Characterisation of voltage shapes acting on the insulation of rotating machines supplied by inverters

Use of frequency inverters for supply of electrical rotating machine is becoming more usual, particularly for applications needing a rotating speed variation. New semiconductors, like Gate Turn-off thyristor (GTO) or Insulated Gate Bipolar Transistor (IGBT), can work at very high frequency, up to 10 kHz, with very short rise times down to less than 0.1 μs. In order to study the ageing associated with these voltage shapes, we have first tried to characterise them. The inverter is a Pulse Width Modulated one (PWM) feeding a 220 V squirrel cage induction motor modified to observe the propagation of voltage shape in the winding. This adaptation results in 36 shielded coaxial wires distributed on the 6 coils and the 17 turns. Turn-to-turn and turn-to-ground voltages are recorded. A spectral analysis shows that the driving frequency is the main frequency propagating in the winding with no attenuation. On the other hand, the motor input turn-to-turn voltages show very short rise time voltage waves (typically 2000V/μs) which are very attenuated and a delay growing during propagation in the winding. Moreover, on the input coils, high frequency oscillations and overvoltages are observed, depending on supply cable length