and negative connections are designated on voltmeters as well as on ammeters.
The greatest damage likely to come from overloading a locomotive is overheating the traction
motors. When the heat generated by the motors is equaled by the rate at which heat is dissipated, a
"continuous rating" is attained. An ammeter indication greater than the continuous rating is called an
overload. Since the motor itself can absorb some heat, a cold motor will not cause an overload condition
as quickly as a warm motor; whereas a motor that has been operating at the "continuous rating" limit for
a long time does not have an overload capacity at all.
Wiring is built to carry a certain load of current; but heavy current loads occasionally are
imposed on it. An overload, if allowed in the wiring, can damage insulation and equipment. Overload
protection is provided by devices, such as fuses and circuit breakers, that break the circuit before damage
occurs. A fuse is inserted in the circuit and carries the full load of current. It has a low melting point
and, when the current becomes excessive, melts and opens the circuit. Since it operates by melting, a
fuse can be used only once. The circuit breaker, a permanent fixture, consists of a switch that is opened
by a magnet coil when the current in the circuit reaches the danger point.
A dynamo is a machine that converts mechanical energy into electrical energy, or electrical
energy into mechanical energy. When the output is mechanical, the dynamo is called a motor; when
electrical, a generator. A generator and a motor have the same basic construction and either may be
called a dynamo. A dynamo is reversible and capable of operation as either a generator or a motor.
Basically, a dynamo consists of a field or stationary coil and an armature or rotating coil.
In locomotives, the fields of the dynamo consist of magnetic pole faces bolted to the frame.
Coils wound around the field poles are called field coils. The amount of current in these coils
determines the strength of the magnetic field. The magnetic field, or flux through the poles, is termed
the field excitation. The stationary pole and frame are called the stator; and the rotating coil, which
carries the load current, is called the armature or rotor.