2018-01-0057 Published 0 3 Apr 2018 © 2018 SAE International. All Rights Reserved.A Review of the State of the Art of Electric Traction Motors Cooling Techniques Alberto Carriero, Matteo Locatelli, Kesavan Ramakrishnan, Gianpiero Mastinu, and Massimiliano Gobbi  Politecnico di Milano Citation: Carriero, A., Locatelli, M., Ramakrishnan, K., Mastinu, G. et al., “A Review of the State of the Art of Electric Traction Motors Cooling Techniques,” SAE Technical Paper 2018-01-0057, 2018, doi:10.4271/2018-01-0057. Abstract 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 pollu - tion 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 extrac - tion 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. Introduction Nowadays, most of the road vehicles are driven by internal combustion engines. Transport sector is changing, the theme of pollution gains increasingly importance as the emission standards become more and more restrictive leading the industry to adopt more refined solutions with an increasing cost in terms of development and price. It follows that electrical machines play an increasingly impor - tant role as the direct pollution issue is completely avoided with a relative low-cost apparatus. These years represent the beginning of a whole new transportation era and philosophy, even if the principles of electric machines are well known, there is still room for improvement in order to replace or cooperate with internal combustion engines as the major traction systems. The most advanced electrical machines used in the auto - motive industry are brushless motors in which windings are the statoric part and permanent magnets form the rotors. Several advantages are offered such as a high efficiency, high power density, low weight, relative small dimensions and higher simplicity with respect to standard IC engines. On the other hand, besides the difficulty in energy storage methods, sensitivity of the magnets to high temperatures and their price is the major drawbacks of permanent magnet machines. The magnets cannot operate at high temperatures since they suffer from demagnetization [ 1], this effect leads to lower performances, especially in terms of mechanical torque [ 2] as Figure 1 states: Moreover, high electrical current can generate significant heat loss in the windings and global efficiency is directly affected by Joule effect. To deal with these problems, efficient rotor/stator cooling systems need to be employed. Numerous cooling systems can be found among industry motors, the classical solutions commonly adopted are defined by proper