Abstract: An AC motor speed controller includes a plurality of capacitors that may be selectively switched, by means of controllably conductive switches, into series electrical connection with an AC motor and an AC voltage source to control the speed of the motor. To change the speed of the motor, a control circuit renders a first switch conductive, in response to a first detected AC voltage zero crossing, to charge a first capacitor to a predetermined voltage. The control circuit then renders a second switch conductive, in response to a subsequent second detected AC voltage zero crossing, to charge a second capacitor to the predetermined voltage. The control circuit then renders both switches simultaneously conductive at a predetermined time after a subsequent third detected AC voltage zero crossing. The capacitors will thereby be charged to the same voltage prior to being switched into series with the motor, thereby resulting in reduced acoustic noise when changing motor speeds.

Abstract: A method for controlling the speed of rotation of a motor by changing the resistance value of a resistor circuit connected in series to the motor and having discontinuous resistance values, including the steps of applying voltage across the motor. Detecting when the applied voltage is lower than a first applied voltage and the resistance value of the resistor circuit is a first resistance value and changing the resistance value of the resistor circuit over to a second resistance value lower than the first resistance value. Detecting when the applied voltage detected is higher than a second applied voltage higher than the first applied voltage and the resistance value of the resistor value is the second resistance value and changing the resistance value of the resistor circuit over to the first resistance value.