[Illustration: 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 BATTERY 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: FIG. 798.—Diagram showing dynamotor connections when
used as an equalizer in the three wire system. DM, dynamotor; G,
generator side; M, motor side.]
=The Five Wire System.=—This system is employed advantageously in many
places in England and Europe, but has not as yet been introduced to any
extent in America. It is very probable that in the future the three
wire 440 volt system will be selected in preference to the five wire
system.
=Dynamotor.=—This is a combination of dynamo and motor on the same
shaft, one receiving current and the other delivering current, usually
of different voltage, the motor being employed to drive the dynamo
with a pressure either higher or lower than that received at the motor
terminals.
The dynamotor in the direct current circuit corresponds to the
transformer in the alternating current circuit.
[Illustration: FIG. 799.—Diagram showing connections of balancing set
in three wire one dynamo system. The set consists of a motor and dynamo
connected, and its operation is practically the same as a dynamotor.]
=Ques. How is the dynamotor used as an equalizer in the three wire
system?=
Ans. When thus used, the machine is connected as in fig. 798. When
both sides of the system are balanced, there will be no current in the
neutral lead N, and a small current will pass through the two armature
windings of the dynamotor in series, both armatures acting as motors.
If the load on one side of the system become larger than the load on
the other side, there will be a greater drop in the leads connected to
the overloaded side and consequently a lower voltage will exist over
the larger load than exists over the smaller load. The armature winding
of the dynamotor connected to the higher voltage will act as a dynamo,
whose pressure will tend to raise the voltage of the more heavily
loaded side.
The direction of the currents in an unbalanced three wire
system that is being supplied with energy from a main dynamo is
shown in the figure. The commutator at G is connected to the
dynamo winding of the dynamotor and is supplying current to the
upper or larger load, and the lower commutator is connected to
the motor winding of the dynamotor and is taking current from
the lightly loaded side.
=Motor-Dynamo or Balancing Set.=—A balancing set or balancer consists
of a motor mechanically connected to a dynamo used to balance a three
wire system. The operation of such a combination is practically the
same as the dynamotor just described. The balancer is connected as
shown in fig. 799.
[Illustration: FIG. 800.—Holzer-Cabot type M motor-dynamo set. This
combination is known as a booster, and is used to raise or lower the
voltage on feeders. The motor is series wound and connected in series
with one leg of the feeder. Thus, the voltage which the booster will
add to the line will be directly in proportion to the current flowing
in the feeder. The regulation is therefore automatic.]
When an unbalanced load comes on, the voltage on the lightly loaded
side rises and on the heavily loaded side drops. The machine on the
light side then takes power from the line and runs as a motor driving
the machine on the heavy side as a dynamo, supplying the extra current
for that side. This action tends to bring the voltage back to normal
and gives good regulation.
In some cases the field of each machine is connected to the
opposite side of the system which gives a quicker action. This
regulation is automatic and the set takes care of unbalanced
loads in either direction without adjustment.
=Balancing Coils.=—Another method of balancing a three wire system
which does away with any additional rotating machines makes use of
balance coils.
=Ques. Describe the type of dynamo used with balancing coils?=
[Illustration: FIG. 801.—Diagram showing connections of balancing coil
system. The dynamo used in this system is provided with both commutator
and collector rings.]
Ans. The regular two wire dynamo is used supplying power to the outside
wires, but there are collector rings connected to the armature. These
rings are much lighter than they would be for a converter as they carry
only about ⅛ of the dynamo load. These rings being light are usually
placed at the end of the commutator and are connected directly to the
commutator bars.
=Ques. How are the balancing coils constructed?=
Ans. They are built of standard transformer parts, and are placed in
cases similar to those of ordinary small transformers.
[Illustration: FIGS 802 and 803.—Distribution by dynamo-motor sets.
Fig. 802, sets in parallel; fig. 803, sets in series. In fig. 802,
current produced by the main dynamo G, is carried to the machines by
the conductors A and B to which the motor portions M are connected
in parallel. These motors are provided with shunt wound field coils
which may be connected to the primary or to the secondary circuit,
consequently the machines run at a practically constant speed. The
dynamo portions D of the transformers are connected to the secondary
circuits which supply the lamps, etc., L, as indicated. The field
magnets of these dynamos may also be fed by the main circuit AB, or
they may be self-excited by shunt or compound winding. In fig. 803, the
motors M are all connected in series with the main dynamo G, and the
dynamo elements D of the transformers, connected to the lamps, etc.,
L. If the current be kept constant (the dynamo G having a regulator
like a series arc dynamo), and the motors M are simple series wound
machines, they will exert a certain torque, or turning effort, which
will be constant. It follows, therefore, that if the dynamos D be
also series wound, each will generate a certain current which will
be constant. If lamps or other devices, designed for that particular
current, be connected in series on the secondary circuits, the dynamos
D will always maintain that current, no matter how many lamps there
may be. When lamps are added, the resistance of the local circuit is
raised, and the current in it decreases, so that the dynamo increases
its speed until it generates sufficient pressure to produce practically
the same current as before. Hence this constitutes a system which is
self-regulating, when lamps, etc., are cut in or out of the secondary
circuits. No harm results even if the secondary be short circuited,
since only the normal current can be generated. But if the secondary
circuit be opened, then the machine will race, and probably injure
itself by centrifugal force, because the torque of the motor M has its
full value, and there is no load upon the dynamos D. To guard against
this danger, some automatic device should be provided to short circuit
the field or armature of the motor when its speed or reverse voltage
rises above a certain point.] The coil has a straight continuous
winding, both ends and a connection from the middle point of the
winding being brought out of the case.
=Ques. How are the coils connected to the dynamo?=
Ans. Two coils are used and are connected to the collector rings as
shown in fig. 801, one coil across each phase. The connections from the
middle points of the coils are connected together and to the neutral
wire of the system.
[Illustration: FIG. 804.—Diagram to show correctness of balancing coil
connection. In the figure, AE, BF, CG, and DH represent the balance
coil and its connection for different positions of the armature of a
bipolar machine.]
=Ques. What is the action of the coils in equalizing the load?=
Ans. On balanced load, the coils take a small alternating exciting
current from the collector rings as any transformer does when connected
to an alternating current line with its secondary open. When an
unbalanced load comes on, the current in the neutral divides, half
going to each coil. This enters the coil at the middle point and half
flows each way through the coil and the slip rings into the armature
winding. The unbalanced current is thus fed back directly into the
dynamo armature continuously.
The coils are small and can be placed back of the switchboard
or below the floor, as they require no attention. The current
flowing to each slip ring is 25% of the direct current in the
neutral wire with the small exciting current taken by the coil
added.
The coils are usually built to take care of current in the
neutral equal to 25% of full load current of the dynamo with a
voltage regulation not to exceed 2 per cent.
=Ques. Upon what does the operation of the balancing coil system
depend?=
Ans. It depends on the following points: First, the _impedance_[1] of
the coils keeps the exciting current which they take from the collector
rings down to a small value as it is alternating current. At the same
time the current from the neutral wire flows through the four half
coils in parallel, and being direct current is impeded only by the
ohmic resistance of the coils, which is low, giving only a slight loss
in the coils. The common point to which the neutral wire is connected
must at all times be neutral to the - and + direct current brushes.
[1] NOTE.—The term _impedance_ means the total opposition in an
electric circuit to the flow of an alternating current, being made up
of the actual or ohmic resistance and the apparent resistance due to
self-induction, or if the circuit contain also capacity, the resultant
apparent resistance due to self-induction and capacity.
That this common point is at all times neutral is readily
shown. Referring to fig. 804, let AE, BF, CG and DH represent
the balance coil and its connection for different positions
of the armature of a bipolar machine. Let O be the tap to the
middle point of the winding.
Take the instant when the balance coil taps are directly
under the direct current brushes as shown at position AE. It
is evident that since the point O is the middle point of the
coil, it is neutral between A and E. When the armature turns so
that the balance coils take the position BF, the voltage drop
between A and E may be divided into 4 parts, AB, BO, OF and FE.
As in the first instance, O is neutral between the ends of the
coil, and the voltage drop over OF equals that over OB.
Since the space AB includes the same number of armature coils
as space FE and they are in fields of equal strength, the
voltages across the two spaces will be equal, and the voltage
over AB equals that over FE. Then adding equals: AB + BO = FE +
FO and O is neutral between A and E as in the first case.
In the same way it can be shown that O is neutral between the
direct current brushes for any position of the balance coil
taps. One coil will operate the system, but two coils, giving
four points spaced 90 electrical degrees apart, give better
distribution of the current to the armature winding and better
regulation of the voltage.
=Boosters.=—A booster may be defined as, _a dynamo inserted in a
circuit at a point when it is necessary to change the voltage_. A
booster is generally driven by a motor, the two armatures being
directly coupled, although boosters are sometimes driven from the
engine or line shaft.
[Illustration: FIG. 805.—Crocker Wheeler motor-dynamo set. There are
numerous cases where such a combination is useful for furnishing a
circuit with a voltage different from that of the main plant or with a
voltage that can be varied independently. For storage battery charging
and electrolytic work, where constant current is desirable, it forms a
simple means of voltage regulation. Where a circuit of special voltage
is required, the set not only supplies current at the desired pressure,
but insulates the special circuit, which may be subject to more severe
requirements than the main system. The advantage of the three wire
distribution can be obtained from any two wire dynamo by means of a
small rotary balancer or balancing transformer, which consists of two
direct current machines of the same voltage, mechanically connected
together with their armatures in series. Multiple voltage systems for
speed regulation can also be obtained by a similar arrangement.]
=Ques. Explain the use of a booster?=
Ans. When a number of feeders run out from a station, the longest
and those carrying the heaviest loads will have so much drop on the
line that the pressure at distant points is too low. It is therefore
necessary to raise the pressure to compensate for the drop and this is
done by inserting a booster in the circuit.
It would not be economical to raise the voltage on all the
lines by supplying current from the main dynamo at higher
pressure, hence the voltage is raised only on the lines which
need it by means of the booster working in series with the main
dynamo.
[Illustration: FIG. 806.—Diagram showing use of auxiliary bus bar. In
order to avoid the necessity for boosters, some stations have an extra
bus bar, which is kept at a higher pressure than the main bus, and to
this are connected the feeders that have an extra large drop.]
=Ques. For what other service are boosters employed?=
Ans. They are used in connection with storage battery plants for
the purpose of raising the voltage of the bus bars to the pressure
necessary for charging storage batteries.
=Ques. What is an auxiliary bus bar?=
Ans. An extra bus bar which is kept at higher pressure than the main
bar.
=Ques. What is the object of an auxiliary bus bar?=
Ans. It is used in place of a booster as shown in fig. 806. One or more
dynamos maintain the pressure between the auxiliary bar and the common
negative bar. The feeders which need boosting are connected to the
common negative bar and the auxiliary bar as shown.