Electrical aging tests on conventional and nanocomposite enamelled wires varying both the relative humidity and the applied voltage amplitude

Electrical machines and in particular electrical motors used in industry applications are usually coupled with inverter drives in order to better control both the motor torque and speed. Due to the presence of these inverter drives, the stator motor windings are subjected to severe voltage waveforms. When the electrical motors having stator windings insulated by means of type I random wire wound are subjected to this kind of severe voltage waveforms, reliability problems, due to the insulating system degradation, increase. In this research work, conventional and nanocomposite enamels have been analyzed: suitable twisted pair specimens insulated by the studied enamels have been prepared and tested. The tests were performed applying sinusoidal voltages having different amplitudes and varying the relative humidity. During the aging tests campaign the amplitude voltage waveform have been always set higher than the partial discharges inception voltage in order to operate in presence of surface partial discharges. The enamel breakdown conditions have been chosen as end-point criterion and the lifetime values have been collected and compared. In order to evidence the behavior of the considered conventional and nanocomposite enamels, the material life curves have been computed. Analyzing the obtained results it has been possible to find out the relations among the time to breakdown, the relative humidity and the voltage amplitude.