Design, Analysis, and Control of Advanced High-Compactness Axial-Flux Permanent Magnet Machines

Abstract

Electric machines work in many areas such as electric transportation and industrial applications as key electromechanical energy conversion devices. It replaces internal combustion engines in many applications, which reduces the environmental pollution and consumption of fossil fuels.

Conventional electric machines are radial-flux permanent magnet machines (RFPMMs) with cylindrical rotors and stators. However, compared with RFPMMs, axial-flux permanent magnet machines (AFPMMs) have more compact structures and higher torque density. They are usually composed of several disc rotors and stators. The large aspect ratio improves the space and material utilization of the AFPMM. These advantages make them welcomed widely in electric aircraft, electric vehicles, electric ships, and industrial robots. Thus, this thesis proposes several double-side AFPMMs including a segmented-Halbach AFPMM, a double-side asynchronous rotor (DSAR) AFPMM, a counter-rotating AFPMM, and a double-stator AFPMM with skew slots. The design, analysis, and control of AFPMMs are conducted based on the proposed AFPMMs. The main work contains modeling and optimization, eccentricity and vibration analysis, harmonic analysis, skew slot investigation, advanced machine control, and application.

Firstly, this thesis not only reviews all kinds of axial-flux machines (AFMs) but also analyzes and compares the features of these AFMs based on academic and commercial data. Besides, potential modeling methods for AFMs and corresponding features are also reviewed.

Secondly, based on the double-rotor segmented-Halbach AFPMM, the magnetic equivalent circuit method (MECM) is proposed. Different PM magnetization patterns in AFPMMs and their effects are introduced. They include conventional axial magnetized PMs and Halbach-array PMs. Then, the analytical modeling principle, machine modeling, and matrix calculation are all illustrated. Compared with the finite element method (FEM), it greatly reduces the computational time as a quasi-3D analytical method. The MECM can be applied in all AFPMMs no matter what kind of PM magnetization patterns are adopted. The experiment validates the accuracy of the MECM in the calculation of flux density, no-load back electromotive force (EMF), and output torque. In addition, the eccentricity detection and vibration analysis in the prototype is conducted. It reflects the effect of eccentricity and provides the diagnosis method in the double-rotor AFPMMs.

Thirdly, the thesis focuses on the optimization and analysis of AFPMMs with the example of the double-side asynchronous rotor (DSAR) AFPMM. Different pole-slot configurations in AFPMMs are introduced and analyzed. Qualitative analysis for harmonics is conducted to investigate the influence between different configurations in a double-side AFPMM. Based on the analysis, the DSAR-AFPMM with two different pole-slot configurations on the two sides is proposed. Then, a hybrid optimization is designed for the machine to reduce the calculation burden. It combines the quasi-3D optimization and 3D parametric study. To verify the analysis and optimization, an experiment is performed. Besides, a novel control strategy with a reduced-switch-count series-end winding (SW) drive is designed for the machine. It broadens the speed range of the two rotors with one less switch.

Fourthly, the influence of skew slots on AFPMMs is investigated. Reduction methods of cogging torque of AFPMMs are clarified. Then, skew slots are designed in the machine with different types of PMs. The asymmetry problem caused by skew slots is also discussed and solved. In order to validate the performance, a prototype is fabricated and a corresponding experiment is conducted.

Finally, two counter-rotating rotors are achieved in an AFPMM based on magnetic-field modulation (MFM). The MFM theory is introduced based on harmonic analysis. According to the theory, the split teeth are designed in the yokeless and segmented armature (YASA) AFPMM. Then, the performance evaluation of the machine with different shapes of split teeth is conducted through FEM. The experiment verifies the performance of the YASA AFPMM. Due to the harmonics in the back EMF, a control method for the multi-phase machine is proposed to suppress harmonic currents in the proposed machine.

Date of Award	8 Jul 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chunhua LIU (Supervisor)