Dealing with motor winding problems caused by inverter drives

Random wound stator windings operating in utility and industrial plants have failed when exposed to the fast rise-time voltage surges coming from inverters. Studies show that the failure is due to a combination of bad luck in specific motor installations (resonance phenomena caused by power cable length and surge impedance ratios) together with the fact that modern inverter-fed drives (IFDs) create tens of thousands of surges per second with rise-times as fast as 50 ns. Measurements on motors show that these surges create partial discharges (also called corona) and these discharges may eventually destroy the turn-to-turn and/or phase-to-phase insulation, resulting in premature motor failure. The paper discusses the specific mechanisms involved in the stator winding failure due to IFDs and present the measurements and analysis from surge monitoring installed on many different motors. Although some motors may experience short rise-time, high magnitudes surges, most motors experience either low magnitude and/or long rise-time surges, which are relatively harmless. Usually, several different magnitudes and rise-times are present from the same IFD. Thus, it seems that conventional motor stators can be safely used in many (but not all) IFD applications. Methods are presented to determine when special IFD duty motors are needed. Voltage surge tests, as well as partial discharge tests, can help the user insure that motors can successfully operate in severe applications