Overvoltage protection of inverter-fed drives with the help of energy varistors - Dimensioning rules for consideration of different cable types

Abstract

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. With varistors it is possible to decrease these overvoltages in combination with an extended rise time of the impulses occurring at the machine terminal. During this service the varistor has to withstand the power losses and has to dissipate heat into the surrounding medium. The operation mode of the varistor strongly depends on the basic parameters like pulse frequency, rise time of the impulses at the inverter, applied cable, size of the motor etc. Here correlations between the voltage-current characteristic (U-I characteristic) of the varistor, the maximum occurring voltage steepness and the applied cable type are given in order to be able to carry out detailed dimensioning for different application cases.