Solid state motor speed control

摘要

A solid state motor speed control is described that is capable of controlling a motor so that it operates quietly, eliminating annoying buzzing noises that would otherwise be generated by the motor's windings. The invention provides for motor speed control with infinitely variable speed control settings. Rather than use triacs that generate RFI noise and cause motor windings to buzz due to their inherent avalanche switching, the invention utilizes power FETs or IGBTs to provide the switching function to control power delivery to the motor load. The power switches are controlled by an oscillator that generates a pulse train with a duty cycle controllable by the user. A filter shapes this signal into a trapezoidal control signal that drives the gate input of the power FET or IGBT switch. A kick start circuit turns the motor on initially for two to three seconds to overcome the inertia of the motor and its load. A zero crossing detector prevents the motor from being turned on except at a zero crossing of the AC source to further reduce noise.