Effect of pulse width modulated voltage on induction motor insulation

Pulse-width modulated adjustable frequency drives are extensively used in the industries for control of induction motors. It has led to significant advantages in terms of the performance, size, and efficiency of variable speed motor controls but the applied voltage and current waveforms become non sinusoidal. These non sinusoidal voltage and current waveforms result into voltage & current peaks, voltage transients and increased harmonics. Voltage peaks and voltage transients increase the insulation stress. Current peaks increase the thermal stress in the insulation. Voltage harmonics increase dielectric and core losses. The increased thermal and voltage stress result into accelerated insulation aging; hence life of the motor is reduced. Paper presents the effect of this voltage overshoot on the insulation of three-phase induction motors. A comparison between PWM drive and conventional sinusoidal voltage is presented by experimental results. The results compare quantitatively the insulation stress parameters due to PWM drive & sinusoidal voltage and current for the motor. The results can be used to estimate the accelerated insulation ageing due to PWM drive in three-phase induction motors.