The impact of humidity on PD inception voltage as a function of rise-time in random wound motors of different designs

Random wound stator windings in motors, operating in utility and industrial plants, have been reported to suffer from premature failures. Although several failure mechanisms exist, dissection of failed stators show some of these failures have been caused by exposure to fast rise-time voltage surges coming from inverter drives. Partial discharges may occur between turns during fast rise-time voltage surges. Previously, the relationship between Discharge Inception Voltage (DIV) and rise-time has been characterized for magnet wire insulation samples and stators of various designs under laboratory conditions. As well, DIV and surge environment measurements performed in the field on motors suspected to exhibit stator winding problems related to surge voltages have been carried out. In this paper, the influence of ambient humidity on DIV for a magnet wire insulation sample is studied. The results are discussed in relation to DIV curves obtained on stators with confirmed problems in regards to withstanding the surge voltage environment applied during normal operation. The paper presents and discusses the results of the experiments and does not attempt to put forward a theoretical model for discharge behaviour under surge conditions in random wound stators.