Field current is supplied by the auxiliary generator. A no-voltage relay, connected in the output circuit,
gives protection against failure of ac power. A fuse or circuit breaker is in the line supplying the
alternator fields, and, if this circuit is open, there is no ac power output. The alternator is attached to the
main generator frame and is driven at engine speed by the main generator shaft. Voltage and frequency
of the alternator, and consequent speed of the motors fed by the alternator, therefore vary with engine
POWER TRANSMISSION SYSTEM
In the engine of the diesel-electric locomotive, as in all internal combustion engines, the relay of
force begins with the push of the piston in the power stroke. Piston force travels through the connecting
rod to the crankshaft, which transmits it to the rotary drive. Cranks of the crankshaft are
counterbalanced and designed to insure an even and smooth distribution of force through the shaft.
Up to this point, power relay has been purely mechanical. If the locomotive were equipped with
gears for its transmission system, the relay of force would be mechanical throughout. Gears in a
transmission similar to that in an automobile, in order to be large enough to control a locomotive, would
be too large and bulky to be practical. A transmission is omitted, therefore, in favor of wires that form
an electrical transmission. These wires lead from the generator to traction motors that change the
electrical power back to mechanical power. Motors are mounted in the locomotive trucks on some
locomotives and are geared to the locomotive axles. About half of the weight of the motor is supported
on the truck frame through a nose on the motor frame and the other half by bearings on the driving axle.
a. Electrical transmission. The task of the electrical transmission system is to receive
mechanical energy from an engine, convert it into electrical energy in a generator, and transmit it by
wires through controllers and relays to traction motors which change it back to mechanical energy at the
wheels. A complete engine-generator set is called a power unit. Some locomotives have two or more
power units, each requiring fuel, water, and oil pumps; radiator fans; and blowers. Besides the main
generator, there is an air compressor and auxiliary generator; also powered by the engine, they supply the
engine-starting equipment, airbrakes, pneumatic controls, and low-voltage light and power circuit.
b. Generator and traction motors.
In diesel-electric transmission, a generator is mounted
directly to the engine