Figure 40 also shows the schematic circuit diagram of the generator circuit. When
the M-G set is up to speed the voltmeter on the panel will show the voltage generated by
the generator, but no current will flow to the battery until the snap
switch at the bottom of the panel is turned "On," and then only if the generator
voltage is high enough to close the reverse-current relay. The action of this relay
is as follows:-When the snap switch is closed the voltage coil is energized, and, if
the voltage is high enough, the relay switch will close, connecting the generator to
the storage battery. As long as the current is in a direction so as to charge the
battery, the current coil of the relay will aid the voltage coil in holding the contacts closed. If the voltage of the generator dropped so as to allow current to flow
from the battery to the generator, the current coil would "buck" the voltage coil,
causing the contacts of the reverse current relay to open, and prevent the battery
from becoming discharged by running the generator as a motor. There are two
fuses of thirty amperes capacity in the battery lines to protect the equipment and
battery in case of a short circuit, or an excessively high charging rate.
When an AC motor is used the starting box is omitted, and an additional snapswitch is provided on the panel of the motor-generator set for starting the motor.
The AC motor used is a single-phase motor of the repulsion starting type described in
Chapter III (section 31), except that the motor used in battery charging sets is
larger in size than the projector motor.