SECTION 1500
POWER SUPPLY
SECTION INDEX
Batteries 1502
Primary Cells 1502
Lead-Acid Storage Cells 1503
Nickel-Cadmium Storage Cells 1504
Battery Chargers 1504
Selenium Rectifiers 1504
Transformers 1505
Effects of Different Frequencies 1505
Voltages and Polarity 1505

1501



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B ATTERI ES
Unless the continuity of operation of the signal
system is to depend entirely upon the continuity
of the a-c supply, batteries must be provided.
Where there are no a-c distribution lines, batteries
are, of course, absolutely necessary. This section
describes briefly various types of cells. Cells are
available with characteristics which differ from the
examples given here. Detailed information on a
particular type of cell should be obtained from its
manufacturer.
Primary Cells
A primary cell, Figure 1501, is a device for the direct
transformation of chemical energy into electrical
energy. Primary cells are used for feeding track
circuits and for operating signals where the ampere-hour
consumption is moderate and where a-c
lines are not practical.
There is a great difference in the voltage or
current capacity of different kinds of cells. The
ordinary dry cell has a voltage of 1.48 volts on open
circuit, but if any appreciable amount of current is
taken from it, the voltage drops very rapidly. Likewise,
a good cell recuperates rapidly on open circuit
and will soon be back to its original voltage.
The primary cell maintains a quite constant voltage
at the discharge rates we commonly find in railway
signal work.
The voltage of any cell depends on the amount
of current that is being delivered by it at the time
the voltage reading is taken and the amount of
energy that has been taken from it. If the current
drain is light, the voltage may be high; but if the
current drain is heavy, the voltage will be substantially
decreased.
The capacity of a primary cell
- and of a secondary
cell also
- is rated in ampere-hours. One ampere
flowing for one hour (2 amps for 1/2 hr., or 1/3
amp. for 3 hrs., etc.) is called an ampere-hour.
Modern primary cells utilize an air-depolarized,
carbon-zinc couple and a caustic potash and lime
electrolyte. They are now commonly furnished dry
and are activated by simply adding water to each
cell.
Primary cells are available in various capacities
from several manufacturers. Some examples are
1100 ampere-hours capacity and 1300 hours. Both
cells are nominally 1.25 volts. The 1100 hours cells pere hours. Figure 1502 shows how such a cell are available as two- or three-cell batteries. Their maintains a quite constant voltage during discells can be connected by the user in parallel or in charge. Note that the curves vary with temperature series. Note that such a two-cell battery, when the as well as with the rate of discharge. If the discharge cells are connected in parallel, is rated at 2200 am- rate were lower, for example 0.15 ampere, the cell

Figure 1501. Typical primary cell batteries.

1502



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AMPERE HOURS PER CELL
Figure 1502. Primary cells maintain a quite constant voltage during discharge.
PB1 140).

(This table is from Edison Bulletin

voltage at 1100 hours would be about 1.26 volt, even at 0°F. The 0.50 ampere rate is a reasonable average fora track circuit.
Lead-Acid Storage Cells
A storage cell, Figure 1503, is a secondary cell used for storing electric energy at one time for use at another. Passing current through a discharged storage cell in the reverse direction causes chemical changes to take place which restore the cell to its charge condition. This is called reversibility. The primary cell is not reversible to any marked extent.
Lead-acid cells are available as lead-antimony cells and as lead-calcium cells. Lead-antimony cells perform somewhat better when exposed to high temperatures, while lead-calcium cells retain their charge better in storage. As many other factors also affect the performance of lead-acid cells, it is recommended that the battery manufacturer be consulted before selecting stationary batteries.
Lead-acid cells are made in many sizes, but the voltage of a cell is the same whatever the size. Only the ampere-hour capacity is increased by increasing the size and number of plates. Lead- acid cells are usually rated on the amount of current that they will deliver in 8 hours. An 80 ampere- hour cell will deliver 10 amperes for eight hours. Such a cell will deliver fewer ampere-hours when discharged in 5 hours, and the total capacity in ampere-hours will be greater if the discharge rate is less. This is the same as in primary cells.
Low temperature reduces the ampere-hour capacity of a storage cell. At 0°F, you would get

Figure 1503. Lead-acid storage cells in the battery room
of a large car classification yard.
-
only 45% of what you would get at 77°F. For example, a 120 ampere-hour battery would give you only around 54 ampere-hours at 0°F.
The electrolyte is diluted sulphuric acid with a specific gravity of from 1.210 to 1.220 (see maker’s specifications). In a fully charged cell, the specific gravity varies from 1.215 to 1.300 at 77°F. A discharged cell reads about 1.140 at 77°F.
Voltage of a fully charged cell is 2 volts. On trickle charge, voltage is 2.15 to 2.17 for lead- antimony and 2.17 to 2.25 for lead-calcium. A discharged cell reads 1 .8 volts.
When charging lead-acid cells, it is necessary to put in considerably more energy than can be taken out. It is best not to discharge them too low or they will be too badly sulphated. When cells are discharged, the sulphate in the electrolyte is taken up by the plates, and when they are charged this sulphate is driven out again into the electrolyte. This

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1503

 

 

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makes the electrolyte heavier (higher specific gravity) when the cells are fully charged and becomes thereby a means for telling when the cells are fully charged.
When cells are left standing, the charge gradually leaks off, and so it is good practice to keep a small current flowing into them all the time to make up for this leakage. This is called trickle charging. Usually, secondary batteries in signaling are always connected to the load. Track relays are a continuous load as are, for example, various relays in block signaling, interlocking, and rail-highway crossing protection circuits. These loads vary, such as when a track circuit is occupied, a switch machine operates, or a crossing gate operates. Secondary batteries in this kind of service are usually kept on floating charge by means of battery-charging rectifiers (described later). The floating charge rate is adjusted to compensate for: (1) the steady load, (2) the intermittent loads, and (3) the losses that normally take place by local action within the secondary cells.

erators driven by gasoline or diesel engines. In most signaling applications, however, dry plate rectifiers of the selenium type are used.
A selenium rectifier, Figures 1505 and 1506, consists of an assembly of selenium plates or cells, heat sinks, and spacers, assembled on an insulated stud. A selenium cell consists of a thin layer of selenium sandwiched between two plates, a back plate and a front electrode. The layer of selenium acts like a one-way valve to the flow of current. It offers low resistance to current flow from the back plate to the front electrode and very high resistance to current flow in the opposite direction. Each rectifier has a maximum current rating which cannot be exceeded without damaging the rectifier.

Nickel-Cadmium Storage Cells
The active material of the positive plate of a nickel- cadmium storage cell, Figure 1504, consists principally of nickel hydrate. That of the negative plate is a cadmium oxide mixture.
The electrolyte is an aqueous solution of potassium hydroxide (but do not add water to it). Its specific gravity is of no value in determining the state of charge of the cells. They should be maintained, on continuous charge, at 1.42 to 1.5 volts at 60-70° F. Nominal voltage of a fully charged cell is 1.21 volts; of a discharged cell, one volt. Always check the battery manufacturer’s recommendations.
Nickel-cadmium cells can withstand temperatures as low as minus 40° F without suffering permanent damage. Their capacity decreases with temperature. The percentage of ampere-hour output at low temperatures is about the same as for lead-acid cells.

BATTERY CHARGERS

Selenium Rectifiers

Batteries of secondary cells are charged from a.c. by means of dry plate rectifiers or by motor-generators. In some instances, such as in car classification yards, batteries may be charged from gen Figur

1504. A typical nickel-cadmium storage cell.

1504



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The number of selenium plates and their connection in series or in multiple depend upon the output voltage and current for which a particular rectifier is designed.
One selenium disc, represented by the rectifier symbol in Figure 1507, comprises a half-wave rectifier. Thus current from only one half of the a-c wave flows through the battery being charged, or other load.
In a full-wave rectifier, Figures 1506 and 1508, the flow of current is directed by the action of four rectifying discs connected in a bridge arrangement so that current from both halves of the a-c wave flows through the load in the same direction.
Selenium rectifiers, besides being used to charge secondary batteries, are also used to operate d-c devices, such as relays, directly from a.c. Half-wave rectifiers are also used in signaling as snubbing rectifiers as, for example, in switch machines and in crossing gate mechanisms.
GRS rectifiers are especially designed for signaling applications and are available in many different arrangements and capacities. A typical GRS selenium rectifier is shown in Figure 1505.
TRANSFORMERS
A transformer is a device for raising or lowering the voltage of an a-c circuit. It transforms” one voltage into another. It consists, in its simplest form, of two windings on an iron core. The purpose of the iron core is to ensure that the magnetic field

set up about the primary coil will flow through the secondary coil with minimum loss. What few lines of force do not link primary and secondary are called leakage lines, and the inductance associated with them is called leakage inductance.
The primary is connected to an a-c line, the secondary to the load, which may be a signal lighting circuit, a rectifier, a motor, or any other device which requiresa.c.
The lines of force from the continually changing alternating current in the primary pass through the secondary and induce voltages in it. The relation between the primary and secondary voltages is simple and definite
- it depends upon the relative number of turns. If there are twice as many secondary turns as there are primary turns, the voltage developed across the secondary terminals will be double that across the primary (ignoring losses, which in a well designed transformer are insignificant).
Transformers used in signal work may be either the dry type, self-cooled, Figure 1509 and 1510, or oil-immersed, self-cooled, depending on the Capacity of the transformer. Dry type transformers are generally used with primary voltages up to 600 volts. Above 600 volts, oil-immersed transformers are usually used.
Effects Of Different Frequencies
The lower the frequency the larger must be the transformer in order to deliver a given output without overheating. The frequencies generally used in signal work are 25, 60, and 100 Hz.
A transformer designed for a certain work at 25 Hz would perform the same work on a 60-Hz transmission line with less heat than on the 25-Hz transmission line. The same applies to 60-Hz transformer used on a 100-Hz transmission Iine.A 25-Hz transformer connected to a 60-Hz transmission line would have its coils cut by the magnetic flux at more than twice the rate they would be cut by a 25- Hz transmission, therefore requiring less lines of force to perform the function of sustaining the voltage in the coils and a smaller irqn core to accommodate them. It is therefore evident that transformers designed for a certain frequency should not be used on lines having a lower frequency, as excessive current will flow in the primary coil, thus causing overheating.
Voltages And Polarity
Figure 1511 shows how the terminal board on a GRS Type UT transformer is marked. This transformer has two primaries and two secondaries.

‘— .._
Figure 1505. The GRS Type ST selenium rectifier is furnished with a transformer with adjustable reactance to facilitate precise setting of the d-c output current.

1505



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AC

+
3

AC
2

Figure 1507. Current flow through a half-wave rectifier.

Figure 1508. Current flow through a full-wave rectifier.

DC +

DC— AC DC+ AC
6 4 3 2
5

AC

DC— AC
4

SYMBOL

6
5

1

WIRING
2
3
4
6[

E

F

5

Figure 1506. Plate assembly and connections for a selenium rectifier.

1506



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NX12
BX 12
Figure 1511. Terminal board markings on the Type UT
transformer, shown with connections for
240 volts input and two 12-volt outputs.
Here, for example, we show the supply at 240 volts. Thus we series connect the two primaries. The secondaries are shown connected to furnish 12 volts to each of two a-c track circuits, or one secondary could be feeding a signal lighting circuit and the other, the power-transfer relay. Also, the two secondaries could be connected in series to supply 24 volts to panel lamps. Note that plus is connected to minus in the usual way.

C4
0120.30.60. 10
+ 20•30•60• 1O—
o 120
÷18
\3 \3
!\,
+ 18
\3 \3 !\ 1
I\I\I
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-0
I

NX12 BX 12

Figure 1509. The GRS Type UT transformer has two

primaries and two secondaries.

Figure 1510. The Type UT transformer can be mounted on standard relay racks.

1507



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I1iHI
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