Electric induction motor safe starter that senses back EMF to determine rotor movement before connection to mains supply

摘要

A starting circuit for an induction motor is provided whereby a stationary rotor flux is established followed by supplying a storage charge to a motor winding (between terminals 6,8,10) in order to provide a starting torque. Before applying a mains power supply (4) to the motor after the starting torque has been applied, a back EMF is sensed to detect an instantaneous back EMF generated on removal of the starting current that was used to provide the starting torque. This detection, using the voltage measurement at capacitor C1 when switch S2 turns off, provides an indication that the rotor is turning before the mains supply is connected to the motor. This provides a reliable and simple method for ensuring that a mains supply (4) can be safely connected to the motor.