Improved modulation techniques for PWM-VSI drives

PWM-VSI based AC motor drives have two main problems. The inverter is nonlinear which causes instability problems in some specific working points of the AC machine and it emits acoustical noise due to the switching frequency. Nonlinearities like dead-time in the power devices, a load dependent DC link voltage ripple and the voltage drop across the switches are modelled and compensated by improved modulation techniques in order to obtain an ideal inverter. Different feedback and feedforward techniques are proposed. The acoustic noise is reduced by using a random modulation strategy. Measurements show a significant improvement by using feedforward and feedback techniques for linearizing the inverter as well as an improvement in reduction of the acoustic noise emission is achieved by using random modulation. It is concluded that a combination of a random modulation strategy and feedforward/feedback techniques gives an almost ideal AC motor drive system.<>