resistance cause a change of torque on the torque motor armature, the armature will move the sectors in
such a direction as to cut short field resistance in or out and re-establish the set voltage at the same
voltage for every position.
b. Battery system. The storage battery supplies power for cranking the engine and also to
supply power to the control and lighting circuits before the engine is started. After the engine starts,
these circuits draw on the auxiliary generator. The battery is beneath the underframe, beneath the cab
floor, or in the compartment next to the cab. A switch in the equipment cabinet or on a nearby panel
connects the battery to the auxiliary generator and control circuits. The battery ammeter indicates the
rate of charge or discharge of the battery; it should indicate zero or various charge readings when the
engine is running. As soon as the engine is cranked, the auxiliary generator starts to restore the current
and the ammeter should register a charging current until the battery is charged. The battery may also be
charged from shop circuits or terminal yard circuits through external charging receptacles. When the
engine is shut down or if the auxiliary generator voltage drops too low to charge the battery, a reverse
current relay opens a battery contactor between the auxiliary generator and the battery, preventing the
battery from discharging back into the auxiliary generator windings.
Measures of electrical pressure, resistance, and quantity are called volts, ohms, and amperes. A
volt is the unit of pressure leaving the generator or battery; an ohm is a unit of resistance; and an ampere
is the unit used to measure power available to the receiving mechanism, such as one of the traction
motors. Ohm's law states the relationship between these: current equals voltage divided by resistance.
Voltage is measured by a voltmeter and amperage by an ammeter.
Wiring diagrams, using lines and standardized symbols and abbreviations, are used in tracing
circuits and locating troubles on diesel-electric locomotives. Wiring in the electric system is built to
carry a specific load of current; current heavier than that specified is called an overload. Since an
overload in the wiring can harm equipment, fuses and circuit breakers are provided to break the circuit
before damage occurs.
Mechanical energy can be changed into electrical energy, or electrical into mechanical, by a
dynamo. If the mechanical energy