The low-voltage electric machine (EM) is a core technology for transportation electrification, and features like high power density and compact volume are essential prerequisites. However, these requirements are usually in conflict with the reliability property of EM, especially in the safety-critical industry such as aviation. Therefore, an appropriate balance between high-performance and reliability needs to be found. Often, the over-engineering method is applied to ensure safety, although it might have a detrimental effect on the EM volume. To address this issue, the EM reliability assessment is included at the EM design stage through the physics of failure (PoF) theory. In EMs, the windings play a key role in electromechanical energy conversion, but their insulation system is subject to frequent failure and represents a reliability bottleneck. Therefore, in-depth research on the root causes of insulation breakdown is beneficial for EM reliability improvement purposes. Indeed, increasing awareness and knowledge on the mechanism of the insulation degradation process and the related lifetime modeling enables the growth of appropriate tools for achieving reliability targets since the first EM design steps. In this work, the main aspects of the insulation system, in terms of materials and manufacturing, are first reviewed. Then, the principal stresses experienced by the winding insulation system are deeply discussed with the purpose of building a profound understanding of the PoF. Finally, an overview of the most common insulation lifetime prediction models is presented, and their use for accomplishing the reliability-oriented design (RoD) and the remaining useful life (RUL) estimation are examined.

Low-voltage electric machines (EMs) with high power density and compact volume represent a key target in transportation electrification. In order to achieve optimal performance and simultaneously keep the high reliability of EM, the lifetime prediction should be merged into the EM design. In the literature, constant temperature accelerated lifetime tests (ALTs) are frequently applied for EM lifetime prediction. However, most EMs experience variable temperature profiles (TPs) during their operations. Thus, the variable temperature ALTs were considered by employing three periodic TPs experiments. The obtained results are postprocessed by Weibull distribution and related life models are constructed. Then, EM lifetimes under different TPs are analyzed by comparing the analytical and experimental results. Finally, the difference between the lifetime value of the model and experimental results is analyzed, and the extra loss of life caused by thermal-mechanical (T-M) stress is derived through experimental evidence.

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.

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.