The rapid development of transportation electrification brings unprecedented opportunities and challenges as well to the technological improvement of high-power density motors. As the major restriction of the motor performance, thermal issues attract more attentions recently and the heat pipe (HP), due to its high thermal conductivity and lightweight characteristics, is identified as a promising solution. Though HPs have been attempted in cooling systems of electrical machines, their thermal benefits still need to be fully understood. This article provides a comprehensive review and performs a series of experimental investigations to fill the technology gaps for the HP application in electrical machines. Firstly, the HP technology is briefly introduced, followed by a detailed review on its applications in different locations of electrical machines with corresponding thermal improvements reported. Then, experimental tests are conducted to investigate the correlation between the HPs’ parameters and their cooling effectiveness, including cross-sectional shape, bending angle, fins arrangement and inclination angle. The obtained results and conclusions could provide a guideline for the promotion of HPs in next generation electrical machines for future transportation.

Heat pipes (HPs) are being extensively explored in motor cooling scenarios for enhanced cooling capacity. HPs are commonly bent to adapt to the compact structure of electrical machines, whereas the bending effect on motor cooling effectiveness still requires further investigation. This article analytically and experimentally studies the effect of the bending process, including bending angle and bending radius, on HP thermal properties. Cooling density is defined and derived to analyze the tradeoff between HP thermal performance and additional volume due to HP installation. Practical guidelines for feasible HP bending solutions under different space constraints are provided to achieve a higher cooling density. Finally, the HP bending effect on motor cooling effectiveness under various cooling methods is quantitatively evaluated through a validated thermal model based on a stator-winding assembly. The bending process can degrade the HP equivalent thermal conductivity by up to 76%, thus leading to a temperature difference of 4.8K under liquid cooling conditions.

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).

Solar-powered aircraft are being upsized and required to carry more payloads, which poses great challenges for propulsion motors, especially for their cooling considering extreme lightweight. Due to remarkable thermal performance, heat pipes (HPs) become one of the most promising solutions for aircraft propulsion motors. However, this may also lead to extra losses, which requires a more intensive study to maximize HP thermal benefits and minimize motor losses. In this paper, a propulsion motor with an HP-based winding cooling system is developed, and the effects of HP insertion on motor performance are investigated. Firstly, a motor topology is proposed, and five HPs of different sizes are selected from the market as candidates, with their thermal properties experimentally measured. Then, an appropriate HP size is determined based on a comprehensive evaluation of the thermal performance and losses of the motors with candidate HPs. Finally, the above results are verified by motor prototyping and testing. Both simulation and test results indicate a significant improvement in cooling capacity, as well as extended motor performance boundaries.

Thermal issue has always been a major concern in motor design. Heat pipes (HPs) have been applied in electrical machines with remarkable cooling effects reported. However, critical factors that affect HP thermal benefits have rarely been studied, such as limited contact area between HPs and heat sources, which may lead to high thermal resistance and degrade motor thermal performance. Due to limited thermal conductivities, conventional motor thermal interface materials (TIMs), such as epoxy resin, fail to maximize HP thermal benefits and minimize motor temperature rise. In this case, new TIMs with outstanding thermal properties that have not been used in electrical machines, such as thermal grease and adhesive, are potential candidates to address this issue, while their thermal impact is still unclear. This article conducts both experiments and thermal analyses to quantify the effects of TIM on motor cooling performance. Firstly, the thermal properties of ten typical TIMs in four categories are experimentally characterized. Then, the selected TIMs are applied to a stator-winding assembly with HP to evaluate their thermal impact on motor cooling performance under different cooling conditions. Finally, practical guidelines are given for TIM selection in electrical machines with HPs or other cooling media.

The demand for high-power density motors has been increasing due to their remarkable output capability and compact construction. To achieve a significant improvement in motor power density, lightweight design methods have been recognized as an effective enabler. Therefore, extensive investigations have been conducted to reduce motor mass and achieve lightweight configurations through the exploration of lightweight materials, structures and manufacturing techniques. This article provides a comprehensive review and summary of state-of-the-art lightweight implementation methods for electrical machines, including the utilization of lightweight materials, structural lightweight design, and incorporation of advanced manufacturing technologies, such as additive manufacturing techniques. The advantages and limitations of each approach are also discussed in this paper. Furthermore, some comments and forecasts on potential future methodologies for motor lightweighting are also provided.