Analysis of spoke-type IPM machine by step-staggered rotor with alternate airspace-barriers

Summary form only given. A significant advantage of spoke-type interior permanent magnet (IPM) machines is the flux concentration effect of adjacent PMs, which enhances the air-gap flux density and the output capability. However, the leakage flux around the rotor inner circumference is difficult to reduce [1]-[2]. In order to ease the manufacturing process, a spoke-type IPM rotor with alternate airspace-barriers is proposed in this paper, as shown in Fig. 1 (a). For comparison, two rotors with conventional and alternate airspace-barriers are employed, and 3% higher torque capability can be obtained by the proposed design due to the airspace in the paths of leakage flux, as shown in Fig. 1 (b). Moreover, the novel design is much less sensitive to the rib width, and therefore simplifies the punching tools and punching process for silicon-steel laminations, which is the bottleneck of spoke-type IPM rotor with conventional airspace-barriers [3], particularly for small machines.However, axial unipolar leakage flux is introduced, resulting in magnetization of the end shaft, which reduces the life expectation of bearings and even poses a threat to the machine system. To solve the problem, the rotor is axially divided into two uniform steps and the barriers of which are tangentially staggered by 1-pole pitch. With the step-staggered rotor employed, unipolar leakage flux can be eliminated. However, analysis of the proposed design requires 3-dimensional (3D) finite element (FE) method, which is very time-consuming. It is proved that similar torque capability can be obtained through rotors with conventional airspace-barriers (iron rib=1mm) and step-staggered alternate barriers (iron rib=2mm). Therefore, the equivalence can simplify the optimization process of the proposed design with the 2D FE method employed. In addition, the mechanical strength is analysed and compared between aforementioned rotors, including the mechanical stress and deformation. Sufficient safety margin c- n be achieved (will be shown in the full paper). As a verification of the foregoing analyses, prototypes are fabricated and tested, as shown in Fig. 2.