A Novel Switching Scheme for Three Phase PWM AC Chopper Fed Induction Motor

AC Voltage Controllers are finding important application in speed control of induction motors, light dimmers, heat controllers and soft starters. AC Chopper is an arrangement of bi-directional switches to control the output voltage by varying the duty-cycle of operation of the switches involved. In this paper, a new switching scheme for three phase ac chopper is discussed which requires only three variables for converting the fixed input ac voltage to a controlled ac voltage. The added advantage is that this scheme takes into account the safe operation of the ac chopper by preventing short-circuit conditions. At the same time it provides a path to the motor current when the terminal voltage falls to zero. Implementation of the proposed scheme leads to much simpler control circuitry than discussed in the literature. A Three phase 3HP induction motor-fed from three phase ac chopper is used for the work. The simulation results confirm the improvement in power factor, leading to power savings.