Influence of PWM-type voltage waveforms on reliability of machine insulation system

This paper presents the results of life tests performed on models of turn insulating materials of AC rotating machines, having the purpose to infer the aging acceleration effects due to distorted voltage waveforms. PWM speed drives, indeed, have been shown to affect considerably the reliability of AC motors, due to the unusual voltage waveforms supplied to them. These waveforms are significantly different, regarding harmonic content, with respect to those generally considered as the reference for insulation design. The results discussed in this paper show that the main factor affecting insulation aging acceleration is the increase of peak supply voltage, due to wave reflection caused by impedance mismatch of drive and motor, which is applied to turn insulation. Of course, commutation frequency plays also a fundamental role, while less evident is the contribution of rise and fall rate of supply-voltage pulses. A life model is proposed, based on a statistical analysis of life data, which allows evaluation of the degradation acceleration factor.