46 motors and inverters, necessitate careful management through multizone or staged cooling strategies. Increased system complexity, involving more valves, sensors, pumps and control logic raises the risk of failure and demands sophisticated thermal controllers. Eliminating heat transfer imbalances, where one component might experience rapid heat spikes while others remain cool, requires dynamic cooling management. Packaging constraints within the tight confines of the vehicle architecture pose a significant design problem. Finally, coordinating thermal control across multiple electronic control units requires complex algorithms that respond to various driving conditions and load profiles. Herman cites Tesla and Chery (with Valeo’s thermal management module) as examples of effective integration, using central thermal control units and active valves to manage coolant flow and temperature across different components. YASA’s Odling reinforces the criticality of vehicle-level integration for efficient EVs and hybrids. He notes that vehicle manufacturers typically employ a water–glycol system as a single thermal management solution for battery, power electronics, and the entire electric drive unit (EDU), including the motor. This creates a two-way interaction, where the motor’s inlet coolant conditions are defined by the vehicle’s thermal management system, while the heat rejected by the motor significantly impacts the vehicle’s overall thermal load. In many modern EDU designs, particularly for axle and hybrid applications, fluids are shared between systems, such as the motor oil also lubricating the gearbox. This necessitates comprehensive understanding of the thermal loads from both the motor and the transmission losses to effectively manage the heat rejection back to the vehicle’s heat exchanger and, consequently, the cooling strategy for the battery, cabin and other systems. TIMs in motors Albeit often unnoticed, TIMs play a crucial role in the efficient operation of EV motor cooling systems. These materials are placed strategically between surfaces that need to transfer heat, such as between the motor windings and the stator housing, or between the power electronics and their heat sinks. Their primary function is to fill the microscopic air gaps that exist between even seemingly smooth surfaces. Air is a poor conductor of heat and so replacing it with a thermally conductive TIM significantly enhances the rate at which heat can be dissipated away from heat-generating components. Grayson’s Stephens highlights the dual functionality that some advanced TIMs, particularly phase change materials, can offer: “Phase change materials that can do solid to liquid perform the dual role of absorbing and rejecting heat, thus being both the heat exchanger and coolant transfer liquid.” This ability to absorb heat during periods of high load and then release it as the system cools can help buffer temperature fluctuations and maintain more stable operating conditions. Advances in TIMs focus on balancing acts such as achieving high thermal conductivity without compromising other key properties. Parker Lord has developed products with high thermal conductivity combined with relatively low viscosity that allow better heat rejection without sacrificing end-winding fill, Wyman explains. This is particularly important in areas with complex geometries, ensuring that the TIM can effectively penetrate and fill all the necessary spaces to maximise thermal contact. A critical characteristic of TIMs is the heat transfer coefficient, which dictates how effectively heat moves from the motor to the cooling fluid. A higher heat transfer coefficient means that less coolant flow is required to remove the same amount of heat. This, in turn, leads to a lighter system with reduced consumption of energy by the coolant pump and lower windage losses within the motor. Herman describes advanced TIMs as “the thermal glue that keeps everything running cool and smooth,” emphasising that “better TIMs mean better heat flow, better performance and longer-lasting EV motors – all without major changes to system architecture.” Valeo’s expert outlines several common types of TIMs used in EV motors, including thermal greases and pastes for tight surfaces, gap fillers for uneven surfaces, phase change materials, thermal adhesives for bonding, and thermally conductive foams or pads for larger interfaces. The benefits of advanced TIMs are manifold. Firstly, they offer higher thermal conductivity. She notes that new TIMs incorporating advanced fillers like boron nitride, graphene, carbon nanotubes and ceramic nanoparticles can achieve thermal conductivities of 10 W/m·K or higher; a significant improvement over older compounds. This enhanced heat flow leads to lower operating temperatures and higher power output. Secondly, they provide improved mechanical properties, such as being softer, more compliant, or electrically insulating, thereby May/June 2025 | E-Mobility Engineering With the right thermally conductive, electrically insulated potting (upper stator), heat passes more easily from windings into the case walls and coolant passages, enabling higher performance from the motor (Image courtesy of Parker Lord)
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