STORAGE BATTERY SYSTEMS
Storage batteries are used for many purposes, such as to supply current
for electric vehicles, gas engine ignition, lighting, and in connection
with power stations and distribution work.
* * * * *
The latter is an important field, the storage battery being used in
connection with the power station for the following purposes:
1. To carry the peak load, during hours of maximum demand;
2. To carry the entire load during hours of minimum demand, or for a
short time in case of emergency;
3. To act as an equalizer;
4. For regulation of load and voltage;
5. As compensation for feeder drop;
6. As a preventive against shut downs.
In almost every electric lighting plant there are long periods during
the day and late at night when the number of lamps lighted is so small
that it may not pay to run the generating machinery. In such cases,
storage batteries may usually be used to advantage to aid in carrying
the maximum load and to supply the entire current at minimum load as
illustrated in fig. 1,179. In other words, batteries are substituted
for a certain portion of the machinery plant or are used in place of
the latter.
=Ques. What provision must be made in power plants when storage
batteries are not used?=
Ans. The capacity of the generating machinery must be sufficient
for the heaviest overloads which may occur, and it must be operated
continuously for 24 hours a day in the majority of central stations
supplying current for lighting and power.
=Ques. What results are obtained with this method of working?=
Ans. The engines working under very variable loads, not only operate
at low efficiency, but are continually subjected to severe mechanical
strains.
[Illustration: FIG. 1,179.—Load curve showing use of storage battery
as an aid to the generating machinery. In the diagram, it is seen that
the battery discharges at minimum and maximum loads and is charged at
other times, the battery furnishing current for the entire minimum load
and part of the maximum load.]
=Ques. How may greater efficiency be secured with steam engines under
variable loads?=
Ans. Judicious selection of the number and sizes of the engines enable
them to be worked in most cases at a considerable fraction of their
full capacity nearly all the time.
=Ques. What further improvement is secured in most cases with the
storage battery?=
Ans. The plant is made more flexible, and the economy of the engines
is increased by making their loads nearer uniform, and nearer to full
capacity while they are running.
=Ques. What is the effect of a battery connected in parallel with a
dynamo, as in fig. 1,180?=
Ans. It is not necessary for the dynamo to have a capacity exceeding
that which is sufficient for the average daily load, at which it may be
worked practically all the time.
[Illustration: FIG. 1,180.—Storage battery connected in parallel with
a dynamo. This arrangement enables the dynamo to be stopped for a
considerable portion of the time, and thus save labor and attention. It
also acts to prevent fluctuations as in a dynamo driven by a gas engine
whose speed varies periodically because of the nature of its cycle of
operation.]
When the load is below the average, the dynamo charges the
battery, and when the load rises above the average, during the
hours of maximum demand, the battery discharges into the line
in parallel with the dynamo. During the hours of minimum demand
the engines may be shut down and the necessary current supplied
from the battery alone, thus not only increasing the efficiency
of the plant, but serving to maintain a steadier pressure under
fluctuating loads.
=Ques. What is understood by the expression "floating the battery on
the line"?=
Ans. A storage battery is said to _float_ on a line when connected
across the circuit at some distance from the power station, so that
a heavy load on the line, within the range of the battery influence,
causes sufficient line drop to allow the battery to discharge, while
with a light load on the line, the drop is small and the impressed
voltage at the battery high enough to charge the battery. This usage
is confined chiefly to electric railway service, where large voltage
changes are permissible.
[Illustration: FIG. 1,181.—Diagram showing effect of storage battery
in regulating the dynamo load in a combined railway and lighting plant.
In this case the average and line loads are about equal and the battery
covers the instantaneous fluctuations. It will be noted that while the
line load fluctuations vary between 780 and 1,420 amperes, those of the
dynamo load are kept at an average between 1,030 and 1,160 amperes.]
=Ques. When the battery is floated on the line, how may the amount of
charge be made to approximately equal the amount of discharge?=
Ans. By properly proportioning the number of cells in series.
=Connections and Circuit Control Apparatus.=—When a storage battery is
used in an electric lighting plant, provision must be made for feeding
the lamps, etc., from either the dynamo or battery separately, or from
the two working in parallel, and it should be possible to charge the
battery at the same time the lamps are being supplied. To accomplish
these results requires three switches, for the following connections:
1. To connect the lamps to the dynamo;
2. To connect the lamps to the battery;
3. To connect the battery to the dynamo.
[Illustration: FIG. 1,182.—Diagram showing action of storage battery
as a reservoir of reserve power. The figure shows an actual load curve
from an Edison station for 24 hours. A sudden storm caused the load
to be thrown on very quickly, the peak of the load being higher than
usual.]
In some plants, the first switch is omitted, because the lamps are
always fed by the battery alone, the latter being charged during the
day, when no lamps are in use.
It is desirable, however, to have all three switches in every plant in
order to be able to supply lamps and charge the battery at any time.
In the battery circuit there should be an ammeter having a scale on
both sides of zero, to show whether the battery is being charged
or discharged, as well as the value of the current. Another similar
ammeter is required in the circuit between the dynamo and the battery,
to show the direction and amount of current. A third ammeter is
desirable in the lamp circuit, to show the total current supplied to
the lamps, but it need only indicate on one side of zero, since the
current there always flows in the same direction.
[Illustration: FIG. 1,183.—Diagram showing three wire system with one
dynamo and storage battery. A 220 volt dynamo charges a storage battery
of corresponding pressure, which in turn subdivides the pressure and
supplies a three wire system, the neutral wire of which is connected to
the middle point of the battery as shown.]
A voltmeter is required with a three-way switch to connect it to the
dynamo, battery or lamps, and a circuit breaker must be inserted in the
battery circuit in order that it may be opened when the current becomes
excessive.
A discriminating cut out or reverse current circuit breaker is required
between the dynamo and the battery to open the circuit when the
charging current falls below a certain value, and thus avoid any danger
of the battery discharging through the dynamo, if from any cause the
voltage of the latter drop below that of the battery. This completes
the ordinary measuring and circuit controlling apparatus employed with
storage batteries.
=Methods of Control for Storage Batteries.=—As the external voltage
of a storage battery varies with the amount of charge it contains and
with the direction of the current, it is necessary to employ some means
for compensating this variation in order to maintain a constant voltage
on the line supplied by the battery. The various devices used for this
purpose are as follows:
1. Variable resistances;
2. End cell switches;
3. Reverse pressure cells;
4. Boosters.
[Illustration: FIG. 1,184.—Diagram showing connections for ignition
outfit. The charging switch has four indications—"Off," "Battery,"
"Dynamo" and "Charge." When engine is at rest switch is turned to
"Off." The first turn brings it to "Battery," enabling the engine to be
started. Next turn cuts battery off and puts "Dynamo" direct on engine.
The next turn brings the switch to "Charge." Dynamo then charges the
battery and surplus current is stored up. Next turn is "Off," which
stops engine and disconnects battery from dynamo. Test the dynamo
wires with test paper (negative makes mark). Put positive of dynamo to
positive of battery. Dynamo should be regulated to charge at about four
amperes.]
The particular method selected will depend upon the size of the
battery, the purpose for which it is used, the allowable limits pf
current and voltage variations, the cost of the system, etc.
=Variable Resistance.=—Regulation by variable resistance may be used
advantageously only with batteries of small capacity, and in small
lighting plants such as those of yachts, where the space available for
battery auxiliaries is limited, and where the cost of energy is so low
that the loss of power in the resistance is not objectionable.
[Illustration: FIG. 1,185.—Variable resistance method of regulation
for storage battery; diagram showing connections for charging two
halves of a battery in parallel.]
The connections for one of the simplest methods is shown in fig. 1,185.
The battery is divided into two halves, which are connected in series
for discharging and in parallel for charging. Since the voltage of each
cell at the end of a discharge should not be lower than 1.8 volts, a
battery intended for use on a 110 volt lighting circuit will require
110 ÷ 1.8 = 62 cells. The voltage necessary, however, for each cell at
the end of a charge is about 2.6 volts, or a total of 2.6 × 62 = 161
volts for the battery, a value which is far above the line voltage. By
dividing the battery into two halves and connecting them in parallel
only 80.5 volts are necessary for charging. The excess voltage of the
line, 29.5 volts is taken up by the resistance, which also controls the
output of the battery on discharge.
=End Cell Switches.=—These may be used to advantage in small
installations where there is not demand for current during the day, or
where the charging is done by means of _boosters_.
[Illustration: FIG. 1,186.—Diagram of connections of a battery
equipment for a residential lighting plant. In the diagram the
voltmeter and voltmeter connections have been omitted. The bus bars on
the battery panel are connected directly to the bus bars on the dynamo
panel. In this installation the dynamos are run during the afternoon on
discharge, being regulated by means of an end cell switch. On charge,
the pressure above that of the bus bars, required to bring all cells
up to full charge, is supplied by means of a motor driven charging
booster, the voltage at the armature being suitably varied by changing
the field excitation.]
=Ques. What is an end cell switch?=
Ans. A form of switch employed in connection with a storage battery in
order to control the end cells for regulating the voltage.
=Ques. Describe the construction of an end cell switch.=
Ans. This is shown in fig. 1,187. The switch contact arm is made in
two parts, A and B, which are insulated from each other as shown, and
connected with each other through the protective resistance R. The end
cell contacts are so spaced that when the main current carrying part
A of the switch arm is squarely on one end cell contact such as X,
the part B, does not touch any other contact such as Y, but when the
switch arm is advanced for cutting into circuit another end cell, the
part B, reaches the contact Y before the part A, leaves the contact X,
thus keeping the battery circuit closed, while the resistance R, limits
the current in the short circuited cell at the instant the switch arm
passes from one end cell contact to the next.
[Illustration: FIG. 1,187.—Diagram of end cell switch. This form of
switch controls several cells at one end of a storage battery and
is used for regulating the voltage. The requirement of an end cell
switch is that in switching from one end cell contact to another, the
discharging circuit must not be opened, neither must the moving arm
touch one contact before leaving the one adjacent, since the joining
of two contacts will short circuit the cells connected thereto. To
accomplish this, the spacings of the two arms and contacts are such
that when the main arm A is squarely on an end cell contact, the
advance or auxiliary arm B touches no other contact, but in passing
from one point to the next, the advance arm reaches the contact
toward which it is moving before the main arm leaves its contact. The
resistance X, between the two points prevents short circuiting, and the
current to the main circuit is never broken.]
=Ques. How should the conductors joining the end cells to the end cell
switch contacts be proportioned?=
Ans. They must have the same sectional area as the conductors of the
main circuit.
The reason for this is that when any end cell is in use, the
conductor connecting it to the switch becomes a part of the
main circuit. An allowance of 1,000 amperes per sq. in., when
the battery is discharging at the two-hour rate, is considered
good practice.
=Ques. Describe some of the features of end cell switch construction.=
Ans. Those of small capacity are made circular; the larger sizes are
made horizontal in form, and both types may be either operated by hand
or motor driven.
[Illustration: FIG. 1,188.—End cell switch control for storage
battery; connections showing main line open when the battery is being
charged.]
=Ques. Where are end cell switches of large capacity located?=
Ans. Generally they are placed as near the battery room as possible to
avoid the cost of running the heavy conductors, and when such switches
are motor driven, the usual practice is to control their operation from
the main switchboard.
In fig. 1,188 is shown the method of regulation with an end
switch. The diagram shows the battery being charged with the
main switch open, and the voltage of the dynamo raised to the
charging pressure. During discharge the cells are connected
in series, and as the voltage of each cell at the beginning
of discharge is at least 2.1 volts, only 52 or 53 cells are
required to give the desired pressure of 110 volts, but as the
discharge continues, and the voltage of each cell decreases,
the end cells, 1, 2, 3, 4, etc., are cut into circuit
successively by means of the end cell switch, thereby adding
to and compensating the drop in the total voltage until, at
the end of discharge when the voltage of each cell has fallen
to 1.8 volts, the entire 62 cells are in series to supply the
required line pressure.
[Illustration: FIG. 1,189.—Diagram of connections arranged for
charging battery in two parallel groups and discharging in series, the
charge and discharge being controlled by variable resistances. In yacht
lighting the limited space generally prohibits the use of a charging
booster, and in such instances this method of charge and discharge
control is the usual practice. In case the dynamo from which the
battery is charged has sufficient range in voltage to charge all cells
in series, a charging booster is not required, nor is it necessary
to connect groups of cells in parallel, as the dynamo voltage may be
varied as charge proceeds.]
For a 110 volt circuit, the number of cells required is 110 ÷
1.8 = 61, and the number in series when the battery begins to
discharge is 110 ÷ 2.1 = 52. Hence, in a 110 volt circuit an
arrangement must be provided whereby 61 - 52 = 9 cells may be
cut out or switched in, one by one.
The number of end cells for any voltage may be obtained by the
following formula:
Number of end cells = (E/1.8) - (E/2.1)
E = voltage of supply circuit;
1.8 = minimum voltage of cell during discharge;
2.1 = voltage of fully charged cell.
=Reverse Pressure Cells.=—These consist of unformed lead plates
immersed in the ordinary electrolyte of dilute sulphuric acid. As
they have no active material, they possess no capacity, but are
capable of setting up an opposing pressure of about 2 volts each to
the discharging current flowing through them, thereby cutting down
the total voltage of the battery, so that the net voltage across the
line depends on the number of reverse current cells in series in the
battery circuit. As the voltage of the battery falls during discharge,
the reverse pressure cells are cut out, successively, thus keeping the
external or line voltage constant.
[Illustration: FIG. 1,190.—Regulation with reverse pressure cells.
These cells are merely lead plates placed in an electrolyte of dilute
sulphuric acid. They have no capacity but set up an opposing or reverse
voltage of approximately 2 volts per cell if current be passed through
them. In using these cells for controlling discharge, the total number
of active cells in the battery will be the same as if the method of
end cell control had been used. Reverse pressure cells represent an
increase in equipment of about 8 per cent. or more. These cells, as
shown, are connected in the circuit in opposition to the main battery,
and conductors are run from each of them to points on a switch similar
to an end cell switch. At the beginning of discharge, all the reverse
cells are in circuit, acting in opposition to the main battery. As
discharge proceeds and the battery voltage falls, the reverse cells
are gradually cut out of circuit. The only advantage in this method of
regulation is that the discharge throughout the battery is uniform, but
this fact alone does not warrant such means of regulation on account of
the additional expense involved, and the energy loss when discharging
against reverse cells is the same as if resistance had been placed in
the circuit.]
It is obvious, that as these cells do not possess any capacity, the
number of active cells required in the battery will be the same as
when end cell control is employed. Therefore, the reverse pressure
cells represent an increase in equipment, which entails an additional
expense of at least 8 per cent. For this reason, and also on account
of the fact that the amount of energy lost in discharging against
reverse pressure cells, is the same as when the resistance methods
of controlling the discharge are employed, the use of cells for this
purpose is now practically obsolete.
[Illustration: FIG. 1,191.—Holzer-Cabot dynamotor (type K). A
dynamotor is a combination of dynamo and motor on the same shaft,
one receiving 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. A machine of the dynamotor
form, with its windings exactly alike, is often used in three wire
systems to balance or equalize the two halves of the circuit as in fig.
798.]
=Boosters.=—In general, a booster may be defined as _a dynamo inserted
in series in a circuit, to change its voltage_. It may be driven by an
electric motor, in which case it is sometimes called a _motor-booster_.
The function of a booster is to add to an electric pressure derived
from another source.
For instance, if a storage battery be used in conjunction with
one or more dynamos to supply current to an electric light
installation, the battery cannot be charged from the machines
which are feeding the lamps, because it requires a pressure
higher than that required for the lamps to complete the
charge. A small dynamo is therefore connected in series, with
the main machines and the battery, acting in conjunction with
the former to provide the necessary pressure.
[Illustration: FIG. 1,192.—Dayton launch lighting outfit. It consists
of an "Apple" dynamo, switchboard and storage battery. The dynamo is
fitted with a bevel friction drive governor. The dynamo gives a three
ampere charging rate on a six volt battery at its normal speed of 1,050
R. P. M. The switchboard is provided with a combination volt-ammeter
which shows the voltage of the battery, the ampere charging rate
of the dynamo and the ampere discharging rate of the battery. The
automatic cut out in the back of the switchboard automatically severs
the connections between the dynamo and the storage battery when the
engine stops and so prevents the storage battery current running back
through the dynamo when the dynamo is not generating current. A 6 volt,
60 ampere hour battery, consisting of 3 five plate units connected in
series, is used with the size dynamo shown in the illustration.]
The power for running such a dynamo is obtained in various ways.
The dynamo or charging booster may be belt driven or arranged on an
extension of the armature shaft of the main dynamo; again, it may
consist of a single armature with a double winding (fig. 1,191), or a
motor and dynamo coupled together on one bed plate as in figs. 800 and
805. Boosters may be divided into several classes as follows:
1. Series boosters;
2. Shunt boosters;
3. Compound boosters;
4. Differential boosters;
5. Constant current boosters;
6. Separately excited boosters.
=Series Boosters.=—The series booster acts so as to compound the
battery, and tends to maintain a constant voltage on the line, whatever
the load may be. Its operation depends on the fact that the dynamo
voltage must rise and fall with the load. It can, therefore, be used
only with a shunt dynamo or its equivalent as the source of supply.
=Ques. What use is made of the series booster system?=
Ans. It is suited to power, but not to incandescent lighting purposes,
being similar in operation to a floating battery. It is not extensively
used as the other types give better service, under the same conditions.
[Illustration: FIG. 1,193.—Diagram of Joseph Bijur's storage battery
system (General Storage Battery Co.). The booster field winding has
one terminal connected to the middle point of the battery and the
other terminal, to the wire joining the resistances A and B. A lever,
pivoted at L, carries at either end a number of contact points which
dip into troughs of mercury when one end of the lever moves upward
or downward. These points are connected to corresponding points on
their respective resistances, and therefore all of the resistances
connected to contact points which are immersed in the mercury are short
circuited. The points are of various lengths, so that when the lever
operates, they contact progressively with the mercury. If more of the
A points than the B points be immersed in the mercury, the resistance
of B is less than that of A, more sections of it being short circuited.
Current will therefore flow from the middle point of the battery,
through the booster field, and through B to the negative side of the
system, exciting the booster field and producing a booster voltage
to charge the battery. Again, if more of the A points be immersed,
the A resistance becomes the smaller, and current then flows from the
positive side of the system through resistance A, through the booster
field to the middle point of the battery, the field excitation and the
booster pressure produced being in a direction opposite to the first
described, and tending to discharge the battery. When the resistances
A and B are equal, there is no pressure to send current in either
direction through the booster field coil. When the load on the external
circuit is normal, the lever is in a horizontal position, A and B being
equal, no current flows through the booster field hence, no current
passes into or out of the battery. With increase of external load, the
pull of the solenoid is strengthened by a small increase in dynamo
current passing through the winding. This draws down the left end of
the lever producing a current in the booster field such as to discharge
the battery and assist the dynamo to supply the load demand. A decrease
in external load is attended by a slight diminution in dynamo current,
the solenoid is weakened and the pull of the spring predominates. This
results in a downward movement of the right side of the lever causing
excitation of the booster field to produce a pressure to send charge
into the battery.]
=Ques. Describe some characteristics of the series booster.=
Ans. It is automatic and adjusts its voltage to produce the proper
ratio of charge or discharge with varying external load, and it also
tends to maintain a constant voltage across the line, under all
conditions of change in circuit.
[Illustration: FIG. 1,194.—Load diagram, showing kind of service to
which the shunt booster is adapted.]
=Shunt Boosters.=—This type of machine is simply a shunt dynamo,
having its armature circuit in series with the line from the main
dynamo to the battery. A rheostat controls the field excitation. Its
function is to send charge into the battery. It is used in plants
where the battery is not designed to take up load fluctuations, but is
in service only to carry the peak of the load, being charged during
periods of light loads and discharged in parallel with the dynamo.
The shunt booster acts to increase the voltage applied to the battery
so that the charging current will flow into the latter.
=Ques. How is a battery used with a shunt booster proportioned?=
Ans. Usually sufficient battery is provided to carry the entire load
during the light load period.
=Ques. Explain the use of the rheostat controlling the field
excitation.=
Ans. It is used to vary the booster voltage so as to hasten the
charging of the battery if desired.
[Illustration: FIG. 1,195.—Entz' carbon pile booster system (Electric
Storage Battery Co.). The booster field winding is connected at one
end to the middle point of the battery. The other end is connected to
the upper contact points of two carbon pile resistances, A and B. The
lower end of A is connected to the negative side of the battery, and
the corresponding end of B, to the positive side. This arrangement
constitutes in effect a potentiometer. If the resistance of A be
equal to that of B, there is no pressure in the booster field to
establish current through it. The drop through A + B is equal to the
total battery voltage, and if A = B, the drop from either side of the
battery through A or B is one-half the total drop, hence the end of the
booster field winding, connected to the upper ends of A and B is also
at the pressure of the middle point of the battery which is likewise
the pressure of the other side of the booster field coil. Accordingly
when A = B, there can be no current through the coil. When the two
resistances are unequal, there will be current through the booster
field, its direction depending on which of the resistances is the less,
and its magnitude will be proportional to the difference between the
two resistances. Variations in the pressure on a carbon pile causes
variations in its resistance and the solenoid, M, opposed by spring S,
both pulling on lever L which rests on the two piles A and B, controls
the relative resistances of the two piles to cause charge and discharge
of the battery. The solenoid winding is in series with the dynamo
circuit and when the load is normal, the spring pull is just equal to
the magnet pull, and the resistance of A and B are equal. When external
load varies, a small but proportional variation in the pull of P
charges the relative resistances of the piles and the booster field is
energized to produce a voltage to cause battery charge or discharge.]
=Ques. For what service is the shunt booster not suited?=
Ans. It is not adapted to circuits where there are sudden fluctuations
that are great compared with the capacity of the dynamo.
=Ques. What is its action in changing from charge to discharge?=
Ans. It is not automatic, the switching must be done by hand.
[Illustration: FIG. 1,196.—Diagram showing usual connections of a
non-reversible shunt booster and battery system. In charging, the
switches A and B are closed, and C put on contact _m_; the end cell
switch D is put on the last contact. Part of the dynamo current will
go into the line and part through the booster into the battery. The
charging current is adjusted by the field rheostat E. To discharge,
throw the end cell switch D to first contact; next turn switch C to
contact _s_. The battery is then in parallel with the dynamo with all
end cells cut out. As the voltage of the battery falls, end cells are
cut in by the end cell switch D.]
=Ques. How may it be used reversibly?=
Ans. It will give a pressure to assist the battery to discharge when
excited from the bus bars and provided with a reversing rheostat.
In this case it will assist the battery to discharge when
the direction of the field magnetization is changed. When so
used, no end cells are necessary, but the booster must be run
continuously during the entire period of discharge.
=Ques. What should be the battery capacity on a 110 volt circuit with a
reversible booster?=
Ans. 56 cells will be sufficient.
The voltage to fully charge is 56 × 2.6 = 146, or 36 volts
above dynamo voltage. Minimum voltage of discharge = 1.8 × 56 =
100 volts, or 10 volts less than that of the line. Hence, the
booster need give only 36 volts maximum, and is required to
add 10 volts to the battery voltage toward the end of battery
discharge. In this case, the booster voltage is only 36/49 or
about ¾ of that required in the preceding case; five cells
less of battery are necessary and the end cell switches and
leads are eliminated.
[Illustration: FIG. 1,197.—Diagram of compound booster connections.]
The machine will be larger, however, than it would be if used
only for charging, because the discharge current is unusually
greater than that of charge, and the current carrying of the
armature must be great enough to take care of the heaviest
currents.
=Compound Boosters.=—These machines are used on railway and power
circuits where there are great fluctuations in load, the battery acting
to prevent excessive drop and to assist the generating machinery in
carrying the load, relieving it from the strain of sudden rushes of
current.
The connections are shown in the diagram fig. 1,197. Under ordinary
working conditions, the shunt field of the booster creates an electric
pressure in the same direction as that of the battery, tending to
discharge it.
[Illustration: FIG. 1,198.—Fairbanks-Morse lighting outfit. The above
cut illustrates a 2 horse power vertical special gasoline or kerosene
oil engine belted to a .9 kw. compound wound 32 volt dynamo. It will
supply a maximum of 42-20 watt, or 50-15 watt 32 volt Tungsten lamps
and is built and balanced, so that current can be taken direct from
the dynamo without flicker in the lights. The storage battery has 16
cells and a capacity of 4½ amperes for 7½ hours at 32 volts. This
will supply seven 20 watt Tungsten lamps for 7½ hours, or nine 15
watt lamps for 7½ hours. The switchboard is arranged so as to give
24 hours service. It is customary to run the engine during most of the
lighting period and to use the battery for lights late at night. If the
whole number of lights be not used when the engine and dynamo are in
operation, the surplus is used to charge the battery.]
When no current is flowing into or out of the battery, the following
relation exists:
Dynamo voltage = booster voltage + battery voltage
In this case the dynamo carries the whole external load. If the load
increase, the dynamo voltage decreases, so that the booster voltage
+ battery voltage is greater than the dynamo voltage, and the battery
begins to discharge.
In discharging, the current passes through the series field of the
booster and produces a proportional pressure acting with the shunt
field to raise the voltage of the booster, thus increasing the battery
discharge and shifting more of the load from the dynamo, until the
system becomes balanced.
[Illustration: FIG. 1,199.—Diagram showing method of charging a
storage battery at one voltage and supplying lights at a different
voltage. As may be seen, two end cell switches are required. The
voltage of the supply current is adjusted by the number of cells in
series on switch S', while switch S is moved to cut out cells as they
become fully charged. In this instance the end cells included between
the contact arms of the two end cell switches must be of sufficient
size to receive the charging current, plus the current to the supply
circuit. If the battery can be charged at times when the dynamo is
supplying no other load, only one end cell switch is required.]
If the load on the external circuit be small, the dynamo
voltage rises and current flows into the battery. In this case
the series field acts against the shunt field and decreases the
booster voltage so that the pressure at the dynamo is greater
than booster and battery voltage combined, thus increasing the
rate of charge of the battery until the load causes the dynamo
voltage to drop to normal and the system is again balanced.
The battery and booster can be placed at the power house or
where the greatest drop is likely to occur. As this system,
like the series booster, depends for its action upon the drop
of voltage with increase of load, it is only adapted to shunt
wound dynamos.
From the foregoing description it will be seen that the
compound booster is automatic within certain limits of battery
charge. Any marked change of battery voltage will be followed
by a corresponding change in dynamo current, unless the
rheostat be manipulated to bring battery voltage + booster
voltage back to normal.
While the theoretical dynamo current variation is small for
a given change of load, there is always a sudden, momentary,
current rush from the dynamo on increase of load, the duration
of which is equal to the time lag of magnetization of the
booster field.
Lights on a circuit with variable load will "wink" on sudden
changes of load. In this respect the compound booster is not so
satisfactory as the constant current booster, as in the latter
_all_ dynamo current passes through the series fields, which,
by reason of their self-induction, oppose and check any sudden
current rush, giving the booster field time to change its
magnetization to the proper degree.
[Illustration: FIG. 1,200.—Diagram of connection of one form of
differential booster. In operation, the dynamo current passes through
the series winding of the booster, and the current in this winding
is to remain practically constant. The shunt coil produces a field
which opposes the field produced by the series coil, the resulting
magnetization being, in direction and amount, the resultant of the
two field strengths. The adjustments are so made that when the normal
dynamo current is passing through the series coil, the shunt field
just neutralizes its effect, and the resultant magnetization is zero.
Since the open current voltage of the battery is equal to that of
the system, neither charge nor discharge takes place. With increased
demand on the line, the slight increase in dynamo current in the series
coil overpowers the shunt field, and causes a pressure in the booster
armature in such direction as to assist discharge. If the external load
fall below the average demand, the current in the series coil decreases
slightly so that the shunt field predominates, producing a booster
armature pressure in a direction to assist charge. Although the voltage
of the battery falls while discharging by an amount proportional to the
outflowing current the increased excitation due to this current through
the series coil is also proportional to it, and the booster voltage
rises as that of the battery falls, their sum being always equal to
that of the system. In other words, the booster serves to compound the
battery for constant pressure.]
=Differential Boosters.=—In this type of booster, a series coil
energized from the main current, tends to discharge the battery, and
a shunt coil, excited from the battery, tends to charge the cells.
These two coils are opposed to one another, and the difference in
their respective strengths represents the net strength available for
boosting. In order to produce quicker reversal, additional compound
coils are sometimes added.
[Illustration: FIG. 1,201.—Diagram of differential booster system
with compensating coil. In operation, the compensating field coil
of the booster opposes the shunt coil and prevents the variation of
the battery voltage disturbing the equilibrium of the system. If the
battery pressure be lower than normal, it will not discharge rapidly
enough to relieve the dynamo from overload fluctuations, unless the
booster voltage be increased, and the dynamo will therefore have
to supply a current greater than normal. If a current greater than
normal flow through the compensating coil, the effect of the shunt
coil opposed by the series coil is decreased, and the compensating
coil, acting in the same direction as the series coil, causes a higher
booster pressure tending to discharge the battery, and thus brings down
the dynamo load to normal. Should the battery voltage be above its
normal value, the battery would discharge too rapidly and carry more
than its share of the load. In operating this system, the varying load
must be beyond the booster equipment. The series and compensating coils
may be temporarily short circuited so that the battery may be charged
more rapidly.]
=Ques. For what service is the differential booster adapted?=
Ans. It is suited to power and railway circuits where the load
fluctuates widely and suddenly.
There are several varieties of this type of booster, and many
patents have been issued covering the different methods of
varying the voltage of the machine.
=Constant Current Boosters.=—In installations where it is desired
to supply both an approximately constant load and a fluctuating load
from the same dynamos (as for instance, in office buildings or hotels,
where it is necessary to supply lights and elevators from the same
source), the fluctuations in the power circuits must not interfere with
the lighting circuits and to prevent this, two sets of bus bars are
provided.
[Illustration: FIG. 1,202.—Diagram of non-reversible or constant
current booster system. The booster armature and field are in series
between one side of the lighting and power bus bars. A shunt field
is also provided, which acts in opposition to the series field. This
booster carries a practically unvarying current from the lighting to
the power bus bars, regardless of the fluctuations of the external
load, which current is equal to the average required by the fluctuating
load. Except under abnormal conditions the shunt field always
predominates giving a voltage which is added to that of the lighting
bus bars, so that the voltage across the power busses is always higher
than that across the lighting by an amount equal to the booster
voltage. If an excessive load come on the power circuits, the increased
excitation of the series coil, due to a slight increase in current from
the lighting to the power bus bars, lowers the booster voltage and
consequently reduces the voltage across the power bus bars. The battery
discharges, furnishing an amount of current equal to the difference
between that required by the load and the constant current through the
booster. If the power load decrease below normal, the slight decrease
in current in the booster series field increases the booster armature
voltage and the excess current goes into the battery. The booster,
therefore, does not in reality give a constant current, but by proper
design the variation may be kept within a few per cent.]
The dynamos are connected in the usual manner to one set of bus bars,
and the lighting circuits are connected across these.
Across the other set of bars are connected the circuits supplying the
fluctuating load, and the battery is also connected directly across
these power bars.
The power bars are supplied with current from the lighting bars, a
non-reversible or so called constant current booster being interposed
between the two as shown in fig. 1,202. Since this permits only a
constant current to pass from the lighting bus bars, the load on the
dynamo does not vary, although the load on the power busses may vary
widely.
[Illustration: FIG. 1,203.—Hubbard's separately excited booster system
(Gould Storage Battery Co.); diagram showing general arrangement.]
=Separately Excited Boosters.=—In some forms of booster the field
excitation is secured by a small exciting dynamo. An example of this
class is shown in fig. 1,203. The exciter is provided with a single
series coil, through which the station output or a proportional
part thereof passes. The armature of the exciter is connected to the
exciting coil on the booster, and thence across the mains as shown.
NOTE.—Reversible boosters should be used where the average
total current to the fluctuating load is greater than the
battery discharge current, and where the pressure of the power
bus bars must not fall off with increase in load. Electric
railway and lighting plants having long feeders are examples
of the systems to which reversible boosters are suited.
Non-reversible boosters should be used where the average total
load is less than the battery discharge current, and where a
drop in the voltage of the power bus bars is of advantage.
Examples of such plants are hotels or apartment houses where
electric elevators are operated from the lighting dynamos.
Boosters are usually driven by electric motors directly
connected to them, though any form of driving power may be used.
With the average current passing through the field coil or the exciter,
its armature generates a voltage which is equal to that of the system,
and in opposition to it. These two opposing pressures balance, and no
current flows in the booster field coils.
[Illustration: FIG. 1,204.—Battery system with regulation for long
feeders, for installing where it is desirable to locate the battery
at a point remote from the station and avoid any equipment requiring
constant attention at the battery end. The compound wound motor,
constant current booster is used and keeps constant the current flowing
through the feeder, the battery taking up all load fluctuations.]
With an increase in external load above the average, the tendency
is for an increase to take place through the exciter series coil,
augmenting its field strength and consequently the exciter armature
voltage. This latter now being higher than that of the line, causes
current to flow in the booster field coil in such a direction as to
produce a pressure in the booster armature which assists the battery
to discharge, and is of a magnitude to compensate for the battery drop
occasioned thereby.
[Illustration: FIG. 1,205.—Diagram illustrating storage battery
system, as applied to an automobile for lighting.]
When the load decreases below the normal, the current in the exciter
field is decreased, and its armature voltage falls below that of the
system. Current will now flow in an opposite direction in the booster
field coil, generating a voltage in the booster armature to assist
charge. Since the exciter always generates a voltage in opposition to
that of the line, this system is known commercially as the counter
pressure system.
HAWKINS PRACTICAL LIBRARY OF
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=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.
=IN WORK (Ready Soon)=
=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.=
[Transcriber's Note:
Where possible, unicode characters are used. Some of the fractions do
not have a unicode character which will cause some inconsistencies in
the display.
Inconsistent spelling and hyphenation are as in the original.]
End of Project Gutenberg's Hawkins Electrical Guide, Vol 4, by Hawkins