Novel Asymmetric Rotor Pole Interior Permanent Magnet Machines with Enhanced Torque Density: An Overview

Abstract

This paper overviews recent developments and novel topologies of a new class of asymmetric rotor pole interior permanent magnet (AIPM) machines for torque enhancement. The principle for torque enhancement by utilizing magnetic-field-shifting is introduced firstly. AIPM machine topologies are then categorized in terms of asymmetries of PM configurations and rotor core geometries. Subsequently, various AIPMs in each category are described and analyzed with respect to merits and demerits. Finally, electromagnetic performances of selected AIPM machines are compared.INDEX TERMS: Asymmetric interior permanent magnet (AIPM), Magnetic-field-shifting (MFS), PM torque, Reluctance torqueI. IntroductionInterior permanent magnet (IPM) machines having PMs buried inside the rotor cores are of growing interests due to superior torque density, high efficiency, wide constant power operating speed range, and high rotor robustness [1]. In IPMs, the reluctance torque resulted from the rotor saliency can be utilized to enhance the torque density without increasing the PM usage. However, in conventional IPM machines with symmetrical rotor structures, the maximum PM and reluctance torque components are achieved at different current angles whose difference is theoretically 45 electrical degrees. Consequently, only a portion of both torque components can be utilized at the current angle of maximum synthetic torque. To deal with this issue, a novel design concept for torque enhancement of IPMs by making the maximum PM and reluctance torque components to be reached with a smaller difference of current angles has been firstly proposed by setting the optimal displacement angle between the surface-mounted PM rotor and the reluctance rotor in a two-part rotor PM machine [2] and by employing asymmetric rotor flux barriers in a V-shape IPM [3], respectively. To utilize the concept designated as magnetic-field-shifting (MFS) effect in this paper, the PM field and reluctance axes of IPM machines need to be shifted, resulting in a new class of novel asymmetric rotor IPM machines. The torque enhancement principle of AIPM is illustrated in Fig. 1. This paper overviews various novel AIPM topologies which are categorized in terms of asymmetries of PM configurations and rotor core geometries, analyses various AIPMs in each category with respect to merits and demerits, and compares electromagnetic performances of selected AIPM machines.II. AIPM Machine TopologiesIn this section, novel AIPM machine topologies in literature are categorized according to whether their PM configurations and rotor core geometries are symmetrical or asymmetric, respectively. The proposed categorization method and some examples reported in references for each category are shown in Fig. 2 (a).An AIPM topology with the asymmetric hybrid-layer PM configuration and asymmetric rotor cavity has been proposed in [4], which achieves torque enhancement with simple structure due to utilizing MFS effect and also shows reduced torque ripples. A hybrid-pole AIPM machine is proposed in [5] that employs V-shape and inset PMs in adjacent poles alternatively, which has asymmetric PMs due to shifted positions of inset PMs but symmetrical rotor core geometry. This hybrid-pole AIPM has higher torque density compared with a V-shape IPM with the same PM usage. An AIPM machine with asymmetric PMs and symmetrical rotor structure is also reported in [6] that can achieve fully aligned maximum PM and reluctance torque components, although the increase of synthetic torque is not notable compared with a PM-assisted reluctance machine because of significant reduction of maximum reluctance torque. The asymmetric flux barrier in one side of symmetrical V-shape PMs in each pole is used in [3] to utilize the MFS effect by shifting the angle of reluctance axis and to clearly increase the synthetic torque. Similar to [2], a two-part rotor design with a SPM rotor and a multi-layer reluctance rotor is proposed in [7] that exhibit symmetrical PM configuration and rotor cavity design, respectively. The MFS effect is utilized by adjusting the displacement angle between both rotor parts, which can also achieve fully aligned maximum PM and reluctance torque components and shows a clear torque enhancement. Four selected AIPM machines with rotor topologies proposed in references [3]-[5] and [7] are redesigned by using the same stator, rotor diameter and PM usage to the conventional Toyota Prius 2010, and their torque performances at 750 r/min and 236A are compared in Fig. 2 (b). Comprehensive overview of more AIPM topologies will be given in the full paper. **