Investigations in evaluating whether frequency inverters endanger low voltage motor windings

Electrical adjustable-speed drives consisting of a pulse width modulated (PWM) frequency inverter and an induction motor have excellent properties as controllability, robustness, small reactive power consumption, efficiency, economy, and so on. For these reasons they are of great importance on all industrial areas and this importance increases permanently. But there are several problems, hardly taken into consideration at the time of their first installations. One must know that under pulse operation, additional voltage stress is given, if the frequency inverter and the motor are connected with a cable of a length of a few meters and more. This stress can lead to partial discharges within the motor winding and so to premature failures. Therefore, it is important either to avoid this discharge or to employ insulation materials resistant to such discharges. In this paper, the authors elucidate some connections concerning the coming into being and avoiding of partial discharges and how they can damage the winding. Since these discharges are not detectable with traditional methods under pulse conditions, a suitable procedure was developed, the capability of which is demonstrated on the basis of several examples.