Acoustic noise and pulsating torques in a triac-controlled permanent-split-capacitor fan motor

A normally quiet single-phase fan motor may become noisy when speed controlled using a conventional triac AC controller. The paper describes an investigation into the cause of this phenomenon. It is shown that the odd-time harmonic voltages introduced by the controller produce pulsating torques at all even multiples of the supply frequency, and that the extra noise components occur at these frequencies. A simple modification to the triac circuit is shown to reduce the effect, at the cost of increased power dissipation in the controller.