Analysis of slanted air-gap structure of interior permanent magnet synchronous motor with brushless field excitation

This paper introduces a new air-gap structure, the slanted air-gap, for increasing the speed limit of an interior permanent magnet synchronous motor (IPMSM) for application in a hybrid electric vehicle. This unique slanted air-gap is intended to maximize the ratio of the back-emf of a machine that is controllable by brushless field excitation (BFE). The BFE structure offers high torque per ampere per core length at low speed and weakened flux at high speed. Although the irregularly shaped air-gap reduces the air-gap flux, it also makes a flux barrier along the d-axis flux path and decreases the d-axis inductance. Therefore, the reluctance torque of the machine increases to compensate for the decreased permanent magnet torque; as a result, the machine achieves a higher ratio of the magnitude of controllable back-emf without losing the high torque capability resulting from the BFE.