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| Characteristic and Magnetic Field Analysis of an Axial Flux Permanent Magnets Maglev Motor with Non-Uniform Air Gap |
| Qin Wei1,2, Ma Yuhua3, Zhang Jielong1, Lü Gang1 |
1. School of Electrical Engineering Beijing Jiaotong University Beijing 100044 China;
2. Beijing Rail Transit Electrical Engineering Technology Research Center Beijing 100044 China;
3. Taiyuan Institute of China Coal Technology and Engineering Group Taiyuan 030006 China |
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Abstract The main disadvantages of the maglev system are the initial construction cost and the maintenance cost caused by the complex guideway compared to the conventional train. To reduce maglev systems costs, many integrated passive guideway designs have been proposed. The electrodynamic wheel (EDW), composed of a permanent magnet (PM) and conductive plate, is a novel low-cost maglev technology. The Axial Flux Permanent Magnets Maglev Motor (AFPMMM), which has more interaction between PMs and conductor plate than the radial topology, is a type of EDW typology. Several studies have been carried out on the electromagnetic characteristics of AFPMMM with a constant airgap between its primary and secondary in recent years. However, AFPMMM should be operated with a non-uniform air-gap to simultaneously create the propulsion, suspension, and guidance forces. Although finite element analysis (FEA) models are often used to analyze the electromagnetic characteristics of AFPMMM, the analytic model is more efficient and more suitable for the initial design. Moreover, 3-D eddy current forces are difficult to be modeled by FEA, particularly when there is more than one mechanical degree of freedom, such as AFPMMM which rotor is simultaneously rotated and translationally. The three-dimensional (3D) analytical model of the magnetic field in an AFPMMM is proposed, and the influence of the structural parameters on electromagnetic characteristics is studied.
Firstly, the basic structure and principle of the proposed motor are introduced. Then, the 3D magnetic field distribution function and its Fourier series of the primary based on the integral approach are proposed. Based on second-order vector potential (SOVP), a 3D analytic model of the motor is proposed in the paper. Thus, the expression of the lift force, thrust force, and power losses can be obtained. The calculations of the magnetic field in the air gap, forces, and power losses are conducted by the finite element method and analytical method. The results show that the analytical model in this paper is correct and practical. Finally, a small-scale prototype is manufactured, and experiments are carried out to verify the predicted results.
The lift-to-weight ratio, which is affected by the main structural parameters such as the number of poles, PM radius, PM thickness, and air gap, reflects the suspension property of the AFPMMM. The relationship between the lift-to-weight ratio and the polepairs shows that when the polepairs is 0~3, the ratio increases with the polepairs. When the polepairs is 3~8, the lift-to-weight ratio decreases with the increase of the number of poles. Since the pole pitch is inversely proportional to the number of poles at a fixed radius of the Halbach rotor, a suitable choice of pole pairs should be 3 pairs. When the PM thickness is 10~20 mm with 3 pole pairs, the lift-to-weight ratio increases with the increase of the magnetic flux density in the air gap. When the thickness is over 20 mm, the growth trend of the flux density gradually slows down, so that the lift-to-weight ratio of the system decreases. Therefore, the thickness of PM must be determined to obtain a maximum lift-to-weight ratio. There are no significant difference in comparing the forces, magnetic field, and power losses obtained by analytical and 3-D FEM simulation.
The following conclusions can be drawn from the analysis: the proposed analytical model, in which the curvature effect, fringing effect and leakage flux can be taken into account, can provide reasonable estimates of the flux density. The forces calculated by the proposed model agree with those obtained by 3D FEA. The results show that the proposed model, compared with 3D FEA model, can give higher accuracy while saving considerable computation time. Thus, the proposed 3D analytical model can be used to analyze and optimize the AFPMMM. Besides, the proposed 3D analytical model can also be used to calculate the performance characteristics of other AFPMs.
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Received: 21 December 2021
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2023, Vol. 38 
