is changed into electrical, the dynamo is called a generator; if the electrical energy is changed into
mechanical, the dynamo is a motor. A generator can be either the alternating-current or the direct-
current type. Current is set up in the generator's armature coil whenever the coil cuts across the lines of
magnetic force between the generator's poles. With an alternating-current generator, the current flows
through the coil first in one direction then the other unless the generator has a commutator to turn the
alternating current into a direct current. If a generator has many coils, connected to form a closed circuit,
a direct current is supplied. Direct current generator coils can be connected in series, in shunt, or in a
combination of series and shunt. Like a generator, a motor can a also be connected in shunt or in series.
Traction motors are series-wound, direct-current motors and are provided with a shunt. Their function is
to convert electrical energy from the generator into mechanical force to turn the locomotive wheels.
Because of the locomotive's size and weight, gears large enough to control it would be too large
and bulky to be practical. Mechanical gear transmission of power to the wheels is therefore replaced by
electrical transmission. To change the mechanical force from the engine into electrical power, a
generator is operated by the engine's crankshaft. Wires from the generator transmit the power to traction
motors and the traction motors turn the wheels.
Electrical circuits in the locomotive are connected in series, in parallel, or in series-parallel, a
combination of the two. Circuits are opened and closed by contactors, operated either by compressed air
in heavy circuits, or by current from the battery in circuits where the current is low. Auxiliary switches
to control the connecting or breaking of circuits are called interlocks. A relay is a device that changes
connections in one part of a circuit in response to changes taking place in another part. Changing
traction motors from series connection to series-parallel or parallel connection is known as transition. It
can be done by connecting the motors in parallel or by shunting off part of the current drawn into the
circuit, forcing more current to be drawn from the generator. A traction motor cutout switch is used to
take the motors out of circuit if there is an electrical failure.
In dynamic braking, the locomotive's wheels are used to drive the traction motors which, acting
like generators, slow the locomotive's speed without causing wear of the wheels. When brakes are
applied, a pneumatic switch stops the engine, stops the fuel pump, and turns on indicating lights.