An EESM (Externally Excited Synchronous Motor) consists of a rotor with wound copper wires. One of its benefits is the ability to control the rotor electromagnetically with the rotor current, which is an advantage over an IPMSM (Internal Permanent Magnet Synchronous Motor). To practically use it and achieve optimal NVH quietness performance, the air- gap shape was redesigned to generate a sinusoidal curved magnetic flux density distribution. This differs from the standard design, in which the air gap has the same circumference as the rotor and stator. There was a significant reduction in the high-order magnetic flux density, which did not affect the torque. In addition, there was a reduction in the excitation force and minimal iron loss.
Unlike an IPMSM, which only uses magnets and produces less heat, the copper wires of the EESM rotor generate heat as current flows through them. To maintain power density, it is important to ensure optimal cooling performance. A new cooling structure was developed to compensate for this increase in generated heat. The new cooling structure includes a direct internal oil projection at the rotor hot points and an indirect one at the stator. Finally, as a side effect of the EESM, the high rotation speed causes a large centrifugal force on the rotor wire. The strength reliability was guaranteed using a slot-wedge shape and precision manufacturing. Synchronous control and high-speed, high-precision, as well as high- density layered winding were done to guarantee the manufacturing speed for multipole wire winding.