Yatai Ji

The high electric stress experienced by insulation systems today continues to pose significant challenges to the reliability of inverter-fed electrical machines (EMs) used in transportation applications. Electrical aging caused by partial discharge is one of the primary factors threatening the reliability of EMs. Moreover, compared to EMs meant for industrial applications, the insulation systems of EMs designed for more-electric aircraft may also be affected by pressure conditions. For instance, EMs used in primary flight control surface actuators may operate during the cruise phase at altitudes of approximately 12 km and pressures as low as 200 mbar. Several studies have highlighted that low-pressure environments can accelerate the electrical aging process. However, a comprehensive lifetime model accounting for both pressure and electrical stress remains unavailable. Such a model would enable machine designers to assess the suitability of insulation systems for various applications across different operating altitudes (i.e., pressure levels). In this paper, accelerated insulation aging tests are conducted under twelve distinct combinations of pressure and voltage conditions, and a multi-stress (i.e., electrical and pressure stress) lifetime model applicable in the range 200÷1000 mbar is developed, accounting for the impact of pressure on inverse power law’s parameters.

Due to the increasing DC bus voltage, electrical machines (EMs) for transportation applications are exposed to a great partial discharge (PD) risk nowadays. Thus, PD risk assessment is essential for fulfilling the reliability requirement. PD inception voltage (PDIV) measurement under sinusoidal voltage is sometimes preferred because it demands an easy experiment setup, both in terms of the voltage source and PD detection system. However, the obtained results may lead to an underestimation of PD risks since, in actual applications, EMs are fed through inverters and pulsewidth modulation (PWM) voltage waveforms are applied. In this article, the difference between PDIV results obtained under sinusoidal and pulse voltage signals is quantified, and different environmental conditions are investigated, considering the potential transportation electrification applications. Finally, based on the experimental results, a stress voltage waveform enhancement factor (EF) is proposed and precisely determined. The study reveals that this factor is not constant under varying environmental conditions, with a maximum value of 1.18 observed at $220~^{\circ }$ C and 200 mbar. Furthermore, the influence of the statistical processing method on the derived factors is analyzed, showing a maximum variation of approximately 10% when different Weibull percentiles are employed.

Room temperature vulcanized silicone rubber (RTV-SiR) is broadly recommended as a high voltage insulator coating (HVIC) for ceramic insulators in polluted zones. However, the rising HVDC ranges impose serious challenges in terms of coating degradation. To increase the acceptability of HVIC, the present work investigates the electrothermal characteristics of RTV-SiR for HVIC using the inclined plane test in conjunction with long-term accelerated aging. Four types of RTV-SiR composites are fabricated by adding various wt% of micro, nano, and micro-nano (hybrid) silica fillers. Inclined plane tests (IPT) are conducted separately for both polarities of DC on pristine and aged samples (treated with accelerated multi-stressing in the lab for 10,000 hours). Unaged hybrid samples (SB4) outperform by achieving the highest initial tracking voltage (ITV) of 3.25 kV for negative polarity DC (–DC) IPT, followed by the counterpart under positive polarity DC (+DC). Similarly, a notable suppression of leakage currents is witnessed in hybrid and 20 wt% silica-added composites throughout the IPT campaign. Also, from time to failure analysis, a relatively shortened time to failure is observed for all blends treated under positive DC, confirming the detrimental impact of positive polarity. For instance, unaged pure SiR samples (SB1) feature 6 hours before failure under –DC stress compared to 5 hours when +DC is applied. From the findings, it is recognizable that hybrid composites depict better overall performance, followed by samples with 20 wt% filler, thus showing high integrity under all test conditions.

The rapid electrification of transportation demands compact, high-power-density low-voltage electric machines (EMs), whose winding insulation life limits safety and uptime. Conventional Arrhenius models consider only thermal–chemical aging and often neglect thermal–mechanical (T-M) fatigue resulting from frequent temperature cycling in vehicle duty cycles. Based on mathematical derivation and previous physical models, this article proposed a unified temperature-cycling life model that couples Arrhenius T-C terms with a phenomenological T-M fatigue term driven by cycle amplitude and mean temperature. The life model’s parameters are tuned using experimental data collected during accelerated temperature profiles (TPs) cycling tests performed on motorette specimens, while the life model is validated across different TPs. The predicted life outcomes demonstrate higher computational efficiency and yield more conservative results compared to the physical model simulations. As proof of concept, the proposed model is employed at the design stage of an EM meant for automotive applications, in order to achieve a mission profile-based reliability-oriented design. Meanwhile, a real-time life consumption methodology, employing online cycle counting, is implemented to facilitate continuous reliability assessment and monitoring. This integrated approach establishes a systematic framework for evaluating and managing the reliability of EM insulation systems, thereby enhancing the methodological foundation for life prediction under realistic and dynamically varying operating conditions.

Electrical insulation systems (EISs) are the principal reliability bottleneck of modern electrical machines (EMs). Among the many stresses acting on insulation, thermal stress is the most pervasive because it accelerates chemical reactions that progressively erode dielectric and mechanical integrity, ultimately dictating service life. As EMs migrate into compact, high-power-density platforms—automotive, aerospace, and industrial drives—designers need lifetime models that are not merely explanatory but actionable, linking operating temperatures and missions to quantified ageing and risk. This review article traces the evolution of thermal-ageing modelling from fundamental chemistry to a practical design tool. The historical empirical lineage of Arrhenius equation, Arrhenius–Dakin model, and Montsinger model is first revisited, clarifying their assumptions, parameter definitions, and the construction of thermal endurance curves. A discussion then follows on extensions that address deviations from first-order kinetics and demonstrate how variable temperature histories can be incorporated through cumulative damage formulations suitable for duty-cycle analysis. Since models are required to be anchored in data, accelerated thermal ageing (ATA) practices on representative specimens are outlined, alongside a description of the Weibull post-processing for deriving percentile lifetimes aligned with design targets. Building upon these foundations, the Physics-of-Failure (PoF) approach is introduced as a reliability-oriented design (ROD) methodology, in which validated lifetime models guide material selection and geometry optimisation while supporting prognostics and health management during operation. The emerging trend towards a hybrid PoF–AI approach is also discussed, which integrates artificial intelligence to identify nonlinear degradation patterns and drifting parameter relationships beyond the reach of empirical models, with physical constraints ensuring that predictions remain consistent with known ageing mechanisms. Such integration enables the learning process to adapt to operational variability and coupled stress effects, thereby improving both the accuracy and physical interpretability of lifetime estimation. The review aims to provide a concise view of models, tests, and workflows that convert thermal-ageing knowledge into robust, design-time decisions. By linking empirical and physics-based insights with modern data-driven learning, these developments support proactive maintenance, sustainable asset management, and extended operational lifetimes for next-generation EMs.

The aging issue of inverter-fed motor windings is becoming increasingly critical. Although C_p (insulation equivalent parallel capacitance) and other insulation parameters are measured and failure modes are investigated in previous offline and online experiments, the reasons behind parameter variation remain unclear. The insulation aging mechanism is not fully elucidated or experimentally confirmed. In this article, an accelerated aging experiment on twisted coil samples is conducted to thoroughly explore the causes of insulation parameter variation. The TGA-FTIR method (Thermalgravimetric Analysis - Fourier Transform Infrared Spectroscopy) is employed to investigate the material change. The experiment demonstrates a correlation between online and offline results. The variation in C_p is confirmed by TGA-FTIR, revealing that chemical aging predominates in the early stage, while physical aging becomes more dominant later. Other aging-related factors are also discussed. Humidity affects the parameter values, though pure water is proved to have no significant long-term aging effect. The twisted coil samples are suitable for numerous repeated experiments, making them effective for studying the insulation aging mechanism in inverter-fed motors.

For electrified transportation applications, the winding insulation of an electric machine (EM) is often thermally aged by variable temperature profiles (TPs). Considering the high-reliability requirement, the overengineering approach is commonly adopted, although it conflicts with the high-power-density demand. Therefore, a reliability-oriented design (RoD) methodology is preferred to find a balance between reliability and performance. In order to apply the RoD, the physics of the failure mechanism of EM windings has to be investigated by performing accelerated lifetime tests (ALTs) on the electrical insulation system (EIS). Life prediction of winding is often based on constant temperature ALTs, which would lead to some inaccuracy for the EM working under variable temperature conditions. Thus, a lifetime model that accounts for aging effects caused by variable TPs is needed. In this article, the ALTs are performed at both constant and variable temperature conditions. Then, a newly developed lifetime model taking into account the thermomechanical stress is introduced based on experiment results and simulations. In this way, the EM life prediction could be obtained and more accurate in any temperature range and period. Meanwhile, the practical application of the lifetime is illustrated through a case study of an aerospace motor. Then, the application paradigm of irregular temperature situations is also illustrated with a counting algorithm. To conclude, this article developed a more comprehensive life model for the variable temperature conditions, which is able to cope with real-world scenarios and improve the EM design.

Inverter-fed electrical machines used in electric vehicle applications are increasingly prone to experiencing partial discharge (PD), particularly in 800 V powertrains. Therefore, a precise assessment of PD risk is crucial for ensuring a PD-free design and guaranteeing reliability. Several enhancement factors are introduced in IEC 60034-18-41 accounting for the effect of influencing factors on PD activities. However, an accurate determination of the thermal aging enhancement factor remains a challenging issue. In this paper, PDIV variations are measured under three accelerated aging temperature levels with more than 10 aging sub-cycles. The data are then used to deduce the changing rate of PDIV at generic temperature conditions and finally, the thermal aging enhancement factor considering lifetime constraints and temperature is derived based on the rate theory and Arrhenius law. With this factor, PD risk could be easily and accurately determined considering thermal lifetime constraints during the design stage, and an automotive study case is presented to effectively illustrate the application of the derived enhancement factor.