The operating coil of each DC line switch is in the same circuit as the operating coil of its associated motor line switch, so that the generator line switch closes
when the motor is started.
If power is applied to the motor of one of the sets and both of the generator
line switches could be closed at the same time, the generator of the other machine
would run as a motor, which is not desired. To avoid this, the "Start" button for
one motor-generator set opens the line switch for the other generator and vice-versa.
As an additional safeguard, a mechanical "inter-lock" is also added to the panel.
This "inter-lock" consists of a bar pivoted between the two contactors (see Figure
25) . When one contactor closes, it pushes one end of this bar down, and the other
end is pushed up against the other contactor so that it cannot close. On account
of these features, in case of emergency it is not necessary to push the "Stop" button
of a machine which has been operating. Simply push the "Start" button for the
other machine. This will automatically turn off the power to the machine which
was being used and start up the stand-by machine.
To adjust the voltage of the DC generators push the "Start" button of one of
the push-button stations. Adjust the voltage at the generator panel, by means of the
generator field rheostat which is directly below the line switch which is closed, so
that the generator panel meter reads 110 volts when two projector motors are
running. After the voltage of one generator is adjusted, stop the machine, start
the other, and adjust it in the same manner. The generator voltages should seldom
require readjustment after they are once set properly.