Medium Voltage PWM ASD Waveform Effects on Large Motors

This paper addresses the effects that modern Medium Voltage (MV) adjustable speed drives (ASDs) have on the large motors they control. Most published technical literature on ASDs describe the issues associated with Low Voltage (LV) ASDs, their voltage waveforms and how they affect small or medium sized motors. This information is not accurate for MV ASDs applied to large motors. The larger horsepower motors utilized on MV ASDs are designed differently (insulation systems, windings, air gaps and bearings) than smaller LV motors, which changes how they react to the PWM waveforms. Specific topics addressed include voltage waveform effects on motor insulation, common mode bearing currents, partial discharge, and harmonic heating of the motor. This paper presents the differences of MV PWM ASDs compared to LV ASDs with respect to output voltage characteristics such as dv/dt, step voltage, and PWM switching frequency. MV ASDs require different guidelines to prevent damage to motors than their LV counterparts. The effect of these factors on motor heating, peak terminal voltage, output filter requirement and bearing currents are discussed. Data from technical papers, standards, and test data are referenced. The goal of the paper is to reduce the confusion on the application of MV ASDs.