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Hawkins Electrical Guide v. 04 (of 10) / 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 cover

Hawkins Electrical Guide v. 04 (of 10) / 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

Chapter 2: CHAPTER XXXVI DISTRIBUTION SYSTEMS
By N. Hawkins
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About This Book

A practical electrical manual provides a progressive question-and-answer treatment of power distribution, wiring, and related apparatus. It explains series, parallel, three-wire and five-wire arrangements and methods for balancing and regulating voltage using dynamotors, motor-dynamo sets, boosters, and balancing coils, accompanied by diagrams. Detailed sections examine conductors, materials, gauges, resistance and ampacity, cable capacities, and calculation methods (circular mils, ampere-foot), and offer wiring tables and installation guidance. The material focuses on applied problem-solving and safe inside-wiring practices for engineers, electricians, students, and others learning direct-current distribution and installation techniques.

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Title: Hawkins Electrical Guide v. 04 (of 10)

Author: N. Hawkins

Release date: July 23, 2015 [eBook #49512]
Most recently updated: October 24, 2024

Language: English

Credits: Produced by Juliet Sutherland, Wayne Hammond and the Online
Distributed Proofreading Team at http://www.pgdp.net

*** START OF THE PROJECT GUTENBERG EBOOK HAWKINS ELECTRICAL GUIDE V. 04 (OF 10) ***

The cover image was created by the transcriber and is placed in the public domain.

THE THOUGHT IS IN THE QUESTION THE INFORMATION IS IN THE ANSWER

HAWKINS
ELECTRICAL GUIDE
NUMBER
FOUR
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, 1914,
BY
THEO. AUDEL & CO.,
New York.

Printed in the United States.

TABLE OF CONTENTS GUIDE NO. 4

DISTRIBUTION SYSTEMS 697 to 720

Classification—series system—danger in series arc light system—constant current system—parallel system—arrangement of feeder and mains in parallel system—series-parallel system—center of distribution—Edison three wire system—evolution of the three wire system—balanced three wire system—balancing of three wire system—copper economy in three wire systems—Dobrowolsky three wire system—modifications of three wire system—three wire storage battery system—three wire double dynamo system—three wire bridge system—three wire three brush dynamo system—Dobrowolsky three wire system—three wire auxiliary dynamo system—three wire compensator system—extension of the three wire principle—five wire system—dynamotor—connections of balancing set—balancing coils—distribution by dynamo motor sets—boosters and their uses—auxiliary bus bar.
WIRES AND WIRE CALCULATION 721 to 764

Preliminary considerations—various wires—copper wire—iron wire—German silver wire—standard of copper wire resistance—relative conductivity of different metals and alloys—conductors—cable for elevator annunciators—covered conductors—rubber covered conductors—rubber insulated telephone and telegraph wires—weather proof conductors—twisted weather proof wires—precautions in using weather proof conductors—slow burning wire; where used—slow burning weather proof wire; where used; how installed—miscellaneous insulated conductors—safe carrying capacity of wire—pothead wires—circular mils—square mils—mil foot—lamp foot—ampere foot—center of distribution—wire gauges—B. & S. standard wire gauge—micrometer screw gauge—calculating gauge—table of various wire gauges—table of lamp feet for rubber covered wires—lamp table for weather proof wires—symmetrical and unsymmetrical distribution—wiring table for light and power circuits—table of wire equivalents; how to use—table of cable capacities—incandescent lamps on 660 watt circuits—"tree" and "modified tree" system of wiring—distribution with sub-feeders—wrong and right methods of loop wiring—table, of amperes per motor; of amperes per dynamo—calculations for three wire circuit—three wire circuit panel board—size of the neutral wire.
INSIDE WIRING 765 to 798

The term "wiring"—open or exposed wiring—selection of wires—installation—disadvantages of open wiring—splicing—pitch of wires—crossing of wires—wiring across pipes—practical points relating to exposed wiring—methods of carrying wires, through floors; on walls—protecting exposed wiring on low ceilings—various porcelain knobs and cleats—wires used in mouldings—standard wooden moulding—kick box—usual character of moulding work—practical points relating to wiring in mouldings—tapping outlets—arc light wiring—arc lamps on low pressure service—circular fixture block—concealed knob and tube wiring; objections; method of installation—arrangement of switch and receptacle outlet in knob and tube wiring—switch boxes—rigid conduit wiring; advantage—types of rigid conduit; requirements—-conduit box—disadvantages of unlined conduit—flexible conduit wiring—Greenfield flexible steel conduit—"fishing"—insulating point—canopy insulator—fish plug and method of insertion—method of installing conduits in fireproof buildings—service entrance to rubber conduit system—condulet outlet to arc lamp—hickey—methods of bending large conduits—armoured cable wiring; features; installation.
OUTSIDE WIRING 799 to 824

Materials for outside conductors—tensile strength of copper wire—pole lines—pole constructing tools—wooden poles—preservation of wooden poles—preservation processes—methods of setting wooden poles in unsuitable soil—reinforced concrete poles—cross arms—lineman's portable platform—poles for light and power wires—spacing of poles—erecting the poles—guy anchors—method of raising a pole—method of pulling anchor into place—guys for poles—head and foot guying—guying corner poles—guy stubs and anchor logs—climbers—wiring the line—pay out reels—method of stringing wires—"come alongs"—tension on wires—sag table—lineman's block and fall with "come alongs"—attaching wire to insulator—splicing; American wire joint; McIntire sleeve and sleeve joint—transpositions—insulators—overhead cable construction—petticoat insulator—Clark's "antihum"—service connections and loops—method of making series, and parallel service connections—joint pole crossing—service wires.
UNDERGROUND WIRING 825 to 844

City conditions—underground systems—various conduits—vitrified clay pipe conduit—vitrified clay or earthenware trough conduit—joints in multiple-duct vitrified clay conduit—concrete duct conduits—methods of laying conduit—method of laying single duct vitrified clay conduit—method of laying multiple duct clay conduit—wooden duct conduits—objection to use of wood; remedy; adaptation wooden built-in conduits; method of installation—wrought iron or steel pipe conduits; method of installation—porcelain bridgework or carriers—cast iron pipe and trough conduit; advantages—fibre conduits; joints—Edison tube system—underground cables—metal sheaths on underground cables—pot heads—General Electric manhole junction box—pot head connections.
WIRING OF BUILDINGS 845 to 864

Preliminary considerations—electrician's instructions—location of receptacles—ceiling buttons—hallway wiring—selection of wiring system—three wire convertible system—method of wiring—location of panel boards—current required on each floor of building—arrangement of feeders—installation of motors—largest size of feeder permissible—method of cutting pocket in floor—outlet baseboard—completed pocket—how to examine partition interiors—house plan of conduit wiring—attachment of mains to knobs—precautions in making joints—wiring for heating appliances—wiring with combination of moulding, flexible tubing or conduit in non-fire proof building—feeder system for large hotel.
SIGN FLASHERS 865 to 884

Classification—Carbon flashers—wiring diagram for Dull's carbon flashers—brush flashers—knife flashers—flasher transmission gearing—simple on and off flashers—flash system of gas lighting—high speed flashers—lighting flashers—wiring diagram for flags—diagram showing method of wiring for high speed effects on single lines—method of wiring for a torch—wiring diagram for high speeds—Dull's lightning—script breakers—chaser flashers—thermo flashers—carriage calls—monogram for carriage calls—wiring diagrams for sign illumination—-National carriage call operating keyboard—clock monogram—Bett's clock mechanism for operating electric monogram time flasher—talking signs—two way thermal flasher.
LIGHTNING PROTECTION 885 to 892

Lightning rods; why sharp points are used; erection—diagram showing principle of air gap arrestor—variable gap arc breaker—location of lightning arrestor—carbon arresters with fuses for telephone lines—ground connection.
STORAGE BATTERIES 893 to 968

Early experiments—theory—description of storage cell—electrolyte—effect of current passing through electrolyte—types of storage batteries—Plant cells—Willard plates; wood separator—parts of "Autex" automobile cells—Faure or pasted type—comparison of Plant and Faure plates—the electrolyte; kind generally used; preparation; test; mixing acid and water; kind of vessel used;—specific gravity table—effect of deep vessel—density of electrolyte—hydrometer syringe—impurities in electrolyte—tests for impurities; chlorine, nitrates, acetic acid, iron, copper, mercury, platinum—old electrolyte—voltage of a secondary cell—charging—connections for charging—charging; first time; period; regulation of voltage—Edison cell data—frequency of charging—cadmium test—emergency connections for weak ignition battery—portable testing instruments—charge indications—two methods of charging from a direct current lighting system—colors of the plates—how best results are obtained in charging—charge voltage—two ways of charging—diagram of charging connections—how to keep charging current constant—tests while charging; after charging—charge indications—behavior of electrolyte during discharge—lead burning outfit—"boiling"; causes—hydrogen gas generator for lead burning—quick charging—charging through the night—charging period for new battery—Willard underhung battery box for automobiles—high charging rates—"National" instructions for taking voltage readings—mercury arc rectifier—capacity—table of capacity variation for different discharge rates—how to increase the capacity—discharging; too rapidly rating; maximum rate—Edison alternating current rectifier—attention after discharging—the battery room—battery attendants—points on care and management—placement of cells—how to avoid leakage—precautions when unpacking cells—assembling cells—connections—precaution in joining terminals—battery troubles—short circuiting; indication; location—overdischarge; buckling—sulphation of plates—data on National cells; American cells; Autex cells—action in idle cell—lack of capacity—how to prevent lead poisoning—low specific gravity without short circuits; causes—treatment of weak cells—pole testing paper—disconnecting cells—sulphuric acid specific gravity table—how to take a battery out of commission—Witham charging board—putting batteries into commission—cleaning jars—table of voltage charge as affected by discharge rate—condensed rules for the proper care of batteries.
STORAGE BATTERY SYSTEMS 969 to 996

Uses of storage batteries; their importance in power plants—load curve showing use of storage battery as aid to the generating machinery—parallel connection of battery and dynamo—"floating the battery on the line"—diagram showing effect of battery in regulating dynamo load—connections and circuit control apparatus—diagram showing action of battery as a reservoir of reserve power—three wire system with battery and dynamo—methods of control for storage batteries—diagram of connections for ignition outfit—variable resistance—end cell switches—diagram of connection of battery equipment for residential lighting plant—end cell switch diagram—features of end cell switch construction—end cell switch control—circuit diagram for charging battery in two parallel groups and discharging in series—reverse pressure cells; regulation—Holzer-Cabot dynamotor—boosters—application of series booster system—Bijur's battery system—load diagram—characteristics of series booster—shunt boosters; with battery—Entz' carbon pile booster system—application of shunt booster—circuit diagram for non-reversible shunt booster and battery system—compound boosters; their connections—method of charging battery at one voltage and supplying lights at a different voltage—connections of one form of differential booster—differential boosters; with compensating coil; adaptation.

CHAPTER XXXVI
DISTRIBUTION SYSTEMS

The selection of the system of transmission and distribution of electric energy from the generating plant to lamps, motors, and other devices, is governed mainly by the cost of the metallic conductors, which in many electrical installations, is a larger item than the cost of the generating plant itself. This is especially true in case of long distance transmission, while in those of the lighting plants, the cost of wiring is usually more expensive than that of the boilers, engines, and generators combined.

The principal distribution systems, are classed as:

1. Series;
2. Parallel;
3. Series-parallel;
4. Parallel-series.

Ques. What is the characteristic feature of each class?

Ans. In the series systems the current is constant, but the voltage varies. In the parallel systems, the voltage is constant, but the current varies.

Series System of Distribution.—A series system affords the simplest arrangement of lamps, motors, or other devices supplied with electric energy. The connections of such a system are shown in fig. 783. The current from the terminal of the dynamo passes through the lamps, L, L, L, L, one after the other and finally returns to the terminal. The current remains practically constant, but the voltage falls throughout the circuit in direct proportion to the resistance, and the difference in pressure between any two points in the circuit is equal to the current in amperes multiplied by the resistance in ohms included between them.

For example. Each open arc lamp requires about 50 volts. In the system shown in fig. 783, the pressure measured across the brushes of the dynamo is assumed to be 1,000 volts. As this current flows through the circuit 45 volts will be actually lost in each lamp, and as the drop on the line wire is usually about 10 per cent. of the total voltage, there will be a drop of 5 volts on the conductor between any two lamps. In the circuit shown, there are twenty lamps, therefore, the difference in pressure between either terminal of the dynamo and middle point A of the circuit will be 10 lamps × 50 volts = 500 volts. Likewise, the difference in pressure between any two points on the circuit will be equal to 50 volts multiplied by the number of lamps included between them.

Fig. 783.—Series system of distribution. This is a constant current system, so called because the current remains practically constant. It is used chiefly for arc lighting.

Ques. Describe the danger in a series arc light system?

Ans. Since the total voltage of the system is equal to the sum of the volts consumed in all of the lamps, it is high enough to be dangerous to personal safety.

This is illustrated in fig. 783. If the line be grounded at B owing to defective insulation, the pressure of the circuit at that point will be zero, and therefore, a man standing on the ground could touch that point without receiving a shock, but if he should touch the line at the point C, he will receive a slight shock of 150 volts, as there are three lamps between the point C, and the ground connection B. Therefore, the danger of touching the circuit increases directly with the resistance between the point touched and the ground connection, so that if a man touch the circuit at the point D, he will receive a dangerous shock of 16 × 50 = 800 volts. In practice, sixty lamps are often placed on a single arc lighting circuit, so that its total pressure is about 3,000 volts, thus greatly increasing the danger of the system.

Ques. What is a constant current system?

Ans. The series system is a constant current system, and is so called because the current remains practically constant, while the voltage falls throughout the circuit in direct proportion to the resistance.

Ques. What are the principal applications of the series system?

Ans. For arc lighting, and telegraphic circuits.

Ques. What are the advantages of the series system?

Ans. In the case of telegraphic circuits only one wire is required, and for lighting and power transmission and distribution, only two wires; therefore, it is simpler and cheaper than any other system.

Ques. What is the disadvantage of the series system?

Ans. The danger due to the high voltage in installations such as arc lighting circuits.

Parallel System.—Parallel or multiple systems are usually more complicated than series systems, but since the voltage can be maintained nearly constant by various methods, practically all incandescent lamps, electric motors, and a large proportion of arc lamps are supplied by parallel systems.

The general principle of the parallel system is shown in fig. 784. With six lamps on the circuit, each requiring one-half ampere of current, at 110 volts, the dynamo will have to supply a current of 3 amperes at a pressure of 112 volts, and this current will flow through the circuit and distribute itself as shown on account of the lesser resistance of the wire relatively to that of the lamps. At the first lamp, the 3 amperes will divide, ½ ampere flowing through the lamp and the remaining 2½ amperes passing on to the next lamp and so on through the entire circuit. The reduction of pressure from 112 volts across the brushes to 110 volts at the last lamp is due to the resistance of the conducting wires.

Ques. What three effects are due to this drop in pressure?

Ans. 1, All the lamps or motors in the circuit receive a lower voltage than that at the dynamo, 2, some lamps or motors may receive a lower voltage than the others, and 3, the voltage at some lamps or motors may vary when the others are turned on or off.

Fig. 784.—Parallel system of distribution. This is a constant voltage system and is used principally for incandescent lighting and electric motor circuits.

The first is the least harmful and may be counteracted by running the dynamo at a little higher voltage; but the second and third are very objectionable and difficult to overcome. They are counteracted successfully in practice, however, by various methods of regulation, the use of boosters, and the operation of dynamos in parallel.

Ques. What are the principal applications of parallel or constant pressure systems?

Ans. They are used on practically all incandescent lamp and electric motor circuits, and on some arc lamp circuits.

Ques. Why is it specially applicable to incandescent lamp circuits?

Ans. Incandescent lamps cannot be made to stand a pressure much over 220 volts, and therefore have to be operated on low voltage systems.

Ques. What is the principal disadvantage of a parallel system as compared with a series system?

Ans. The greater cost of the copper conductors.

Fig. 785.—Arrangement of feeder and mains in parallel system. By locating the feeder at the electrical center, less copper is required for the mains. The cut does not show the fuses which in practice are placed at the junction of feeder and main.

Ques. What is the usual arrangement of parallel systems?

Ans. Conductors known as a feeder run out from the station, and connected to these are other conductors known as a main to which in turn the lamps or other devices are connected as shown in fig. 785.

Ques. In what two ways may feeders be connected?

Ans. They may be connected at the same end of the mains, known as parallel feeding, or they may be connected at the opposite end of the main, called anti-parallel feeding.

The main may be of uniform cross section throughout, or it may change in size so as to keep the current density approximately constant. The above condition gives rise to four possible combinations:

1. Cylindrical conductors parallel feeding, fig. 786;
2. Tapering conductors, parallel feeding, fig. 787;
3. Cylindrical conductors, anti-parallel feeding, fig. 788;
4. Tapering conductors, anti-parallel feeding, fig. 789.

Figs. 786 to 789.—Various parallel systems. Fig. 786, cylindrical conductors parallel feeding; fig. 787, tapering conductors parallel feeding; fig. 788, cylindrical conductors anti-parallel feeding; fig. 789, tapering conductors anti-parallel feeding. The term "tapering" is here used to denote a conductor made up of lengths of wire, each length smaller than the preceding length, the object of such arrangement being to avoid a waste of copper by progressively diminishing the size of wire so that the relation between circular rails and amperes is kept approximately constant. In an anti-parallel system, the current is fed to the lamp from opposite ends of the system.

Series-Parallel System.—This is a combination of the series and parallel systems, and is arranged as indicated in fig. 790. Several lamps are arranged in parallel to form a group, and a number of such sets are connected in series, as shown. It is not necessary for the groups to be identical, provided they are all adapted to take the same current in amperes, which should be kept constant, and provided the lamps of each set agree in voltage. For example, on the ordinary 10-ampere arc circuit, one group might consist of 5 lamps, each requiring 2 amperes at 50 volts; the next might be composed of 10 lamps, each taking 1 ampere at 100 volts, and so on.

Fig. 790.—Series-parallel system of distribution. It consists of groups of parallel connected receptive devices, the groups being arranged in the circuit in series.

Parallel-Series System.—In this method of connection, one or more groups of lamp are connected in series and the groups in parallel as shown in fig. 791.

Fig. 791.—Parallel-series system of distribution. It consists of groups of series connected receptive devices, the groups being arranged in the circuit in parallel.

Ques. When is a parallel-series system used?

Ans. When it is desired to operate a number of lamps or motors on a line where voltage is several times that required to operate a single lamp or motor.

The parallel-series system is employed chiefly in the lighting circuit on electric traction lines; here, usually five 110 volt lamps are connected to the source of supply which has a pressure of 550 volts.

Center of Distribution.—It is important to determine the point at which the feeders should be attached to the mains in order to minimize the amount of copper required. The method employed is similar to that used in determining the best location of a power plant as regards amount of copper required. The center of distribution may be called the electrical center of gravity of the system, and is found by separately obtaining the center of gravity of straight sections and then determining the total resultant and point of application of this resultant of the straight sections.

Feeders (feeding cables or conductors) are run from the source of supply to the distributing centers, and, as these feeders are in many cases of considerable length, a substantial loss of pressure generally occurs in them. The pressure at the source of supply, however, is so regulated as to compensate for the drop in the feeders, and the pressure at the distributing centers is thus kept constant; or the same result is obtained by the use of regulating devices in the feeders. The essential condition in most systems is that the pressure at the distributing centers shall be kept practically constant, irrespective of the load.

Edison Three Wire System.—In electric lighting systems used up to about 1897, it was not considered practicable to use incandescent lamps requiring a pressure exceeding 120 volts. This limited the operating voltage of parallel systems, and necessitated the use of conductors of large size and weight, especially where the current had to be transmitted a considerable distance.

The effect of this limiting voltage is more apparent when it is clearly understood that the size of wire required to carry a current depends upon the amperes and not upon the volts.

A wire capable of carrying a current of 10 amperes at 20 volts, can carry 10 amperes at 20,000 volts or any other voltage. Therefore, since the amount of electric energy or power transmitted through a conductor is equal to the amperes multiplied by the volts, it is clear that by increasing the voltage, the power transmitting capacity of a current can be almost indefinitely increased without increasing the size of the conducting wire. This is the reason why considerations of economy dictate the use of the highest voltages possible in long distance transmissions. The voltage of the current is determined, however, by the requirements of the apparatus to be operated.

Incandescent lamps usually require a pressure of 110 volts, and the current required by a 16 candle power lamp at that voltage is about ½ ampere. Therefore if the lamp be designed for a pressure of 220 volts, the current will be reduced to ¼ ampere, and the same size of wire could be used to feed twice as many lamps.

Figs. 792 and 793.—Evolution of the three wire system. Fig. 792 shows two dynamos supplying two independent circuits. These may be connected in series as in fig. 793, thus operating the two circuits of fig. 792 with two wires instead of four. To balance the system in case of unequal loading, a third or neutral wire is used as shown in fig. 794.

The saving of copper is the sole merit of the three wire system, and the object which led to its invention was to effect this economy with the use of 110 volt lamps.

Principle of the Three Wire System.—In fig. 792, two dynamos A and B are shown supplying two independent incandescent lighting circuits, each circuit receiving 3 amperes of current at a pressure of 110 volts. It is evident that the dynamos could be connected with each other in series, and the lamps connected in series with two each, as shown in fig. 793, thus making the two wires K and L of the two independent circuits unnecessary, as the pressure will be increased to 220 volts while the current will remain at 3 amperes, and each lamp will require ¼ ampere.

Fig. 794.—Balanced three wire system. The middle conductor, known as the neutral wire, keeps the system balanced in case of unequal loading, that is, a current will flow through it, to or from the dynamos, according to the preponderance of lamps on the one side or the other. These current conditions are shown in fig. 797.

The amount of copper saved will be 100 per cent., but this arrangement is open to the objection, that when one of the lamps is turned off, or burned out, its companion will also go out. This difficulty is avoided in the three wire system by running a third wire N, from the junction O, between the two dynamos, as shown in fig. 794, thus providing a supply or return conductor to any one of the lamps, and permitting any number of lamps to be disconnected without affecting those which remain. If the system be exactly balanced, no current will flow through the wire N, because the pressure toward the - terminal of the dynamo A, will be equal to the pressure from the + terminal of dynamo B, thus neutralizing the pressure in the wire. For this reason the middle wire of a three wire system is called the neutral wire, and is usually indicated by the symbol O or ± the latter meaning that it is positive to the first wire and negative to the second. If the system be unbalanced, a current will flow through the neutral wire, to or from the dynamos, according to the preponderance of lamps in the upper or lower sets. When the number in the lower set is the greater, the current in the neutral wire will flow from the dynamos as shown in fig. 797, and toward the dynamos under the reverse condition.

In the case represented in fig. 797, there are five lamps in circuit, requiring 2½ amperes of current at a pressure of 110 volts. The two lamps in the upper set will require 1 ampere, and the three lamps in the lower set, 1½ amperes. Since a pressure of 110 volts can force only a current of one ampere through resistance of the two lamps in the upper set, it is evident, that the additional ½ ampere required by the three lamps in the lower set will have to be supplied through the neutral wire, as shown.

Balancing of Three Wire System.—In practice it is impossible to obtain an exactly balanced system, as the turning on and off of lamps as required results in a preponderance of lamps in the upper or lower sets, and furthermore, even when the number of lamps in the two sets are equal, they may be located irregularly, thereby causing the currents to flow for short distances in the neutral line. Therefore, the larger the number of lamps in the circuit, the easier it will be to keep the system in a balanced condition.

Copper Economy in Three Wire Systems.—Theoretically, the size of the neutral wire has to be only sufficient to carry the largest current that will pass through it. A large margin of safety, however, is allowed in practice so that its cross section ranges from about one-third that of the outside line, in large central station systems, to the same as that of each outside line in small isolated systems.

If the neutral wire be made one-half the size of the outside conductor, as is usually the case in feeders, the amount of copper required is 5/16 of that necessary for the two wire system. For mains it is customary to make all three conductors the same size increasing the amount of copper to ⅜ of that required for the two wire system.

Fig. 795.—Dobrowolsky three wire system with self-induction coil. It consists of an ordinary direct current dynamo, the armature A and pole pieces N and S of which are shown. A self-induction coil D, is connected to two diametrically opposite points of the winding of the armature A. The coil D may be carried by and revolve with the armature; but in the construction represented, it is stationary, being connected to the armature winding through the brushes CC, rings and wires JJ. The middle point of the self-induction coil D, is connected to the neutral conductor O of the three wire system, the outside conductors + and - being supplied from the brushes BB in the usual manner. The pressure at the terminals of the coil D is alternating; hence the latter, on account of its self-induction, does not act as a short circuit to the armature. Furthermore, the inductances of the two halves of the coil D being equal, the pressure of the neutral wire O is kept midway between the pressures of the outside wires + and -. When the two sides of the system are unbalanced in load, the difference in current carried in one direction or the other by the neutral wire passes freely through the coil D, since the current is steady, or varies slowly, and is therefore unimpeded by the self-induction. It is evident that the ohmic resistance of D should be as low and its self-induction as high as possible, in order that the loss of energy and the difference in voltage on the two sides of the system shall be as small as possible under all conditions.

Modifications of the Three Wire System.—By the employment of suitable arrangements, it is possible to operate a three wire system with only one dynamo. Some of the various arrangements which have been used or proposed in this connection may be briefly mentioned as follows:

Three Wire Storage Battery System, in which a storage battery is connected between the two outside wires, and the pressure of the neutral wire varied to balance the system by shifting the point at which it is connected to the battery.

Three Wire Double Dynamo System, in which a double dynamo having two armature windings upon the same core, connected to two separate commutators, is used in the same manner as two separate dynamos connected in series.

Three Wire Bridge System, in which a resistance is connected across the two outside wires, and the neutral wire is brought to a point on the resistance through a movable switch. The pressures on the two sides of the circuit are equalized by adjusting the arm of the switch for any change of load.

Fig. 796.—Three wire compensator system. A and B are the compensators or equalizers. They consist of auxiliary dynamos coupled together and connected to the system as shown. D is the main dynamo, and E, a booster.

Three Wire Three Brush Dynamo System, in which the neutral wire is connected to a third brush on the dynamo.

Dobrowolsky Three Wire System, in which a self-induction coil is connected to two diametrically opposite points of the armature of an ordinary direct current dynamo. The principle of this system is illustrated in fig. 795.

Three Wire Auxiliary Dynamo System, in which the neutral wire is connected to an auxiliary dynamo which supplies a pressure one-half as great as that of the main dynamo. The auxiliary dynamo is usually belt driven by the main dynamo, and acts as a dynamo when the load is greater on the negative side of the circuit, and as a motor when the excess of load is on the positive side.

Three Wire Compensator System, in which two auxiliary dynamos A and B called compensators or equalizers, are coupled together and connected to the system as shown in fig. 796. Each compensator generates one-half as much pressure as the main dynamo D, and serves to equalize the pressure and the load, the compensator on the lightly loaded side operating as a motor and driving the other as a dynamo. When the system is exactly balanced, both compensators run as motors under no load, therefore, consume very little energy. In this arrangement only one booster E, is required for both sides of the system, as the compensators are connected to the outside wires at a point beyond the boosters, and therefore, sub-divide the increased difference of pressure equally between the two sides of the system.

Fig. 797.—Three wire double dynamo system having two separate windings on the same core and separate commutators A and B as shown.

Extension of the Three Wire Principle.—In order to attain still greater economy in copper, the principles of the three wire system may be extended to include four, five, six, and seven wire systems. The comparative weights of copper required by such systems are as follows:

Two wire system 1.000
Three " " all wires of equal size .370
Three " " neutral wire one-half size .313
Four " " all wires of equal size .222
Five " " " " " " " .156
Seven " " " " " " " .096

The four wire system requires about two-ninths as much copper, and the seven wire system about one-tenth as much copper, as an equivalent two wire system; but neither is desirable, as their operation involves too much inconvenience, too many unavoidable complications, and create a possibility of accident, which more than offsets the saving in copper.