1.34. MAIN CIRCUITS
Power from the main generator is carried by electric cables to the traction motors, which are
geared to the driving axles. The main power circuit is this path of current flowing from the generator
through the cables and motors, and back to the generator. Locomotives have three types of circuits:
series, parallel, and series-parallel. In a series circuit, the same current passes through each device and
connection in completing its path to the source of supply, and the total resistance of the circuit is equal
to the sum of the resistance of all its portions. In a parallel circuit, the current from the source divides
through two or more parallel paths and the total current from the source equals the sum of the current in
the parallel paths. The resistance of a parallel circuit is always lower than the resistance of any of its
individual parallel paths. A series of Christmas tree lights in which all go out when any one burns out
(opens the circuit) is a familiar example of a series circuit; each bulb can stand only a portion of the
voltage from a house circuit. The lights in which the other bulbs continue to burn when one burns out is
a parallel circuit; each bulb operates on house voltage. If motors are connected in the same manner as
the lamps, the same circuit characteristics apply. The difference in operating characteristics of motors at
different voltages is a chief reason for the relatively greater number of methods used for connecting
traction motors.
Many different designs of main power circuits are used. Each circuit is independent of the
others, except for the small wires in the wheel slip relay circuit. Connections in many locomotives are
permanently joined in series-parallel. A tie between the motor circuits exists when portions of the
circuits are in series-parallel and no contactors are used. In order that full generator voltage can be
applied when the motors are in series connection, a contactor will close if the tying circuit is in use and
other contactors will open the circuit to and from the generator.
In electric drive, power can be easily varied. The throttle adjusts voltage, current, and tractive
effort. While the throttle remains in the first notch, the current, or amperage, decreases rapidly because
the motors, which are increasing speed, develop a greater counter-voltage. If the throttle is moved to a
higher position, the fuel supply is increased and the engine and generator can deliver more power.
When the throttle is advanced as far as possible, engine speed and generator voltage are at their
maximum.
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