Time to Breakdown for Type I Motor Winding Insulation Under High-Frequency High-Slew-Rate Square Wave Voltages

Abstract

The emergence of (ultra)wide bandgap ((U)WBG)-based devices has significantly enhanced performance in high-frequency switching applications, enabling breakthroughs in electric transportation, aerospace, and renewable energy systems. However, it also presents considerable challenges to insulation systems, such as electric motor winding insulation, which are subjected to these high-frequency high-slew-rate voltage pulses. This paper explores the impact of high-frequency, high-slew-rate square wave voltages on degradation and the time required for breakdown of Type I electric motor winding insulation. Although past studies reported in the literature were conducted at frequencies of a few kHz and low slew rates. The question remains unanswered: what occurs at higher frequencies, up to a few hundred kHz and at elevated slew rates? This paper addresses this technical gap. Utilising a state-of-the-art custom-built high-voltage, high-frequency high-slew-rate square wave generator, this study reports and analyses experimental results up to 200 kHz and short rise times down to 50 ns, which have not been reported to date. Two types of specimens, twisted and perpendicular pairs, are considered to simulate the motor winding configuration, and the test results are analysed using the Weibull model and multi-factor lifetime modelling. This paper corrects the common misconception that the higher the frequency of the applied voltage, the shorter the insulation lifetime. We found that this is incorrect when operating at frequencies above a few 100 kHz (> 100 kHz), where saturation or critical frequency exists, and beyond that point, the lifetime may even increase.