Equivalent air gap shortening by three-dimensional gap structure for torque improvement of electric machines

The importance of reducing the amount of rare earth materials, used in high-performance permanent magnets (PMs) has been established. However, the performance of such motors will not be easy to match without the use of rare earth materials. Although the torque characteristic of the motor can be improved by shortening the air gap length of the motor, there is a mechanical limitation on the gap length. In the present paper, a three-dimensional air gap structure of the motor is proposed to improve the torque characteristics without shortening the gap length. The 3-D air gap structure is demonstrated to be equivalent to shortening the actual gap length to approximately 70% of the original length.