An overvoltage suppression scheme for AC motor drives using a half DC-link voltage level at each PWM transition

Abstract

Passive filters are conventionally used to suppress overvoltage in the motor terminal, either by reducing the voltage rising rate at the inverter output, or by decreasing the motor terminal impedance. We propose an overvoltage suppression scheme that renders the use of passive filters unnecessary. This approach differs from general filter methods, in that it is independent of dv/dt and does not try to reduce dv/dt. Our scheme utilizes the middle voltage level V-DC/2 at each pulsewidth modulation voltage transition, where V-DC represents the de-link voltage. The duration of the middle voltage level is controlled in such a way that reflected voltages are cancelled out at the motor terminal. Optimal cancellation is achieved when the duration of V-DC/2 is equal to twice the transport delay of the cable. Further, if reflection coefficients at the motor terminal and the inverter output are equal to +/-1, no overvoltage takes place. The proposed scheme requires the use of six auxiliary insulated gate bipolar transistor switches. Simulation as well as experimental results are presented here.