Effect of surge voltage risetime on the insulation of low-voltage machines fed by PWM converters

This paper investigates the repetitive surge-voltage withstand of low-voltage mush-wound machines operated on adjustable-speed drives (ASDs) using insulated gate bipolar transistor (IGBT) semiconductor technology. Historical work on surge testing of motor insulation has focused on one or more of the following aspects: (1) large horsepower motors; (2) medium-voltage form-wound motors; (3) single-shot impulse-type transients; or (4) low-voltage machines with surge risetimes >200 ns. IGBT drives can have risetimes of 50-200 ns. Thus, a new study on electrical stress of insulation systems due to the nonlinear voltage distribution of mush-wound motors when subjected to repetitive steep dV/dt square-pulse waveforms (rather than impulse wave testing) is presented. Magnitude and risetime of the repetitive ASD surge-voltage transient induced on the machine terminals is reviewed first. Next, surge propagation into the winding was investigated to identify maximum voltage stress points on the conductor insulation. Potential failure mechanisms observed at these points are then discussed. The significance of decreasing surge risetime and increasing cable lengths on internal nonlinear voltage distribution is studied with experimental results from a 7.5-hp motor with a tapped stator winding.