Traction motors are series-wound, direct-current motors geared to the locomotive axles and
wheels, as shown in figure 1.1, item 33. They convert the electrical energy of the generator into tractive
effort or mechanical energy. Half of the motors' weight is supported on the truck frame and half on the
axle. The motors are either connected permanently in series-parallel or arranged for transition from
series to series-parallel connection. The armature turns on roller bearings in the motor housing. The
axle bearing is a split-sleeve type.
Traction motor fields are provided with shunts which divert a portion of the field current when
the motors cannot absorb the full power output. Field shunting contactors can be operated
pneumatically by magnet valves or by electric relays. A relay operation is one of the easiest to
understand. Relays open and close at certain generator voltages, closing the field-shunting contactors
and permitting some of the current to flow through the shunts and weaken the fields.
Blowers are provided in larger locomotives to force a large quantity of cooling air through the
traction motors and thus prevent them from overheating. A locomotive must not haul a train unless the
traction motors are adequately cooled because heat generated by heavy currents will damage the motors
if it is not carried away. Blowers are mounted on the floor of the locomotive and ducts carry the air
through the underframe to the motors; they may be mechanically driven by the engine or electrically
driven.
1.30. EXCITER
The exciter supplies the main generator field current. It is a small generator that maintains
power output as nearly constant as possible. To avoid overloading the engine, the strength of the main
generator field must be varied so that the generator voltage is decreased as the current demand is
increased.
The exciter's own field is supplied by several sets of windings which are independent of each
other. One set of windings is the four-pole field, energized by the battery or auxiliary generator at a
strength determined by a load regulator installed in the circuit. Another set of windings is a two-pole
field fed by the exciter armature. A third set of windings is a differential, or reversed, field that is
energized by current from the main generator. When current from the main generator to the traction
motors is high, the differential field opposes the other exciter fields and reduces the exciter output.
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