Traction motor




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.