crankshaft and an auxiliary generator is either coupled to the main generator or driven by a belt. One or
more traction motors and their reduction gears are mounted on the driving axles of trucks and a final
reduction gear is located between the traction motor and the axle. No mechanical parts are needed to
reverse the engine, as is necessary with a totally mechanical transmission system.
The power produced by the generator is converted by a traction motor into mechanical
driving force at the wheels. The motor can produce very high torque at start which decreases as the load
on it increases. Adjustment of the generator field strength is made by a regulator that introduces
resistance into the circuit as indicated by the governor. The governor acts on a valve which controls oil
under pressure in its cylinder. During increased engine load, the governor's piston rises, fuel is injected,
and the contacts are closed, causing the motor to rotate in the opposite direction and to introduce
The traction motor is cooled by filtered air forced in either electrically or mechanically.
The electrical method is preferred because it provides a full blast of air at all speeds while the
mechanical system blows at slow speeds when the highest electrical current is in the motor.
c. Connecting-gear ratio. Ratio of the gears connecting the motors to the axles is selected on
the basis of the service the locomotive is to perform. High-speed service, such as passenger service,
calls for a ratio which keeps motor speed low. When power is more important than speed, such as for
freight service, the ratio used is one that requires many revolutions of the motor for only slight
movement of the locomotive. Intermediate ratios are used for general all-purpose service. Gear ratio is
expressed by two numbers: one is the number of teeth on the axle-mounted gear; the other the number of
teeth on the traction motor pinion. For example, if the axle-mounted gear has 60 teeth and the traction
motor pinion 17, the gear ratio is 60-17 or 3.53:1.
The main generator, shown in figure 1.17, converts the power output of the diesel engine into
electric power for the operation of the traction motors. Directly connected to the diesel engine, the
generator's speed varies with engine speed. The generator is self-ventilated by a fan mounted on its
shaft. The main field of the generator is supplied by a battery or by an exciter controlled by a load
regulator. A differential field, in series with the generator, is