Methods to determine which inverter drives need upgraded motor stator windings

Random wound stator windings in motors operating in pulp and paper plants have failed when exposed to the fast-risetime 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 risetimes as fast as 50 ns. Measurements on motors show that these surges create partial discharges (also called corona), and these discharges eventually destroy the turn-to-turn and/or phase-to-phase insulation, resulting in premature motor failure. This paper discusses the specific mechanisms involved in the stator winding failure due to IFDs and presents the measurements and analysis from surge monitoring installed on many different motors. Although some motors may experience short risetime, high magnitude surges, most motors experience either low magnitude and/or long risetime surges, which are relatively harmless. Usually several different magnitudes and risetimes 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. Surge voltage tests, as well as partial discharge tests can help the user insure that motors can successfully operate in severe applications.