Overvoltage protection of inverter-fed drives with the help of energy varistors — dimensioning and lifetime considerations

This contribution deals with overvoltage protection in inverter-fed drives by varistors. Several kinds of overvoltages in combination with very short rise times impose extreme dielectric stress on the insulation of electrical machines. With varistor discs it is possible to limit these overvoltages in combination with an increased rise time of the impulses occurring at the machine terminal. During this service the varistor has to withstand the power losses and to dissipate the resulting heat to the surrounding medium. Here, currents and power losses are focused in order to get better understanding of the mechanisms that lead to a reduction of the overvoltage at the machine terminal. Finally, a proposal for an accelerated aging procedure will be made that takes the given service conditions into account. First results are presented, and the chances of this application for overvoltage protection will be discussed as well.