Low cost speed controller for small AC motors

The article introduces simple and low cost PWM speed controller for the fractional power AC motors (less than 500 W). Variable speed settings are normally required to match the load requirement, reduce acoustical noise, and energy savings in blenders, air blower etc. In the proposed circuit, one transistor and a diode bridge are configured to vary the AC supply across the motor. Switching the transistor ON and OFF at a fast rate results in variable AC voltage. By using the proposed circuit. the PWM modulated AC supply has high frequency harmonic of which the first harmonic is dependent on the operating switching frequency. This in turn makes filtering the harmonics a simple task. Comparison between the phase controls circuits normally adopted for speed control with the proposed controller was investigated using PSpice models.