Partial Discharge Mitigation in Motor Windings Under High-Voltage and Low Pressure

High-voltage motors powered by pulse width modulated (PWM) inverters are increasingly utilized in industrial and transportation systems, including electric aircraft propulsion, for their energy efficiency and controllability. However, rapid switching in inverter outputs generates steep voltage transitions that result in localized electric field concentrations, often exceeding the dielectric strength of motor winding insulation. This initiates partial discharge (PD), leading to insulation degradation and premature motor failure. The challenge intensifies in harsh conditions such as high voltage and low pressure, where the Partial Discharge Inception Voltage (PDIV) is further reduced. This paper proposes and validates a hybrid mitigation approach that combines functional capacitive field grading coatings (FCFGC) with extended rise-time PWM control to suppress PD in motor windings. The FCFGC improves field uniformity via nonlinear permittivity, while the longer voltage rise time reduces <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$d v / d t$</tex>, collectively mitigating electric stress. Experimental results using twisted pair samples under different voltages and pressures demonstrate that the hybrid strategy significantly increases PDIV and reduces PD magnitude, outperforming either method alone. Finite element simulations further support the observed field suppression. This scalable and cost-effective method enhances insulation reliability under the extreme electrical and environmental conditions encountered in electric aircraft systems.