Smooth speed control of three-phase induction motor by integral-cycle switching

The speed of three-phase induction motors can be controlled in steps by controlling the applied voltage using an integral-cycle switching method. However, the OFF period causes large pulsations in generated torque. Moreover the induced EMF in the windings and decelerating rotor during the OFF period, cause large reswitching currents as well as large transient-negative torques. They cause abnormal performance of the motor and even proportional speed variation in steps becomes difficult. A combination of integral-cycle control and switching-instant control on the applied voltage wave (in the first half-cycle during ON period) is employed for the fine voltage variation of one-phase of the stator input. This reduces peak re-switching current as well as transient-negative torque and therefore smooth and proportional speed variation is obtained