The new "Insight" hybrid vehicle (HEV) that Honda Motor Co Ltd released Dec 14, 2018, in Japan uses a magnet containing no heavy rare earth elements for the rotor of its motor.
As a result, it became possible to improve heat resistance while keeping the strength of its magnetic force without using dysprosium (Dy) or terbium (Tb), which are high-priced and cannot be always stably procured.
Honda has already employed the magnet containing no heavy rare earth elements for the "i-DCD" one-motor hybrid mechanism. But this is the first time that the company has applied it to the "i-MMD" two-motor hybrid mechanism.
For the rotor of HEV motor, a Nd-Fe-B (neodymium-iron-boron)-based magnet (neodymium magnet) with heavy rare earth elements (e.g. Dy and Tb) added is used. Dy, Tb, etc are added to increase the coercive force of magnet, which is an indicator of heat resistance.
For the new Insight's motor, which improves heat resistance without using heavy rare earth elements, the method to cool the neodymium magnet embedded in the rotor was improved. Specifically, Honda used a new method called "axis core cooling."
The previous motor of the i-MMD uses a neodymium magnet with heavy rare earth elements added. And its rotor is cooled by dropping ATF (automatic transmission fluid) from the upper part of the motor's stator. However, with this method, it is difficult to keep the heat resistance of a magnet using no heavy rare earth elements.
Therefore, for the motor of the new Insight, Honda employed a method that supplies ATF to the inside of the rotor's rotating shaft and directly cools the magnet by using a centrifugal force (axis core cooling). Cooling performance is improved by using both the new cooling method and the conventional dropping method, making it possible to prevent the heat resistance of magnet from being lowered, Honda said.