Analysis of overvoltage on motor winding insulation fed by PWM pulses

Abstract

The introduction of Adjustable/Variable Speed Drives (ASD/VSD) caused new exposures for motor insulation systems. Particularly the application of fast transistor switches and pulse-width modulated (PWM) train of pulses may result in the overvoltages on motor insulation. The mechanism of partial discharges in insulating system has been widely investigated for sinusoidal voltage shape. However, fast rising PWM pulses may result in another mechanism of partial discharges initiation and further evolution and thus the degradation of insulating material. The paper discuses the influence of PWM pulses shape on expected exposures on electric motor insulation. For this aim the voltage pulse shape may be represented generally as the trapezoidal one. Voltage pulse of such a shape is propagated through feeding cable to the motor insulation input. As the effect the time transient voltage response arises at this point and acts on the motor insulating system. The shape and value of such an overvoltage depend on the cable parameters, the equivalent impedance of the winding, and on the parameters of feeding voltage shape, i.e. the slope of voltage increase and decrease.