Design and analysis of reluctance-coupled high-torque-density electric motors

Abstract

Electric machines play a pivotal role in energy conversion, particularly in areas such as industrial robotics and electrified transportation propulsion systems. These applications require electric motors that can provide high torque density. Reluctance-coupled motors, known for their flux modulation effect, have gained attention for their high torque density and efficiency. To further enhance torque density in reluctance-coupled machines, two promising approaches are improving the armature field by current and strengthening the magnetic field excited by permanent magnets (PMs). Consequently, this thesis embarks on exploring innovative reluctance-coupled machines with a dedicated emphasis on achieving increased torque density through implementing these two strategies. A novel dual-stator Vernier permanent magnet machine (DSVPMM) is proposed to improve torque density and power factor. Conventional Vernier machines face challenges in increasing the electric load without compromising power efficiency. In response, DSVPMMs efficiently utilize previously unused inner space within the machine to enhance electric load. This design also addresses the low power factor issue found in Vernier machines. The proposed machine features a unique configuration with a complementary stator design and phase-shifted dual stator winding. This innovative design enables an increase in the electric load without sacrificing power efficiency and effectively reduces non-­working even-order harmonics generated by the armature winding. A comprehensive analysis of the electromagnetic performance, mechanical structure, and thermal analysis is conducted through finite element analysis (FEA). The FEA results demonstrate that the proposed design achieves higher torque density and power factor while mitigating unbalanced forces compared to the conventional Vernier machines. Finally, a prototype is fabricated to validate the proposed design. Furthermore, additional PMs are properly incorporated into a stator PM machine to improve the air-gap flux density. A novel stator dual-PM flux-switching (SDPM-FS) machine is proposed to enhance torque density. The SDPM-FS machine utilizes a unique arrangement of PMs on the stator tooth region, adding radially magnetized PMs into the dummy slots to enhance the third-order harmonic and introduce the fourth-order harmonic of the stator PM excited magnetomotive force (MMF). Consequently, the magnitudes of low-order working harmonics of air-gap flux density are significantly improved, resulting in notable enhancements in torque density and efficiency compared to the conventional machine. Moreover, a dual-side PM (DSPM) machine is proposed to further improve torque density. The topologies of conventional DSPM machines limit the structure of stator PM and stator teeth, thus restricting torque production on both the rotor and stator sides. To overcome this constraint, a novel DSPM machine with a decoupled topology of PM and stator core is proposed. This design allows for optimal topologies of both components. Additional even-order harmonics of the equivalent stator MMF are generated, thereby improving torque production on the stator side. The proposed DSPM machine demonstrates superior torque density and efficiency compared to a conventional DSPM machine in performance comparison. Finally, a prototype of the DSPM machine is fabricated and tested to validate the analysis.