A Study on the Impacts of Wide-Bandgap Devices on Turn-to-Turn Insulation Performance in Hairpin Winding for Electric Vehicle Traction Motors

Abstract

Electric Vehicles (EVs) are one of the important levers of transportation electrification. However, charging time and limited mileage per charge remain significant barriers to widespread EV adoption. Continuous advancements in EV technology aim to mitigate these challenges. Three major improvements addressing these issues include increasing the operating voltage level, replacing random-wound motors with hairpin winding, and utilizing wide-bandgap (WBG)-based power converters to drive the motors. These advancements, however, raise concerns regarding motor reliability, particularly in the winding insulation system. Therefore, it is crucial to study and characterize the effects of higher voltage levels and WBG device-based drives on hairpin winding insulation. Turn-to-turn insulation is the most vulnerable point in motor stators. Power electronic converters employ pulse-width modulation (PWM) techniques to generate AC output waveforms, producing pulses with fast (short) rise times, overshoot, high frequency, and variable duty cycles. These PWM-driven systems subject insulation to greater electrical stress than conventional AC-fed machines. The adoption of WBG device-based drives exacerbates this stress due to their inherently fast switching characteristics and high-frequency components. Increased electrical stress may lead to partial discharge (PD) activity, which accelerates insulation degradation. Consequently, evaluating turn-to-turn insulation under WBG device-based drive operation and PD exposure is critical. This study develops a high-voltage SiC-MOSFET pulse generator to investigate the impact of WBG device-based drives on turn-to-turn insulation. A comprehensive analysis is conducted by examining the effects of pulse rise time, overshoot, frequency, and duty cycle. Three rise times (40 ns, 500 ns, and 800 ns) are considered to assess the influence of fast-switching transients inherent to WBG devices. Overshoot effects are examined using 10% and 20% overshoot pulses, while frequency effects are evaluated at 5 kHz and 10 kHz. Additionally, the impact of duty cycle is studied at 20% and 50%. Since traction motors operate under elevated thermal conditions, this study also evaluates the effect of increased temperature on insulation degradation to more accurately replicate in-service stress conditions. To assess turn-to-turn insulation performance, back-to-back test samples replicating hairpin winding structures are developed using actual flat wires employed in EV motors. Two wire types, corona-resistant and non-corona-resistant, are evaluated and compared. Experimental tests are designed based on Design of Experiment (DOE) principles, with samples subjected to 24-hour aging under high-voltage pulses generated by the SiC-MOSFET pulse generator in the presence of PD activity. Insulation performance is assessed before and after aging by measuring partial discharge inception voltage (PDIV) and conducting dielectric frequency response (DFR) analysis. Wire surface temperature is continuously monitored during the aging process, and PD activity is confirmed through the detection of PD electromagnetic wave emissions using a UHF antenna. Furthermore, optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM) images, and EDX analysis are utilized to examine wire coating integrity before and after aging. Results indicate that corona-resistant wires exhibit superior performance under PD conditions compared to non-corona-resistant wires. Additionally, frequency is identified as the dominant factor influencing PDIV drop, whereas overshoot has the most significant effect on the increase in dissipation factor after aging. Microscopy, AFM, SEM, and EDX analysis reveal clear evidence of PD-induced wire coating damage. The combined impact of thermal and electrical stress is examined, with findings compared to room-temperature test results. This research provides critical insights into the reliability of turn-to-turn insulation in hairpin-wound EV motors under WBG device-based drive operation, offering valuable guidance for motor reliability improvement in next-generation EV powertrains.