Smart Electrical Grid Interface Using Floating H-Bridges to Improve the Performance of Induction Motors

Three-phase floating H-bridges can be inserted between the utility grid and an induction motor to inject a voltage in series with the grid voltage. The magnitude and phase of this voltage is used to control the motor voltage under steady-state operation so as to lower the motor power losses over its entire load range. This is made possible because the three-phase H-bridge can be used to both increase and decrease the motor voltage relative to the grid voltage as required. This feature affects the performance of the overall system, more specifically, the motor power conversion efficiency is improved, lowering its operating temperature and, hence, improving reliability and lifetime expectancy. A variety of motor voltage control options exist, the controller presented is suitable for applications using loads such as fans and pumps, where variable frequency is not required. Readily available machine nameplate data are used to identify the motor output power associated with its maximum efficiency operating point when operated under rated voltage. This data are then used as the basis to control the motor voltage according to the square root of the measured motor input power. The controller and the performance of the three-phase H-bridge are described. The benefits of the variable voltage control are assessed by comparing the motor performance with the machine operated at its rated voltage. The chosen controller also results in the H-bridge dc voltages being relatively low and constant over a wide load range, hence lowering the power losses and electrical stress in the power electronics. Both experimental results and theoretical predictions are used to illustrate the performance of both the three-phase H-bridge and the motor.