wound to oppose the main field. The purpose is to vary the total field strength to obtain a constant
kilowatt output. Other fields, such as starting fields, are used when cranking.
The main pole pieces of the generator are of laminated steel riveted together and bolted to the
frame. The field coils are impregnated and baked with insulating compounds to guard against
movement and chafing within the coil and to permit flow of heat to the surfaces. Built to withstand high
speed and vibrations, the armature of this generator is balanced both before and after winding to reduce
vibration. It is supported at one end by an antifriction bearing and at the other by the engine crankshaft.
Figure 1.17. Main Generator.
Similar to a generator, a direct-current motor such as the traction motor can be connected in
shunt or in series. Shunt motors are used where a constant speed is desired; output of the motor varies
little with a change in load. Series motors have good traction, but must always be connected to a load;
otherwise they will speed up so fast that they will be damaged.