Application of varistors for overvoltage protection of machine windings in inverter-fed drives

This contribution deals with overvoltage protection in inverter-fed drives by varistors. Several kinds of overvoltage in combination with very low rise times impose extreme dielectric stress on the insulation of electrical machines. Here a novel approach of overvoltage protection in inverter-fed drives is presented. After a discussion of the origins of the overvoltages and of overvoltage limitation by varistors in general, the dimensioning for the particular application, which is quite unusual for varistors, will be presented. Two special cases of actual overvoltages are addressed. Oscillograms are analyzed, and comparisons of measurements and calculations are made as well. Finally, possible applications for certain cases are discussed.