Partial discharge characteristics in motor insulations under exposure to multi-level inverters

The use of variable speed drives fed by pulse width modulated (PWM) inverters allows a more efficient use of electric energy. The drawback is however that the stress imposed on motor winding insulation increases and partial discharges (PDs) appear under such conditions, being considered the major contributor to the reduction in the insulation life time. We present in this paper changes observed in the PD characteristics of motor insulation systems aimed for different voltage levels. These were a twisted pair test object as well as a higher voltage level motor insulation wire, fed from either two- or three-level inverters. Each of the objects was subjected to PWM waveforms of different rise-times. The twisted pair test objects were insulated by a chromium oxide filled enamel, whereas the motor insulation objects were micaepoxy based. To compare the performances, measurements of extinction voltages and PD characteristics were conducted. Specifically, the total number of PDs and their amplitude per cycle were obtained at 2.5 and 4.5 kV
pp
, dependent on the system tested. The experimental results showed that the total summed PD magnitude drops considerably when applying a three level inverter. However the maximum PD magnitude remain at the same level for the same rise time in both cases. Also the extinction voltage remains the same for a specific rise time. The PD magnitude decreases for increased rise time, similarly for both the cases. Thus the presented observations suggest that a change to a multilevel inverter alone may not be sufficient for increasing the motor insulation system life.