SR Motor Made Small Enough for Hybrid Cars

Dec 17, 2009 12:35 Motohiko Hamada, Nikkei Automotive Technology

A Japanese university researcher succeeded in downsizing a switched reluctance motor (SR motor). Because SR motors do not use a permanent magnet or rare-earth material, they are expected to be used as motors for next-generation vehicles such as hybrid electric vehicles (HEVs).

The small-size SR motor was developed by Akira Chiba, professor at the Department of Electrical Engineering, Faculty of Science & Technology, Tokyo University of Science.

Currently, HEVs are equipped with a synchronous motor whose rotor is embedded with a permanent magnet. However, 90% of the rare-earth material needed for making permanent magnets is exported by China, and its output is limited. Moreover, its price is now doubling or even tripling due to the increasing demand for it. Therefore, the commercialization of an SR motor has been waited for.

SR motors are rotated by utilizing the difference in magnetic reluctance. They have excellent heat resistance and robustness because of their simple structure. But the torque and energy use efficiency of SR motors are inferior to those of synchronous motors; therefore, to ensure enough performance for HEVs, they become too large to be mounted on vehicles.

The prototyped SR motor has the same size as the 50kW synchronous motor equipped in the second-generation Toyota Prius. Also, it has a maximum torque of 403N·m and an energy use efficiency of 86%, which are equivalent to those of the Prius' motor (400N·m and 83%, respectively). The torque density of the SR motor is 45N·m/L.

This time, Chiba found that the numbers of rotors and stators are correlated with the torque. So, he increased the numbers to 12 and 18, respectively. Also, the torque was increased by ramping the stators.

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