Soft start and voltage control of grid connected induction motors using floating capacitor H-bridge converters

A system is described whereby floating capacitor H-bridge converters are located in each phase between the utility grid and a squirrel cage induction motor. Injecting a series voltage in each phase, the power electronics can be used to control the motor voltage during starting and hence limit the motor starting current to the desired level. The motor voltage can also be used under steady-state operation either to control the grid VAR or to compensate for grid voltage sags and swells. The paper presents the theory necessary to understand how to adjust the phase and magnitude of the motor voltage relative to the utility grid. Hardware experimental results show that the proposed system can successfully soft start a standard squirrel cage induction machine under various load conditions. Moreover, a range of objectives can be achieved including: power factor correction and motor voltage control.