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.

Increasing current density is a common way to improve the power density of electrical machines (EMs) which also boosts the winding temperature. Higher winding temperature accelerates the thermal degradation of insulation and also increases the risk of partial discharge (PD). Commonly, the winding hotspot temperature constraint is decided based on an over-engineering approach (i.e., the winding hotspot temperature is 30°C below the wire thermal class) constraining the EM’s performance. In this paper, the margin of winding hotspot temperature is precisely determined focusing mainly on the insulation degradation issues through experimental investigation. The partial discharge inception voltage and partial discharge extinction voltage have been measured under different temperature conditions for samples with different thermal aging statuses. Rectangular wires which find application in automotive motors are selected as test samples. The results link the temperature with the PD risk and the thermal lifetime of the insulation, and the winding hotspot temperature margin is accordingly determined. An automotive case study is presented to prove the applicability of the investigation outcomes.

For aerospace electrical machines (EMs), the partial discharge (PD) risk is becoming much more severe due to higher DC link voltage levels and faster switching devices. Traditionally, EMs for safety-critical applications mainly rely on over-engineering methods to guarantee reliability. However, a recent shift towards reliability-oriented design, which is based on understanding the physics of failure, allows for meeting the reliability requirements while ensuring performance. From the perspective of the reliability-oriented methodology, the PD-free design of a starter/generator (S/G) with a 540V DC bus is presented and the main steps for a comprehensive PD risk evaluation are discussed. First, the PD inception mechanism is experimentally investigated under pulse voltage excitation for several rise times, temperatures, and pressure values. Then the measured partial discharge inception voltage (PDIV) values are used to perform an accurate PD risk assessment, which includes the winding hotspot temperature resulting from the S/G mission profile, the environment operating conditions, and the connection cable length. Finally, the PD risk of a S/G with a higher DC bus (i.e., 800V) is examined to comply with the future aircraft voltage level.

Higher DC link voltage and shorter rise time keep challenging the insulation system of inverter-fed low-voltage electrical machines. In more electric aircraft applications, the low-pressure working environment faced by electrical machines further increases the risk of partial discharge (PD) endangering the electric drive reliability. Through an additional inorganic insulation layer, corona-resistant (CR) wires reveal the ability to withstand PD and represent a potential solution for aerospace applications. In this paper, a CR wire is characterized and its performance is compared against that of a non-corona-resistant (NCR) counterpart in the perspective of partial discharge inception voltage (PDIV), electrical endurance, and thermal properties. A starter/generator is considered as a study case to highlight the influence of machine design when CR is employed. Two machine designs, each using CR and NCR wires are presented and analyzed in terms of rated current, copper slot fill factor, and reliability performance (PDIV and lifetime extension).

This article endeavors to illuminate the variations of various streamer inception parameters (SIPs) with respect to air pressure based on Schumann’s streamer inception criterion (SCSIC). The results based on measured partial discharge inception voltage (PDIV) values and using electric field distribution obtained via electrostatic simulations and ionization swarm parameters reveal that the Schumann constant, ${K}$ , which is the natural logarithm of the threshold number of electrons determining the transition from Townsend to streamer discharge, and consequently, the critical avalanche size ( ${N} _{\text {c}}$ ) increase with air pressure reduction. Different SIPs such as critical field line length (CFLL), effective ionization coefficient of air ( $\alpha _{\text {eff}}$ ), PD inception field ( ${E} _{\text {inc}}$ ), firing voltage ( ${V} _{\text {firing}}$ ) across the critical field line (CFL), and ${K}$ and ${N} _{\text {c}}$ are analyzed extensively as a function of air pressure. In light of the findings of this contribution, it is demonstrated that the derived ${K}$ functions as a function of air pressure can improve drastically the accuracy of PDIV prediction in particular for low air pressures rather than a single ${K}$ parameter value obtained at ground level. The study’s findings represent a guideline for electrical machine designers to improve the insulation design of electrical machines employed in the more electric aircraft (MEA) applications.

Considering the turn-to-turn insulation as the most susceptible part to the failure of the electrical machine’s insulation system in inverter-fed motors, this paper aims to elucidate the effect of temperature ( T ) on the streamer inception parameters (SIPs), including the Schumann constant ( K ), critical field line length (CFLL), air effective ionization coefficient (α _eff ), and streamer inception field ( E _inc ). Particular attention is also given to extending the partial discharge inception (PDIV) modelling at cruising altitude (CA), equivalent to the air pressure of 200 mbar, as a function of T based on the Schumann streamer inception criterion (SCSIC), compared with the results obtained at 1000 mbar, i.e., ground level (GL). Furthermore, the ratios of SIPs and PDIV values at CA to those attained at GL are analysed as a function of T . The investigation findings serve as a prescription for electrical machine designers to enhance the insulating design of electrical machines used in More Electric Aircraft (MEA) applications.