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.
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.