Squirrel cage induction motors switching

The most recent development in the starting of squirrel-cage induction motors is the introduction of the electronic soft-start. This principle has been derived from variable-frequency speed controllers using switched Thyristor or power transistor bridges. The supply sine wave is chopped so that a reduced voltage and frequency is applied to the motor.These are gradually increased so that the motor speed rises in a controlled manner, with the starting current limited to any chosen value. 

In this group there are three alternative possibilities that are practical. The first possibility uses an intermediate device between the sinusoidal supply and the motor to vary the magnitude of the voltage applied to the motor. Secondly, simple switching methods are available when two or three discrete speeds are required. These are usually obtained by special stator winding arrangements for squirrel-cage induction motors. 

AC4 - Stringent inductive. switching, such as the switching of a squirrel cage motor with inching and plugging operations (Section 6.20.1(B))... 

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