The pulse width modulation (PWM) technique (also known as a chopper drive) is used to adjust the voltage applied to the motor. With the variation of the Pulse 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 an AC mains is the source), a power switch and a fast power diode for motor current free-wheeling at switch-off. The advantage of PWM modulation with respect to phase-angle partialization method is higher efficiency, less acoustic noise and better EMC behavior, but it can have an impact on the brush’s duration.
To improve the efficiency of universal motors, a DC control can be implemented. Indeed, applying a DC voltage to the motor will reduce the iron losses due to the eddy current. The easiest way to implement such a DC control is to use a Triac in series with the motor inside a diode bridge. The motor speed control is then still achieved with phase-control, thanks to a low-cost Diac circuit or with a low-end microcontroller.
In this application, the rate of decrease of the Triac current is only limited by the mains spurious inductor, this is why an inductor has to be added in series with the AC switch. Using AC switches with a very high (di/dt) parameter allows this inductor size to be drastically reduced. The advantages versus a PWM drive control is increased robustness in regards to overvoltage, higher overcurrent capability and reduction of EMI noise due to high-frequency device switching.