PWM Voltage Frequency Impacts EV Motor Insulation Life
Electric vehicles (EVs) are no longer just a symbol of sustainable mobility—they represent a technological frontier where performance, efficiency, and reliability converge. As the automotive industry accelerates toward electrification, one critical yet often overlooked component remains under intense scrutiny: the insulation system of electric drive motors. A recent study published in High Voltage Engineering has shed new light on a crucial challenge facing next-generation EVs—how high-frequency pulse width modulation (PWM) voltages influence the long-term durability of motor insulation, particularly under varying fundamental and carrier frequencies.
The research, led by Peng Zeng, Wang Peng, Zhu Yingwei, and Yu Chaofan from the College of Electrical Engineering at Sichuan University, in collaboration with Zhao Anran from CRRC Zhuzhou Electric Locomotive Research Institute and Lin Xiyun from Xiandeng High-Tech Electric Co., Ltd., presents a comprehensive investigation into the effects of frequency parameters on insulation endurance lifetime. As modern EVs increasingly adopt silicon carbide (SiC) power semiconductors, the resulting high-speed switching generates PWM waveforms with rapid voltage transitions and elevated frequencies—conditions that place unprecedented stress on motor insulation systems.
Unlike traditional internal combustion engine vehicles, EVs rely on inverters to convert DC battery power into AC for the motor. These inverters use PWM techniques to precisely control motor speed and torque. However, this efficiency comes at a cost. The fast rise and fall times of PWM voltages—often below 100 nanoseconds—can create voltage overshoots at the motor terminals, sometimes reaching up to twice the DC bus voltage due to impedance mismatches in cabling. When these transient voltages exceed the partial discharge inception voltage (PDIV) of the insulation, localized electrical discharges occur within microscopic voids or at material interfaces.
Partial discharges (PD), though small in energy, are insidious. Over time, they erode insulation materials through physical bombardment, chemical degradation, and thermal stress. This progressive damage leads to the formation of conductive tracks—known as electrical trees—that eventually bridge insulation layers, causing catastrophic failure. In EVs, where motor reliability directly impacts vehicle safety and warranty costs, understanding and mitigating PD-induced aging is paramount.
Previous studies on insulation lifetime under repetitive voltage stresses have typically used simplified square or impulse waveforms. While informative, these do not accurately replicate the complex voltage environment inside an actual EV motor, where both the fundamental frequency (related to motor speed) and the carrier frequency (determined by inverter switching) interact dynamically. The team from Sichuan University recognized this gap and designed an experimental setup that closely mimics real-world operating conditions.
Their test platform utilized a field-programmable gate array (FPGA) to generate authentic PWM waveforms, allowing precise control over both fundamental and carrier frequencies. The core of the experiment involved twisted pairs of 0.7 mm diameter polyimide-insulated enameled wires—representative of actual stator winding configurations. These samples were subjected to accelerated aging under high-temperature (120°C) and controlled humidity (40%) conditions, simulating the harsh thermal environment inside a motor housing.
One of the most significant findings was the consistent localization of partial discharges at the polarity reversal points of the fundamental waveform. Regardless of whether the fundamental frequency was set at 50 Hz or 500 Hz, or the carrier frequency varied between 5 kHz and 20 kHz, PD activity clustered around the zero-crossing regions of the sinusoidal base signal. This behavior stems from the superposition of high-frequency switching components on the low-frequency envelope, creating transient voltage peaks that momentarily exceed the PDIV.
Interestingly, the study revealed that while the intrinsic PDIV of the material remained largely unchanged with frequency, the repetitive partial discharge inception voltage (RPDIV) decreased as carrier frequency increased. This phenomenon is attributed to space charge accumulation on or within the insulation. At higher switching frequencies, there is insufficient time for surface charges generated during one discharge event to dissipate before the next voltage transition occurs. These residual charges effectively lower the threshold for subsequent discharges, making PD more frequent and intense.
This insight challenges the assumption that insulation performance can be evaluated solely based on static or low-frequency PDIV measurements. In real EV applications, it is the dynamic, repetitive nature of the stress that governs degradation. The research underscores the need for test standards that account for RPDIV rather than just PDIV when qualifying insulation systems for inverter-fed motors.
Beyond identifying discharge patterns, the team quantified the impact of frequency on insulation lifetime. Their data showed a clear trend: as either fundamental or carrier frequency increased, the time to failure decreased dramatically. More precisely, the relationship followed a power-law decay—meaning that doubling the frequency resulted in more than a proportional reduction in life. For instance, raising the fundamental frequency from 50 Hz to 500 Hz reduced median insulation life by over 70%, even when all other parameters were held constant.
Similarly, increasing the carrier frequency from 5 kHz to 20 kHz at a fixed fundamental frequency of 500 Hz led to a more than threefold reduction in endurance. This confirms that high switching frequencies, while beneficial for reducing motor losses and audible noise, come with a hidden trade-off in insulation longevity.
To model these observations, the researchers employed a frequency-lifetime inverse power model—a well-established approach in accelerated life testing. The model demonstrated excellent fit with experimental data, achieving a coefficient of determination (R²) greater than 0.99. This high accuracy suggests that the power-law relationship is not merely empirical but reflects the underlying physics of discharge-driven degradation.
A particularly valuable contribution of the study lies in its analysis of the interaction between fundamental and carrier frequencies. When the researchers plotted voltage-lifetime curves for different carrier frequencies under a fixed fundamental frequency and applied a power-law transformation, the resulting lines were nearly parallel. This parallelism implies that changing the carrier frequency does not alter the fundamental degradation mechanism but instead shifts the entire lifetime curve vertically—akin to applying a constant scaling factor.
This finding has profound implications for insulation qualification and lifetime prediction. It suggests that the effects of fundamental frequency and carrier frequency can be decoupled in modeling efforts. Engineers can first establish a baseline lifetime curve at a reference frequency and then apply correction factors for deviations in either parameter. Such an approach simplifies what would otherwise be a multidimensional and computationally intensive problem.
The study also addressed the role of temperature, a key variable in motor operation. Using infrared thermography, the team measured surface temperature rises under various voltage and frequency combinations. As expected, higher voltages led to greater heating due to increased dielectric losses and discharge activity. However, when conducting lifetime tests at different ambient temperatures under high-stress conditions (3.35 kV peak-to-peak), they found that temperature had a relatively minor effect on failure time.
This counterintuitive result highlights the dominance of electrical stress over thermal stress in the final stages of insulation breakdown. Once partial discharges become vigorous, the localized energy deposition at discharge sites far exceeds the bulk thermal effects. In other words, under severe electrical stress, the insulation fails due to rapid electro-erosion rather than gradual thermal aging. This reinforces the idea that for EV motors operating under high PWM stresses, electrical endurance should be the primary design criterion, with thermal management playing a secondary, albeit still important, role.
The implications of this research extend beyond academic interest. For automakers and motor suppliers, it provides a robust framework for evaluating insulation materials and system designs. As EVs push toward higher power densities and faster charging, the demand for compact, high-efficiency motors will only intensify. SiC-based inverters, capable of switching at tens of kilohertz, are already being deployed in premium models and will likely become mainstream in the coming years.
However, without proper insulation systems designed for these extreme electrical environments, the benefits of wide-bandgap semiconductors could be undermined by premature motor failures. The work by Peng, Wang, Zhu, and colleagues offers a pathway to avoid such pitfalls. By establishing predictive models based on real PWM waveforms, manufacturers can now simulate insulation life under various drive cycles and operating conditions, enabling more informed material selection and design optimization.
Moreover, the study supports the development of new international standards for insulation testing. Current protocols often lack specificity regarding waveform parameters, leading to inconsistent qualification results across laboratories. The detailed methodology and findings presented in this paper could inform updates to standards such as IEC 60034-18-42, which already addresses partial discharge-resistant insulation systems for inverter-fed machines.
Another area of impact is in the design of test equipment and monitoring systems. The use of ultra-high-frequency (UHF) antennas in the study allowed for precise detection of PD signals while filtering out electromagnetic interference from power electronics. This technique could be adapted for online condition monitoring in production vehicles, enabling predictive maintenance and early fault detection—key capabilities for ensuring long-term reliability and customer satisfaction.
From a materials science perspective, the research highlights the need for next-generation insulation materials that are not only thermally robust but also highly resistant to partial discharge under high-frequency excitation. Traditional enamel coatings may need to be supplemented or replaced with nanocomposites, multilayer structures, or self-healing polymers that can better manage space charge and resist erosion.
The study also raises questions about motor manufacturing processes. The researchers noted that hand-wound twisted pairs exhibited asymmetric PD patterns due to geometric imperfections. In mass-produced motors, especially those using automated winding techniques, ensuring uniform insulation stress distribution will be critical. Designers may need to revisit winding layouts, slot fill factors, and impregnation processes to minimize localized overstress.
For EV owners and fleet operators, the practical takeaway is reassurance that the industry is actively addressing reliability challenges at a fundamental level. While consumers rarely think about motor insulation, its performance directly affects vehicle longevity, warranty claims, and overall ownership experience. Studies like this one help ensure that EVs are not just environmentally friendly but also built to last.
In conclusion, the research conducted by Peng Zeng, Wang Peng, Zhu Yingwei, Yu Chaofan, Zhao Anran, and Lin Xiyun represents a significant advancement in understanding the complex interplay between power electronics and electrical insulation in electric vehicles. By moving beyond idealized test conditions and embracing the realities of PWM-driven operation, they have provided both experimental evidence and theoretical models that will guide future development in the EV sector.
Their work underscores a central truth in engineering: progress in one domain—such as semiconductor switching speed—must be matched by corresponding advances in supporting technologies, including insulation systems. As the automotive world transitions to electrification, such interdisciplinary research will be essential to building vehicles that are not only powerful and efficient but also durable and trustworthy.
As the industry continues to innovate, studies like this serve as a reminder that behind every breakthrough in performance lies meticulous scientific inquiry into the materials and mechanisms that make it possible. The journey toward sustainable transportation is not just about batteries and motors—it is also about the invisible layers of insulation that keep those motors running smoothly, mile after mile.
Peng Zeng, Wang Peng, Zhu Yingwei, Yu Chaofan, Zhao Anran, Lin Xiyun, College of Electrical Engineering, Sichuan University; High Voltage Engineering, DOI: 10.13336/j.1003-6520.hve.20231420