Soft start of induction motors using floating capacitor H-bridge converters

This paper presents the results of a study to investigate the use of IGBT hard-switched power electronics to soft start a squirrel cage induction motor when operating from the utility grid. The paper presents the theory necessary to understand how three floating capacitor H-bridge converters can be used to control the motor voltage during starting and hence limit the motor starting current to the desired level. The proposed soft-start system can also be used during steady state operation for other purposes such as making the utility power factor to be less lagging, unity or leading, hence enabling VAR control and grid voltage support in large motor applications. Hardware implementation is presented together with experimental results that show that the proposed control strategy can successfully soft start a standard squirrel cage induction machine under a range of load conditions. The experimental results demonstrate the ability to implement current limit control during starting.