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Thermal management on electrical machines

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The demand for electrification in various transportation fields, being it for railway, marine, aerospace or automobiles, is increasing at an unprecedented high rate to achieve a fully electrified drive system, due to the increasingly stringent requirements on carbon emissions reduction from various governments. In the field of new energy vehicles, the "New Energy Vehicle Industry Development Plan (2021-2035)" from China Ministry of Industry and Information Technology emphasizes the technologies such as the high-efficiency and high-power-density drive motor system, fiber-reinforced composite materials for lightweight vehicles and points out that by 2025, the average power consumption of all-electric passenger cars will be reduced to 12 kWh/100 kilometers.

In our previous articles, we introduced the recent developments on power electronic, controllers at the Nottingham  Electrification Centre and the research study on synchronous excitation motors, which effectively improve the performance of the aforementioned drive system. This article will focus on the background, content and future development of another significant challenge faced in the drive system --- the thermal management on electrical machines, which is an important aspect to push the boundaries of motor technology and solve the current bottleneck of further increase in power density (kW/kg, kW/L).


Electrical machines mainly include stator, rotor, and housing, as shown in Fig.1 of a surface mount permanent magnet synchronous motor. Copper losses are caused in the coil due to the current flowing inside, including DC losses and AC losses, which occupy a large amount of the total motor losses. Other losses include iron loss, magnet loss, friction loss, which increase the motor temperature, such as the hot spot in the lower bottom of the stator slot in Fig.1. Inappropriate thermal management results into the insulation system aging and magnet performance degradation problems, which shorten the motor life expectancy or even cause the motor failure. Effective thermal techniques help to increase the motor maximum current density, and thus improve the motor power density.


 There are mainly two methodologies to improve the motor thermal performance: i) reduce the motor losses; ii) enhance the heat dissipation. Based on the different cooling medium type, the motor cooling techniques are divided into oil cooling, air cooling and water cooling. For example, the synchronous excitation motor described in earlier article adopts the air forced cooling method. Based on the way the cooling medium interact with the heat source, the cooling system is divided into direct cooling and indirect cooling. The current density that can be achieved and the pros/cons of different cooling techniques on the market are shown in the following table.

There are three major trends in the research of thermal improvement on electrical machines: 1) new material research and development, such as the investigation on materials application to the stator slots with low electrical resistivity materials but high thermal conductivity to shorten the heat transfer path between the coil and the cooling medium. 2) direct cooling, such as spray cooling or flooded oil cooling. 3) new motor cooling structure. The hybrid cooling structure of at least two cooling methods also provide a broad prospect to improve the motor thermal performance.


Based on a water-jacket-cooled 75kW electric vehicle in Fig. 2, back-iron extension (BIE) in Fig. 3 is firstly introduced and investigated, which is extended from the stator back-iron along the slot centerline to the slot opening. With much higher thermal conductivity (30W/(m∙K)) than the equivalent slot thermal conductivity (0-2 W/(m∙K)), BIE greatly shortens the heat transfer path between the slot center to the cooling water jacket and reduces the motor peak temperature by up to 26.7%.


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