Universal Motor High frequency PWM Drive
The pulse width modulation (PWM) technique (also known as chopper drive) is used to adjust the voltage applied to the motor. With the variation of the PWM duty cycle, the effective voltage seen by the motor can be changed. Compared to a phase-angle drive, a chopper drive requires a more complicated power stage with an input rectifier (if AC mains is the source), a power switch and a fast power diode for motor current recirculation at switch-off. The advantage of PWM modulation with respect to phase-angle partialization is higher efficiency, less acoustic noise and better EMC behavior, but it can have an impact on brush life duration.
AN422: Improved Universal Motor Drive