The effect of impulsive voltage rise time on insulation endurance of inverter-fed motors

Endurance tests under repetitive impulse voltages should be performed on inverter-fed motor insulation to evaluate its insulation performance according to the technical specification (TS) released by the International Electrical Commission (IEC), i.e. IEC TS 60034-18-42. To simplify testing procedures in the application, the TS allows life tests to be performed under sinusoidal waveform, as it assumes that sinusoidal and impulsive voltages will provide comparable results if they have same peak-to-peak voltage and frequency. However, we have found that voltage rise time can have significant effect on PD features in both time and frequency domains, so that if PD are deemed the cause of accelerated aging, rise time must be taken into account and sinusoidal waveform may be not the best choice for accelerated aging procedures which resemble the real stress experienced in service by insulation systems of rotating machines controlled by Pulse Width Modulation (PWM). This study aims at summarize the effect of impulsive voltage rise time on insulation life based on endurance tests on crossed enameled wires used for type I motors and turn/turn insulation for Type II motors. Repetitive voltages of 6 rise times (i.e. 50 ns, 200 ns, 1 μs, 16 μs, 200 μs and 500 μs, sinusoidal voltage with same peak-to-peak voltage and frequency) and 3 rise times (i.e. 150 ns, 2 μs and sinusoidal voltage with same peak-to-peak voltage and frequency) are used in the endurance tests on Type I and Type II motor insulation, respectively. Statistical analysis indicate that, due to the different PD features caused by the impulsive voltage rise time, impulsive voltage rise time can have significant effect on insulation lifetime. Accordingly, when performing endurance tests on high-voltage inverter-fed motor insulation, the rise time influence on final life of inverter-fed motor insulation should be carefully considered.