A traction motor is an electric motor used to drive a piece of machinery. Consisting of heavy wire wound inside of the motor's case known as the field windings and wire wound around the motor's inner shaft called the armature, the electric power is transported between the two elements through brushes. The brushes are small, spring-loaded metallic components upon which the armature's exposed shaft rides. Many systems are used in the motor to redirect excess power as well as to use the electricity to reverse the motors and act as brakes.
There are many sizes and styles of traction motors in use around the world; however, the most commonly recognized form is found in a diesel locomotive. The diesel engine in a locomotive turns a large generator that supplies the power to the electric motor that actually propels the train. The official name for the motor that powers escalators, electric automobiles and even clothes-washing machines is a traction motor. Both alternating current (AC) and direct current (DC) electric motors have been used as traction motor designs on a vast array of machines, with the most successful design using the AC motor.
One reason for the use of a traction motor on a diesel locomotive is the ease with which the power can be applied to the wheels. Unlike an internal combustion engine, an electric motor begins making power in the form of torque, as soon as the motor receives electrical power. This allows the motor to begin easily pulling the load at very slow speeds. In the case of an internal combustion engine, the engine typically makes the majority of its pulling power in the mid to upper revolutions per minute (RPM) engine speed. With the electric motor coming on so strong at such a slow speed, the engineer is able to better control the starting of the train into motion without damaging the train, the cargo or the train's crew.
In an effort to reduce weight, gear boxes used to direct power from the motor to the wheels were replaced by placing the wheels directly onto the motor's center shaft. With the motor working as its own axle, the locomotive's chassis as well as the tracks were given a longer life due, in part, to the reduction of weight being applied to both components. By placing specially designed resistors in the motors, the speed control is much easier than the early traction motor models in which the engineer was continually forced to manually reduce the amount of current being sent to the motors in order to maintain a steady speed.