The impact of surge voltage rise-time on PD inception voltage 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, some of these failures have been caused by exposure to fast rise-time voltage surges coming from inverter drives. Dissection of failed stator windings has indicated that partial discharges have been a predominate cause of the stator failure. Voltage surges of a sufficient magnitude and rise-time can result in PD, degrading the organic magnet wire enamel insulation and eventually resulting in turn-to-turn, phase-to-ground or phase-to-phase failures. Specialized instrumentation allows for the full characterization of the surge environment when applied to a random wound stator winding. To help interpret these results, experiments relating surge rise-time to PD inception voltage for various stator designs were performed. The paper presents and discusses the results of these initial experiments. This paper does not include a proposal for, or a discussion about a model for discharge behavior under surge conditions in random wound stators.