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. 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.Ī 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. 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. 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. By arranging the field winding with multiple taps, the speed characteristic can be varied, allowing relatively smooth operator control of acceleration. 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. Direct-current motors with series field windings are the oldest type of traction motors.
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