An experimental optimization of preliminary tests on enameled-copper wires adopted for motor windings of PWM controlled solid state drives

The enamelled copper wires of electrical machines supplied by IGBT converters are electrically stressed by PWM square voltages, having components at a fundamental and at a switching frequency. A series of tests has been implemented to answer the following question: does the switching frequency value play a role in the ageing process? Monitoring the partial discharges (PD), twisted pairs were aged at two test temperatures applying voltages having the same fundamental frequency component and different switching frequencies. The life curves obtained at both test temperatures were independent on the switching frequency and were similar. However PD were present only at the voltage polarity reversal of the fundamental component: theoretically PD should have been present also at each switching event. The latter finding would be consistent with the above life curves behaviour, under the assumption that the ageing process is driven by the PD. Further investigations are under way