1.36. ENGINE STARTING CIRCUIT
Activated by the starting button on the operator's panel, the starting circuit provides power to
start the engine. The starting button closes the starting contactors, used to connect the storage battery to
the main generator. Starting contactors consist of coil-operated main contacts, carrying the heavy
starting current, and smaller auxiliary contacts, used as interlocks. Those in the generator field circuit
close first, followed by contactors in the generator armature circuit. If this sequence were not
established, the armature would short circuit the battery when its contactors closed. Interlocks on the
contactors control the sequence automatically when the start button is pressed.
The fuel supply system must be functioning and, since lubricating oil pressure does not exist in a
shutdown engine, the starting switch or related connections must be interlocked to prevent a fuel
shutdown until lubricating oil pressure builds up. The main power controls must be interlocked in an
"off" position to stop power flow when the engine fires. Since the voltage of the main generator is too
high for the control circuit immediately after firing, the starting circuit must be opened automatically.
1.37. POWER CONTROL CIRCUITS
Control circuits are low-voltage circuits that control the switches necessary for handling the
high-voltage power circuits. A typical control circuit is both manually and automatically operated.
Power is generally fed through a master control switch from the source. The operating coils in the
switches appear next to the negative return line of the control circuit. The intermediate equipment
between the control switch and the operating coils consists of manual controls, automatic controls, and
interlocks which determine when the switches are energized.
In an automobile, gears enable you to vary your use of the power being put out by the engine; for
example, they make it possible for you to go up steep hills at a slow speed. But gears large enough to
give the same effect on a locomotive would be much too large to be practical. Another method is used.
Called transition, it is varying the power put out by the generator.
The amount of current required by the traction motors constitutes the load for the generator. By
controlling the generator