Comparative Analysis of Axial-Radial, Axial, and Radial Flux Permanent Magnet Machines

Abstract

I. IntroductionConventional radial flux permanent magnet machines (RFPMMs) and axial flux permanent magnet machines (AFPMMs) are always the research hotpot high performance drive systems. RFPMMs have simple structure and magnetic circuit, which makes them easy to design and analyze [1]. As for the AFPMMs, they are paid attention for their compactness [2]. The merit makes them more suitable for direct-drive system [3][4]. However, for some special applications such as industrial robotic arms, machines with higher torque density are required [5]. In this case, the axial-radial flux permanent magnet machines (ARFPMMs) may meet the requirements. Compared with the AFPMM and RFPMM, the proposed ARFPMM has the merits of compactness, high utilization of windings and high torque density.In this paper, The structures of ARFPMM, AFPMM and RFPMM are introduced. A detailed performance comparison between the three topologies is also conducted. 3D models of different topologies are built and 3D finite element method (FEM) is used to analyze them. The performances such as back electromotive force (EMF), torque density, and torque ripple are compared to specify the best topology for particular applications.II. Three TopologiesIn order to exclude factors which may affect the comparison results, some parameters of the machines are kept consistent. The inner/outer diameter and axial length of three machines are the same. As shown in Fig. 1, the pole/slot combinations of three topologies are all 20 poles 24 slots. In addition, toroidal windings are also adopted in the RFPMM to keep consistent with the AFPMM and ARFPMM. They are all placed in the opening slots. Permanent magnets which are mounted on the surface of rotors are arranged as Halbach array. It can suppress the harmonics of air-gap flux density [6]. Each pole pair contains six segments, so the angle of magnetization direction between permanent magnets is 60°.Fig. 1 shows that the numbers of rotors of three machines are different. The RFPMM and the AFPMM has a radial rotor and two axial rotors, respectively, while the ARFPMM has a radial rotor and two axial rotors. It makes the active part of windings of the ARFPMM the longest and that of the RFPMM the shortest. Those differences lead to different performances.III. Operation Principles of Three TopologiesDue to the different configurations, the equivalent magnetic circuits of three machines are different. The magnetic flux of RFPMM goes from rotor to stator in radial direction. There are circumferential and radial components of magnetic flux in the stator core. The electromagnetic torque is generated by the reaction between the radial magnetic field and the outside components of windings.As for the AFPMM, the magnetic flux goes through air gap in axial direction. There are circumferential and axial components in the stator core. The electromagnetic torque is due to the reaction between the axial magnetic field and the radial components of windings.The magnetic flux of ARFPMM is the combination of that of RFPMM and AFPMM. It flows in the stator core in circumferential, radial, and axial direction. Similarly, the electromagnetic torque is also the combination of that of RFPMM and AFPMM.IV. Performance ComparisonThe three machines are analyzed through FEM. Considering the ending effect, a 3D model is also built for the RFPMM. Fig. 2(a) shows the back EMFs of three machines obtained through 3D finite element analysis. The amplitude of ARFPMM is the largest, while that of RFPMM is the smallest. The waveforms show that the distortion of back EMF in the ARFPMM is the most serious. However, as shown in Fig. 2(b), the largest harmonic of back EMF in the ARFPMM is the third harmonic, which can be neglected due to the star connection of three phase windings.V. ConclusionThis paper has described the structures and operation principles of ARFPMM, AFPMM, and RFPMM. The results of 3D FEA show that the proposed ARFPMM has the highest torque density. Because the slots and PMs of RFPMM are all rectangular, while the PMs of AFPMM are fan-shaped and slots are rectangular, the percentage of torque ripple to average torque of the RFPMM is the largest. The ARFPMM is equipped with the characteristics of both AFPMM and RFPMM. Thus, the percentage of torque ripple of ARFPMM is in the middle.AcknowledgeThis work is supported in part by NSFC (52077186&51677159), in part by STIC of Shenzhen Municipality (JCYJ20180307123918658), and in part by ITF (ITP/027/19AP), HK. **