POWER PLANTS
The power plant is an organization of devices to furnish to a telephone
system the several kinds of current, at proper pressures, for the
performance of the several general electrical tasks within the exchange.
=Kinds of Currents Employed.= Sources of both direct and alternating
current are required and a single exchange may employ these for one or
more of the following purposes:
_Direct Current._ Current which flows always in one direction whether
steady or varying, is referred to as direct current, and may be required
for transmitters, for relays, for line, supervisory, and auxiliary
signals, for busy tests, for automatic switches, for call registers, for
telegraphy, and in the form of pulsating current for the ringing of
biased bells.
_Alternating Current._ Sources of alternating current are required for
the ringing of bells, for busy-back and other automatic signals to
subscribers, for howler signals to attract the attention of subscribers
who have left their receivers off their hooks, and for signaling over
composite lines.
=Types of Power Plants.= Clearly the requirements for current supply
differ greatly for magneto and common-battery systems. There is,
however, no great difference between the power plants required for the
automatic and the manual common-battery systems.
In the simplest form of telephone system--two magneto telephones on a
private line--the power plant at each station consists of two elements:
one, the magneto generator, which is a translating device for turning
hand power into alternating current for ringing the bell of the distant
station; and the other, a primary battery which furnishes current to
energize the transmitter. In such a system, therefore, each telephone
has its own power plant. The term power plant, however, as commonly
employed in telephone work, refers more particularly to the organization
of devices at the central office for furnishing the required kinds of
current, and it is to power plants in this sense that this chapter is
devoted.
_Magneto Systems._ If magneto lines be connected to a switchboard, the
current for throwing the drop at the switchboard is furnished by the
subscriber's generator, and the current for energizing the subscriber's
transmitter is furnished by the local battery at his station; but
sources of current must be provided for enabling the central-office
operator to signal or talk to the subscribers. These are about the only
needs for which current must be furnished in an ordinary magneto central
office. If a multiple board is employed, direct current is also needed
for the purpose of the busy test and also for operating the drop
restoring circuits, if the electrical method of restoring the drops is
employed.
_Common-Battery Systems._ In common-battery systems the requirements are
very much more extensive. The subscribers' telephones have no power
plants of their own, but are provided with a common source of direct
current located at the central office for supplying the talking current,
and for operating the central-office signals, and the operators are
provided with one or more common sources of alternating or pulsating
current for ringing the subscribers' bells. Common-battery equipment
requires the use of currents of different kinds for a greater number of
auxiliary purposes than does magneto equipment. These facts make the
power plant in a common-battery office much more important than in a
magneto office.
=Operators' Transmitter Supply.= In a small magneto exchange, the
transmitter current may be had from primary batteries, a separate
battery being employed for each operator's set. When there are more than
three or four operators, however, it is usual, even in magneto offices,
to obtain the transmitter current from a common storage battery. A
storage battery has the fortunate quality of very low internal
resistance, therefore a number of operators' transmitters may be
actuated by one source without introducing cross-talk. In other words, a
storage battery is a current-furnishing device of good regulation, the
variation of consumption in one circuit leading from it causing slight
variation in the currents of other circuits leading from it. If this
were not so, cross-talk would exist between the telephones of the
operators' positions connected to the same battery. This regulating
quality enables the multiple feeding of telephone circuits to be carried
further than the mere supplying of operators' sets and is the quality
which makes possible the successful use of a storage battery as the
single source of transmitter current for common-battery central-office
equipment.
In furnishing a plurality of operators' transmitters from a common
battery, the importance of low resistance and inductance in the portion
of the path that is common to all of the circuits must not be
overlooked. Not only is a battery of extremely low resistance required,
but also conductors leading from it that are common to two or more of
the circuits should be of very low resistance and consequently large in
cross-section and as short as possible. In common-battery offices there
is obviously no need of employing a separate battery for the operators'
transmitters, since they may readily be supplied from the common storage
battery which supplies direct current to the subscribers' lines.
=Ringing-Current Supply.= _Magneto Generators._ As a central-office
equipment is required to ring many subscribers' bells, only the small
ones find it convenient to ring them by means of hand-operated magneto
generators. Small magneto switchboards are usually equipped so that each
operator is provided with a hand-generator, but even where such is the
case some source of ringing current not manually operated is desirable.
In larger switchboards the hand generators are entirely dispensed with.
The magneto generator may be driven by a belt from any convenient
constantly moving pulley, and the early telephone exchanges were often
equipped with such generators having better bearings and more current
capacity than those in magneto telephones. These were adapted to be run
constantly from some source of power, delivering ringing current to the
operators' keyboards at from 16 to 20 cycles per second.
_Pole Changers._ Vibrating pole changers were also used in the early
exchanges, but passed out of use, partly because of poor design, but
more because of the absence of good forms of primary batteries for
vibrating them and for furnishing the direct currents to be transformed
into alternating line current for ringing the bells. The pole changer
was redesigned after the beginning of the great spread of telephony in
the United States in 1893. Today it is firmly established as an element
of good telephone practice. Fig. 411 illustrates the principle upon
which one of the well-known pole changers--the Warner--operates. In
this _1_ is an electromagnet supplied by a constant-current battery _2_
to keep the vibratory system continually in motion. This motor magnet
and its battery work in a local circuit and cause vibration in exactly
the same manner as the armature of an ordinary electric door bell is
caused to vibrate. The battery from which the ringing current is derived
is indicated at _3_, and the poles of this are connected, respectively,
to the vibrating contacts _4_ and _5_. These contacts are merely the
moving members of a pole changing switch, and a study of the action will
readily show that when these moving parts engage the right-hand
contacts, current will flow to the line supposed to be connected to the
terminals _6_ and _7_ in one direction, while, when these parts engage
the left-hand contacts, current will flow to the line in the reverse
direction. The circuit of the condenser shown is controlled by the
armature of the relay _8_.
The winding of this relay is put directly in the circuit of the main
battery _3_, so that whenever current is drawn from this battery to ring
a distant bell, this relay will be operated and will bridge the
condenser across the circuit of the line. The purpose of the condenser
is to make the impulses flowing from the pole changer less abrupt, and
the reason for having its bridged circuit normally broken is to prevent
a waste of current from the battery _3_, due to the energy which would
otherwise be consumed by the condenser if it were left permanently
across the line.
[Illustration: Fig. 411. Warner Pole Changer]
[Illustration: Fig. 412. Pole Changers for Harmonic Ringing]
Pole changers for ringing bells of harmonic party lines are required to
produce alternating currents of practically constant frequencies. The
ideal arrangement is to cause the direct currents from a storage battery
to be alternated by means of the pole changers, and then transformed
into higher voltages required for ringing purposes, the transformer
also serving to smooth the current wave, making it more suitable for
ringing purposes. In Fig. 412 such an arrangement, adapted to develop
currents for harmonic ringing on party lines, is shown. The regular
common battery of the central office is indicated at _1_, _2_ being an
auxiliary battery of dry cells, the purpose of which will be presently
referred to. At the right of the battery _1_ there is shown the calling
plug with its associated party-line ringing keys adapted to impress the
several frequencies on the subscribers' lines. The method by which the
current from the main storage battery passes through the motor magnets
of the several vibrators, and by which the primary currents through the
transformers are made to alternate at the respective frequencies of
these vibrators, will be obvious from the drawing. It is also clear that
the secondary currents developed in these transformers are led to the
several ringing keys so as to be available for connection with the
subscribers' lines at the will of the operator. The condensers are
bridged across the primary windings of the transformers for the purpose
of aiding in smoothing out the current waves. The use of the auxiliary
battery _2_ and the retardation coil _3_ in the main supply lead is for
the purpose of preventing the pulsating currents drawn from the main
battery _1_ from making the battery "noisy." These two batteries have
like poles connected to the supply lead, and the auxiliary battery
furnishes no current to the system except when the electromotive force
of the impulse flowing from the main battery is choked down by the
impedance coil and the deficiency is then momentarily supplied for each
wave by the auxiliary battery. This is the method developed by the Dean
Electric Company for preventing the pole-changer system from causing
disturbances on lines supplied from the same main battery.
[Illustration: Fig. 413. Multi-Cyclic Generator Set]
_Ringing Dynamos._ Alternating and pulsating currents for ringing
purposes are also largely furnished from alternating-current dynamos
similar to those used in commercial power and lighting work, but
specially designed to produce ringing currents of proper frequency and
voltage. These are usually driven by electric motors deriving their
current either from the commercial supply mains or from the
central-office battery. In large exchanges harmonic ringers are usually
operated by alternating-current generators driven by motors, a separate
dynamo being provided to furnish the current of each frequency. Fig. 413
shows a set of four such generators directly connected to a common
motor. As no source of commercial power for driving such generators is
absolutely uniform, and since the frequency of the ringing current must
remain very close to a constant predetermined rate, some means must be
employed for holding the generators at a constant speed of revolution,
and this is done by means of a governor shown at the right-hand end of
the shaft in Fig. 413. The principle of this governor is shown in Fig.
414. A weighted spring acts, by centrifugal force, to make a contact
against an adjustable screw, when the speed of the shaft rises a
predetermined amount. This spring and its contact are connected to two
collector rings _1_ and _2_ on the motor shaft, and connection is made
with these by the brushes _3_ and _4_. The closing of the governor
contact serves, therefore, merely to short-circuit the resistance _5_,
which is normally included in the shunt field of the motor. This
governor is based on the principle that weakening the field increases
the speed. It acts to insert the resistance in series with the field
winding when the speed falls, and this, in turn, results in restoring
the speed to normal.
[Illustration: Fig. 414. Governor for Harmonic Ringing Generators]
=Auxiliary Signaling Currents.= Alternating currents, such as those
employed for busy signals to subscribers in automatic systems, those for
causing loud tones in receivers which have been left off the hook
switch, and those for producing loud tones in calling receivers
connected to composite lines, all need to be of much higher frequency
than alternating current for ringing bells. The simplest way of
producing such tones is by means of an interrupter like that of a
vibrating bell; but this is not the most reliable way and it is usual to
produce busy or "busy-back" currents by rotating commutators to
interrupt a steady current at the required rate. As the usual busy-back
signal is a series of recurrent tones about one-half second long,
interspersed with periods of silence, the rapidly commuted direct
current is required to be further commuted at a slow rate, and this is
conveniently done by associating a high-speed commutator with a
low-speed one. Such an arrangement may be seen at the left-hand end of
the multicyclic alternating machine shown in Fig. 413. This commuting
device is usually associated with the ringing machine because that is
the one thing about a central office that is available for imparting
continuous rotary motion.
=Primary Sources.= Most telephone power plants consume commercial
electric power and deliver special electric current. Usually some
translating device, such as a motor-generator or a mercury-arc
rectifier, is employed to transform the commercial current into the
specialized current required for the immediate uses of the exchange.
_Charging from Direct-Current Mains._ In some cases commercial direct
current is used to charge the storage batteries without the intervention
of the translating devices, resistances being used in series with the
battery to regulate the amount of current. Commercial direct current
usually is available at pressures from 110 volts and upward, while
telephone power plants contain storage batteries rarely of pressures
higher than 50 volts. To charge a 50-volt storage battery direct from
110-volt mains results in the loss of about half the energy purchased,
this lost energy being set free in the form of heat generated in the
resistance devices. Notwithstanding this, it is sometimes economical to
charge directly from the commercial direct-current power mains, but only
in small offices where the total amount of current consumed is not large
and where the greatest simplicity in equipment is desirable. It is
better, however, in nearly all cases, to convert the purchased power
from the received voltage to the required voltage by some form of
translating device, such as a rotary converter or a mercury-arc
rectifier.
_Rotary Converters._ Broadly speaking, a rotary converter consists of a
motor adapted to the voltage and kind of current received, mechanically
coupled to a generator adapted to produce current of the required kind
and voltage. The harmonic ringing machine shown in Fig. 413 is an
example of this, this particular one being adapted to receive direct
current at ordinary commercial pressure and to deliver four different
alternating currents of suitable pressures and frequencies. It is to be
understood, however, that the conversion may be from direct current to
direct current, from alternating to direct, or from direct to
alternating. Such a device where the motor is a separate and distinct
machine from the generator or generators is called a _motor-generator_.
It is usual to connect the motors and the generators together directly
by a coupling having some flexibility, as shown in Fig. 413, so as to
prevent undue friction in the bearings.
[Illustration: THE POWER AND WIRE CHIEF'S ROOM OF THE EXCHANGE AT WEBB
CITY, MISSOURI]
As an alternative to the converting device made up of a motor coupled to
a generator, both motor and generator windings may be combined on the
same core and rotate within the same field. Such a rotary converter has
been called a _dynamotor_. As a rule the dynamotor is only suitable for
small power-plant work. It has the following objectionable features:
(_a_) It is difficult to regulate its output, since the same field
serves for both the motor and the dynamo windings. For this reason its
main use is as a ringing machine where the regulation of the output is
not an important factor. (_b_) Furthermore, the fact that the motor and
dynamo armature windings are on the same core makes it difficult to
guard against breakdowns of the insulation between the two windings,
especially when the driving current is of high voltage.
_Charging Dynamos._ The dynamo for charging the storage battery is, of
course, a direct-current machine and may be a part of a motor generator
or it may derive its power from some other than an electric motor, such
as a gas or steam engine. It should be able to develop a voltage
slightly above that of the voltage of the storage battery when at its
maximum charge, so as always to be able to deliver current to the
charging battery regardless of the state of charge. A 30-volt generator,
for example, can charge eleven cells in series economically; a 60-volt
generator can charge twenty-five cells in series economically.
Battery-charging generators are controlled as to their output by varying
a resistance in series with their fields. Such machines are usually
shunt-wound. Sometimes they are compound-wound, but compounding is less
important in telephone generators than in some other uses. A feature of
great importance in the design of charging generators is smoothness of
current. If it were possible to design generators to produce absolutely
even or smooth current, the storage battery would not be such an
essential feature to common-battery exchanges, because then the
generator might deliver its current directly to the bus bars of the
office without any storage-battery connection and without causing noise
on the lines. Such generators have been built in small units. Even if
these smooth current generators were commercially developed to a degree
to produce absolutely no noise on the lines, the storage battery would
still be used, since its action as a reservoir for electrical energy is
important. It not only dispenses with the necessity of running the
generators continuously, but it also affords a safeguard against
breakdowns which is one of its important uses.
The ability to carry the load of a central office directly on the
charging generator without the use of a storage battery is of no
importance except in an emergency which takes the storage battery wholly
out of service. Since the beginning of common-battery working such
emergencies have happened a negligible number of times. Far more
communities have lacked telephone service because of accidents beyond
human control than because of storage-battery failures.
In power plants serving large offices, the demand upon the storage
battery is great enough to require large plate areas in each cell. The
internal resistance, therefore, is small and considerable fluctuations
may exist in the charging current without their being heard in the
talking circuits. The amount of noise to be heard depends also on the
type of charging generator. Increasing the number of armature coils and
commutator segments increases the smoothness of the charging current.
The shape of the generator pole pieces is also a factor in securing such
smoothness.
If, with a given machine and storage battery, the talking circuits are
disturbed by the charging current, relief may be obtained by inserting a
large impedance in the charging circuit. This impedance requires to be
of low resistance, because whatever heat is developed in it is lost
energy. This means that the best conditions exist when the resistance is
low and the inductance large. These conditions are satisfied by using in
the impedance coil many turns of large wire and an ample iron core.
Dynamotors are not generally suitable for charging purposes. Not only is
the difficulty in regulating their output a disadvantage, but the fact
that the primary and secondary windings are so closely associated on the
armature core makes them carry into the charging current, not only the
commutator noises of the generator end, but of the motor end as well.
_Mercury-Arc Rectifiers._ In common-battery offices serving a few
hundred lines, and where the commercial supply is alternating current,
it is good practice to transform it into direct-battery charging current
by means of a mercury-arc rectifier. It is a device broadly similar to
the mercury-arc lamp produced by Peter Cooper Hewitt. It contains no
moving parts and operates at high efficiency without introducing noises
into the telephone lines. It requires little care and has good length of
life.
[Illustration: Fig. 415. Mercury-Arc Rectifier Circuits]
The circuit of a mercury-arc rectifier charging outfit is shown in Fig.
415. The mercury-arc rectifier proper consists of a glass bulb
containing vacuum and a small amount of mercury. When its terminals are
connected, as indicated--the two anodes across an alternating-current
source and the cathode with a circuit that is to be supplied with direct
current--this device has the peculiarity of action that current will
flow alternately from the two anodes always to the cathode and never
from it. The cathode, therefore, becomes a source of positive potential
and, as such, is used in charging the storage battery through the series
reactance coil and the compensating reactances, as indicated. The line
transformer shown at the upper portion of Fig. 415, is the one for
converting the high-potential alternating current to the comparatively
low-potential current required for the action of the rectifier. The
transformer below this has a one-to-one ratio, and is called the
insulating transformer. Its purpose is to safeguard the telephone
apparatus and circuits against abnormal potentials from the line, and
also to prevent the ground, which is commonly placed on the neutral wire
of transformers on commercial lighting circuits, from interfering with
the ground that is commonly placed on the positive pole of the
central-office battery.
=Provision Against Breakdown.= In order to provide against breakdown of
service, a well-designed telephone power plant should have available
more than one primary source of power and more than one charging unit
and ringing unit.
_Duplicate Primary Sources._ In large cities where the commercial power
service is highly developed and a breakdown of the generating station is
practically impossible, it is customary to depend on that service alone.
In order to insure against loss of power due to an accident to portions
of the distributing system, it is the common custom to run two entirely
separate power leads into the office, coming, if possible, from
different parts of the system so that a breakdown on one section will
not deprive the telephone exchange of primary power. In smaller places
where the commercial service is not so reliable, it is usual to provide,
in addition to the commercial electric-power service, an independent
source of power in the form of a gas or steam engine. This may be run as
a regular source, the commercial service being employed as an emergency
or _vice versa_, as economy may dictate. In providing a gas engine for
driving charging dynamos, it is important to obtain one having as good
regulation as possible, in order to obtain a charging current of
practically constant voltage.
_Duplicate Charging Machines._ The storage batteries of telephone
exchanges are usually provided of sufficient capacity to supply the
direct-current needs of the office for twenty-four hours after a full
charge has been given them. This in itself is a strong safeguard against
breakdown. In addition to this the charging machines should be in
duplicate, so that a burnt-out armature or other damage to one of the
charging units will not disable the plant.
_Duplicate Ringing Machines._ It is equally important that the ringing
machines, whether of the rotary or vibrating type, be in duplicate. For
large exchanges the ringing machines are usually dynamos, and it is not
unusual to have one of these driven from the commercial power mains and
the other from the storage battery. With this arrangement complete
failure of all sources of primary power would still leave the exchange
operative as long as sufficient charge remains in the storage battery.
_Capacity of Power Units._ In designing telephone switchboards it is the
common practice to so design the frameworks that the space for multiple
jacks is in excess of that required for the original installation. In a
like manner, the power plant is also designed with a view of being
readily increased in capacity to an amount sufficient to provide current
for the ultimate number of subscribers' lines for which the switchboard
is designed. The motor generators, or whatever means are provided for
charging the storage batteries, are usually installed of sufficient size
to care for the ultimate requirements of the office. The ringing
machines are also provided for the ultimate equipment. However, in the
case of the storage battery, it is common practice to provide the
battery tanks of sufficient size to care for the ultimate capacity,
while the plates are installed for a capacity only slightly in excess of
that required for the original installation. As the equipment of
subscribers' lines is increased, additional plates may, therefore, be
added to the cells without replacing the storage battery as a whole, and
without making extraordinary provisions to prevent the interruption of
service. It is also customary to provide charging and supply leads from
the storage battery of carrying capacity sufficient for the ultimate
requirements of the office.
=Storage Battery.= The storage battery is the power plant element which
has made common-battery systems possible. The common-battery system is
the element which has made the present wide development of telephony
possible.
A storage-battery cell is an electro-chemical device in which a chemical
state is changed by the passage of current through the cell, this state
tending to revert when a current is allowed to flow in the opposite
direction. A storage cell consists of two conductors in a solution, the
nature and the relation of these three elements being such that when a
direct current is made to pass from one conductor to the other through
the solution, the compelled chemical change is proportional to the
product of the current and its duration. When the two conductors are
joined by a path over which current may flow, a current does flow in the
opposite direction to that which charged the cell.
All storage batteries so far in extensive use in telephone systems are
composed of lead plates in a solution of sulphuric acid in water called
the _electrolyte_. In charging, the current tends to oxidize the lead of
one plate and de-oxidize the other. In discharging, the tendency is
toward equilibrium.
The containers, employed in telephone work, for the plates and
electrolyte are either of glass or wood with a lead lining, the glass
jars being used for the smaller sized plates of small capacity cells,
while the lead-lined wooden tanks are employed with the larger capacity
cells. The potential of a cell is slightly over two volts and is
independent of the shape or size of the plates for a given type of
battery. The storage capacity of a cell is determined by the size and
the number of plates. Therefore, by increasing the number of plates and
the areas of their surfaces, the ampere-hour capacity of the cell is
correspondingly increased. The desired potential of the battery is
obtained by connecting the proper number of cells in series.
Storage-battery cells used in telephone work vary from 2 plates having
an area of 12 square inches each, to cells having over 50 plates, each
plate having an area of 240 square inches. The ampere-hour capacity of
these batteries varies from 6 ampere hours to 4,000 ampere hours,
respectively, when used at an average 8-hour discharge rate. In Fig. 416
is illustrated a storage cell employing a glass container and having
fifteen plates. Each plate is 11 inches high and 10-1/2 inches wide,
with an area, therefore, of 115.5 square inches. Such a cell has a
normal capacity of 560 ampere hours. The type illustrated is one made by
the Electric Storage Battery Company of Philadelphia, Pa.[A]
[Illustration: Fig. 416. Storage Cell]
_Installation._ In installing the glass jars it is customary to place
them in trays partially filled with sand. They are, however, at times
installed on insulators so designed as to prevent moisture from causing
leakage between the cells. The cells using wooden tanks are placed on
glass or porcelain insulators, and the tanks are placed with enough
clearance between them to prevent the lead lining of adjacent tanks from
being in contact and thereby short-circuiting the cells. After the
positive and the negative plates have been installed in the tanks, their
respective terminals are connected to bus bars, these bus bars being,
for the small types of battery, lead-covered clamping bolts, while in
the larger types reinforced lead bus bars are employed, to which the
plates are securely joined by a process called lead burning. This
process consists in melting a portion of the bus bar and the terminal
lug of the plate by a flame of very high temperature, thus fusing each
individual plate to the proper bus bar. The plates of adjacent cells are
connected to the same bus bar, thus eliminating the necessity of any
other connection between the cells.
_Initial Charge._ As soon as the plates have been installed in the tanks
and welded to the bus bars, the cell should be filled with electrolyte
having a specific gravity of 1.180 to 1.190 to one-half inch above the
tops of the plates and then the charge should be immediately started at
about the normal rate. In the case of a battery consisting of cells of
large capacity, it is customary to place the electrolyte in the cells as
nearly simultaneously as possible rather than to completely fill the
cells in consecutive order. When the electrolyte is placed in the cells
simultaneously, the charge is started at a very much reduced rate before
the cells are completely filled, the rate being increased as the cells
are filled, the normal rate of charge being reached when the cells are
completely filled. Readings should be taken hourly of the specific
gravity and temperature of the electrolyte, voltage of the cells, and
amperage of charging current. A record or log should be kept of the
specific gravity and voltage of each of the cells of the battery
regularly during the life of the battery and it is well to commence this
record with the initial charge.
The initial charge should be maintained for at least ten hours after the
time when the voltage and specific gravity have reached a maximum. If
for any reason it is impractical to continue the initial charge
uninterrupted, the first period of charging should be at least from
twelve to fifteen hours. However, every effort should be made to have
the initial charge continuous, as an interruption tends to increase the
time necessary for the initial charge, and if the time be too long
between the periods of the initial charge, the efficiency and capacity
of the cells are liable to be affected. In case of a large battery,
precaution should be taken to insure that the ventilation is
exceptionally good, because if it is not good the temperature is liable
to increase considerably and thereby cause an undue amount of
evaporation from the cells.
The object of the temperature readings taken during the charge is to
enable corrections to be made to the specific gravity readings as
obtained by the hydrometer, in order that the correct specific gravity
may be ascertained. This correction is made by adding .001 specific
gravity for each three degrees in temperature above 70 deg. Fahrenheit,
or subtracting the same amount for each three degrees below 70 deg.
Fahrenheit. At the time the cells begin to gas they should be gone over
carefully to see that they gas evenly, and also to detect and remedy
early in the charging period any defects which may exist. If there is
any doubt in regard to the time at which the cells reach a maximum
voltage and specific gravity, the charge should be continued
sufficiently long before the last ten hours of the charge are commenced
to eliminate any such doubt, as in many cases poor efficiency and low
capacity of a cell later in its life may be traced to an insufficient
initial charge.
_Operation._ After the battery has been put in commission the periodic
charges should be carefully watched, as excessive charging causes
disintegration and decreases the life and capacity of the battery;
while, on the other hand, undercharging will result in sulphating of the
plates and decrease of capacity, and, if the undercharge be great, will
result in a disintegration of the plates. It is, therefore, essential
that the battery be charged regularly and at the rate specified for the
particular battery in question. In order to minimize the chance of
either continuously overcharging or undercharging the battery, the
charges are divided into two classes, namely, regular charges and
overcharges. The regular charges are the periodic charges for the
purpose of restoring the capacity of the battery after discharge. The
overcharges, which should occur once a week or once in every two weeks,
according to the use of the battery, are for the purpose of insuring
that all cells have received their proper charge, for reducing such
sulphating as may have occurred on cells undercharged, and for keeping
the plates, in general, in a healthy condition. The specific gravity of
the electrolyte, the voltage of the battery, and the amount of gasing
observed are all indications of the amount of charge which the battery
has received and should all be considered when practicable. Either the
specific gravity or voltage may be used as the routine method of
determining the proper charge, but, however, if the proper charge is
determined by the voltage readings, this should be frequently checked by
the specific gravity, and _vice versa_.
During the charging and discharging of a battery the level of the
electrolyte in the cells will fall. As the portion of the electrolyte
which is evaporated is mainly water, the electrolyte may be readily
restored to its normal level by adding distilled water or carefully
collected rain water.
_Pilot Cell._ As the specific gravity of all the cells of a battery,
after having once been properly adjusted, will vary the same in all the
cells during use, it has been found satisfactory to use one cell,
commonly termed the pilot cell, for taking the regular specific gravity
readings and only reading the specific gravity of all the cells
occasionally or on the overcharge. This cell must be representative of
all the cells of the battery, and if the battery is so subdivided in use
that several sets of cells are liable to receive different usage, a
pilot cell should be selected for each group.
_Overcharge._ If the battery is charged daily, it should receive an
overcharge once a week, or if charged less frequently, an overcharge
should be given at least once every two weeks. In making an overcharge
this should be done at a constant rate and at a rate specified for the
battery. During the overcharge the voltage of the battery and the
specific gravity of the pilot cell should be taken every fifteen minutes
from the time the gasing begins. The charge should be continued until
five consecutive, specific-gravity readings are practically the same.
The voltage of the battery should not increase during the last hour of
the charge.
As the principal object of the overcharge is to insure that all of the
cells have received the proper charge, it must, therefore, be continued
long enough to not only properly charge the most efficient cells, but
also to properly charge those which are lower in efficiency. The longer
the interval between overcharges, the greater will be the variation
between the cells and, therefore, it is necessary to continue the
overcharge longer when the interval between overcharges is as great as
two weeks. Before the overcharge is made the cells should be carefully
inspected for short circuits and other abnormal conditions. These
inspections may best be made by submerging an electric lamp in the cell,
if the cell be of wood, or of allowing it to shine through from the
outside, if it be of glass. By this means any foreign material may be
readily detected and removed before serious damage is caused. In making
these inspections it must be borne in mind that whatever tools or
implements are used must be non-metallic and of some insulating
material.
_Regular Charge._ Regular charges are the periodic charges for restoring
the capacity of the battery, and should be made as frequently as the use
of the battery demands. The voltage of the cells is a good guide for
determining when the battery should be recharged. The voltage of a cell
should never be allowed to drop below 1.8 volts, and it is usually
considered better practice to recharge when the battery has reached 1.9
volts. If a battery is to remain idle for even a short time, it should
be left in a completely charged condition.
The regular charges for cells completely equipped with plates should be
continued until the specific gravity of the pilot cell has risen to five
points below the maximum attained on the preceding overcharge, or, if
only partially equipped with plates, until it has risen to three points
below the previous maximum. The voltage per cell at this time should be
from .05 volts to .1 volts below that obtained on the previous
overcharge. At this time all the cells should be gasing, but not as
freely as on an overcharge.
_Low Cells._ An unhealthy condition in a cell usually manifests itself
in one of the following ways: Falling off in specific gravity or voltage
relative to the rest of the cells, lack of gasing when charged, and
color of the plates, either noticeably lighter or darker than those of
other cells of the battery. When any of the above conditions are found
in a cell, the cell should receive immediate attention, as a delay may
mean serious trouble. The cell should be thoroughly inspected to
determine if a short-circuit exists, either caused by some foreign
substance, by an excess of sediment in the bottom of the tank, or by
portions of the plates themselves. If such a condition is found, the
cause should be immediately removed and, if the defect has been of short
duration, the next overcharge will probably restore it to normal
condition. If the defect has existed for some time, it is often
necessary to give the cell a separate charge. This may be done by
connecting it directly to the charging generator with temporary leads
and thus bring it back to its normal condition. It is sometimes found
necessary to replace the cell in order to restore the battery to its
normal condition.
_Sediment._ The cells of the battery should be carefully watched to
prevent the sediment which collects in the bottom of the jar or tank
during use from reaching the bottom of the plates, thereby causing short
circuits between them. When the sediment in the cell has reached within
one-half inch of the bottom of the plates, it should be removed at once.
With small cells using glass jars this can most easily be done directly
after an overcharge by carefully drawing off the electrolyte without
disturbing the sediment and then removing it from the jar. The plates
and electrolyte should be replaced in the jar as soon as convenient to
prevent the plates from becoming dry. If the plates are large and in
wooden tanks, the sediment can most easily be removed by means of a
scoop made especially for the purpose. The preferable time to clean the
tanks is just before an overcharge.
_Replacing Batteries._ There comes a time in the life of nearly every
central-office equipment when the storage battery must be completely
renewed. This is due to the fact that the life of even the best of
storage batteries is not as great as the life of the average switchboard
equipment. It may also be due to the necessity for greater capacity than
can be secured with the existing battery tanks, usually caused by
underestimating the traffic the office will be required to handle.
Again, it is sometimes necessary to make extensive alterations in an
existing battery, perhaps due to the necessity for changing its
location. To change a battery one cell at a time, keeping the others in
commission meanwhile, has often been done, but it is always expensive
and unsatisfactory and is likely to shorten the life of the battery, due
to improper and irregular forming of the plates during the initial
charge. The advent of the electric automobile industry has brought with
it a convenient means for overcoming this difficulty. Portable storage
cells for automobile use are available in almost every locality and may
often be rented at small cost. A sufficient number of such cells may be
temporarily installed, enough of them being placed in multiple to give
the necessary output. By floating a temporary battery so formed across
the charging mains and running the generators continuously, a temporary
source of current supply may be had at small expense for running the
exchange during the period required for alterations. Usually a time of
low traffic is chosen for making the changes, such as from Saturday
evening to Monday morning. Very large central-office batteries, serving
as many as 6,000 lines, have thus been taken out of service and replaced
without interfering with the traffic and with the use of but a
comparatively few portable cells. One precaution has to be observed in
such work, and that is not to subject the portable cells to too great an
overcharge, due to the great excess of generator over battery capacity.
This is easily avoided by watching the ammeters to see that the input is
not in too great excess of the output, and if necessary, by frequently
stopping the machines to avoid this.
=Power Switchboard.= The clearing-house of the telephone power plant is
the power board. In most cases, it carries switches, meters, and
protective devices.
_Switches._ The switches most essential are those for opening and
closing the motor and the generator circuits of the charging sets and
with these usually are associated the starting rheostats of the motors
and the field rheostats of the generators. The starting rheostats are
adapted to allow resistance to be removed from the motor armature
circuit, allowing the armature to gain speed and increase its
counter-electromotive force without overheating. The accepted type has
means for opening the driving circuit automatically in case its voltage
should fall, thus preventing a temporary interruption of driving current
from damaging the motor armature on its return to normal voltage.
[Illustration: Fig. 417. Power-Plant Circuits]
_Meters._ The meters usually are voltmeters and ammeters, the former
being adapted to read the several voltages of direct currents in the
power plant. An important one to be known is the voltage of the
generator before beginning a battery charge, so that the generator may
not be thrown on the storage battery while generating a voltage less
than that of the battery. If this were done, the battery would discharge
through the generator armature. The voltmeter enables the voltage of the
charging generator to be kept above that of the battery, as the latter
rises during charge. It enables the performance of several cells of the
battery to be observed. A convenient way is to connect the terminals of
the several cells to jacks on the power board and to terminate the
voltmeter in a plug.
The ammeter, with suitable connections, enables the battery-charge rate
to be kept normal and the battery discharge to be observed. In order to
economize power, it is best to charge the battery during the hours of
heavy load. The generator output then divides, the switchboard taking
what the load requires, the battery receiving the remainder.
In systems requiring the terminal voltage of the equipment to be kept
constant within close limits, either it is necessary to use two
batteries--never drawing current from a battery during charge--or to
provide means of compensating for the rise of voltage while the battery
is under charge. The latter is the more modern method and is done either
by using fewer cells when the voltage per cell is higher or by inserting
counter-electromotive force cells in the discharge leads, opposing the
discharge by more or fewer cells as the voltage of the battery is
higher or lower. In either method, switches on the power board enable
the insertion and removal of the necessary end cells or
counter-electromotive force cells.
_Protective Devices._ The protective devices required on a power board
are principally _circuit-breakers_ and _fuses_. Circuit-breakers are
adapted to open motor and generator circuits when their currents are too
great, too small, or in the wrong direction. Fuses are adapted to open
circuits when the currents in them are too great. The best type is that
in which the operation of the fuses sounds or shows an alarm, or both.
=Power-Plant Circuits.= The circuit arrangement of central-office power
plants is subject to wide variation according to conditions. The type of
telephone switchboard equipment, whether magneto or common-battery,
automatic or manual, will, of course, largely affect the circuit
arrangement of the power plant. Fig. 417 shows a typical example of good
practice in this respect for use with a common-battery manual
switchboard equipment. Besides showing the switches for handling the
various machines and the charge-and-discharge leads from the storage
battery, this diagram shows how current from the storage battery is
delivered to various parts of the central-office equipment.
[Footnote A: The instructions given later in this chapter are for
batteries of this make, although they are applicable in many respects to
all types commonly used in telephone work.]