Effects of inverter pulses on the electrical insulation system of motors

The Pulse Width Modulated (PWM) voltage waveforms produce complex overvoltages that stress the insulation of cables and rotating machines more severely than normal sinusoidal voltage. The overvoltages have a form of fast switching pulses with very short rise time, composed of repetitive sequences. Such conditions have influence on inception and activity of partial discharges (PD) and space charge accumulation in insulating systems. Paper presents some aspects of this problem investigated for twisted-pair samples of low voltage motor wires. Results of electric field modeling for wire-to-wire insulation and PD activity for testing voltage registered in controlled laboratory conditions are shown. The phase-resolved PD patterns registered by means of the Phase Resolved Partial Discharge (PRPD) acquisition method are used for PD activity assessment. Times-to-breakdown for wires of different diameters were also evaluated for trapezoidal voltages with 10 kHz frequency. Results analysis indicates correlation between wire diameter, electric field, PD activity and life-time of twisted-pair insulating systems.