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Progress in understanding the nature of PD in stators created by inverter drives

Random wound stator windings in motors operating in industrial plants have been subjected to premature failures. Dissection of failed stator windings have indicated that partial discharges have been the predominate cause of failure. Partial discharge (PD) may occur between turns during fast rise time voltage surges of sufficient magnitude and may eventually result in turn-to-turn or phase-to-phase failure. By presenting actual measurements of surge environments and the characteristics of partial discharges measured during these surges, this paper discusses the specific mechanisms, and the specific conditions, under which partial discharges may occur in low voltage motors subjected to inverter fed drives. Emphasis is put on recent development outlining progress in the area of surge testing from a diagnostic point of view.