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 conveyors, as well as vehicles with electrical transmission systems (diesel-electric locomotives, electric hybrid vehicles), and battery electric vehicles. Motor types and control 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 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 series off 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 voltage 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 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