Inverter-Fed Drives: Which Motor Stators Are at Risk?

Since the 1930s, engineers have known that fast rise-time voltage surges from a circuit breaker closing or lightning can lead to turn-insulation breakdowns in motor stator windings [1], If the turn insulation is of insufficient thickness or has degenerated in service, it punctures when a short rise-time voltage surge occurs. Punctured turn insulation allows a very high circulating current to flow in the affected copper turn, melting the copper conductors and, consequently, burning/melting the slot-liner insulation. This then, leads to a stator-winding ground fault [2], [3].