A
traction motor is an
electric motor used for propulsion of a vehicle, such as
locomotives,
electric or
hydrogen vehicles, or
electric multiple unit trains.
Traction motors are used in electrically powered railway vehicles (
electric multiple units) and other
electric vehicles including electric
milk floats,
trolleybuses,
elevators,
roller coasters, and
conveyor systems, as well as vehicles with electrical transmission systems (
diesel–electric locomotives, electric
hybrid vehicles), and
battery electric vehicles.
Direct-current motors with series
field windings are the oldest type of
traction motors. These provide a speed-torque characteristic useful for propulsion, providing high torque at lower speeds for the acceleration of the vehicle, and declining torque as speed increases. By arranging the field winding with multiple taps, the speed characteristic can be varied, allowing relatively smooth operator control of acceleration. A further measure of control is provided by using pairs of motors on a vehicle in
series-parallel control; for slow operation or heavy loads, two motors can be run in a series of the direct-current supply. Where higher speed is desired, these motors can be operated in parallel, making a higher voltage available at each motor and so allowing higher speeds. Parts of a rail system might use different voltages, with higher voltages in long runs between stations and lower voltages near stations where only slower operation is needed.
A variant of the DC system is the AC series motor, also known as the
universal motor, which is essentially the same device but operates on
alternating current. Since both the armature and
field current reverse at the same time, the behavior of the motor is similar to that when energized with direct current. To achieve better operating conditions, AC railways are often supplied with current at a lower
frequency than the commercial supply used for general lighting and power; special
traction current power stations are used, or
rotary converters used to convert 50 or 60 Hz commercial power to the 25 Hz or 16+2⁄3 Hz frequency used for AC traction motors. Because it permits the simple use of
transformers, the AC system allows efficient distribution of power down the length of a rail line, and also permits speed control with switchgear on the vehicle.
AC induction motors and
synchronous motors are simple and low maintenance, but up until the advent of
power semiconductors, were awkward to apply for traction motors because of their fixed speed characteristic. An AC induction motor generates useful amounts of power only over a narrow speed range determined by its construction and the frequency of the AC power supply. The advent of power semiconductors has made it possible to fit a
variable frequency drive on a locomotive; this allows a wide range of speeds, AC power transmission, and the use of rugged induction motors that do not have wearing parts like brushes and commutators.
[11]