Abstract – Partial discharge (PD) testing has long been an important tool for assessing the condition of the high voltage insulation in motor and generator stator windings. In the past several years, many motors have been powered from inverters which facilitate variable speed motor operation. The most common drive used today is the voltage-source, pulse width modulation (VS-PWM) type. VS-PWM drives rated up to 13.8 kV are becoming more common in natural gas processing plants, as well as in other petrochemical facilities. Such drives generate high voltage impulses in the kV range with risetimes in the sub-microsecond range. These impulses are a form of severe electrical interference that can make the on-line detection of partial discharge (with magnitudes 1000 times smaller) difficult due to the overlapping frequency content in PD and in the impulses. Thus, PD detection on medium voltage VS-PWM systems has been a challenge in spite of the serious stator winding insulation aging that such drives may cause to these motors. This paper discusses the stator winding failure mechanisms which produce PD, including the insulation problems that VS-PWM drives can accelerate. A research project that lasted several years is reviewed. It culminated in a prototype on-line PD monitoring system suitable for motors fed by VS-PWM drives.