Control of the power output of the main generator is known as load control. As load is imposed
on the engine, the governor admits more fuel until the fuel limit for that speed is reached. Any attempt
to increase the load beyond this point will cause the engine to slow down. At such a time, a load
regulator in the generator excitation circuit automatically reduces the power output of the generator by
reducing its field excitation and the diesel engine continues to run at normal speed.
The reverser, shown later in figure 2.17, is an electropneumatic switching device that reverses the
direction of current in the traction motor field windings. When the fields are reversed, the traction
motors change their direction of rotation thereby changing the direction of travel of the locomotive. The
main switching element of the reverser is a rotating drum; on its surface are copper segments that contact
different combinations of sliding fingers as the drum rotates. Magnet valves on the reverser are fed
through interlocks on the main power switches to keep the reverser from turning except when the power
circuits are open. When a magnet valve (explained in par. 1.34c) is energized, air is applied to the
pneumatic mechanism to turn the drum and to establish the proper electrical connections. If the coil of a
magnet valve is burned out, or if the air supply fails, the reverser may be thrown manually by depressing
a button on the magnet valve. If the engine is dead, the drum can be turned by hand.
A traction motor cutout switch is used to take the motors out of the circuit if there is an electrical
failure in the circuit, such as a short circuit or ground. Three types of cutout switches are used: a toggle
switch in each motor circuit; a multipole rotary switch which controls all motor circuits; and contacts
mounted on the reverser. With rotary or toggle cutout switches, the control circuit to the main power
switches passes through the traction motor cutout switch. When the cutout switch is open, the main
power switches are therefore open, and the corresponding motors are disconnected from the generator.
The switch setting should not be changed under heavy load because heavy currents are likely to arc and
burn the contactors. Before opening a power circuit, idle the engine by the throttle or by an isolating