Electrical stresses applied to stator insulation in low-voltage induction motors fed by PWM drives

The authors report experiments to identify real voltage stresses applied by pulsewidth modulated (PWM) variable-speed drive on the stator insulation of a low-voltage induction motor. The idea that PWM drives impose very high stresses on machine parts, especially the insulating material, is widely accepted. Many theories and simulations have been carried out on the subject for nearly a decade, but too few experimental confirmations have been undertaken. The authors have processed a standard motor to access the copper wires inside the stator winding. Observation of real electrical stresses applied to insulation was performed thanks to these tapping points. The ground insulation is first identified putting in value the inductive effect of winding and the complexity of the three-phase voltage combination at the star point. Monitoring of the voltages applied to interturn insulation reveals much higher surges than expected. Finally, the impact of PWM drives on insulating material is discussed with reference to the resulting experimental data.