Pulse-Width Modulation DC Motor Control

In order to control the speed of a dc series field motor at different required torque levels, it is necessary to adjust the voltage applied to the motor. For any particular constant voltage the motor speed is determined solely by the torque requirements and top speed is reached under minimum torque conditions. When a series motor is used as a traction drive for vehicles, it is desirable to control the voltage to the motor to fit the various torque requirements of grades, speed, and load. The common method of varying the speed of the motor is by inserting resistance in series with motor to reduce the supplied power. This type of motor speed control is very inefficient and wasteful of battery power due to the I2R loss, especially under high current, high torque conditions.