Abstract— On-line partial discharge (PD) measurement is now widely applied to motors rated 6 kV and above that are fed from the 50/60 Hz power system. However, during the past several years, many motors are being powered from inverters which facilitate variable speed motors to improve process efficiencies. One of the drives now being used is of the voltage source, pulse width modulation (VS-PWM) type. Such drives generate high voltage impulses in the kV range with risetimes in the sub-microsecond range. These high voltage spikes are a form of severe electrical interference which can make the detection of PD (with magnitudes 1000 x smaller) difficult due to the overlapping frequency content in PD and the spikes. Thus, PD detection on VS-PWM fed motors has been a challenge in spite of the serious insulation aging that such drives may cause. This paper outlines the challenges in measuring the stator winding PD in motors fed by VS-PWM drives and describes a newly-developed measurement system. Results from 12.5 MW and 45 MW compressor motors are presented.