purchase of -additional wire for feeders, the connection of a booster in the old -feeders, or the installation of both larger feeders and a booster. -Of these, it is generally found that either the second or the third -mentioned alternative meets the conditions most satisfactorily. - - A booster installed in a railway system for the purpose just - mentioned, would have a series wound motor, and the conditions - to which it must conform would be as follows: The motor having a - series winding must provide for the full feeder current passing - through both armature and field windings. - - Owing to the varying loads on a railway system, due to the - frequent starting and stopping of cars, the feeder current varies - between zero and some such value as 150 amperes. This fluctuation - of current through the field winding will, in ordinary cases, vary - the magnetization of the pole pieces from zero almost to the point - of saturation; that is, the maximum feeder current will so nearly - fill the magnet cores with lines of force that it would be quite - difficult to cause more lines of magnetic force to pass through - them. - - So long as the point of saturation is not reached, however, the - proportion of current to field strength remains constant, and - therefore the ratio of amperes to volts will not vary. - - The severe fluctuations of the feeder current would, if the - motor were shunt or compound wound, cause most serious sparking - and various other troubles, but in a series motor where the back - ampere turns on the armature that react on the field vary in - precisely the same proportion as the ampere turns in the field, - there exists at all times a tendency to balance the active forces - and produce satisfactory operation. If, however, the field magnet - cores be very large, they cannot so quickly respond, magnetically, - to changes in the strength of the current, and there is then - greater liability of the armature reaction momentarily weakening - the field and thereby producing temporary sparking. - -=Ques. Are motor generators always composed of direct current sets?= - -Ans. No. - -=Ques. Describe conditions requiring a different combination.= - -Ans. For purposes where for instance direct currents of widely -different voltages are to be obtained from an alternating current -circuit, and it is desired to install but one set, a motor generator -consisting of an alternating current motor such as an induction -motor, and a dynamo must necessarily be employed. - - In such sets, it is common to find both motor and dynamo - armatures mounted on a common shaft, and the respective field - frames resting on a single base, although for connection on a - very high pressure alternating current circuit, separate armature - shafts insulated from each other but directly connected together, - and separate bases resting on a single foundation, are usually - employed to afford the highest degree of insulation between the - respective circuits of the two machines. - -=Ques. What is the objection to a set composed of alternating current -motor and alternator?= - -Ans. The commercial field that would be naturally covered by such a -set is better supplied by a transformer. - -=Ques. Why?= - -Ans. Because a transformer contains no moving parts, and is therefore -simpler in construction, cheaper in price, and less liable to get out -of order. - -=Dynamotors.=--A dynamotor differs from a motor generator in that -the motor armature and the generator armature are combined into one, -thereby requiring but one field frame. Since the motor and generator -armature windings are mounted on a single core, the armature reaction -due to the one winding is neutralized by the reaction caused by the -other winding. There is, consequently, little or no tendency for -sparking to occur at the brushes, and they therefore need not be -shifted on this account for different loads. - -=Ques. How is a dynamotor usually constructed?= - -Ans. It is usually built with two pole pieces which are shunt wound. - -=Ques. Why does the voltage developed fall off slightly under an -increase of load?= - -Ans. Because a compound winding cannot be provided. - -[Illustration: FIGS. 2,862 and 2,863.--Method of putting on belts -when the driver is in motion, and device used. The latter is called a -_belt slipper_, and consists, as shown in fig. 2,862, of a cone and -shield, which revolve upon the stem, B, thus yielding easily to the -pull of the belt. A staff or handle C of any convenient length can be -fastened to the socket. The mode of operation is illustrated in fig. -2,863, which is self explanatory.] - -=Ques. Describe the armature construction and operation.= - -Ans. It consists of two separate windings; one of which is joined to -a commutator mounted on one side of the armature for motor purposes, -and the other to the commutator on the other side of the armature for -generator purposes. - - By means of two studs of brushes pressing on the motor - commutator, current from the service wires is fed into the winding - connected to this commutator, and since the shunt field winding - is also excited by the current from the service wires, there is - developed in the generator winding on the rotating armature a - direct voltage which is proportional to the speed of rotation of - the armature in revolutions per second, the number of conductors - in series which constitute the generator winding, and the total - strength of the field in which the armature revolves. This - pressure causes current to pass through the generator winding and - the distributing circuit when the distributing circuit to which - this winding is connected by means of its respective commutator, - brushes, etc., is closed. - -[Illustration: FIGS. 2,864 to 2,866.--Converter connections; fig. -2,864 double delta connection; fig. 2,865 diametrical connection; -fig. 2,866 two circuit single phase connection. For six phase -synchronous converter, two different arrangements of the connections -are generally used. One is called the _double delta_, and the other -the _diametrical_ connection. Let the armature winding of the -converter be represented by a circle as in figs. 2,864 and 2,865, -and let the six equidistant points on the circumference represent -collector rings, then the secondary of the supply transformers can -be connected to the collector rings in a _double delta_ as in fig. -2,864, or across diametrical pairs of pointer as in fig. 2,865. -In the first instance, the voltage ratio is the same as for the -three phase synchronous converter and simply consists of two delta -systems. The transformers can also be connected in double star, and -in such a case the ratio between the three phase voltage between -the terminals of each star, and the direct voltage will be the same -as for double delta, while the voltage of each transformer coil, -or voltage to neutral, is 1 ÷ √3 times as much. With the -diametrical connection, the ratio is the same as for the two ring -single phase converter, it being analogous to three such systems. - _ _ -Hence six phase double delta E_{1} = √3E ÷ 2√2 = .612E. -Six phase diametrical, E_{1} = E ÷ √2 = .707E. The ratio of the -virtual_voltage E_{0} between any collector ring and the neutral -point is always E_{0} = (E ÷ 2) √2 = .354E. For single phase -synchronous converters, consisting of a closed circuit armature -winding tapped at two equidistant points to the two collector rings -the virtual voltage is 1 ÷ √2 × the direct current voltage. While -such an arrangement of the single phase converter is the simplest, -requiring only two collector rings, it is undesirable, especially -for larger machines, on account of excessive heating of the armature -conductors. In fig. 2,866, which represents the armature winding of a -single phase converter, the supply circuits from two secondaries of -the step down transformers are connected to four collector rings, so -that the two circuits are in phase with each other, but each spreads -over an arc of 120 electrical degrees instead of over 180 degrees -as in the single phase circuit converter. To distinguish the two -types, it is generally called a two circuit single phase synchronous -converter. The virtual voltage E_{2} bears to the direct voltage the -same relation as in the three phase converter, that is single phase -two circuit, E_{1} = √3 ÷ 2√2 =.612E.] - -=Ques. How is a dynamotor started?= - -Ans. It is connected at its motor end and started in the same manner -as any shunt wound motor on a constant pressure circuit. - -=Ques. What precautions should be taken in starting a dynamotor?= - -Ans. The necessary precautions are, to have the poles strongly -magnetized before passing current through the motor winding on the -armature; to increase gradually the current through this winding, and -not to close the generating circuit until normal conditions regarding -speed, etc., are established in the motor circuit. - -=Ques. How is the current developed in the machine regulated?= - -Ans. It can be regulated by the introduction of resistance in one or -the other of the armature circuits, or by a shifting of the brushes -around the commutator. - -=Ques. Are dynamotors less efficient than motor generators of a -similar type?= - -Ans. No, they are more efficient. - -=Ques. Why?= - -Ans. Because they have only one field circuit and at least one -bearing less than a motor generator. - - A motor generator has at least three bearings, and occasionally, - four, where the set consists of two independent machines directly - connected together. - -=Rotary Converters.=--An important modification of the dynamotor -is the rotary converter. This machine forms, as it were, a link -between alternating and direct current systems, being in general a -combination of an alternating current motor and a dynamo. - -[Illustration: FIG. 2,867.--Skeleton diagram showing wiring of -alternator, exciter, transformer and converter. The cut also shows -switchboard and connections.] - -It has practically become a fixture in all large electric railway -systems and in other installations where heavy direct currents of -constant pressure are required at a considerable distance from the -generating plant. In such cases a rotary converter is installed -in the sub-station, and being simpler in construction, higher in -efficiency, more economical of floor space, and lower in price than -a motor generator set consisting of an alternating current motor and -a dynamo which might be used in its place, it has almost entirely -superseded the latter machine for the class of work mentioned. - -=Ques. What is the objection to the single phase rotary converter?= - -Ans. It is not self-starting. - -=Ques. What feature of operation is inherent in a rotary converter?= - -Ans. A rotary converter is a "reversible machine." - - That is to say, if it be supplied with direct current of the - proper voltage at its commutator end, it will run as a direct - current motor and deliver alternating current to the collector - rings. While this feature is sometimes taken advantage of in - starting the converter from rest, the machine is not often used - permanently in this way, its commercial application being usually - the conversion of alternating currents into direct currents. - -=Ques. How does a rotary converter operate when driven by direct -current?= - -Ans. The same as a direct current motor, its speed of rotation -depending upon the relation existing between the strength of the -field and the direct current voltage applied. - - If the field be weak with respect to the armature magnetism - resulting from the applied voltage, the armature will rotate at a - high speed, increasing until the conductors on the armature cut - the lines of force in the field so as to develop a voltage which - will be equal to that applied. - - Again, if the field be strong with respect to the armature - magnetism, resulting from the applied voltage, the armature will - rotate at a low speed. If, therefore, it be desired to operate the - converter in this manner and maintain an alternating current of - constant frequency, the speed of rotation must be kept constant by - supplying a constant voltage not only to the brushes pressing on - the commutator, but also to the terminals of the field winding. - -[Illustration: FIG. 2,868.--General Electric synchronous converter -with series booster. This type of converter generally consists of -an alternator with revolving field mounted on the same shaft as the -converter armature. The armature of the alternator, or booster, as it -is usually called, is stationary and connected electrically in series -between the supply circuit and the collector rings of the synchronous -converter. The booster field has the same number of pole as the -converter and is generally shunt wound. A change in the booster -voltage will correspondingly change the alternating voltage impressed -on the converter and this regulation can, of course, be made so as -to either increase or decrease the impressed voltage by means of -strengthening or weakening the booster field. The voltage variation -can be made either non-automatic or automatic, and in the latter -case, it becomes necessary to provide a motor operated rheostat -controlled by suitable relays, or the booster can be provided with -a series field. By means of a booster, it is possible to vary the -direct voltage of the converter with a constant alternating supply -voltage, and this voltage regulation is obtained without disturbance -of the power factor or wave shape of the system. Synchronous -converters are frequently installed in connection with Edison -systems, where three wire direct current is required. The three wire -feature is obtained either by providing extra collector rings and -compensator, as with ordinary direct current generators, or also by -connecting the neutral wire directly to the neutral point of the -secondary winding of step down transformers, if such be furnished.] - -=Ques. How does it operate with alternating current drive?= - -Ans. The same as a synchronous motor. - -=Ques. What is the most troublesome part and why?= - -Ans. The commutator, because of the many pieces of which it is -composed and the necessary lines along which it is constructed, its -peripheral speed must be kept within reasonable limits. - -=Ques. What should be the limit of the commutator speed?= - -Ans. The commutator speed, or tangential speed at the brushes should -not exceed 3,000 feet per minute. - -[Illustration: FIG. 2,869.--Wiring diagram for General Electric -synchronous converter with series booster as illustrated in fig. -2,868.] - -=Ques. Name another limitation necessary for satisfactory operation.= - -Ans. The pressure between adjacent commutator bars should not exceed -eight or ten volts. - - If the commutator bars be made narrow in order to obtain - the necessary number for the desired voltage with the minimum - circumference and therefore low commutator speed, the brushes - employed to collect the current are liable to require excessive - width in order to provide the proper cross section and yet not - cover more than two bars at once. - -=Ques. How can the commutator speed be kept within reasonable limits, -other than by reducing the width of the commutator bars?= - -Ans. By using alternating current of comparatively low frequency. - - For a rotary converter delivering 500 volt direct current, the - proper frequency for the alternating current circuit has been - found to be 25 cycles per second. - -=Ques. When a rotary converter is operated in this usual manner on an -alternating current circuit, how can the direct current be varied?= - -Ans. It may be varied (from zero to a maximum) by changing the value -of the alternating pressure supplied to the machine, or it may be -altered within a limited range by moving the brushes around the -commutator, or in a compound wound converter by changing the amount -of compounding. - - Under ordinary conditions, varying the voltage developed by - changing the voltage at the motor end is not practical, hence - the voltage developed can be varied only over a limited range. - In addition to this, the voltage developed at the direct current - end bears always a certain constant proportion to the alternating - current voltage applied at the motor end; this is due to the same - winding being used both for motor and generator purposes. In all - cases the proportion is such that the alternating current voltage - is the lower, being in the single phase and in the two phase - converters about .707 of the direct current voltage, and in the - three phase converter about .612 of the direct current voltage. - It is thus seen that whatever value of direct current voltage be - desired, the value of the applied alternating current voltage must - be lower, requiring in consequence the installation of step down - transformers at the sub-station for reducing the line wire voltage - to conform to the direct current pressure required. - -=Ques. What is the efficiency of a rotary converter?= - -Ans. It may be said to have approximately the same efficiency as that -in the average of the same output, although in reality the converter -is a trifle more efficient on account of affording a somewhat shorter -average path for the current in the armature, reducing in consequence -the resistance loss and the armature reaction. - -=Ques. May a converter be overloaded more than a dynamo of the same -output, and why?= - -Ans. Yes, because there is usually less resistance loss in the -armature of the converter than in the armature of the dynamo. - -[Illustration: FIG. 2,870.--Wiring diagram for three wire synchronous -converter with delta-Y connected step down transformer with the -neutral brought out. It is evident that in this case each transformer -secondary receives ⅓ of the neutral current, and if this current -be not so small, as compared with the exciting current of the -transformer, it will cause an increase in the magnetic density.] - - Thus, a two phase converter may be overloaded approximately 60 - per cent., and a three phase converter may be overloaded about 30 - per cent. above their respective outputs if operated as dynamos. - -=Ques. Describe how a converter is started.= - -Ans. There are several methods any one of which may be employed, the -choice in any given case depending upon which of them may best be -followed under the existing conditions. - - If it be found advisable to start the converter with direct - current, the same connections would be made between the source of - the direct current and the armature terminals on the commutator - side of the converter as would be the case were a direct current - shunt motor of considerable size to be started; this naturally - means that a starting rheostat and a circuit breaker will be - introduced in the armature circuit. - - The shunt field winding alone is used, and this part of the - wiring may be made permanent if, as is usually the case, the same - source of direct current is used normally for separate field - excitation. - -[Illustration: FIG. 2,871.--Wiring diagram of three wire synchronous -converter with distributed Y secondary. This system eliminates the -flux distortion due to the unbalanced direct current in the neutral. -Two separate interconnected windings are used for each leg of the Y. -The unbalanced neutral current flowing in this system may be compared -in action to the effect of a magnetizing current in a transformer. -The effect of the main transformer currents in the primary and -secondary is balanced with regard to the flux in the transformer -core, which depends upon the magnetic current. When a direct current -is passed through the transformer, unless the fluxes produced by -the same neutralize one another, its effect on the transformer iron -varies as the magnetizing current. For example, assume a transformer -having a normal ampere capacity of 100 and, approximately, 6 amperes -magnetizing current, and assume that three such transformers are -used with Y connected secondaries for operating a synchronous -converter connected to a three wire Edison system. Allowing 25 per -cent. unbalancing, the current will divide equally among the three -legs giving 8.33 amperes per leg, which is more than the normal -magnetizing current. The loss due to this current is, however, -inappreciable, but the increased core losses may be considerable. -If a distributed winding be used, the direct current flows in the -opposite direction, around the halves of each core thus entirely -neutralizing the flux distortion. Whether the straight Y connection -is to be used is merely a question of balancing the increased core -loss of the straight Y connection against the increased copper loss -and the greater cost of the interconnected Y system. The straight -Y connection is much simpler, and it would be quite permissible to -use it for transformers of small capacities where the direct current -circulating in the neutral is less than 30 per cent. of the rated -transformer current.] - - The direct current may be derived from a storage battery, from a - separate converter, or from a motor generator set installed in the - sub-station for the purpose. - - An adjustable rheostat will, of course, be connected in the - field circuit for regulation. Before starting the converter, - however, it is necessary to do certain wiring between the - terminals on the collector side of the machine and the alternating - current supply wires, in order that the change over from direct - current motive power to alternating current motive power may be - made when the proper phase relations are established between - the alternating current in the supply wires and the alternating - current in the armature winding of the converter. - - In order that proper phase relations exist, the armature - of the converter must rotate at such a speed that each coil - thereon passes its proper reversal point at the same time as the - alternating current reverses in the supply wires. This speed may - be calculated by doubling the frequency of the supply current and - then dividing by the number of pole pieces on the converter, but a - far more accurate method of judging when the converter is in step - or in synchronism with the supply current consists in employing - incandescent lamps as shown in fig. 2,872. - -=Ques. How is a polyphase converter started with alternating current?= - -Ans. This may be done by applying the alternating pressure directly -to the collector rings while the armature is at rest. There need -be no field excitation; in fact the field windings on the separate -pole pieces should be disconnected from each other before the -alternating voltage is applied to the armature, else a high voltage -will be induced in the field windings which may prove injurious to -their insulation. The passage of the alternating current through the -armature winding produces a magnetic field that rotates about the -armature core, and induces in the pole pieces eddy currents, which, -reacting on the armature, exert a sufficient torque to start the -converter from rest and cause it to speed up to synchronism. - -=Ques. How much alternating current is required to start a polyphase -converter?= - -Ans. About 100 per cent. more than that required for full load. - -=Ques. How may this starting current be reduced?= - -Ans. Transformers may be switched into circuit temporarily to reduce -the line wire voltage until the speed become normal. - -[Illustration: FIG. 2,872.--Wiring diagram showing arrangement of -incandescent lamps for determining the proper phase relations in -starting a rotary converter. The alternating current side of a -three phase converter is shown at C. The three brushes, D, T and G -pressing on its collector rings are joined in order to the three -single pole switches H, L and B which can be made to connect with -the respective wires M, R, and V, of the alternating current supply -circuit. Across one of the outside switches, H, for example, a number -of incandescent lamps are joined in series as indicated at E, while -the three pole switch (not shown) in the main circuit, between the -alternator and the single pole switches is open. If then the main -switch just mentioned and the middle switch L be both closed, and -the armature of the alternator be brought up to normal speed by -running it as a direct current motor, the lamps at E will light up -and darken in rapid succession; the lighting and darkening of the -lamps will continue until, by a proper adjustment of the speed, -the correct phase relations be established between the alternating -current in the supply circuit and the alternating current developed -in the armature of the converter. As this condition is approached, -the intervals between the successive lighting up and darkening of -the lamps will increase until they remain perfectly dark. There is -then no difference of pressure between the supply circuit M R V and -the rotary converter armature circuit, so the source of the direct -current may at that instant be disconnected from the machine, and the -switches H and B, closed. If the change over has been accomplished -before the phase relations of the two circuits differed, the -converter will at once conform itself to the supply circuit and run -thereon as a synchronous motor without further trouble. The opening -of the direct current circuit and the closing of the alternating -current supply circuit may be done by hand, but preferably by -employing a device that will automatically trip the circuit breaker -in the direct current circuit at the instant the switches in the -alternating current circuit are closed.] - - In conjunction with this method, the method of synchronizing - shown in fig. 2,872 may be used, thus, in starting, there is - an alternating current between the brushes which pulsates very - rapidly, but when synchronism is approached, the pulsations - become less rapid until finally with the converter in step with - the alternator the pulsations entirely disappear. - - The light given by the lamps thus connected indicates accurately - the condition of affairs at any one time, varying from a rapidly - fluctuating light at the beginning to one of constant brilliancy - at synchronism. - -[Illustration: FIG. 2,873.--Diagram of motor converter. This machine -which is only to be used for converting from alternating to direct -current, consists of an ordinary induction motor with phase wound -armature, and a dynamo. The revolving parts of both machines are -mounted on the same shaft and from the figure it is seen that the -armature of the motor and the armature of the dynamo are also -electrically connected. The motor converter is a synchronous machine, -but the dynamo receives the current from the armature of the motor at -a frequency much reduced from that impressed upon the field winding -of the motor. Assuming that the motor and the converter have the same -number of pole, the motor will rotate at a speed corresponding to -one-half the frequency of the supply circuit. The motor will operate -half as a motor and half as a transformer, and the converter, half -as a dynamo and half as a synchronous converter, in that one-half of -the electrical energy supplied to the motor will be converted into -mechanical power for driving the converter, while the other one-half -is transferred to the secondary motor windings and thereby to the -converter armature in the form of electrical power. The capacity of -the motor is theoretically only half what it would be if it were to -convert the whole of the electrical energy into mechanical power -because the rating depends upon the speed of the rotating field and -not on that of the rotor. If the two machines have a different number -of pole, or are connected to run at different speeds, the division of -power is at a different but constant ratio. The machine starts up as -an ordinary polyphase induction motor and the field of the converter -is built up as though it were an ordinary dynamo. Motor converters -are occasionally used on high frequency systems, as their commutating -component is of half frequency, and thus permits better commutator -design than a high frequency converter. The advantage of this type -of machine is that for phase control it requires no extra reactive -coils, the motor itself having sufficient reactance. It is, however, -larger than standard converters, but smaller than motor generators, -as half the power is converted in each machine. Its efficiency is -less than for synchronous converters, and the danger of reaching -double speed in case of a short circuit on the direct current side -is very great. It has been used abroad to some extent for 60 cycle -work, in preference to synchronous converters, but with the present -reliable design of 60 cycle converters, and the general use of 25 -cycles, where severe service conditions are met, as in railroading, -motor converters should not be recommended.] - -=Ques. If the armature of the starting motor have a starting -resistance, how must this be connected?= - -Ans. It should be connected in series with the armature inductors -before the alternating voltage is applied. - - As the motor increases in speed, the starting resistance is - gradually short circuited until it is entirely cut out of circuit. - -[Illustration: FIG. 2,874.--Sectional view of General Electric -vertical synchronous converter. In this construction, the field frame -carrying the poles is mounted on cast iron pedestals and is split -vertically. This allows the two halves of the frame to be separated -for inspection or repairs of the armature. The armature, including -commutator and collector rings, is mounted on a vertical stationary -shaft, which is rigidly supported from the foundation. The thrust -of the armature is carried on a roller bearing attached to the top -of the shaft and upper side of the armature spider. The under side -of the lower plate of the roller bearing is made spherical and fits -into a corresponding spherical cup on the end of the shaft, making -the bearing self aligning. The armature spider has a babbitted sleeve -along the fit of the vertical shaft, which acts as a guide bearing -and has to take only the thrust due to the unbalancing effect of -the rotating parts. A circulating pump furnishes oil to the roller -bearing, the oil draining off through the guide bearing. A marked -advantage of this type of construction is the accessibility of the -commutator for adjustment of the brushes, etc., as there is no pit or -pedestal bearing to interfere.] - - NOTE.--Some converters are provided - with a small induction motor for starting - mounted on an iron bracket cast in the converter - frame, and whose shaft is keyed to that of the - converter. Allowing for a certain amount of - slip in the induction motor, the field of this - machine must possess a less number of magnet - poles than the converter in order to enable the - latter machine to be brought to full synchronism. - To start the induction motor, it is simply - necessary to apply to its field terminals the - proper alternating voltage. The bracket, and - therefore the motor, is usually mounted outside - the armature bearing on the collector side of the - converter. - -[Illustration: FIG. 2,875.--Resistance measurement by "drop" method. -The circuit whose resistance is to be measured, is connected in -series with an ammeter and an adjustable resistance to vary the -flow of current. A voltmeter is connected directly across the -terminals of the resistance to be measured, as shown in the figure. -According to Ohm's law I = E ÷ R, from which, R = E ÷ I. If then the -current flowing in the circuit through the unknown resistance be -measured, and also the drop or difference of pressure, the resistance -can be calculated by above formula. In order to secure accurate -determination of the resistance such value of current must be used -as will give large deflections of the needle on the instruments -employed. A number of independent readings should be taken with some -variation of the current and necessarily a corresponding variation -in voltage. The resistance should then be figured from each set -of readings and the average of all readings taken for the correct -resistance. Great care must be taken, however, in the readings, and -the instruments must be fairly accurate. For example, suppose that -the combined instrument error and the error of the reading in the -voltmeter should be 1 per cent., the reading being high, while the -corresponding error of the ammeter is 1 per cent. low. This would -cause an error of approximately 2 per cent. in the reading of the -resistance. In making careful measurements of the resistance, it is -also necessary to determine the temperature of the resistance being -measured, as the resistance of copper increases approximately .4 -of 1 per cent. for each degree rise in temperature. Use is made of -this fact for determining the increase in temperature of a piece of -apparatus when operating under load. The resistance of the apparatus -at some known temperature is measured, this being called the cold -resistance of the apparatus. At the end of the temperature test the -hot resistance is taken. Assume the resistance has increased by 15 -per cent. This would indicate a rise in temperature of 37½ degrees -above the original or cold temperature of the apparatus. Suppose then -that in measuring the cold resistance, results are obtained which are -2 per cent. low, and that in measuring the hot resistance, there be -2 per cent. error in the opposite direction. This would mean that a -total error of 4 per cent. had been made in the difference between -the hot and cold resistances, or an error of 10 degrees. The correct -rise in temperature is, therefore, about 27½ instead of 37½ degrees. -In other words, an error of 2 per cent. in measuring each resistance -has caused an error of approximately 36½ per cent. in the measurement -of the rise in temperature. The constant .4 which has been used above -is only approximate and should not be used for exact work. For detail -instructions of making calculations of resistance and temperature, -see "Standardization Rules of the A.I.E.E."] - -=Ques. Describe the usual wiring for the installation of a rotary -converter in a sub-station.= - -Ans. Commencing at the entrance of the high pressure cables, first -there is the wiring for the lightning arresters, then for the -connection in circuit of the high tension switching devices, from -which the conductors are led to bus bars, and thence to the step down -transformers. - -[Illustration: Figs. 2,876 to 2,879.--How to connect instruments -for power measurement. There are several ways of connecting an -ammeter, voltmeter and wattmeter in the circuit for the measurement -of power. A few of the methods are discussed below. With some of the -connections it is necessary to correct the readings of the wattmeter -for the losses in the coil, or coils, of the wattmeter, or for losses -in ammeter or voltmeter. This is necessary since the instruments may -be so connected that the wattmeter not only measures the load but -includes in its indications some of the instrument losses. If the -load measured be small, or considerable accuracy is required, these -instrument losses may be calculated as follows: Loss in pressure -coils is E^{2} ÷ R, in which E is the voltage at the terminals of -the pressure coil and R is the resistance. Loss in current coil is -I^{2} R in which I is the current flowing and R the resistance of the -current coil. In general let E_{v} = voltage across terminals of the -voltmeter; E_{_w_} = voltage across the terminals of the pressure -coil of the wattmeter; I_{_w_} = current through current coil of -wattmeter; I_{_a_} = current through current coil of ammeter; R_{_v_} -= resistance of pressure coil of voltmeter; R_{_w_} = resistance of -pressure coil of wattmeter; R^{1}_{w} = resistance of current coil -of wattmeter; R_{_a_} = resistance of current coil of ammeter. Then -the losses in the various coils will be as follows: E^{2}_{_v_} ÷ -R_{_v_} = loss in pressure coil of voltmeter. E^{2}_{_w_} ÷ R_{_w_} -= loss in pressure coil of wattmeter. I^{2}_{_w_} ÷ R_{_v_} = loss -in current coil of wattmeter. I^{2}_{_a_}R_{_a_} = loss in current -coil of ammeter. If connection be made as in fig. 2,876, the correct -power of the circuit will be the wattmeter reading W-(E^{2}_{_v_} ÷ -R_{_v_} + E^{2}_{_w_} ÷ R_{_w_}) in which E_{_v_} = E_{_w_}. In fig. -2,877, the power is W-E^{2}_{_w_} ÷ R_{_w_}. In fig. 2,878, the power -is W-I^{2}_{_w_}R^{1}_{_w_}, or the correct power is the wattmeter -reading minus the loss in the current coil of the wattmeter. In fig. -2,879, the power is W-(E^{2}_{_w_} ÷ R_{_w_} + I^{2}_{_a_}R_{_a_})· -The usual method of connection is either as in fig. 2,876 or fig. -2,877. In either case the current reading is that of the load plus -the currents in the pressure coils of the voltmeter and wattmeter. -Unless the current being measured, however, is very small, or extreme -accuracy is desired, it is unnecessary to correct ammeter readings. -In fig. 2,877 a small error is introduced due to the fact that the -actual voltage applied to the load is that given by the voltmeter -minus the small drop in voltage through the current coil of the -wattmeter. If an accurate measure of the current in connection with -the power consumed by the load be required, the connections shown -in fig. 2,879 are used, and if extreme accuracy is required, the -wattmeter reading is reduced by the losses in the ammeter and in -the pressure coil of the wattmeter. The loss in the pressure coil -of a wattmeter or voltmeter may be as high as 12 or 15 watts at 220 -volts. The loss in the current coil of a wattmeter with 10 amperes -flowing may be 6 or 8 watts. It can be easily seen that if the core -or copper losses of small transformers are being measured, it is -quite necessary to correct the wattmeter readings, for the instrument -losses. In measuring the losses of a 25 or 50 H.P. induction motor, -the instrument losses may be neglected. A careful study of the above -will show when it becomes necessary to correct for instrument losses -and the method of making these corrections. Connections are seldom -used which make it necessary to correct for the losses in the current -coils of either ammeter or wattmeter, as the losses vary with the -change in the current. On the other hand, the voltages generally -used are fairly constant at 110 or 220, and when the losses of the -pressure coils at these voltages have once been calculated, the -necessary instrument correction can be readily made.] - - On a three phase system the transformers should be joined in - delta connection, as a considerable advantage is thereby gained - over the star connection, in that should one of the transformers - become defective, the remaining two will carry the load without - change except more or less additional heating. Between the - transformers and rotary converter the circuits should be as short - and simple as possible, switches, fuses, and other instruments - being entirely excluded. The direct current from the converter is - led to the direct current switchboard, and from there distributed - to the feeder circuits. - - =WATTMETER ERROR FOR A LOAD OF 1,000 VOLT-AMPERES= - (For a lag of 1 degree in the pressure coil) - - +------------+----------+-------+-------------------+ - | | | |Error of indication| - |Power factor|True watts| Error | in per cent | - | | | | of true value | - +------------+----------+-------+-------------------+ - | 1. | 1,000 | .3 | 0.03 | - | .9 | 900 | 7.6 | 0.85 | - | .8 | 800 | 10.5 | 1.31 | - | .7 | 700 | 12.5 | 1.78 | - | .6 | 600 | 13.9 | 2.32 | - | .5 | 500 | 15.1 | 3.02 | - | .4 | 400 | 15.9 | 3.98 | - | .3 | 300 | 16.6 | 5.54 | - | .2 | 200 | 17.1 | 8.55 | - | .1 | 100 | 17.3 | 17.30 | - +------------+----------+-------+-------------------+ - - NOTE.--In the iron vane type instrument when - used as a wattmeter, the current of the series - coil always remains in perfect phase with - the current of the circuit, provided series - transformers are not introduced. The error, - then, is entirely due to the lag of the current - in the pressure coil, and this error in high - power factor is exceedingly small, increasing as - the power factor decreases. In the above table - it should be noted that the value of the error - as distinguished from the per cent. of error, - instead of indefinitely increasing as the power - factor diminishes, rapidly attains a maximum - value which is less than 2 per cent. of the power - delivered under the same current and without - inductance. It should also be noted that the - above tabulation is on the assumption of a lag of - 1 degree in the pressure coil. The actual lag in - Wagner instruments for instance, is approximately - .085 of a degree, and the error due to the lag - of the pressure coil in Wagner instruments is, - therefore, proportionally reduced from the - figures shown in the above tabulation. - -=Ques. In large sub-stations containing several rotary converters how -are they operated?= - -Ans. Frequently they are installed to receive their respective -currents from the same set of bus bars; that is, they may be operated -as alternating current motors in parallel. They are also frequently -operated independently from single bus bars, but very seldom in -series with each other. - -[Illustration: FIG. 2,880.--Single phase motor test. In this method -of measuring the input of a single phase motor of any type, the -ammeter, voltmeter and wattmeter are connected as shown in the -illustration. The ammeter measures the current flowing through the -motor, the voltmeter, the pressure across the terminals of the motor, -and the wattmeter the total power which flows through the motor -circuit. With the connections as shown, the wattmeter would also -measure the slight losses in the voltmeter and the pressure coil of -the wattmeter, but for motors of ¼ H.P. and larger, this loss is so -small that it may be neglected. The power factor may be calculated by -dividing the true watts as indicated by the wattmeter, by the product -of the volts and amperes.] - -=Ques. How may the direct current circuit be connected?= - -Ans. In parallel. - - NOTE.--In motor testing, by the methods - illustrated in the accompanying cuts, it is - assumed that the motor is loaded in the ordinary - way by belting or direct connecting the motor - to some form of load, and that the object is to - determine whether the motor is over or under - loaded, and approximately what per cent. of full - load it is carrying. All commercial motors have - name plates, giving the rating of the motor and - the full load current in amperes. Hence the - per cent. of load carried can be determined - approximately by measuring the current input - and the voltage. If an efficiency test of the - apparatus be required, it becomes necessary to - use some form of absorption by dynamometer, such - as a Prony or other form of brake. The output of - the motor can then be determined from the brake - readings. The scope of the present treatment - is, however, too limited to go into the subject - of different methods of measuring the output of - the apparatus, and is confined rather to methods - of measuring current input, voltage, and watts. - The accuracy of all tests is obviously dependent - upon the accuracy of the instruments employed. - Before accepting the result obtained by any test, - especially under light or no load, correction - should be made for wattmeter error. See table of - wattmeter error on page 2,075. - -[Illustration: FIG. 2,881.--Three phase motor test; voltmeter and -ammeter method. If it be desired to determine the approximate load -on a three phase motor, this may be done by means of the connections -as shown in the figure, and the current through one of the three -lines and the voltage across the phase measured. If the voltage be -approximately the rated voltage of the motor and the amperes the -rated current of the motor (as noted on the name plate) it may be -assumed that the motor is carrying approximately full load. If, on -the other hand, the amperes show much in excess of full load rating, -the motor is carrying an overload. The heat generated in the copper -varies as the square of the current. That generated in the iron -varies anywhere from the 1.6 power, to the square. This method is -very convenient if a wattmeter be not available, although, it is, of -course, of no value for the determination of the efficiency or power -factor of the apparatus. This method gives fairly accurate results, -providing the load on the three phases of the motor be fairly well -balanced. If there be much difference, however, in the voltage of -the three phases, the ammeter should be switched from one circuit to -another, and the current measured in each phase. If the motor be very -lightly loaded and the voltage of the different phases vary by 2 or 3 -per cent., the current in the three legs of the circuit will vary 20 -to 30 per cent.] - -=Ques. What provision should be made against interruption of service -in sub-stations?= - -Ans. There should be one reserve rotary converter to every three or -four converters actually required. - -=Ques. Why does a rotary converter operate with greater efficiency, -and require less attention than does a dynamo of the same output?= - -Ans. There is less friction, and less armature resistance, the -latter because the alternating current at certain portions of each -revolution passes directly to the commutator bars without traversing -the entire armature winding as it does in a dynamo; there is no -distortion of the field and consequently no sparking, or shifting of -the brushes, since the armature reaction resulting from the current -fed into the machine and that due to the current generated in the -armature completely neutralizes each other. - -[Illustration: FIG. 2,882.--Three phase motor test by the two -wattmeter method. If an accurate test of a three phase motor be -required, it is necessary to use the method here indicated. Assume -the motor to be loaded with a brake so that its output can be -determined. This method gives correct results even with considerable -unbalancing in the voltages of the three phases. With the connections -as shown, the sum of the two wattmeter readings gives the total power -in the circuit. Neither meter by itself measures the power in any -one of the three phases. In fact, with light load one of the meters -will probably give a negative reading, and it will then be necessary -to either reverse its current or pressure leads in order that the -deflection may be noted. In such cases the algebraic sums of the two -readings must be taken. In, other words, if one read plus 500 watts -and the other, minus 300 watts, the total power in the circuit will -be 500 minus 300, or 200 watts. As the load comes on, the readings of -the instrument which gave the negative deflection will decrease until -the reading drops to zero, and it will then be necessary to again -reverse the pressure leads on this wattmeter. Thereafter the readings -of both instruments will be positive, and the numerical sum of the -two should be taken as the measurement of the load. If one set of the -instruments be removed from the circuit, the reading of the remaining -wattmeter will have no meaning. As stated above, it will not indicate -the power under these conditions in any one phase of the circuit. The -power factor is obtained by dividing the actual watts input by the -product of the average of the voltmeter readings × the average of the -ampere readings × 1.73.] - -=What electrical difficulty is experienced with a rotary converter?= - -Ans. Regulation of the direct current voltage. - -=Ques. How is this done?= - -Ans. It can be maintained constant only by preserving uniform -conditions of inductance in the alternating current circuit, and -uniform conditions in the alternator. - - While changes in either of these may be compensated to a certain - extent by adjustment of the field strength of the converter, they - cannot be entirely neutralized in this manner; it is therefore - necessary that both the line circuit and the alternator be - given attention if the best results are to be obtained from the - converter. - -=Ques. What mechanical difficulty is experienced with rotary -converters?= - -Ans. Hunting. - -=Ques. What is the cause of this?= - -Ans. It is due to a variation in frequency. - - The inertia of the converter armature tends to maintain a - constant speed; variations in the frequency of the supply circuit - will cause a displacement of phase between the current in the - armature and that in the line wires, which displacement, however, - the synchronizing current strives to decrease. The synchronizing - current, although beneficial in remedying the trouble after it - occurs, exerts but little effort in preventing it, and many - attempts have been made to devise a plan to eliminate this trouble. - - NOTE.--Three phase motor test; polyphase - wattmeter method. This is identical with the test - of fig. 2,882, except that the wattmeter itself - combines the movement of the two wattmeters. - Otherwise the method of making the measurements - is identical. If the power factor be known to be - less than 50 per cent., connect one movement so - as to give a positive deflection; then disconnect - movement one and connect movement two so as to - give a positive deflection. Then reverse either - the pressure or current leads of the movement, - giving the smaller deflection, leaving the - remaining movement with the original connections. - The readings now obtained will be the correct - total watts delivered to the motor. If the power - factor be known to be over 50 per cent., the same - methods should be employed, except that both - movements should be independently connected to - give positive readings. An unloaded induction - motor has a power factor of less than 50 per - cent., and may, therefore, be used as above - for determining the correct connections. For a - better understanding of the reasons for the above - method of procedure, the explanation of the two - wattmeter method, fig. 2,882, should be read. The - power factor may be calculated as explained under - fig. 2,882. Connect as shown in fig 2,882. The - following check on connection may be made. Let - the polyphase induction motor run idle, that is, - with no load. The motor will then operate with a - power factor less than 50 per cent. The polyphase - meter should give a positive indication, but - if each movement be tried separately one will - be found to give a negative reading, the other - movement will give a positive reading. This can - be done by disconnecting one of the pressure - leads from the binding post of one movement. When - the power factor is above 50 per cent. then both - movements will give positive deflection. - -=Ques. What are the methods employed to prevent hunting?= - -Ans. 1, the employment of a strongly magnetized field relative to -that developed by the armature; 2, a heavy flywheel effect in the -converter; 3, the increasing of the inductance of the armature by -sinking the windings thereon in deep slots in the core, the slots -being provided with extended heads; and 4, the employment of damping -devices or amortisseur winding on the pole pieces of the converter. - -[Illustration: FIG. 2,883.--Three phase motor test; one wattmeter -method. This method is equivalent to the two wattmeter method with -the following difference. A single voltmeter (as shown above) with -a switch, A, can be used to connect the voltmeter across either one -of the two phases. Three switches, B, C and D, are employed for -changing the connection of the ammeter and wattmeter in either one -of the two lines. With the switches B and D in the position shown, -the ammeter and wattmeter series coils are connected in the left hand -line. The switch C must be closed under these conditions in order to -have the middle line closed. Another reading should then be taken -before any change of load has occurred, with switch A thrown to the -right, switch B closed, switch D thrown to the right and switch C -opened. The ammeter and the current coil of the wattmeter will then -be connected to the middle line of the motor. In order to prevent -any interruption of the circuit, the switches B, D and C should be -operated in the order given above. With very light load on the motor -the wattmeter will probably give a negative deflection in one phase -or the other, and it will be necessary to reverse its connections -before taking the readings. For this purpose a double pole, double -throw switch is sometimes inserted in the circuit of the pressure -coil of the wattmeter so that the indications can be reversed -without disturbing any of the connections. It is suggested, before -undertaking this test, that the instructions for test by the two -wattmeter and by the polyphase wattmeter methods be read.] - -=Ques. What method is the best?= - -Ans. The damping method. - - The devices employed for the purpose are usually copper shields - placed between or around the pole pieces, although in some - converters the copper is embedded in the poles, and in others it - is made simply to surround a portion of the pole tips. - - In any case its action is as follows: The armature rotating at a - variable speed has a field developed therein which is assumed to - be also rotating at a variable speed; the magnetism of this rotary - field induces currents in the copper which, however, react on the - armature and oppose any tendency toward a further shifting of the - magnetism in the armature and therefore prevent the development - of additional currents in the copper. Since copper is of low - resistance, the induced currents are sufficient in strength to - thus dampen any tendency toward phase displacement, and so exert a - steadying influence upon the installation as a whole. - -[Illustration: FIG. 2,884.--Three phase motor, one wattmeter and Y -box method. This method is of service, only, provided the voltages of -the three phases are the same. A slight variation of the voltage of -the different phases may cause a very large error in the readings of -the wattmeter, and inasmuch as the voltage of all commercial three -phase circuits is more or less unbalanced, this method is not to be -recommended for motor testing. With balanced voltage in all three -phases, the power is that indicated by the wattmeter, multiplied by -three. Power factor may be calculated as before.] - -=Electrical Measuring Instruments.=--In the manufacture of most -measuring instruments, the graduations of the scale are made at the -factory, by comparing the deflections of the pointer with voltages -as measured on standard apparatus. The voltmeters in most common -use have capacities of 5, 15, 75, 150, 300, 500 and 750 volts each, -although in the measurement of very low resistances such as those of -armatures, heavy cables, or bus bars, voltmeters having capacities as -low as .02 volt are employed. - -[Illustration: FIG. 2,885.--Test of three phase motor with neutral -brought out; single wattmeter method. Some star connected motors -have the connection brought out from the neutral of the winding. In -this case the circuit may be connected, as here shown. The voltmeter -now measures voltage between the neutral and one of the lines, and -the wattmeter the power in one of the three phases of the motor. -Therefore, the total power taken by the motor will be three times the -wattmeter readings. By this method, just as accurate results can be -obtained as with the two wattmeter method. The power factor will be -the indicated watts divided by the product of the indicated amperes -and volts.] - -The difference between the design of direct current voltmeters of -different capacities lies simply in the high resistance joined in -series with the fine wire coil. This resistance is usually about 100 -ohms per volt capacity of the meter, and is composed of fine silk -covered copper wire wound non-inductively on a wooden spool. - -In the operation of an instrument, if the pointer when deflected do -not readily come to a position of rest owing to friction in the -moving parts, it may be aided in this respect by gently tapping -the case of the instrument with the hand; this will often enable -the obstruction, if not of a serious nature, to be overcome and an -accurate reading to be obtained. - -[Illustration: FIG. 2,886.--Temperature test of a large three -phase induction motor. Temperature tests are usually made on small -induction motors by belting the motor to a generator and loading the -generator with a lamp bank or resistance until the motor input is -equal to the full load. If, however, the motor be of considerable -size, such that the cost of power becomes a considerable item in -the cost of testing, the method here shown may be employed. For -this purpose, however, two motors, preferably of the same size and -type, are required. One is driven as a motor and runs slightly below -synchronism, due to its slip when operating with load. This motor is -belted to a second machine. If the pulley of the second machine be -smaller than the pulley of the first machine, the second machine will -then operate as an induction generator, and will return to the line -as much power as the first motor draws from the line, less the losses -of the second machine. By properly selecting the ratio of pulleys, -the first machine can be caused to draw full load current and full -load energy from the line. In this way, the total energy consumed -is equivalent to the total of the losses of both machines, which is -approximately twice the losses of a single machine. The figure shows -the connection of the wattmeters, without necessary switches, for -reading the total energy by two wattmeter method. Detailed connection -of the wattmeter is shown in fig. 2,883. It is usual, in making -temperature tests, to insert one or more thermometers in what is -supposed to be the hottest part of the winding, one on the surface -of the laminae and one in the air duct between the iron laminae. The -test should be continued until the difference in temperature between -any part of the motor and the air reaches a steady value. The motor -should then be stopped and the temperature of the armature also -measured. For the method of testing wound armature type induction -motors of very large size, see fig. 2,890. For the approved way -of taking temperature readings and interpreting results, see the -"Standardization Rules of the A.I.E.E."] - -=Ques. Describe a two scale voltmeter.= - -Ans. In this type of instrument, one scale is for low voltage -readings and the other for high voltage readings; on these scales -the values of the graduations for low voltages are usually marked -with red figures, while those for high voltages are marked with -black figures. A voltmeter carrying two scales must also contain two -resistances in place of one; a terminal from each of these coils must -be connected with a separate binding post, but the remaining terminal -of each resistance is joined to a wire which connects through the -fine wire coil with the third binding post of the meter. The two -first mentioned binding posts are usually mounted at the left hand -side of the meter and the last mentioned binding post and key at the -right hand side. - -[Illustration: FIG. 2,887.--Alternator excitation or magnetization -curve test. The object of this test is to determine the change of -the armature voltage due to the variation of the field current when -the external circuit is kept open. As here shown, the field circuit -is connected with an ammeter and an adjustable resistance in series -with a direct current source of supply. The adjustable resistance is -varied, and readings of the voltmeter across the armature, and of -the ammeter, are recorded. The speed of the generator must be kept -constant, preferably at the speed which is given on the name plate. -The excitation or magnetization curve of the machine is obtained by -plotting the current and the voltage.] - - The resistance corresponding to the high reading scale is - composed of copper wire having the same diameter as that - constituting the resistance for the low reading scale, but as the - capacity of the former scale is generally a whole number of times - greater than that of the latter scale, the resistances for the two - must bear the same proportion. - -[Illustration: FIG. 2,888.--Three phase alternator synchronous -impedance test. In determining the regulation of an alternator, it -is necessary to obtain what is called the _synchronous impedance_ of -the machine. To obtain this, the field is connected, as shown above. -Voltmeters are removed and the armature short circuited with the -ammeters in circuit. The field current is then varied, the armature -driven at synchronous speed, and the armature current measured by -the ammeters in circuit. The relation between field and armature -amperes are then plotted. The combination of the results of this -test, with those obtained from the test shown in fig. 2,887, are used -in the determination of the regulation of an alternator. Engineers -differ widely in the application of the above to the determination -of regulation, and employ many empirical formulae and constants for -different lines of design.] - -=Ques. How is a two scale voltmeter connected?= - -Ans. In the connection of a two scale voltmeter in circuit, the -single binding post is always employed regardless of which scale is -desired. If, then, the voltage be such that it may be measured on the -low reading scale, the other binding post employed is that connected -to the lower of the two resistances contained within; if, however, -the pressure be higher than those recorded on the low reading scale, -the binding post connected to the higher of the two resistances -contained within is used. - - NOTE.--Three phase alternator load test. By - means of the connection shown in fig. 2,888, - readings of armature current and field amperes - can be obtained with any desired load. The field - current can be varied also so as to maintain - constant armature voltage irrespective of load; - or the field current may be kept constant and - the armature voltage allowed to vary as the load - increases. The connections may also be used to - make a temperature test on the alternator by - loading it with an artificial load. In some cases - after the alternator is installed the connection - may be used to make a temperature test, using - the actual commercial load the alternator is - furnishing. - - Inasmuch as the capacities of the scales are - usually marked on or near the corresponding - binding posts, there will generally be no - difficulty in selecting the proper one of the two - left hand binding posts. - -[Illustration: FIG. 2,889.--Three phase alternator or synchronous -motor temperature test. In this test two alternators or synchronous -motors of same size and type are used, and are belted together, one -to be driven as a synchronous motor and the other as an alternator. -The method employed is to synchronize the synchronous motor with -the alternator or alternators on the three phase circuit, and then -connect to the line by means of a three pole single throw switch. -The alternator is then similarly synchronized with the alternator -of the three phase circuit and thrown onto the line. By varying the -field of the alternator it can be made to carry approximately full -load, and the motor will then be also approximately fully loaded. -The usual method is to have the motor carry slightly in excess of -full load, and the alternator slightly less than full load. Under -these conditions the motor will run a little warmer than it should -with normal load, while the alternator will run slightly cooler. -Temperature measurements are made in the same way as discussed -under three phase motors. The necessary ammeters, voltmeters and -wattmeters for adjusting the loads on the motors and generator are -shown in above figure. If pulleys be of sufficient size to transmit -the full load, with, say one per cent. slip, the pulley on the motor -should be one per cent. larger in diameter than the pulley on the -alternator, so as to enable the alternator to remain in synchronism -and at the same time deliver power to the circuit. With very large -machines under test, it is inadvisable to use the above method as -it is sometimes difficult to so adjust the pulleys and belt tension -that the belt slip will be just right to make up for the difference -in diameter of the pulleys, and very violent flapping of the belt -results. To meet such cases, various other methods have been devised. -One which gives consistent results is shown in fig. 2,890.] - -=Ques. How is a two scale voltmeter connected when the binding posts -are not marked?= - -Ans. If only an approximate idea is possessed of the voltage to be -measured, it is always advisable to connect to the binding post -corresponding to the high reading scale of the meter in order -to determine if the measurement may not be made safely and more -accurately on the low reading scale. In any case, some knowledge must -be had of the voltage at hand, else the high reading portion of the -instrument may be endangered. - -[Illustration: FIG. 2,890.--Three phase alternator or synchronous -motor temperature test. Supply the field with normal field current. -The armature is connected in open delta as illustrated, and full load -current sent through it from an external source of direct current, -care being taken to ground one terminal of the dynamo so as to avoid -danger of shock due to the voltage on the armature winding. The field -is then driven at synchronous speed. If the armature be designed to -be connected star for 2,300 volts, the voltage generated in each leg -of the delta will be 1,330 volts, and unless one leg of the dynamo -were grounded, the tester might receive a severe shock by coming -in contact with the direct current circuit. The insulation of the -dynamo would also be subjected to abnormal strain unless one terminal -were grounded. By the above method the field is subjected to its -full copper loss and the armature to full copper loss and core loss. -Temperature readings are taken as per standardization rules of the -A.I.E.E. This method may also be used with satisfactory results on -large three phase motors of the wound rotor type. If the alternator -pressure be above 600 volts, a pressure transformer should be used in -connection with the voltmeter.] - - _Too much care cannot be taken to observe these precautions_ - whenever the voltmeter is used, for the burning out or charring - of the insulation either in the fine wire coil or in the high - resistance of the meter by an excessive current, is one of the - most serious accidents that can befall the instrument. - - If a voltmeter has been subjected to a voltage higher than that - for which it was designed, yet not sufficiently high to injure the - insulation, but high enough to cause the pointer to pass rapidly - over the entire scale, damage has been done in another way. The - pointer being forced against the side of the case in this manner, - bends it more or less and so introduces an error in the readings - that are afterward taken. - - The same damage will be done if the meter be connected in - circuit so the current does not pass through it in the proper - direction, although in this case the pointer is not liable to - be bent so much as when it is forced to the opposite side of - the meter by an abnormal current, since then it has gained - considerable momentum which causes a severer impact. The extent - of the damage may be ascertained by noting how far away from the - zero mark the pointer lies when no current is passing through - the instrument. If this distance be more than two-tenths of a - division, the metal case enclosing the working part should be - removed and the pointer straightened by the careful use of a pair - of pinchers. - -[Illustration: FIG. 2,891.--Direct motor or dynamo magnetization -test. The object of this test is to determine the variation of -armature voltage without load, with the current flowing through the -field circuit. The armature should be driven at normal speed. The -adjustment resistance in the field circuit is varied and the voltage -across the armature measured. The curve obtained by plotting these -two figures is usually called magnetization curve of the dynamo. It -is usual to start with the higher resistance in the field circuit so -that very small current flows, gradually increasing this current by -cutting out the field resistance. When the highest no load voltage -required is reached, the field current is then diminished, and what -is called the descending (as opposed to the ascending) magnetization -curves are obtained. The difference in the two curves is due to the -lag of the magnetization behind the magnetizing current, and is -caused by the hysteresis of the iron of the armature core.] - -=Ques. What should be noted with respect to location of instruments?= - -Ans. If they be placed near conductors carrying large currents, -the magnetic field developed thereby will produce a change in the -magnetism of the instruments and so introduce an error in the -readings. - -[Illustration: FIG. 2,892.--Shunt dynamo external characteristic -test. The external characteristic of a shunt dynamo is a curve -showing the relation between the current and voltage of the external -circuit. This is obtained by the connection as here shown. The shunt -field is so adjusted that the machine gives normal voltage when -the external circuit is open. The field current is then maintained -constant and the external current varied by varying the resistance -in the circuit. By plotting voltage along the vertical, against the -corresponding amperes represented along the horizontal, the external -characteristic is obtained.] - -=Ques. How should portable instruments be wired?= - -Ans. The wires must be firmly secured to the supports on which they -rest, so as to reduce the possibility of their being pulled by -accident, and so causing the instruments to fall. - - A fall or a rough handling of the meter at once shows its effect - on the readings, for as much harm is done as would result from a - similar treatment of a watch. - - The hardened steel pivots used in all high grade voltmeters are - ground and polished with extreme care so as to secure and maintain - a high degree of sensitiveness. The jewels on which the moving - parts revolve are of sapphire, and they too must necessarily be - made with skill and carefulness; if, therefore, the jewels become - cracked and the pivots dulled by careless handling, the meter at - once becomes useless as a measuring instrument. - -=Ques. How should readings be taken?= - -Ans. The deflection of the pointer should be read to tenths of a -division; this can be done with considerable accuracy, especially -after a little practice. - -[Illustration: FIG. 2,893.--Load and speed test of direct current -shunt motor. The object of this test is to maintain the voltage -applied to the motor constant, and to vary the load by means of a -brake and find the corresponding variation in speed of the machine -and the current drawn from the circuit. If the motor be a constant -speed motor, the field resistance is maintained constant. The above -indicates the method of connecting instruments for the test alone; -for starting the machine the ordinary starting box, should, of -course, be inserted.] - - For very accurate results, a temperature correction should be - applied to compensate the effect which the temperature of the - atmosphere has upon the resistance of the meter when measurements - are being taken. In ordinary station practice the temperature - correction is negligible, being for resistance corresponding to - the high scale in first class meters, less than one-quarter of 1 - per cent. for a range of 35 degrees above or 35 degrees below 70 - degrees Fahrenheit. - -=Ques. What attachment is sometimes provided on station voltmeters -used for constant pressure service?= - -Ans. A normal index. - -[Illustration: FIG. 2,894.--Temperature test of direct current -motor or dynamo; loading back method. In making temperature tests -on a small dynamo it is usual to drive the dynamo with a motor and -load the dynamo by means of a lamp bank or resistance, the voltage -across the dynamo being maintained constant, and the current through -the external circuit adjusted to full load value. The temperatures -are then recorded, and when they reach a constant value above the -temperature of the atmosphere, the test is discontinued. Similarly, -in making a test on a small motor, the motor is loaded with a dynamo -and the load increased until the input current reaches the normal -full load value of the motor, the test being conducted as for a small -dynamo. When, however, the apparatus, either motor or dynamo, reaches -a certain size, it becomes necessary, in order to economize energy, -to use what is called the =loading back method=, as here illustrated. -The motor is started in the usual way, with the dynamo belted to it, -the circuit of the dynamo being open. The field of the dynamo is then -adjusted so that the dynamo voltage is equal to that of the line. -The dynamo is then connected to the circuit and its field resistance -varied until it carries normal full load current. Under these -conditions, if the motor and dynamo be of the same size and type, -the motor will carry slightly in excess of full load, the difference -being approximately twice the losses of the machines. Under these -conditions the total power drawn from the line is equal to twice the -loss of either machine. Temperature readings are taken as in other -temperature tests.] - -=Ques. What precaution must be taken in connecting station -voltmeters?= - -Ans. Care must be taken to guard against any short circuiting of the -voltmeter, which, would mean a short circuiting of the generator, and -as a result the probable burning out of its armature. - - The high resistance of the voltmeter prevents any such - occurrence when it is connected in the proper way, but should one - side of the circuit be grounded to the metal case or frame of the - meter, a careless handling of the lead connected with the other - side of the circuit would produce the result just mentioned. - -[Illustration: FIG. 2,895.--Compound dynamo external characteristics -test; adjustable load. The object of this test is to determine the -relation between armature voltage and armature current. Shunt field -is adjusted to give normal secondary voltage when the external -circuit is open. The load is then applied by means of an adjustable -resistance or lamp bank, and readings of external voltage and current -recorded. If the machine be normally compounded, the external voltage -will remain practically constant throughout the load range. If the -machine be under-compounded, the external voltage will drop with -load, while if over-compounded, there will be a rise in voltage with -increase in load.] - -=Ques. Why do station voltmeters indicate a voltage slightly lower -than actually exists across the leads?= - -Ans. Since they are usually connected permanently in circuit; a -certain amount of heat is developed in the wiring of the instrument. - -[Illustration: FIGS. 2,896 and 2,897.--Transformer core loss and -leakage, or exciting current test. With the primary circuit open, -the ammeter indicates the exciting or no load current. It should -be noted that all instruments are inserted on the low voltage -side, for both safety of the operator and because the measurements -are more accurate. The no load primary current, if the ratio of -transformation be 10: 1, will be one-tenth of the measured secondary -current. The wattmeter connected, as shown, measures the sum of the -losses, in the transformer, in the pressure coil of the wattmeter, -and in the voltmeter. On all standard makes of portable instruments, -the resistance of the wattmeter pressure coil and of the voltmeter -is given, and the loss in either instrument is the square of the -voltage at its terminals, divided by its resistance. Subtracting -these losses from the total indicated upon the wattmeter, gives the -true core or iron loss. It should be noted that in this diagram is -shown an auxiliary transformer with a number of taps for obtaining -the exact rated voltage of the transformer under test. In fig. 2,897 -is shown, in general, the same connections as in fig. 2,896, except -that the auto-transformer has been replaced by a resistance. If the -line voltage available be not much in excess of the rated voltage of -the transformer under test, very little error is introduced by the -use of the resistance method. However, if the difference be 10 per -cent. or more the auxiliary transformer shown in fig. 2,896 should be -used. Measurements made under the resistance method always give lower -results than those obtained with the auxiliary transformer.] - - The effect of this heat increases the voltmeter resistance and - consequently reduces the current below that which otherwise would - pass through the meter; since the deflections of the pointer - are governed by the strength of the current, station voltmeters - invariably indicate a voltage slightly lower than that which - actually exists across their leads. - -[Illustration: FIG. 2,898.--Diagram of connections for calibrating a -wattmeter. The calibration of a portable wattmeter is accomplished -with direct current of constant value which is passed through the -series winding by connecting the source thereof with the current -terminals. A direct current voltage which may be varied throughout -the range of the wattmeter is also applied to the instrument -between the middle and right hand pressure terminals A and E the -wiring in the meter between these terminals being such that its -differential winding is then cut out of circuit. The method of -procedure consists in comparing the deflections on the wattmeter at -five of six approximately equidistant points over its scale with the -corresponding products of volts and amperes used to obtain them. The -changes in the wattmeter deflections are effected by merely varying -the voltage, the value of the current being maintained constant at a -value which represents the full current capacity of the meter.] - - NOTE.--=Checking up of a recording wattmeter.= - This may conveniently be done by noting the - deflections at short intervals on an ammeter - connected in circuit, and also the readings - on the dial of the recording wattmeter during - this period. If this test be continued for an - appreciable time, the product of the pressure - in volts, the current in amperes, and the time - in hours, should equal the number of watthours - recorded on the counters of the dial. - - NOTE.--=Transformer testing.= In the early - days of transformer building, before the - commercial wattmeter had been perfected, leakage - or exciting current was the criterion of good - design. After the introduction of the wattmeter, - core loss became the all important factor, and - for a long time the question of leakage current - was lost sight of. With the introduction of - silicon steel, leakage or exciting current again - assumed prominence. Keeping in mind the fact - that all characteristics of a transformer are of - more or less importance, it is essential that - the user of such apparatus have at hand the - necessary facilities for making tests of all such - variable quantities. The tests which all users - of transformers should make, are given in this - chapter. - -=Ques. Can direct current be measured by an alternating current -voltmeter?= - -Ans. Yes. - -[Illustration: FIG. 2,899.--Transformer copper loss by wattmeter -measurement and impedance. At first glance, this method would -seem better than the calculation of loss after measurement of the -resistance. However, it should be noted that the wattmeter is, in -itself, subject to considerable error under the low power factor -that will exist in this test. The secondary of the transformer is -short circuited, and a voltage applied to the primary which is just -sufficient to cause full load primary current. If full current pass -through the primary of the transformer with the secondary short -circuited, the secondary will also carry full load current. With -connections as shown, and with the full load current, the voltmeter -indicates the impedance volts of the transformer. This divided by -the rated voltage gives what is called the _per cent. impedance of -the transformer_. In a commercial transformer of 5 kw., this should -be approximately 3 per cent. The iron loss of the transformer under -approximately 3 per cent. of the normal voltage will be negligible, -and the losses measured will be the sum of the primary and secondary -copper losses. As in the discussion of the core loss measurements, -the wattmeter readings must be corrected for the loss in its pressure -coil, the method of correction being the same as that discussed under -the core loss measurement. If the impedance volts, as measured, be -divided by the primary current, the impedance of the transformer -is obtained. The reciprocal of this quantity is known by the term -"admittance." _When two or more transformers are connected in -parallel they divide the load in proportion to their admittance._ -It is, therefore, important that the users of transformers know the -impedance of the apparatus used, in order to determine whether two -or more transformers will operate satisfactorily in parallel. For -discussion of wattmeter error on low power factor, see note on page -2,075. For accurate measurement of impedance, the voltmeter should -be connected directly across the terminals of the transformer rather -than as shown in the diagram.] - - NOTE.--=Transformer copper loss test.= The - usual and best method of obtaining copper - losses is to separately measure the primary and - secondary resistance and calculate from these - the primary and secondary copper losses. For - general diagram of connections and discussion - of the drop method, see fig. 2,875. The current - should be kept well within the load current of - the transformer to avoid temperature rise during - the test. In other words, the resistance of the - coil is the voltage across its terminals divided - by the current. The resistance of the primary - coil can be measured similarly. The copper loss - in watts in each coil will then be the product - of the resistance and the square of the rated - current for that coil. The total copper loss will - be the sum. - -=Ques. What would be the effect of placing a direct voltmeter across -an alternating current circuit, and why?= - -Ans. There would be no deflection of the pointer owing to the rapid -reversals of the alternating current. - -=Ques. What are the usual capacities of alternating current -voltmeters?= - -Ans. They are 3, 7.5, 10, 12, 15, 20, 60, 75, 120, 150, 300 and 600 -volts, but these capacities may each be increased by the use of a -multiplier. - -[Illustration: FIG. 2,900.--Temperature test of transformer with -non-inductive load. The figure shows the simplest way of making the -test. Connect the primary of the transformer to the line as shown, -and carry normal secondary load by means of a bank of lamps or other -suitable resistance, until full load secondary current is shown by -the ammeter in the secondary circuit. The transformer should then be -allowed to run at its rated load for the desired interval of time, -temperature readings being made of the oil in its hottest part, and -also of the surrounding air. Where temperatures of the coil rather -than temperatures of the oil are desired, it is necessary to use the -resistance method. This is obtained by first carefully measuring the -resistance of both primary and secondary coils at the temperature -of the room, and then, after the transformer has been under heat -test for the desired time, disconnect it from the circuit and again -measure the resistance of primary and secondary. For proper method -of calculating the temperature rise from resistance measurements, -the reader is referred to the standardization rules of the A.I.E.E. -In making resistance measurements of large transformers by the drop -method care should be taken to allow both ammeter and voltmeter -indications to settle down to steady values before readings are -taken. This may require several minutes. Each time the current is -changed it is necessary in order to obtain check values on resistance -measurements, to wait until the current is again settled to its -permanent value before taking readings. All resistance measurements -must be taken with great care, as small errors in the measurement of -the resistance may make very large errors in the determination of -the temperature rise. The method above described is satisfactory for -small transformers. Where large units are to be tested, the cost of -current for testing becomes an important item. The "bucking test" as -in fig. 2,901, is more economical.] - -=Ques. How are station voltmeters usually attached to the -switchboard?= - -Ans. They are usually bolted to the switchboard by means of four -iron supports mounted on the back of the instrument; two of these are -fastened near each side of the case. - - Under certain conditions, however, as in paralleling of - alternators, it is convenient to have the alternating current - voltmeter mounted on a swinging bracket at the side of the - switchboard. The voltmeter may then be swung around in any desired - direction so as to enable the attendant to keep informed of the - voltage while switching in each additional alternator. - -[Illustration: FIG. 2,901.--Transformer temperature "bucking test." -For this purpose two transformers of the same size and ratio are -required. The connections are as shown. Full secondary voltage is -applied, and rheostats or auxiliary auto-transformers are inserted -in the circuit to properly regulate the voltage. The primaries -are connected with one bucking the other, and a voltage equal to -twice the impedance voltage of either transformer inserted in the -primary circuit. It should be noted that when the secondaries are -subjected to the full secondary voltage, a full primary voltage -exists across either primary, but with the primaries connected so -that the voltage of one is bucked against the voltage of the other, -the resultant voltage in the circuit will be zero. By applying to the -primary circuit twice the impedance voltage of either transformer, -full primary and secondary current will circulate through both -transformers. On the other hand, by subjecting the secondaries to -the full secondary voltage, the iron of the transformer will be -magnetized as under its regular operating conditions, and the full -iron loss of the transformer introduced. This method permits the -operation of two transformers under temperature test with their -full losses, without taking energy from the line equal to the rated -capacity. Measurements of temperature are taken in exactly the -same way as above. This method is successfully employed for making -temperature tests on transformers of all sizes.] - -=Ques. How should an ammeter be operated to get accurate readings, -and why?= - -Ans. It should be cut out of circuit except while taking a reading, -because of the error introduced by the heating effect of the current. - -[Illustration: FIG. 2,902.--Transformer insulation test. In applying -a 10,000 volt insulation test between the primary and secondary of -a transformer, the testing leads should be disconnected from the -transformer under test, and a spark gap introduced as shown, with the -test needle set at a proper sparking distance for 10,000 volts. A -high resistance should be connected in the secondary before closing -its circuit, and the voltage gradually increased by cutting out -this secondary resistance until a spark jumps across the spark gap. -When the spark jumps across the spark gap, the voltmeter reading -should be recorded and the testing transformer disconnected. The -spark gap should then be increased about 10 per cent. and the high -tension leads connected to the transformer under test as indicated -in the diagram. In order to equalize the insulation strains, all -primary leads should be connected together, all secondary leads not -only connected together, but to the core as well. All resistance -in the rheostat in the low tension circuit should then be inserted -and the switch closed. Gradually cut out secondary resistance until -the voltmeter shows the same voltage as was recorded previously -when the spark jumped across the gap, and apply this voltage to the -transformer for one minute. Insulation tests for a period of over -one minute are very unadvisable, as transformers with excellent -insulation may be seriously damaged by prolonged insulation tests. -The longer the strain to which any insulation is subjected, the -shorter the subsequent life of the insulation. Also the greater the -applied voltage above the actual operating voltage of the apparatus, -the shorter the subsequent life of the insulation. In testing small -transformers, the spark gap may be omitted and the voltage of the low -pressure coil of the testing transformer measured. This multiplied by -the ratio of transformation gives the testing voltage.] - - In an ammeter having a capacity of 50 amperes, the error thus - introduced will be less than 1 per cent. if connected continuously - in circuit with a current not exceeding three-quarters this - capacity. - - An ammeter of 100 amperes capacity may be used indefinitely - in circuit with less than 1 per cent. error up to one-half its - capacity, and for five minutes at three-quarters capacity without - exceeding the 1 per cent. limit. - -[Illustration: FIG. 2,903.--Transformer insulation test as made -when a special high tension transformer be not available. In this -method a number of standard transformers, connected as shown may be -employed, but great care should be taken to have such transformer -cases thoroughly insulated from the ground and from one another, in -order to minimize the insulation strains in the testing transformers. -Care should be taken to insert in the circuit of each testing -transformer a fuse, not in excess of the transformer capacity, which -will blow, in case of a break down in the apparatus under test. -In testing insulation between secondary and core, disconnect the -primary entirely, apply one terminal of the testing transformer to -the secondary terminals of the transformer under test, and the other -terminal of the testing transformer to the core of the transformer -under test. This test should also not be in excess of one minute.] - - The 150 scale ammeter may be left in circuit for an indefinite - length of time at one-third its full capacity, and for three - minutes at one-half its full capacity, with a negligible error. - - Ammeters of 200 and of 300 ampere capacities must not - continuously carry more than one-quarter of these loads - respectively if the readings are to have an accuracy within 1 per - cent. nor more than one-half these respective number of amperes - for three minutes if the same degree of accuracy be desired. - - In order to cut or shunt the ammeter out of circuit when not - in use, it is customary when wiring the instrument in place, to - introduce a switch as a shunt across it; this switch is kept - closed except when a measurement is being taken. - - When currents larger than 300 amperes have to be measured, - ammeter shunts are generally employed, although ammeters up to - 500 amperes capacity are manufactured. - -[Illustration: FIG. 2,904.--Transformer internal insulation test, -sometimes called double normal voltage test, from the fact that most -transformers are tested with double normal voltage across their -terminals. If either the primary or secondary of the transformer -be connected to some source of current with voltage double that of -the voltage of the transformer under test, the insulation between -adjacent turns, and also the insulation between adjacent layers -will be subjected to twice the normal operating voltage. It is good -practice to employ high frequency for this test in order to prevent -an abnormal current from passing through the transformer. Sixty -cycle transformers are usually tested on 133 cycles, and 25 cycle -transformers on 60 cycle circuits for this double normal voltage -test. It is necessary to insert the resistance in the circuit of the -transformer and bring the voltage up gradually, the same as applying -other high insulation tests in order to prevent abnormal rises in -pressure at the instant of closing the circuit.] - -=Ques. What is used in place of instrument shunts for high pressure -alternating current measurements?= - -Ans. Instrument transformers. - -=Ques. What important attention should be periodically given to -measuring instruments?= - -Ans. They should be frequently tested by comparison with standards -that are known to be correct. - - Electrical measuring instruments, owing to the nature of their - construction and the conditions under which they must necessarily - be used, are subject to variations in accuracy. This feature is an - annoying one on account of the difficulty of detecting it; a meter - may, as far as appearances go, be in excellent working order and - yet give readings which are not to be relied upon. - - Ridiculous as it may appear, the average station attendant - may frequently be seen straining his eyes to read to tenths of - a division on the scale of a meter which, if subjected to test, - would show an inaccuracy of over 2 per cent. - - In testing a meter, by comparing it with a standard, in order to - obtain the best results there should be one man at each meter so - that simultaneous readings may be taken on both instruments, and - the man at the standard meter should maintain the voltage constant - while a reading is being taken, by means of a rheostat in the - field circuit of the generator supplying the current. - -[Illustration: FIG. 2,905.--Transformer insulation resistance -test. The insulation, besides being able to resist puncture, due -to increased voltage, must also have sufficient resistance to -prevent any appreciable amount of current flowing between primary -and secondary coils. It is, therefore, sometimes important that the -insulation resistance between primary and secondary be measured. This -can be done, as here shown. Great care should be taken to have all -wires thoroughly insulated from the ground, and to have an ammeter -placed as near as possible to the terminals of the transformer under -test, in order that current leaking from one side of the line to the -other, external to the transformer, may not be measured. Great care -is required in making this measurement, in order to obtain consistent -results.] - - Each meter should be checked or calibrated at five or six - approximately equidistant points over its scale; the adjustable - resistance being varied each time to give a deflection on the - standard meter of an even number of divisions and the deflection - on the other meter recorded at whatever it may be. Having obtained - the necessary readings, the calculation of the constant or - multiplying factor of the meter undergoing test is next in order. - - This may best be shown by taking an actual case in which a 150 - scale voltmeter is being tested to determine its accuracy. The - data and calculations are as follows: - - Readings on Readings on Constant - standard meter meter tested - 150 149.2 150 ÷ 149.2 = 1.005 - 125 125.0 125 ÷ 125.0 = 1.000 - 100 98.9 100 ÷ 98.9 = 1.011 - 75 73.6 75 ÷ 73.6 = 1.019 - 50 50.0 50 ÷ 50.0 = 1.000 - 25 24.8 25 ÷ 24.8 = 1.008 - ------ - 6.043 - - Average constant for six readings, 6.043 ÷ 6 = 1.007. - -[Illustration: FIG. 2,906.--Transformer winding or ratio test. The -object of this test is to check the ratio between the primary and -the secondary windings. For this purpose a transformer of known -ratio is used as a standard. Connect the transformer under test with -a standard transformer as shown. Leave switch S_{2} open. With the -single pole double throw switch in position S_{1}B, the voltmeter -is thrown across the terminals of the standard transformer. With -the switch in position S_{1}A, the voltmeter is thrown across the -terminals of the transformer under test. The voltmeter should be read -with the switch in each position. If the winding ratio be the same as -that of the standard transformer, the two voltmeter readings will be -identical.] - - It may be stated in general that before taking the readings for - this test, the zero position of the pointer on the meter tested - should be noted, and if it be more than two-tenths of a division - off the zero mark, the case of the meter should be removed and the - pointer straightened. - - Furthermore, it will be noticed from the readings here recorded - that the test is started at the high reading end of the scale; - this is done in order that the pointer may gradually be brought - up to this spot, by slowly cutting out of circuit the adjustable - resistance, and thus show whether or not the pointer has a - tendency to stick at any part of the scale. If the meter seem to - be defective in this respect, it should be remedied either by - bending the pointer or scale, or by renewing one or both of the - jewels, before the comparison with the standard is commenced. - - It is obvious from the readings recorded for the 150 scale - voltmeter, that as compared with the corresponding deflections of - the standard, the former are a trifle low. - - In order to determine for each observation how much too low - they are, it is necessary to divide each reading on the standard - by the corresponding reading on the meter tested. The result is - the amount by which a deflection of this size on the meter tested - must be multiplied in order to obtain the exact reading. This - multiplier is called a constant, and as shown, a constant is - determined for each of the six observations. - - The average constant for the six readings is then found, and - this is taken as the constant for the meter as a whole; that is, - whenever this 150-scale voltmeter is used, each reading taken - thereon must be multiplied by 1.007 in order to correct for its - inaccuracy. - - The most convenient and systematic way of registering the - constant of a meter is to write it, together with the number of - the meter and the date of its calibration, in ink on a cardboard - tag and loop the same by means of a string to the handle or some - other convenient part of the meter. - - NOTE.--=Transformer polarity test.= A test of - importance in the manufacture of transformers, - and sometimes necessary for the user, is the - so called _banking_ or _polarity_ test. The - transformers from any particular manufacturer - have the leads brought out in such a manner that - a transformer of any size can be connected to - primary and secondary lines in a given order - without danger of blowing the fuses due to - incorrect connections. All manufacturers of - transformers, however, do not bank transformers - in the same way, so that it is necessary in - placing transformers of different makes to test - for polarity. This is done as shown in fig. - 2,906. One transformer is selected as a standard - and the leads of the second transformer connected - as indicated in the diagram. If the transformers - be 1,100-2,200 volts to 110-220, two 110 volt - lamps are connected in the secondaries of the - transformers as indicated, while the primary of - the transformer is connected across the line. - In transformers built for two primary and two - secondary voltages, it is necessary to test each - primary and each secondary. The diagram shows the - method of connecting one 2,200 volt coil and one - 110 volt coil to the transformer to be tested. - When the primary circuit of the transformer under - test is closed, and if the secondary leads of - the 110 volt coil under test be brought out of - the case properly, the two 110 volt lamps should - be brightly illuminated. If, on the other hand, - the two 110 volt terminals have been reversed, - no current will flow through the lamps. If - these two terminals be found to be brought out - correctly, transfer the secondary leads of the - transformer under test to the second 110 volt - coil. Upon closing the primary circuit, the lamp - should again be brightly illuminated. Repeat this - process with each of the secondary coils and the - other primary coil, and if the lamps show up - brightly in every case on closing the primary - circuit, all leads have been properly brought - out. If on any tests the lamps do not light up - brightly, the leads on the transformer must be so - changed as to produce the proper banking. - -=Ques. What are the usual remedies applied to a voltmeter to correct -a 3 or 4 per cent. error?= - -Ans. They consist of straightening the pointer, varying the tension -of the spiral springs, renewing the jewels in the bearings, altering -the value of the high resistance, and, in the case of a direct -current instrument, strengthening the permanent magnet. - -=Ques. How is the permanent magnet strengthened?= - -Ans. After detaching it from the instrument, wrap around several -turns of insulated wire, and pass through this wire for a short time -3 or 4 amperes of direct current in such a direction as to reinforce -the magnet magnetism. - -=Ques. How may the value of the high resistance of a voltmeter be -altered?= - -Ans. Determine the resistance of the voltmeter and add or subtract, -according as the reading is high or low, a certain length of wire -whose resistance is in per cent. of the voltmeter resistance the same -as the per cent. of error. - - NOTE.--The complete calibration of a two - scale voltmeter does not, as might be supposed, - necessitate that the readings on both scales be - checked with standards, for since the resistance - corresponding to the one scale is always some - multiple of the resistance of the other, the - constants of the two scales are proportional. For - instance, if S = the reading at the end of the - high scale of the voltmeter; S^{1} = the reading - at the end of the low scale of the voltmeter; R = - the resistance in the meter corresponding to the - high scale; R^{1} = the resistance in the meter - corresponding to the low scale; K = the constant - for the high scale, and K^{1} = the constant for - the low scale. Then - - SK ÷ R = S^{1}K^{1} ÷ R^{1} - -from which - - K^{1} = SKR ÷ S^{1}R - - That is to say, if the respective resistances corresponding to - the two scales be known, and the constant of the high scale be - determined by comparison with a standard, then by aid of these - known values and the maximum readings on the two scales, the - constant of the low scale may be calculated. It is also possible - to calculate the constant of the high scale if the constant of the - low scale be known, together with the values of the resistances - corresponding to the two scales; for from the equation previously - given. - - K = RS^{1}K^{1} ÷ R^{1}S - -=Ques. What is a frequent cause of error in an alternating current -meter, and why?= - -Ans. The deterioration of its insulation, which permits the working -parts of the instrument coming in contact with the surrounding metal -case. - - A convenient method of testing for deterioration of insulation - is shown in fig. 2,905. - -[Illustration: FIG. 2,907.--Diagrams showing various synchronous -converter transformer connections. The diametrical connection is -used most frequently as it requires only one secondary coil on each -transformer, this being connected to diametrically opposite points on -the armature winding. The middle points can be connected together and -a neutral obtained the unbalanced three wire direct current having no -distorting effect. With diametrical secondaries, the primaries should -preferably be connected delta, except with regulating pole converters -where they must be connected Y. Diametrical secondaries with delta -primaries should not be used with regulating pole converters. Double -star connection of secondaries may, however, be used with delta -primaries, and is free from the trouble of the triple harmonic of the -transformer appearing in the primary. In this case, however, the two -secondary neutrals must not be connected with each other.] - -=How to Test Generators.=--In the operation of electrical stations, -many problems dealing with the generators installed therein can be -readily solved by the aid of characteristic curves, which bear a -relation to the generators similarly as do indicator diagrams to -steam engines. - -[Illustration: FIG. 2,908.--General form of characteristic curves -for a series dynamo. The general curve that may be expected is OA. -It is obtained practically in the same manner as for the shunt -characteristic curve, except that no field rheostat is employed. -Commencing with no load or amperes, there will probably be a -small deflection noticeable on the voltmeter, due to the residual -magnetism. The other readings are taken with successive reductions -of main current resistance. The curve OA thus obtained for a certain -series generator is practically a straight line at the beginning, -representing thereby a proportional increase of voltage with -increase of current, but after a certain current is reached (about -20 amperes in this case) the curve flattens and takes a downward -direction. The turning point occurs in the characteristic curves -of all series generators, and it denotes the stage at which the -iron magnet cores become so saturated with lines of magnetic force -that they will not readily allow more to pass through them; this -turning point is technically known as the point of saturation, and -the current corresponding (20 amperes in this case) is called the -critical current of the dynamo. The point of saturation in any given -series machine is governed by the amount of iron in the magnetic -circuit; its position in the curve therefore varies according to -the design of the generator as does also the critical current. The -value of the latter is important inasmuch as the valuable features -of a series generator assert themselves only when the machine is -supplying a greater number of amperes than that of the critical -current, for if the series generator be worked along that part M A of -the curve to the right of the point of saturation it becomes nearly -self-regulating as regards current, because as the current increases -the voltage drops. In the diagram in addition to the characteristic -curve O A, which may more definitely be called an external -characteristic curve on account of representing the conditions -external to the generator, there is shown a total characteristic -curve, O C B. The latter curve represents the relation between the -current and the total voltage developed in the armature, and may be -plotted from the external characteristic curve if the resistance of -the armature between brushes and the resistance of the series field -winding be known. For example, assume these combined resistances -amount to .6 ohm. At 30 amperes there would be required 30 × .6 = 18 -volts to force this current through the armature and field windings. -At 30 amperes the external pressure is 65 volts, as shown by the -curve O A; the total voltage developed for 30 amperes is, therefore, -the external voltage plus the internal voltage or 65 + 18 = 83 -volts. Plotting 83 volts for 30 amperes will give one point for the -external characteristic curve of this machine, and by determining in -like manner the total voltages developed for six or eight different -currents over the scale, sufficient data will be at hand for plotting -and drawing in the curve O C B.] - -In steam engineering, a man who did not fully understand the method -of taking an indicator diagram would be considered not in touch with -his profession, and in electrical engineering the same would be true -of one ignorant of the method of obtaining characteristic curves. - - The necessary arrangement or connection of the generator from - which it is desired to obtain a characteristic curve, consists in - providing a constant motive power so that the machine may be run - at a uniform speed, and when the field magnets of the generator - are separately excited the field current from the outside source - must also be maintained constant, preferably by a rheostat - connected in the field of the auxiliary exciting machine. It is - also necessary in every case that means be provided for varying - the main current of the generator step by step from zero to - maximum. This may best be done by employing a water rheostat, as - shown in fig. 2,909. - -=Ques. What instruments are needed in making a test of dynamo -characteristics?= - -Ans. A voltmeter, ammeter, speed indicator, the usual switches and -rheostats. - -=Ques. How is the apparatus connected?= - -Ans. It is connected as shown in fig. 2,910. - -=Ques. Describe the test.= - -Ans. Having completed the preliminaries as in fig. 2,910, the test -should be started with the main circuit of the generator open. Then, -in the case of the shunt machine, the speed should be made normal and -the field rheostat adjusted until the voltmeter reading indicates -the rated voltage of the machine at no load and readings taken. The -electrodes of the water rheostat should be adjusted for maximum -resistance and main circuit closed, and a second set of readings -taken. Several sets of readings are taken, with successive reductions -of water rheostat resistance. The results are then plotted on -coordinate paper giving the characteristic curve shown in fig. 2,908. - -[Illustration: FIG. 2,909.--Water rheostat. It consists essentially -of a tank of suitable size containing salt water into which are -placed two electrodes having means of adjustment of the distance -separating them. The solution depends on the voltage. Pure water is -seldom used for pressures under 1,000 volts. The size of the tank is -determined by the size of the electrodes, and roughly the size of the -latter equal the number of amperes. With a current density of one -ampere per square inch, a water solution gives a drop of 2,500 to -3,000 volts per inch distance between the plates. Where high voltage -is used, the water must be circulated through and from the tank by -rubber hose allowing for 2,500 volts, a length of 15 to 20 feet of -1 inch hose to prevent grounding. In place of the arrangement shown -above, a barrel may be used for the tank, and for the electrodes, -coils of galvanized iron wire. This is the simplest form and is -satisfactory.] - -=Ques. What does the characteristic curve (fig. 2,911) show?= - -Ans. An examination of the curve shows that the highest point of -the curve occurs at no load or 0 amperes; that as the current is -increased, the voltage drops, first slightly to the point B and then -rapidly until the point E is reached, when any further lowering of -resistance in the main circuit to increase the current causes not -only a rapid decline in the voltage but also of the current until -both voltage and current become approximately zero. - -[Illustration: FIG. 2,910.--Connections for test of dynamo. During -the test, one man should be assigned to the tachometer, another man -to the water rheostat, and there should preferably be one man at -each of the electrical measuring instruments. In order to enable the -man at the tachometer to keep the speed constant, he should be in -communication either directly or indirectly with the source of the -driving power, and the man at the water rheostat should be in plain -view of the man reading the ammeter so that the latter party may -signal him for the proper adjustment of the rheostat in order that -the desired increase of current be obtained for each set of readings.] - - In some generators, a very slight current results even when the - terminals of the machine are actually short circuited; that is, - due to residual magnetism in the pole pieces, the lower portion of - the curve often terminates, not exactly at zero, but at a point - some distance along the current line. - - The working portion of the curve is from A to C, at which time - the machine is supplying a fairly constant voltage. From C to E - shows a critical condition of affairs, while the straight portion - D O represents the unstable part of the curve caused by the field - current being below its proper value. - - The position of the point C determines the maximum power the - machine is capable of developing, being in this case (47.5 × 25) ÷ - 746 = 1.59 horse power. - -=Ques. How may the commercial efficiency of a generator be -determined?= - -Ans. To obtain the commercial efficiency, the _input_ and _output_ -must be found for different loads. - - The input may be found by running the generator as a motor - at its rated speed, loading it by means of a Prony brake. The - generator must be stripped of all belting or other mechanical - connections, supplied with its normal voltage and full load - current, and the pressure of the Prony brake upon its armature - shaft or pulley adjusted until the rated speed of the armature is - obtained. The data thus obtained is substituted in the formula. - - 2π L W R - input in brake horse power = ---------- (1) - 33,000 - -in which - - L = length of Prony brake lever; - W = pounds pull at end of lever; - R = revolutions per minute. - -The output or electrical horse power for the same load is easily -calculated from the formula - - amperes × volts - output in electrical horse power = --------------- (2) - 746 - -After obtaining value for (1) and (2) the commercial efficiency for -the load taken is obtained from the formula - - output - commercial efficiency = ------ (3) - input - -Having obtained the commercial efficiency, the difference between the -ideal 100 per cent. and the efficiency found will be due to certain -losses in the generator. These losses may be classified as - - 1. Mechanical. - 2. Electrical. - - The mechanical losses are the friction of the bearings and - brushes, and air friction. The electrical losses consist of the - eddy current loss, hysteresis loss, armature resistance loss, and - field resistance loss. - - In testing for these losses, the generator to be tested should - be belted to a calibrated motor which latter machine should - preferably be of the constant pressure, shunt wound type. - - The friction of the bearings and belt of the generator are - determined together by raising the brushes off its commutator and - running it at the rated speed by means of the calibrated motor. - -[Illustration: FIG. 2,911.--Characteristic curve of shunt dynamo. -Suppose in making the test, the deflections on the meters for the -first readings be 63 volts and 0 amperes, the plotting of these -values will give the first point on the curve. Similarly, the second -readings with main circuit closed and maximum resistance in the water -rheostat may be assumed to be 62.5 volts and 7.5 amperes, which -plotted gives the second point B. A still further lowering of the -plate will permit a stronger current in the main circuit, and the -value of this together with its corresponding voltage will give a -third point for the curve. Neither for this reading, however, nor -for the following readings of the test should the field rheostat be -altered. When six or eight points ranging from zero to a maximum -current have been obtained and plotted, a curved line should be drawn -through them such as shown through ABCDEFG0, the _characteristic -curve_ of the dynamo. While the curve may be sketched in free hand, -it should preferably be drawn by the aid of French curves. In case -the French curve cannot be exactly made to coincide with all the -points as for instance C and D, it should be run in between giving an -average result, and smoothing out irregularities, or small errors due -to the "personal equation." The meter of course must be correct or -calibrated and the readings corrected by the calibration coefficient.] - - The amount of power as ascertained from the calibration curve of - the motor for the voltage and current used therein when driving - the generator as just explained, is a measure of these two losses. - The power thus used is practically constant at all loads and is - about 2 per cent. of that necessary to drive the generator at full - load. - -[Illustration: FIG. 2,912.--Characteristic curves for a compound -dynamo. If the machine be over compounded, the characteristic curve -has the form of the curve A B, which curve was obtained from a -machine over-compounded from 118 to 123 volts, and designed to give -203 amperes at full load. The preliminary arrangements for testing -a compound dynamo are similar to those for a shunt generator, and -if the shunt across the series field winding be already made up and -in position, the readings are taken precisely in the same manner. -It is generally considered sufficient if observations be recorded -at zero, ¼, ½, ¾ and full load. If it be desired to ascertain the -effect which residual magnetism has upon the field magnets the -current is decreased after the full load point is reached without -opening the circuit, and readings are taken in succession at ¾, -½, ¼ and zero load giving in this case the curve B C D E S. It is -thus seen that residual magnetism exerts no small effect upon the -voltage obtained at the different loads, for had there been no -residual magnetism in the field magnets the curve B C D E S would -have coincided with the curve A B. The curve A B, and the straight -line A X drawn through the points A and B, are almost identical, -and as A X represents the theoretical characteristic curve for the -machine, it is seen that compounding is practically perfect. In -order to insure such accurate results being obtained, providing -the machinery be correctly designed, requires considerable care in -taking the readings; for example, each step or load on the ascending -curve should not be exceeded before the corresponding deflection is -taken, else the residual magnetism will cause the pressure reading -to be higher than it actually should be, and the following pressure -readings will also be affected in the same manner. In case the shunt -to be employed across the series field has not been made up, it is -advisable to perform a trial test before taking the readings for the -curve as previously described. The trial test consists in taking -two readings,--one at no load and the other at full load, the shunt -being so adjusted as to length and section that the desired amount -of compounding will be obtained in the latter reading with normal -voltage at no load. If the first trial fail to produce the desired -result by giving too low a voltage at full load, the length of the -shunt across the series field should be increased, or its section -should be reduced by employing a less number of strips in its makeup; -again, if the voltage at full load be higher than that desired, there -must be made a decrease in length or an increase of section in the -shunt employed.] - - The friction of the brushes can very conveniently be determined - next by lowering them on the commutator and giving them the proper - tension. - - The increase in power resulting from the greater current that - will now be taken by the motor to run the dynamo at its rated - speed, will be a measure of this loss. In general, its value will - be about .5 per cent. of the total power required to drive the - dynamo at full load, and this also will remain constant at all - loads. - - The friction of the air upon the moving armature of the dynamo - cannot be determined experimentally, but theoretically this loss - is small and may be estimated as .5 per cent.; it is also constant - at all loads. - - The core loss may be determined experimentally by exciting - the field magnets of the dynamo with the normal full load field - current through the magnet coils, and noting the increase of power - required by the motor to maintain the rated speed of the dynamo - thus excited under no load, over that necessary under the same - conditions with no field excitation. This increase of power will - be the value of the core loss. The core loss is approximately 3 - per cent. of the power required to operate the dynamo at full - load, and it is constant at varying loads. If it be desired to - divide the core loss into its component parts, it is necessary - also to run the dynamo under the same conditions as before with - field excitation but at half its rated speed. If, then, - - H = the power lost in hysteresis at rated speed, - E = the power lost in eddy currents at rated speed, - T = the power lost in hysteresis and eddy currents at rated speed, - S = the power lost in hysteresis and eddy currents at half speed. - -there may be formed the two following equations: - - H E - T = H + E, and S = --- + ---, - 2 2 - - from which the elimination of H will give E = 2T - 4S. - - The value of the eddy current loss thus found will be about 1½ - per cent., and constant at all loads. - - Having previously ascertained the power lost in both eddy - currents and hysteresis, and knowing now the power lost in eddy - currents alone, it is easy to find that lost in hysteresis by - simply subtracting the latter known value from the former. The - value of the hysteresis loss is therefore approximately 1½ per - cent., and it is constant at different loads. - - There yet remains to be determined the armature resistance - loss and the field resistance loss. As for the calibrated motor, - this may be disconnected from the dynamo, as it need not be used - further in the test. - - The armature resistance is the resistance of the armature - winding of the dynamo, between the commutator bars upon which - press the positive and negative brushes. Assume that the value - of the armature resistance be known, call this value R ohms, - together with that of the full load armature current, which is - also known and which call I amperes, this is sufficient data for - calculating the armature resistance loss at full load. It is - evident that to force the full load current I through the armature - resistance R will require a pressure of R volts, and that the - watts lost in doing so will be the voltage multiplied by the - current. The armature resistance is consequently - - IR × I = I^{2}R watts - -or, expressed in horse power it is - - I^{2}R ÷ 746 - - At full load it is usually about 2 per cent. of the total - power required to drive the generator fully loaded. The armature - resistance loss varies in proportion to the load, in fact, as the - last expression shows, it increases as the square of the armature - current. - - The field resistance loss is calculated in the same manner as - just explained for the armature resistance loss, it being equal - in horse power to the square of the full load field current - multiplied by the resistance of the field winding and divided by - 746. In a shunt dynamo it is practically constant at 2 per cent. - of the total power at full load, but in a series or in a compound - generator it will vary in proportion to the load. - - - - - HAWKINS PRACTICAL LIBRARY OF ELECTRICITY - - IN HANDY POCKET FORM PRICE $1 EACH - - -_They are not only the best, but the cheapest work published on -Electricity. Each number being complete in itself. Separate numbers -sent postpaid to any address on receipt of price. They are guaranteed -in every way or your money will be returned. Complete catalog of -series will be mailed free on request._ - -=ELECTRICAL GUIDE, NO. 1= - Containing the principles of Elementary Electricity, Magnetism, - Induction, Experiments, Dynamos, Electric Machinery. - -=ELECTRICAL GUIDE, NO. 2= - The construction of Dynamos, Motors, Armatures, Armature - Windings, Installing of Dynamos. - -=ELECTRICAL GUIDE, NO. 3= - Electrical Instruments, Testing, Practical Management of Dynamos - and Motors. - -=ELECTRICAL GUIDE, NO. 4= - Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers, - Storage Batteries. - -=ELECTRICAL GUIDE, NO. 5= - Principles of Alternating Currents and Alternators. - -=ELECTRICAL GUIDE, NO. 6= - Alternating Current Motors, Transformers, Converters, Rectifiers. - -=ELECTRICAL GUIDE, NO. 7= - Alternating Current Systems, Circuit Breakers, Measuring - Instruments. - -=ELECTRICAL GUIDE, NO. 8= - Alternating Current Switch Boards, Wiring, Power Stations, - Installation and Operation. - -=ELECTRICAL GUIDE, NO. 9= - Telephone, Telegraph, Wireless, Bells, Lighting, Railways. - -=ELECTRICAL GUIDE, NO. 10= - Modern Practical Applications of Electricity and Ready Reference - Index of the 10 Numbers. - - =Theo. Audel & Co., Publishers. 72 FIFTH AVENUE=, - =NEW YORK.= - - - - - -End of the Project Gutenberg EBook of Hawkins Electrical Guide Vol. 8 (of 10), by -Nehemiah Hawkins - -*** END OF THIS PROJECT GUTENBERG EBOOK HAWKINS ELECTRICAL GUIDE, VOL 8 *** - -***** This file should be named 50068-0.txt or 50068-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/5/0/0/6/50068/ - -Produced by Juliet Sutherland, Paul Marshall and the Online -Distributed Proofreading Team at http://www.pgdp.net - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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