Traction motor consists of two
parts, a rotating armature and fixed field windings surrounding the rotating
armature mounted around a shaft. The fixed field windings consist of tightly
wound coils of wire fitted inside the motor case. The armature is another set
of coils wound round a central shaft and is connected to the field windings
through "brushes" which spring-loaded contacts pressing against an
extension of the armature are called the commutator. The commutator collects
all the terminations of the armature coils and distributes them in a circular
pattern to allow the correct sequence of current flow. It has a low resistance
field and armature circuit. Because of this, when voltage is applied to it, the
current is high. The advantage of high current is that the magnetic fields
inside the motor are strong, producing high torque (turning force), so it is
ideal for starting a train. The disadvantage is that the current flowing into
the motor has to be limited, otherwise the supply could be overloaded and its cabling could be damaged. At best, the
torque would exceed the friction and the driving wheels would slip. Resistors
are use to limit the initial current. As the DC motor starts to turn, the
interaction of the magnetic fields inside causes it to generate a reverse
voltage internally. This "back-EMF" (electromagnetic force) opposes
the applied voltage and the current that flows is governed by the difference
between the two. As the motor speeds up, the back-EMF rises, the resultant EMF
falls, less current passes through the motor and the torque drops. The motor
naturally stops accelerating when the forces due to weight of the train match
the torque produced by the motors. To continue accelerating the train, series
resistors are switched out step by step, each step increasing the effective
voltage and thus the current and torque for a little bit longer until the motor
catches up. When no resistors are left in the circuit, full line voltage is
applied directly to the motor. If the train starts to climb an incline, the
speed reduces because forces due to weight of the train is greater than torque
and this reduction in speed causes the back-EMF to fall and thus the effective
voltage to rise - until the current through the motor produces enough torque to
match the new require force.