47 Motor cooling | Insight allowing them to withstand vibration, expansion and contraction without degrading thermal performance. Thirdly, modern TIMs offer easier application in manufacturing through pre-formed pads, dispensable pastes or curing compounds that are compatible with automated processes, reducing production time and defects, she explains. Finally, many advanced TIMs offer electrical isolation with thermal conductivity, which is crucial in highvoltage EV systems, enabling compact motor–inverter integration without compromising safety. Herman also highlights the adoption of advanced TIMs by leading EV manufacturers such as Tesla, Lucid Motors and BMW in their motor housings to enhance thermal management. Tesla utilizes highperformance gap fillers and thermal pastes to optimize heat dissipation from the stator to the housing. Lucid Motors, with its power-dense drive units, likely employs custom-designed TIMs, potentially including phase change materials or liquid-dispensed thermal gels, to manage the high heat loads generated by high-speed operation. BMW incorporates thermally conductive adhesives and gap pads in their motors, emphasizing durability for modular motor construction. Odling connects the benefits of TIMs to the challenges of heat transfer in conventional radial flux motors. He explains that in such motors, heat must travel through multiple interfaces – from the copper windings to the stator iron and then to the cooling jacket – with each interface presenting thermal resistance. By using a thermal medium such as a high thermal conductivity polymer to intimately fill the space around the copper windings, heat can be transferred to the stator more quickly, YASA’s expert notes. Similarly, a good thermal interface between the stator iron and the cooling jacket can further reduce the overall thermal resistance, leading to lower operating temperatures in the core of the motor that, in turn, enable improved performance. While axial-flux specialist YASA’s current market motors do not require TIMs owing to their direct cooling approach, they are actively exploring the use of TIMs between the copper and iron in future high-power density designs to further enhance thermal management within the stator. In a potted motor, heat flows from the stator core toward the end windings and out through the potting and housing, not into the back iron as is often assumed. Since stator slots already contain thermally conductive copper, fully potting them is unnecessary for effective cooling, says Parker Lord’s Wyman. Stephens explains that electric motors operate most efficiently at their nominal rpm range. During high-speed driving, however, motors often run closer to their maximum rpm, leading to reduced efficiency and a greater proportion of the electrical energy being converted to heat. Using the example of a 100 kW motor operating at 85% efficiency, Grayson’s expert points out that a significant 15 kW of heat is generated. To effectively dissipate this heat, it is crucial to specify coolant flow rates that are high enough to transfer the heat without excessive temperature differentials within the coolant, but not so high as to cause significant pressure drops across the motor’s cooling channels. Peak heat While fast charging is primarily a thermal challenge for the battery system, as Siemens’ Uppuluri notes, it can indirectly impact motor performance, particularly during regenerative braking. Although regenerative braking events are typically short cycles, they can generate substantial heat within the motor. However, Uppuluri suggests that the transient nature of this heat generation might not always necessitate immediate intensive cooling, because the coolant temperature might experience only a temporary rise. The more critical thermal challenge for batteries during fast charging lies in the sustained high level of power input over extended periods, leading to significant heat build-up that requires robust cooling solutions to prevent derating and prolonged charging times. For high-performance motors themselves, Uppuluri revisits the fundamental choice between air and liquid cooling. While lower-powered motors can often be adequately air-cooled, high-performance E-Mobility Engineering | May/June 2025 End winding potting has been shown to provide significant improvements in motor efficiency during start/stop cycles involving low speed and high torque operation (Image courtesy of Parker Lord)
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