Repetitive Control Based Pwm Inverter Dead-Time Compensation For Ac Servo Drive

Abstract

AC servomotor speed or current controller based on PI control and/or optimal control, can not eliminate the cyclical fluctuations of speed or current when subject to periodic disturbances. However, repetitive control performs well when the disturbance is cyclical with constant period (frequency). For a PWM (Pulse Width Modulated) inverter driven AC servomotor, a time delay is inserted between the switches of the same leg, to prevent the phase shortage of inverter arms. This time-lag causes serious distortions ofthe output current of the inverter. This is known as dead-time effect. This effect is cyclical as it is repeated at every period ofthe sinusoidal motor current. Conventional dead-time compensation method based on current reference polarity, is widely used in industry to improve the output current waveform. The improved waveforms however still suffer from the zero current crossing phenomenon which is cyclical in angle domain not in time domain. This paper analyses the origins of waveforms distortions around the zero current crossing in PWM inverters. A proposed method, based on angle domain repetitive control to reduce the distortions in the PWM inverters output waveforms caused by the dead-time and the zero crossing problem is described. Theoretical analysis as well as simulation results, to verify the proposed method, are described in this paper.