Design and analysis of high torque density machines for aerospace applications

• Main Author: Shen, Fawen
• Other Authors: Christopher H. T. Lee
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering
• Online Access: https://hdl.handle.net/10356/180982
• Institution: Nanyang Technological University

Owing to the growing demands for low carbon emissions and the soaring concerns over the energy crisis, the more electric aircraft (MEA), which is regarded as the most desirable substitute for conventional aircraft, has drawn increasing interest. As the key components of the propulsion system, the electric motors directly affect the performance of MEA, thus requiring more attention. To fulfill the requirements of MEA, the electric motors should possess high power density, high torque density, high reliability, and high efficiency. Of all the electric machines, permanent magnet (PM) machines stand out as the most promising solutions owing to their inherent advantages, which include exceptional torque density, power density, and efficiency. Generally, the PM machines with gearboxes are adopted in the MEA to achieve high-torque low-speed output. However, the gearbox suffers from inevitable mechanical loss and frequent maintenance, thus reducing the system’s efficiency. To address the above issues, the PM field-modulated machines with high torque density have drawn increasing interest since the high-torque low-speed output can be achieved without employing a mechanical gearbox, consequently improving the system efficiency. Three types of PM field-modulated machines: stator-PM field-modulated machine, rotor-PM field-modulated machine, and dual-PM field-modulated machine are reviewed in Chapter 2 to evaluate their potential for aerospace applications. It is found three representative machines, including flux switching permanent magnet machine (FSPM), vernier permanent magnet machine (VPMM), and consequent-pole (CP) dual-PM machine, have better torque performances than other candidates. By reviewing the development of three representative PM field-modulated machines, it is concluded that further torque enhancement is still restricted by existing machine topologies. To face the great challenges for aerospace applications, new machine topologies aiming at further torque enhancement are required. Chapter 3 explores a novel flux-switching machine named the star-array flux-switching permanent-magnet machine or the star-FSPM. Compared to the traditional spoke-FSPM, this innovative design offers two advantages: a heightened flux-focusing effect and a larger stator-slot area. These improvements result in a more efficient utilization of permanent magnets (PM) and an increased torque density. The harmonic characteristics of the magnetic field under no-load conditions and the armature reaction field in the star- FSPM are investigated by employing the field modulation theory. Furthermore, the chapter delves into the torque generation mechanism of the star-FSPM and makes a comparative analysis with its spoke-FSPM counterpart. The findings reveal that this machine enhances positive electromagnetic torque-generating harmonics while reducing the negative electromagnetic torque-generating harmonics, thereby effectively boosting overall torque. Following this investigation, a comprehensive performance comparison among four PM machines is conducted via finite-element analysis (FEA). The outcomes underscore the star- FSPM achieves 88% higher PM utilization ratio and 29% higher torque density in contrast to the spoke- FSPM. To substantiate these theoretical analyses and FEA results, experimental measurements are conducted, validating the overall findings. A flux-switching machine with U-V-array permanent magnet arrangement, termed UV-FSPM, is presented in Chapter 4. The key design is to construct parallel magnetic circuit by adopting U-V-array PMs. Based on the parallel magnetic circuit, the operating principle of the UV-FSPM is illustrated. To provide a deeper insight into the torque enhancement mechanism, the analytical model of the UV-FSPM is built by using the general air-gap field modulation theory. It is found that the U-V-array PMs can significantly enhance the dominant working harmonic which effectively contributes to torque enhancement of the investigated UV-FSPM. To reveal the impact of key parameters on torque performances, the parametric optimization of the UV-FSPM is carried out. The leading performances of three FSPMs are comparatively studied by FEA. It is proved that the investigated UV-FSPM possesses the highest torque density and highest PM utilization ratio within the pool. A prototype of the UV-FSPM is fabricated and tested for verification of the investigated concepts. In Chapter 5, a novel vernier machine with Spoke-V (SV) array permanent magnet arrangement, referred to as SV-VPMM, is investigated. This innovative machine merges the unique attributes of spoke- and V-array permanent magnets, creating the distinctive SV- VPMM configuration. Employing the comprehensive airgap flux modulation theory, we delve into the operational principles underpinning the SV-VPMM. The investigation reveals a remarkable enhancement in the primary operational harmonics, facilitated by the SV-array permanent magnets. To offer effective design insights, a thorough parametric analysis of the SV-VPMM is carried out. Furthermore, the leading performances of three vernier machines are discussed through finite-element analysis (FEA). The results imply that the SV-VPMM achieves 44% higher torque density than the conventional VPMM. Dual-permanent-magnet machines (DPMMs) also exhibit the merit of high torque density. A DPMM with asymmetric stator teeth-tips, termed as AST-DPMM, is presented in Chapter 6. The key of the proposed design is to arrange wide and narrow stator teeth-tips alternatively. By building an analytical model of the AST-DPMM, the impact of asymmetric stator teeth-tips is investigated. It is found the asymmetric stator teeth-tips can enhance the working harmonics by generating magnetomotive force harmonics and permeance harmonics. Hence, it can achieve high torque density and improve PM utilization ratio simultaneously. The performance comparisons together with existing DPMM are conducted to evaluate the investigated AST-DPMM. A prototype of the AST-DPMM is fabricated, and the experimental testing is conducted to verify the proposed ideas. To comprehensively evaluate the performances of four proposed PM machines, the performance comparison of four proposed PM machines including star-FSPM, UV-FSPM, SV-VPMM, and AST-DPMM are conducted and discussed in Chapter 7. The results reveal that the proposed SV-VPMM exhibits higher torque density than other candidates. Based on the comparison results, a high-power SV-VPMM is designed and optimized. The key electromagnetic performances of the high-power SV-VPMM are compared with those of the permanent magnet synchronous machine (PMSM), which is one of the most popular industrial benchmark machines. The results imply that the SV-VPMM outperforms the PMSM regarding torque density, efficiency, and torque ripple. Finally, Chapter 8 summarizes the research work presented in this thesis. It can be concluded that the further torque enhancement of four PM machines including star-FSPM, UV-FSPM, SV-VPMM, and AST-DPMM are correspondingly achieved. Based on the presented research work, several future working focuses are discussed.