43 Motor cooling | Insight E-Mobility Engineering | May/June 2025 operating temperatures. The adoption of direct oil cooling by manufacturers such as Lucid Motors in their compact drive units underscores its effectiveness in achieving high power output in a compact form factor, she notes. The development and application of advanced coolant materials and nanofluids also represent significant strides forward in motor cooling technology. Herman highlights that the development of high-performance coolants with nanoparticles (such as graphene or alumina) improves thermal conductivity significantly over conventional coolants. This allows for more effective heat dissipation without the need for increased pump size or flow rates, facilitating more aggressive driving without derating, and potentially increasing motor lifespan owing to more stable thermal environments. Furthermore, the integration of 3D printed cooling channels directly into motor housings or components offers unprecedented design flexibility for thermal management. Herman notes that additive manufacturing enables complex internal geometries for directing cooling channels straight into motor housings or components, which improves surface area contact with the coolant and enables custom-designed flow paths to eliminate hot spots. While challenges related to serial manufacturing costs and mechanical strength – you don’t want too many holes though the material – are still being addressed, this technology holds immense promise for creating highly optimised cooling solutions. Hello TIM Thermal interface materials (TIMs) have also played a crucial role in enhancing heat transfer within e-motors. Herman explains that TIMs “fill microscopic air gaps between two solid surfaces, such as the copper windings and aluminium housings.” By replacing air, a poor thermal conductor, with materials exhibiting higher thermal conductivity, TIMs “help create a smooth thermal path,” thereby improving the overall efficiency of heat dissipation from the motor’s internal components to the cooling system. Simon Odling, chief of new technology at YASA, details the company’s specific approach to liquid cooling, focusing on an ‘oil-flooded stator’ design. In this configuration, the copper windings are fully immersed in oil within a sealed polymer structure. Odling emphasizes that “it’s all about maximising surface area and contact between, primarily, the copper and the oil, which means we get a very fast cooling effect. So, we can have a very low time constant in terms of thermal mass in the stator and we can get the stator hot or cool very quickly.” The continuous circulation of oil within the stator, facilitated by strategically placed ‘blockers and diverters’ and optimized through computational fluid dynamics analysis, ensures efficient and even temperature distribution, allowing YASA motors to operate at high-performance levels with rapid thermal response. Achieving optimal performance in EVs hinges significantly on the effective is that the industry has gotten a lot more efficient at cooling the motor with less oil.” A crucial aspect of this involves precise identification and targeting of high-temperature areas within the motor. Uppuluri elaborates on the efforts to “identify where these hot spots are, where we need oil nozzles to spray so that we don’t have temperature disparities that are large enough to cause distortion, all done with a minimum of oil.” In addition to minimising the amount of coolant and oil within the system, development looks to reduce the complexity of the delivery system through optimising the design, the number and location of nozzles, and the quantity of oil that should be sprayed to ensure sufficient cooling. This meticulous approach is intended to provide effective cooling without unnecessary energy expenditure and while minimising the weight of the cooling fluid. Targeted cooling Valeo’s Pascale Herman, director of product planning at the company’s power division, provides a broader perspective on the evolution of motor cooling, highlighting the transition from air and water-jacket cooling toward more direct and efficient methods. She emphasises that “with e-motor speed and power getting higher and higher, cooling becomes more important and challenging for performance.” She adds that getting the cooling medium closer to the hottest areas and shortening heat transfer paths are important trends in motor thermal management, citing slot cooling as an example. Furthermore, direct oil cooling, where the coolant is in direct contact with the stator windings and even the rotor, has emerged as a significant advancement. Herman explains that this “enables higher power density and continuous power output without overheating” and “reduces motor size and weight by improving thermal management,” ultimately leading to increased efficiency by maintaining optimal Representative integrated cooling system schematic illustrating how several subsystems including fuel cells, EDUs, converters and inverters can share a cooling loop, which can improve energy efficiency (Image courtesy of Grayson Thermal Systems)
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