Characteristic analysis of a consequent-pole motor with vernier structure using ferrite magnets

Recently, the research on permanent magnet vernier machine (PMVM) is increasing due to its features of high power density and effi- ciency for low speed application. The machine utilizes slot harmonics of air gap permeance to develop the modulation flux, so it is beneficial to use of thin magnets in a surface permanent magnet(SPM) structure because it causes effective air gap shorter, resulting in the increment of slot harmonic of the stator permeance. Therefore, considering the demagnetization and torque density, it is hard to avoid using expensive Nd-magnets [1]– [4]. Meanwhile, a consequent pole machine has the advantage of being able to achieve nearly same levels of air gap flux density, although it reduce the used magnet quantity by half as compared to conventional machine [5]– [7]. If the consequent-pole structure is applied to a surface PMVM, the decrement of average air gap length is expected due to removal of half area of PM, resulting in incre- ment of the air gap permeance harmonics as well. Consequently, even though thick ferrite magnet is used for the vernier motor, this motor is expected to resolve the problem of the average air gap length greatly and to get the higher vernier effect and demagnetization characteristics. In this paper, the character- istics of a consequent-pole motor with vernier structure using ferrite magnet analyzed and the characteristics are compared with those of general SPM and vernier machines by using finite element(FE) simulations in order to verify the advantages of proposed structure. Proposed Idea To get the vernier effects, common relation of $\mathrm {Z}_{r}- \mathrm {Z}_{s}= -\mathrm {p}$ is used, where Zr is the number of rotor pole pairs, Zs is the number of stator slots, p is the number of winding pole pairs. The vernier structure can be converted into consequence pole structure by replacing the S pole of magnet with iron core, which will reduce the average air gap length. Fig.1 shows general structure of a PM vernier machine and the proposed structure of the consequent pole vernier motor. The both prototypes given in Fig. 1 are able to create vernier effect and the induced voltage can be expressed by (1) [1].