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Soft Start and Voltage Control of Induction Motors Using Floating Capacitor H-Bridge Converters

An application of H-bridges that enable soft start and power factor correction of cage induction machines is presented. The proposed power electronics employs floating capacitor H-bridges connected in series with utility grid and a cage induction motor to provide series voltage compensation. Injecting a series voltage in each phase, the proposed system can be used to control the motor voltage during starting and hence limit the motor starting current. In addition, an algorithm is proposed to accurately predict the worst case scenario for bridge dc capacitor voltage before motor starting. Thus, a dc capacitor overvoltage problem can be avoided. Experimental results show that the proposed system can successfully soft start a standard squirrel cage induction machine under different modes and load conditions. A simple representation of the worst case scenario for dc capacitor voltage before motor starting is established.