Stator insulation problems caused by variable speed drives

About 1990, low voltage motor stator winding insulation failures apparently caused by short rise-time voltage surges from voltage source variable speed drives started to be noticed. The root cause was found to be the large number of pulses from the converter, which caused partial discharges between turns in the stator. These discharges eventually erode the winding wire insulation, resulting in premature failure. Medium voltage motors rated up to 13,200 V and driven by pulse width modulated, voltage-source converters are now being introduced. Early experience with 4160 volt motors driven by such converters again shows that the voltage pulses from the drives may adversely affect the stator insulation. However, it seems that different aging processes tend to occur in medium voltage stators compared to those in low voltage motors. In particular, the aging can be accelerated as a result of dielectric heating of the main wall insulation caused by the high frequency components of the drive voltage. In addition, the voltage stress control coatings often used in medium voltage motors rated 3.3 kV and above have much higher capacitive currents flowing in them when driven by converters - leading to more rapid thermal destruction of the coatings. In this paper, the processes involved in random wound and form wound stator insulation degradation will be described, as well as new IEC standards (IEC 60034-18-41 and IEC 60034-18-42) that allow motor manufacturers to qualify low and medium voltage insulation systems intended for variable speed applications.