A Review of the State of the Art of Electric Traction Motors Cooling Techniques

This paper provides a review on state-of-art modern cooling systems employed for thermal cooling of electric motors for vehicle applications. In recent years, the pursue of a more sustainable and ecofriendly mobility has pushed the research towards the development of electric vehicle powertrain systems. Besides the evident advantages of the adoption of electric traction systems in terms of pollution and efficiency, the need of an effective cooling system for the electric machine components gained more and more importance in order to maintain high efficiency and ensure high durability. In fact, it is known that high temperatures can be harmful for the electric motor: besides the evident damages for mechanical parts, the influence on the permanent magnet properties is not negligible [1] [2]. In this fast-evolving environment, different solutions for the thermal problem have been researched and adopted, each one with its own pros and cons. Those who face the development of a PM machine can found plenty of these solutions in literature; so, the purpose of this paper is to draw a first qualitative comparison among the most important mechanisms available to extract heat from the electric machine and to guide the reader to an efficient and effective solution. Various methodologies for heat extraction are here described: resilient thermal pads for a conductive cooling; forced air and liquid loops, spray cooling and hollow rotor shaft for gases/fluids convective cooling methods. Finally, it is provided a table for a qualitative comparison among the various cases.