Design of a ferrite magnet vernier machine for an in-wheel machine

In the field of electric vehicles, outer-rotor in-wheel machines require higher torque at lower speed compared with typical machines. Thus, the permanent magnet vernier machine, which has this property, is considered for this application. The NdFeB sintered magnet used in many machines is expensive and a very low resistance, leading to the danger of demagnetization at high temperature due to heat caused by the eddy current in the magnet. Therefore, the authors proposed a design using a ferrite magnet. However, the efficiency characteristics when using ferrite magnet are poor because the fundamental component of the air-gap flux density is low. The present paper proposes an improved design in which the magnet arrangement on the rotor side is V-type in order to increase the air-gap flux density. Results of calculations using finite element method demonstrate that the proposed design increases the air-gap flux density and obtains high efficiency in overall operation area.