A STUDY OF THE OVER-VOLTAGE STRESS WITH IGBT INVERTER WAVEFORMS ON MOTOR AND SUPPLY CABLING AND THEIR REMEDIAL MEASURES

The proliferation of IGBT inverters in low voltage motors drives in recent years has manifested itself in the gradual degradation of insulation of both the supply cable and the motor windings closest to the terminals. This effect of the steep fronted IGBT inverter waveforms on the supply leads to the motor and the motor itself are now well known. IGBT switches have many desirable properties, however, their fast switching edges can easily cause a total reflection of the inverter out put voltage waveforms from the motor terminals, when the motor is at the end of a cable more than about 20m long. This may lead to a doubling of the supply voltage to the motor and to the cable. This phenomenon has been modelled at the UNSW, taking into account the IGBT characteristics and the switching frequency of the inverter. The model predicts the voltage stress at the motor terminals and along the length of the cable reasonably well. Remedial measures which contain these stresses are also modelled. Experimental results are included which confirm the accuracy of the model and the suitability of the remedial measures suggested. I