MOTOR WINDING PROBLEMS Caused by Inverter Drives

fast rise-time voltage surges surges from a circuit breaker closing can lead to an electrical breakdown of the turn insulation in motor srator windings [1]. If the turn insulation is of an insuffcient thickness or has aged in service, the insulation will puncture when a short rise-time voltage surge occurs. Punctured turn insulation allows for a very high circulating current to flow into the affected copper turn, rapidly melting the copper conductors, which, in turn, results in a consequent burning/melting of the slot liner insulation, thus leading to a stator winding ground fault [2], [3]. Rapid advances in power electronic components in the past decade have lead to a new source of voltage surges. Inverter-fed drivers (IFDs) of the pulse width modulated (PWM) type that use insulated-gate bipolar junction transistors (IGBTs) can create tens of thousands of fast rise-time voltage surges per second. Anecdotal evidence suggests that the large number of voltage surges from IFDs can lead to gradual deteriorations and eventual failure of the turn insulation—both in low voltage (less than 1,000V) and medium voltage (2.3 to 4.16kV) motors [4]–[6]. This article describes measurements of the surge voltage characteristics from a group of eight low-voltage motors driven by IFDs. As described here, two motors have repeatedly been subjected ro unexpected stator winding failures. Inspection of the windings after failure indicated that the main cause of the failures is the voltage surge environment applied to the stator winding.