Simplified model and corrective measures for induction motor instability caused by PWM inverter blanking time

Abstract

Blanking time in PWM (pulse width modulation) inverter switching is known to cause distortion of the AC voltage waveform and can induce instability in induction motors. A fundamental component equivalent circuit of the inverter in which the effect of the waveform distortion is represented as an equivalent source resistance is derived. The added resistance is shown to be the cause of instability. The equivalent circuit is shown to be useful in predicting the current and speed waveforms of induction motors fed by PWM inverters. This circuit is useful for simulating the effect of the blanking time, and requires far less computation than a full PWM inverter model. A simple corrective measure that adds an offset to the PWM reference signal according to the polarity of the load current is described. The compensation corrects the distortion due to the blanking time, and reduces the rotor oscillations due to the motor instability. >