Review of stator insulation problems in medium voltage motors fed from voltage source PWM drives

In the past 10 years, motors rated 3.3 kV and above are increasingly being fed from voltage source, pulse width modulated converters to control motor speed and power output. Such converters produce short risetime voltage impulses that can accelerate the aging of the stator winding turn insulation, mainwall insulation and the surface electric stress relief coatings. This paper reviews the mechanisms that may lead to this premature aging, and outlines the status in developing IEC voltage endurance tests which enable motor manufacturers to design insulation systems that reduce the rate of deterioration by such drives. Experience with an on-line partial discharge monitoring system to detect premature stator winding insulation degradation in 45 MW natural gas compressor motors is summarized.