SCR protection from stalled motor without current sensing

摘要

A motor controller controls a motor operated by an AC line voltage. The controller includes an SCR connectable between an AC line and a motor terminal for controlling application of AC line voltage to the motor. A voltage sensor is operatively associated with the SCR to sense voltage across the SCR. A control circuit is connected to the sensor and the SCR for varying relative duration of on time and off time during each cycle of line voltage to control motor voltage at a reference level. The control circuit determines if a motor stall condition occurs responsive to the sensed SCR voltage and, in response to determining that a motor stall condition occurs, reducing motor voltage by decreasing duration of on time to protect the solid state switch means SCR.