Cooling of electric machines

Why is an effective cooling important?

The thermal behavior of an electric machine is an aspect often neglected in its design process. The machine is designed only using an electro-magnetic point of view. The design of the cooling system is often the last step and must adapt to all the electro-magnetic constraints.

Temperature levels have a direct impact on the lifespan of a machine insulation system. Each machine has an insulation class for its windings corresponding to an operating temperature. The lifespan of a machine is 20,000 hours at its operating temperature. A basic model to predict the lifespan of a machine is to divide it by 2 for each 10°C above the operating temperature [1]. If the temperature of the machine is under its operating temperature, the lifespan is multiplied by 2 for each 10°C difference.

Temperature has also a direct impact on the electric performances of the machine: as an example it increases the winding resistances, and changes the remanent flux density of permanent magnets. Several studies [2][3][4] show how the electromagnetic performances decrease when the internal temperature rises. When the temperature increases from 25°C to 100°C the efficiency decreases of 1% up to 5%. These changes can be considerable considering the complete lifetime of a machine (e.g. wind turbine generator annual energy production). A better cooling of a machine gives more durability and more efficiency. Investing in the cooling system optimization at the design stage of the machine can give significant long-term cost savings.

What are the critical parts of the machines?

Magnets. The temperature of the magnets must be under a certain threshold. Over this threshold, magnets lose their magnetic properties which are critical for the electric performances.

Windings. The temperature of the windings is also very important for a machine. Due to the generation of Joule losses, it is often the highest temperature. The critical temperature for a winding depends on the material used for its insulation.

Bearings. Bearings are the location of mechanical losses which can be very important in case of high rotational speed. High temperature in bearings will reduce the durability of the machine.

What are the different cooling architectures?

Passive/Active cooling. Two options exist for the cooling. The first option is passive: no external source of power is needed. Different mechanisms can be used for passive cooling: the rotation of the rotor induces air movements, the natural convection around the exterior of the machine, fins to increase the exchange surface, etc. In case of active cooling, the flow in the machine is induced by an external source, a fan for example.

Open/closed machines. In an open machine, the air is directly taken from the ambience, circulates inside the machine, and is rejected in the environment. In a closed machine, air circulates in a closed circuit and must be cooled by a heat exchanger. The two configurations have pros and cons. An open circuit is more compact and need less ventilation power. A closed circuit will be more independent from the external conditions. On the contrary, the open circuit will be more affected by outside elements (humidity, dusts, etc.). The closed circuit will need more space and more power due to the heat exchanger.

What are the best strategies for optimization?

To have a good strategy for optimization, it is important to have a clear vision of the objectives and the constraints of the problem. A good cooling system is not necessarily a system with the lowest temperature levels. Those systems would generally have high energetic cost: the ventilation power needed would be very high. As a result, an optimization of a cooling system is often a multi-objective problem with two goals: increases the heat dissipation and decreases the ventilation power (also known as hydraulic/aeraulic power). Some objectives can also be translated in terms of constraints. For example, instead of trying to reduce the temperature of magnets, a constraint on their temperature can be used.

References

[1] O. Barré and B. Napame, “The Insulation for Machines Having a High Lifespan Expectancy, Design, Tests and Acceptance Criteria Issues,” Machines, vol. 5, no. 1, p. 7, Feb. 2017. http://dx.doi.org/10.3390/machines5010007

[2] Aimeng Wang, Heming Li, and Cheng-Tsung Liu, “On the Material and Temperature Impacts of Interior Permanent Magnet Machine for Electric Vehicle Applications,” IEEE Trans. Magn., vol. 44, no. 11, pp. 4329–4332, Nov. 2008. https://doi.org/10.1109/TMAG.2008.2001527

[3] T. Sebastian, “Temperature effects on torque production and efficiency of PM motors using NdFeB magnets,” in Conference Record of the 1993 IEEE Industry Applications Conference Twenty-Eighth IAS Annual Meeting, 1993, vol. 1, pp. 78–83. https://doi.org/10.1109/IAS.1993.298907

[4] M. Beniakar, T. D. Kefalas, and A. G. Kladas, “Investigation of the Impact of the Operational Temperature on the Performance of a Surface Permanent Magnet Motor,” Mater. Sci. Forum, vol. 670, pp. 259–264, Dec. 2010. https://doi.org/10.4028/www.scientific.net/MSF.670.259

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