Surge related partial discharges resulting from IFDs applied to random wound motors

Random wound stator windings in motors operating in utility and industrial plants through out the world have been reported to suffer from premature winding 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 have indicated that partial discharges have been a predominate cause of the stator failure. Partial discharges may occur between turns during fast rise-time voltage surges and result in degradation of the organic magnet wire enamel impregnants, eventually resulting in turn-to-turn or phase-to-phase failures. In this paper, measurements of the surge environment for several motors are presented, some of which have suffered stator winding failures. Inspection of the winding after failure showed local melting caused by puncture of the insulation. Utilizing custom PD detection instrumentation and a variable rise-time surge generator, measurements of partial discharge inception voltage (DIV) have been made. Combining the DIV data; with the actual surge environment from inverter drive allows assessment of the risk of motor failure due to PD related insulation breakdown.