Stator insulation problems associated with low voltage and medium voltage PWM drives

About 15 years ago, the short risetime pulses caused by voltage source, pulse width modulated converters were first recognized as a cause of stator winding insulation failure in low voltage motors. The root cause was found to be the large number of PWM voltage pulses from the drive, which caused partial discharges between turns in the stator. These discharges eventually eroded the magnet wire insulation, resulting in failure. Since the first reports of failures, motor manufacturers have developed many ways to eliminate or slow down this process. Medium voltage motors rated up to 7200 V and driven by pulse width modulated, voltage-source converters are now being introduced. Although relatively rare to date, such systems are expected to play a larger role in cement plants in the future. Early experience with 4160 volt motors driven by such converters again shows that the voltage pulses from the drives may also 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 groundwall insulation caused by the high frequency components of the drive voltage. In addition, the voltage stress control coatings often used in medium voltage motors have much higher capacitive currents flowing in them when driven by converters - leading to more rapid thermal destruction of the coatings. The processes involve are described. The paper also reviews the development of new IEC standards that for the first time will define the stator insulation requirements in a converter duty motor