Investigations of magnet wire enamels and impregnants especially for inverter-fed motors

On the area of adjustable-speed drives the importance of pulse width modulated inverters is permanently increasing. Because of connecting cables between such inverters and motors on the one hand and pulses with steep-fronted voltage waveforms on the other hand there are overvoltages at the motor terminals and a nonlinear distributed phase voltage, both additional deterioration factors for the motor insulation. Since the prevailing cause of this deterioration are partial discharges there is a need for to have insulation materials with a high partial discharge inception voltage and a high resistance to such discharges. In the first part of this paper a new developed wire enamel is presented on the basis of several investigation results as endurance behavior under inverter pulse conditions as function of thickness, structure, composition, mechanical prestress in comparison to traditional enamels. In the second part of this paper the influence of impregnating materials and procedures on the partial discharge behavior and lifetime is demonstrated. The applied samples were twisted pairs of enameled wires and motorettes, respectively. At these investigations a new created measuring technique for partial discharge indication was employed, able to detect discharges under inverter pulse conditions, not possible with pertinent commercial devices