France



— Reluctance machines with DC field coil in stator is a competitive candidate for electric vehicle propulsion due to the elimination of rare-earth permanent magnets (PM), robust structures and controllable excitation. However, their torque density cannot be compared with permanent magnet (PM) machines. Introducing rare-earth PMs into reluctance machines can be an effective way to boost torque density, but the costs will greatly increase as well. To solve this problem, this paper proposes a hybrid reluctance motor using ferrite magnets for torque density improvement. The key is to evoke flux modulation effect by introducing ferrite magnets in stator slots. Based on finite element analysis, with ferrite assisted design, machine torque density can be improved by 25% while the efficiency boosts up by 5%, respectively.&lt;br/&gt;&lt;br/&gt;&lt;i&gt;Index Terms&lt;/i&gt;— hybrid reluctance machine (HRM), ferrite magnets, flux modulation, torque density&lt;br/&gt;&lt;br/&gt;I.Introduction&lt;br/&gt;&lt;br/&gt;Permanent magnet (PM) machines are widely researched for electrical vehicle (EV) applications because of their characteristics of high performance in power factor and torque density. However, PM, as a rare-earth resource, greatly increases the costs of the machines. Therefore, the research of non-PM reluctance machines in EV applications have attracted extensive attention and became an important research hotspot[1].&lt;br/&gt;&lt;br/&gt;As a typical non-PM machine, switched reluctance machine (SRM) has been studied widely due to its simple structure, high reliability and low cost. However, since each phase of the SRM is only energized in first quadrant of BH curve, the torque ripple is serious and the torque density is poor[2]. To improve torque density, rare-earth PM auxiliary SRM in analyzed by introducing PMs into stator slots[3], thus the extended BH working range can be acquired.&lt;br/&gt;&lt;br/&gt;Doubly-fed doubly salient motor has a similar structure with SRM, while it has both DC and AC coils in the stator. Alternating excitation flux can be established by DC field coils, which enables it to work in a whole electrical period[4]. But the torque density is lower than that of SRM since the space of the stator slot is shared by DC and AC coils. Variable flux reluctance motors have symmetrical voltage and inductance characteristics and the torque ripples and noises can be decreased[5]. But the torque density is still lower than that of SRMs.&lt;br/&gt;&lt;br/&gt;An innovative method of embedding PMs in the slots of a hybrid reluctance machine (HRM) has been confirmed as an important strategy to increase the torque density through flux modulation effect[6]. The field windings and the PMs produce the flux linkages with the same phase angle.&lt;br/&gt;&lt;br/&gt;As rare-earth resources, PMs are still desired to be replaced by more accessible materials completely. The application of ferrite magnets in electrical machines is also another effective way to replace rare-earth PM materials[6]. Although its coercivity and remanence are lower than thoese of widely applied NdFeB35. As an artificial material, ferrite magnets have quite lower prices than NdFeB35 and are expected to be widely used in HRMs. Based on the comprehensive consideration of production costs and torque density performance, a HRM using ferrite magnets for electric vehicle propulsion was researched under finite element analysis. Optimal design and prototype experiment are going to be conducted.&lt;br/&gt;&lt;br/&gt;II.Configuration and principle&lt;br/&gt;&lt;br/&gt;The structure of the proposed HRM is shown in Fig .1. It contains DC field coils, AC armature coils, ferrite magnets, 12 stator teeth and 11 rotor teeth. Ferrite magnets are magnetized in the radial outward direction. In order to further explain the flux modulation principle of the field winding flux linkage and the ferrite magnets flux linkage, the field excitation winding and the ferrite magnets magnetic circuit are shown. it is clear to see that the magnetic circuit of the DC field winding is mainly along the wounded stator teeth, meshing rotor teeth and adjacent rotor teeth. The magnetic circuit of ferrite magnets is along the stator slot and the rotor slot, which has a longer magnetic circuit than that of DC. However, at the stator teeth, the magnetic circuit generated by the two excitation methods has the same trend in amplitude changes. This effect can enhance the magnetic circuit. In addition, the torque performance is also improved by flux harmonics.&lt;br/&gt;&lt;br/&gt;III.Performance analysis&lt;br/&gt;&lt;br/&gt;The finite element analysis results flux distributions of ferrite magnets only and DC (6A) excitation only are shown in Fig .2(a) and Fig .2(b), respectively. Consistent with the above analysis, the DC excitation magnetic circuit mainly follows the wounded stator teeth, adjacent stator teeth and meshing rotor teeth. Due to the bipolarity of the ferrite magnets magnetic circuit and the phase consistency with the DC excitation magnetic circuit, the variation range of the composite magnetic circuit is enhanced. This appearance is more obvious in the synthetic magnetic circuit which is shown in Fig .2(c). It can be seen from the torque curves that ferrite magnets HRM can effectively increase the torque density compared with dc excitation only.&lt;br/&gt;&lt;br/&gt;&lt;br/&gt;Conclusion&lt;br/&gt;&lt;br/&gt;This paper proposes a ferrite-assisted hybrid reluctance machine. With the effect of flux modulation, the torque density and the efficiency can be improved by 25% and 5%, respectively.

**References:**

&lt;br/&gt;&lt;br/&gt;[1] I. Boldea, L. N. Tutelea and L. Parsa, IEEE Trans. Ind. Electron&lt;i&gt;.&lt;/i&gt;, Vol. 61, p. 5696-5711 (2014)&lt;br/&gt;[2] J. Ye, B. Bilgin and A. Emadi, IEEE Trans. Power Electron., Vol. 30, p. 1457-1470 (2015)&lt;br/&gt;[3] S. Ullah, S. P. McDonald and R. Martin, IEEE Trans. Ind. Appl., Vol. 55, p. 298-305 (2019)&lt;br/&gt;[4] Z. Bian, Z. Zhang and L. Yu, IEEE Trans. Power Electron., Vol. 35, p. 6070-6081 (2020)&lt;br/&gt;[5] X. Liu and Z. Q. Zhu, IEEE Trans. Ind. Appl., Vol. 50, p. 3675-3684 (2014)&lt;br/&gt;[6] X. Zhao, S. Niu and W. Fu, IEEE Trans. Ind. Electron&lt;i&gt;.&lt;/i&gt;, Vol. 66, p. 4209-4219 (2019)&lt;br/&gt;[7] A. Fasolo, L. Alberti and N. Bianchi, IEEE Trans. Ind. Appl., Vol. 50, p. 3708-3716 (2014)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/20fd31e438b781dcdc88793d1610a11e.jpg)
Fig. 1.  Structure of the proposed HRM.

![](https://assets.underline.io/uploads/markdown_image/1/image/60a1c008720342baa415fa31befac323.png)
Fig. 2. (a) Flux distribution of ferrite magnets only. (b) Flux distribution of DC only. (c) Flux distribution of DC and ferrite magnets. (d) Steady torque curves.

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 Design and Analysis of a Ferrite-Assisted Hybrid Reluctance Machine for Electric Vehicle Propulsion

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— Reluctance machines with DC field coil in stator is a competitive candidate for electric vehicle propulsion due to the elimination of rare-earth permanent magnets (PM), robust structures and controllable excitation. However, their torque density cannot be compared with permanent magnet (PM) machines. Introducing rare-earth PMs into reluctance machines can be an effective way to boost torque density, but the costs will greatly increase as well. To solve this problem, this paper proposes a hybrid reluctance motor using ferrite magnets for torque density improvement. The key is to evoke flux modulation effect by introducing ferrite magnets in stator slots. Based on finite element analysis, with ferrite assisted design, machine torque density can be improved by 25% while the efficiency boosts up by 5%, respectively. Index Terms— hybrid reluctance machine (HRM), ferrite magnets, flux modulation, torque density I.Introduction Permanent magnet (PM) machines are widely researched for electrical vehicle (EV) applications because of their characteristics of high performance in power factor and torque density. However, PM, as a rare-earth resource, greatly increases the costs of the machines. Therefore, the research of non-PM reluctance machines in EV applications have attracted extensive attention and became an important research hotspot[1]. As a typical non-PM machine, switched reluctance machine (SRM) has been studied widely due to its simple structure, high reliability and low cost. However, since each phase of the SRM is only energized in first quadrant of BH curve, the torque ripple is serious and the torque density is poor[2]. To improve torque density, rare-earth PM auxiliary SRM in analyzed by introducing PMs into stator slots[3], thus the extended BH working range can be acquired. Doubly-fed doubly salient motor has a similar structure with SRM, while it has both DC and AC coils in the stator. Alternating excitation flux can be established by DC field coils, which enables it to work in a whole electrical period[4]. But the torque density is lower than that of SRM since the space of the stator slot is shared by DC and AC coils. Variable flux reluctance motors have symmetrical voltage and inductance characteristics and the torque ripples and noises can be decreased[5]. But the torque density is still lower than that of SRMs. An innovative method of embedding PMs in the slots of a hybrid reluctance machine (HRM) has been confirmed as an important strategy to increase the torque density through flux modulation effect[6]. The field windings and the PMs produce the flux linkages with the same phase angle. As rare-earth resources, PMs are still desired to be replaced by more accessible materials completely. The application of ferrite magnets in electrical machines is also another effective way to replace rare-earth PM materials[6]. Although its coercivity and remanence are lower than thoese of widely applied NdFeB35. As an artificial material, ferrite magnets have quite lower prices than NdFeB35 and are expected to be widely used in HRMs. Based on the comprehensive consideration of production costs and torque density performance, a HRM using ferrite magnets for electric vehicle propulsion was researched under finite element analysis. Optimal design and prototype experiment are going to be conducted. II.Configuration and principle The structure of the proposed HRM is shown in Fig .1. It contains DC field coils, AC armature coils, ferrite magnets, 12 stator teeth and 11 rotor teeth. Ferrite magnets are magnetized in the radial outward direction. In order to further explain the flux modulation principle of the field winding flux linkage and the ferrite magnets flux linkage, the field excitation winding and the ferrite magnets magnetic circuit are shown. it is clear to see that the magnetic circuit of the DC field winding is mainly along the wounded stator teeth, meshing rotor teeth and adjacent rotor teeth. The magnetic circuit of ferrite magnets is along the stator slot and the rotor slot, which has a longer magnetic circuit than that of DC. However, at the stator teeth, the magnetic circuit generated by the two excitation methods has the same trend in amplitude changes. This effect can enhance the magnetic circuit. In addition, the torque performance is also improved by flux harmonics. III.Performance analysis The finite element analysis results flux distributions of ferrite magnets only and DC (6A) excitation only are shown in Fig .2(a) and Fig .2(b), respectively. Consistent with the above analysis, the DC excitation magnetic circuit mainly follows the wounded stator teeth, adjacent stator teeth and meshing rotor teeth. Due to the bipolarity of the ferrite magnets magnetic circuit and the phase consistency with the DC excitation magnetic circuit, the variation range of the composite magnetic circuit is enhanced. This appearance is more obvious in the synthetic magnetic circuit which is shown in Fig .2(c). It can be seen from the torque curves that ferrite magnets HRM can effectively increase the torque density compared with dc excitation only. Conclusion This paper proposes a ferrite-assisted hybrid reluctance machine. With the effect of flux modulation, the torque density and the efficiency can be improved by 25% and 5%, respectively.

**References:**

 [1] I. Boldea, L. N. Tutelea and L. Parsa, IEEE Trans. Ind. Electron., Vol. 61, p. 5696-5711 (2014) [2] J. Ye, B. Bilgin and A. Emadi, IEEE Trans. Power Electron., Vol. 30, p. 1457-1470 (2015) [3] S. Ullah, S. P. McDonald and R. Martin, IEEE Trans. Ind. Appl., Vol. 55, p. 298-305 (2019) [4] Z. Bian, Z. Zhang and L. Yu, IEEE Trans. Power Electron., Vol. 35, p. 6070-6081 (2020) [5] X. Liu and Z. Q. Zhu, IEEE Trans. Ind. Appl., Vol. 50, p. 3675-3684 (2014) [6] X. Zhao, S. Niu and W. Fu, IEEE Trans. Ind. Electron., Vol. 66, p. 4209-4219 (2019) [7] A. Fasolo, L. Alberti and N. Bianchi, IEEE Trans. Ind. Appl., Vol. 50, p. 3708-3716 (2014)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/20fd31e438b781dcdc88793d1610a11e.jpg)
Fig. 1. Structure of the proposed HRM.

![](https://assets.underline.io/uploads/markdown_image/1/image/60a1c008720342baa415fa31befac323.png)
Fig. 2. (a) Flux distribution of ferrite magnets only. (b) Flux distribution of DC only. (c) Flux distribution of DC and ferrite magnets. (d) Steady torque curves.

poster

I. Introduction The multi-tooth flux-switching permanent magnet (PM) (FSPM) machines have attracted wide attention due to the advantages of fewer permanent magnet materials and higher torque density, compared with the conventional FSPM machines[1]. Recently, a 6-stator-coil/17-rotor-pole (6/17) multi-tooth FSPM machine with concentrated windings, namely, the C-multi-tooth FSPM machine, was proposed in paper [2], as shown in Fig 1(a). In this paper, in order to evaluate the electromagnetic performance of the multi-tooth FSPM machines with different winding configurations, the full-pitch windings are adopted as is shown in Figure 1 (b), called the FP-multi-tooth machine. II. Performance comparisons In this paper, the electromagnetic performance of the 6/17 multi-tooth machine with different winding configurations is evaluated by finite element analysis (FEA), which is mainly in terms of no-load back EMF, harmonic components, air-gap flux density, cogging torque, electromagnetic torque, etc. Figure 2(a) shows the no-load phase back-EMF per turn waveforms of the two multi-tooth machines at 1500N/min. It can be seen that the winding configuration has an impact on the amplitude of the back-EMF for the multi-tooth machine. The amplitude of the FP-multi-tooth machine is much larger than the C-multi-tooth one with concentrated windings. Figure 2(b) presents the FFT results of the no-load phase back-EMF waveform of the 6/17 multi-tooth machine shown in Figure 2(a), where it is adopted different winding configurations. The fundamental wave amplitudes are respectively 2.45 V (C-multi-tooth machine) and 4.47V(FP-multi-tooth machine), moreover, the total harmonic distortion (THD) is 2.21%, 1.38%, and that the harmonic components are the same, mainly odd-numbered harmonics. Hence, in terms of the 6/17 multi-tooth FSPM machine, the motor with the full-pitch winding exhibits larger amplitude of the fundamental component than the concentrated winding one, and the THD is slightly smaller. Figure 2 (c) shows the two multi-tooth machines air-gap flux density distribution in the air-gap region of coil A1and A2. Furthermore, for the C-multi-tooth and the FP-multi-tooth machine, the average values of Brδ are -0.391T and -0.603T respectively, which means the no-load PM fluxes flowing through the coil A1 and A2 of the FP-multi-tooth machine are larger. Correspondingly, its back EMF should be larger than the C-multi-tooth one, since part of the magnetic flux of the C-multi-tooth machine will flow through other phases to form a closed-loop magnetic circuit. Figure 2(d) presents the cogging torque waveforms of the two multi-tooth machines. The result indicates that the cogging torque waveforms phase of two multi-tooth machines is not the same, since their rotor position is different when the peak value of the maximum back EMF is obtained. To compare the torque performance of the 6/17 multi-tooth machines with different winding configurations, the study is conducted by 2-D-FEA with Flux software under id=0 control, where the slot package factor is kpf =0.45, as shown in Fig. 2(e) and (f). When the current density is changed from 1A/mm2 to 10A/mm2, the FP-multi-tooth machine with full-pitch windings exhibits greater average torque and smaller torque ripples than the C-multi-tooth one. Figure 2(g) shows the electromagnetic torque waveform at the rated current density Jsa=5A/mm2, where the load is 1.0. It can be seen that the average torque of the FP-multi-tooth machine with full-pitch windings (16.8Nm, 4.96%) are greater than that of the C-multi-tooth one (12.7Nm, 11.1%), and that the torque ripple of the FP-multi-tooth machine is smaller than the C-multi-tooth one. III. Conclusion In this paper, the study on the electromagnetic performance of the multi-tooth machine with different winding configuration is conducted by FEA. The results indicate that for the 6/17 multi-tooth FSPM machine, the FP-multi-tooth machine with full-pitch windings exhibits higher no-load phase back EMF, torque density, and smaller harmonic distortion and torque ripple, hence, the FP-multi-tooth machine with full-pitch windings is superior to the multi-tooth one with concentrated windings. A comparative study of other types of multi-tooth machines, such as the 6/10 multi-tooth FSPM machine, will be shown in the full paper.

**References:**

[1]Y. Du, C. Zou,andX. Zhu, "A Full-Pitched Flux-Switching Permanent-Magnet Motor," in IEEE Transactions on Applied Superconductivity, vol. 26, no. 4, pp. 1-5, June 2016, Art no. 0604505, doi: 10.1109/TASC.2016.2541308. [2]G. Zhao and W. Hua, "Comparative Study Between a Novel Multi-Tooth and a V-Shaped Flux-Switching Permanent Magnet Machines," in IEEE Transactions on Magnetics, vol. 55, no. 7, pp. 1-8, July 2019, Art no. 8104908, doi: 10.1109/TMAG.2019.2900749.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/49124426790a017123757c7bd3f86a52.jpg)
Fig. 1 Topologies of 6/17 multi-tooth FSPM machines. (a) C-multi-tooth machine. (b) FP-multi-tooth machine.

![](https://assets.underline.io/uploads/markdown_image/1/image/1bb200063963f8258738af158c0e488b.jpg)
Fig. 2 Performance comparisons. (a) Phase back-EMF per turn waveforms at 1500r/min. (b) Harmonics of back-EMF. (c)Radial air-gap flux density Brδ in the air-gap region of coil A1and A2(d) cogging torque. (e) Electromagnetic torque waveforms versus current densities. (f) Torque ripple waveforms versus current densities. (g) Electromagnetic torque waveforms under Jsa=5A/mm2.

 Comparative Study of Winding Configuration on a Multi-tooth Flux Switching Permanent Magnet Machine

This paper proposed a new slot-PM-assisted reluctance generator with self-excited DC source for stand-alone wind power generation. The key is that several ac armature coils are used to provide electrical power to dc field winding through a rectifier. Compared with separately excited dc source, self-excited dc source is more suitable for stand-alone wind power generation and greatly reduces device costs since no independent dc source is needed. In this paper, the machine configuration and operation principle are introduced. Some leading dimension parameters are optimized to obtain maximum torque and minimum torque ripple. the feasibility of the optimal design with this novel structure is verified by finite element analysis and prototype testing. 1. Introduction: When it comes to provide energy to remote area, stand-alone wind power system with battery appeals to an excellent solution with relatively maintenance simplicity and low system cost [1-2]. The most important part of wind power generation system is the generator. Permanent magnet (PM) machine has been widely used as wind power generator due to simple structure, high efficiency, as well as high power density [3-4]. However, PM machines have no flux regulation ability. In this case hybrid excited machine, which excited by both PMs and dc field source, has been applied. The problem is that they all need independent dc excitation sources, which are not suitable for stand-alone wind power system. To solve abovementioned problem, several ac armature coils are used to supply electrical power to dc field winding through a rectifier, which are called ac rectifier winding. In this way, a novel slot-PM-assisted reluctance generator (SPMA-RG) with self-excited DC source (SEDC) is proposed. 2. Machine configuration and operation principle As shown in Fig. 1, a novel slot-PM-assisted reluctance generator (SPMA-RG) with self-excited DC source (SEDC) has three sets of stator windings, namely dc field winding, ac armature winding as well as ac rectifier winding. Radial magnetized PMs are placed in the slot opening of stator to enhance the torque density compared with traditional variable flux reluctance machine. Reluctance rotor consists of only iron core which provides mechanical robustness. The current produced by ac rectifier winding passes through controlled rectifier to provide excitation current to dc field winding. The advantages of the proposed generator are listed as following. (a) Since all excitation sources are placed in the stator side and the rotor comprises only iron, there are no brush and slip rings, which provides mechanical robustness. (b) Radial magnetized PMs are mounted in the slot openings which enhances the torque density compared with traditional variable flux reluctance machines. (c) Flux produced by dc field winding does not pass through the slot PMs, which will not lead to demagnetization of slot PMs. (d) Output voltage of ac armature winding can be controlled through regulating the dc field excitation, thus variable speed constant voltage can be realized in wind power generation system. (e) Several ac armature coils, namely ac rectifier winding, are used to provide excitation current to dc field winding through a controllable rectifier. In this way, no extra dc source is needed, which not only improves independent characteristic of wind power generation system, but also realizes energy feedback utilization. 3. Electromagnetic performance Fig. 2 shows rectified voltage and current generated from ac rectifier winding and rectified by the controllable rectifier. Furthermore, output voltage of the ac armature winding appears to be stable and achieves three-phase symmetric when rectified voltage and current arrive a stable value. It verified the feasibility of proposed wind power generation system. The ac rectifier winding can be used to provide dc excitation current to dc field winding through the controllable rectifier. 4. Conclusion In this paper, a novel slot-PM-assisted reluctance generator with self-excited DC source is proposed for wind power generation. Several ac armature coils, namely ac rectifier winding, provide electricity to dc field winding through a rectifier. This self-excited dc source greatly reduces device cost and improves the independent characteristic of the wind power system. In the full paper, the electromagnetic performance of proposed SPMA-RG is analyzed in detail. The prototype of proposed generator is manufactured and relative testing is conducted. This work was supported in part by grants from the Hong Kong Research Grants Council, Hong Kong, China, under Project PolyU 152058/17E and PolyU 152143/18E.

**References:**

[1] Ming-Shun Lu, Chung-Liang Chang, Wei-Jen Lee and Li Wang, “Combining the Wind Power Generation System with Energy Storage Equipment”, IEEE Trans. Ind. Appl., Vol.45, No.6, pp.2109-2115. November / Dec. 2009. [2] Ronan Doherty, Ronan Doherty and Mark O’Malley, “Establishing the Role That Wind Generation May Have in Future Generation Portfolios”, IEEE Trans. Power Sys., Vol.21, No.3, pp.1415-1422. Aug. 2006. [3] Y. Fan, K. T. Chau, and M. Cheng, “A new three-phase doubly salient permanent magnet machine for wind power generation,” IEEE Trans. Ind. Appl., vol. 42, no. 1, pp. 53–60, 2006. [4] Y. Chen, P. Pillay, and A. Khan, “PM wind generator topologies,” IEEE Trans. Ind. Appl., vol. 41, no. 6, pp. 1619–1626, 2005.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/a6eb1bf5fe4031c1e8acfed478176d03.jpg)
Fig. 1. Wind power generation system based on the proposed SPMA-RG.

![](https://assets.underline.io/uploads/markdown_image/1/image/b824cd8942dc7188e35ae3accd9e65e0.jpg)
Fig. 2. Rectified voltage/current and Back-EMF.

 Development of Slot-PM-assisted Reluctance Generator with Self-excited DC Source for Stand-alone Wind Power Generation

This paper proposes a novel hybrid-excited outer-rotor machine with skewed permanent magnets (PM) placed at the stator slot opening. The key novelty is that the skew-magnetized PMs can achieve both torque production promotion and stator core saturation relieving. Compared with DC-excited reluctance machine, the proposed machine with skewed PMs has higher torque production and efficiency. Compared with synthetic slot PM excited machine, it is easier for machine assembly and manufacture, and the manufacture cost is reduced. In this paper, the machine structure of the prosed machine is presented. Then, the working principle are introduced briefly, including the relieving-DC-saturation effect and the flux modulation effect. The derivation will be presented in more detail in the full-page paper. Finally, the machine performance under different DC field current is presented. I. Introduction The DC-excited reluctance machine is a promising solution for electric vehicle because of its inherent reliable and simple rotor structure, good thermal dissipation ability and wide speed range [1]. However, the torque production and efficiency are not satisfying, because the excitation ability of DC winding is usually lower than PM [2]. Besides, the saturation in stator core limits the DC current due to the biased field [3]. To address the two problems, this paper proposes a novel skewed PM structure. The radical component of the PMs can provide an effective flux to the armature winding, so that the torque production can be promoted directly. The tangential component of the PMs can relieve the saturation in the stator core, so that higher DC field current can be applied, and hence the torque production can be raised accordingly. II. Machine structure The structure of the proposed machine is presented in Fig. 1(a). The rotor consists of iron core only, so, it is reliable and easy for manufacture. In the stator side, there are two groups of windings, one is the DC field winding and another one is the 3-phase armature winding. The skewed PMs are placed at the slot-openings. The radical component of magnetization of every PM is outwards direction, but the tangential component of magnetization has alternate polarity in every tooth. The purpose of radical component is to provide an additional field linked to armature windings, so that the torque production can be raised directly. The purpose of tangential component is to provide a constant flux in the stator core, of which the direction is opposite with the flux from DC current, so that higher DC current can be applied. The armature winding configuration is presented in Fig. 1(b). The concentrated single-layer winding configuration is used. II. Working principle In this part, to introduce the working principle of skew-magnetized PM clearly, the tangential magnetization component and the radical magnetization component are introduced respectively. For the radical magnetization component of PMs, the following relationship should be satisfied. NPM-Nr+Nw=0 NPM, Nr and Nw represent the pole pair number of PM, rotor teeth number and the order of working harmonic. In this case, the values of NPM, Nr and Nw are 12, 22 and 10, respectively. The pole pair number of armature winding is the same with the order of working harmonic. Hence, 10-pole-pair armature winding is used. The flux path of the tangential magnetization component is presented in Fig. 2(a) together with the flux from DC current. The tangential component contributes little torque production directly if the current is kept unchanged, because the flux from the tangential component of PM is a constant biased flux. But the stator saturation is largely relieved because the directions of flux of DC current and the tangential component of PM are opposite. Hence, higher DC current can be applied when the PMs are used, and hence the torque production can be raised accordingly. III. Machine performance To show the effectiveness of the skew-magnetized PMs, an FEA model is built. The maximum diameter is 150 mm, stack length is 80 mm, and air gap length is 0.5 mm. DC windings have the same number of turns and rated current as armature windings, the values are 52 turns and 6 A respectively. The torque production under different field excitation is presented in Fig. 2(b). Either the DC field current or skew-magnetized PMs can be used for excitation individually. The torque production from skewed PMs is higher, but the torque production from DC field current is easier to regulate. IV. Conclusion In this paper, a novel hybrid-excited machine with skew-magnetized PM is proposed. The radical magnetization component of PM is used to promote the torque directly, and the tangential magnetization component of PM is used to relieve the saturation. The flexible DC excitation current provide a wide speed range, and the additional skewed PM raises the torque production. An FEA model is built and the torque production is presented to verify the effectiveness of the proposed machine. The other features of the proposed machine will be presented and analyzed in the full-page paper. Acknowledgement This work was supported in part by grants from the Hong Kong Research Grants Council, Hong Kong, China, under Project PolyU 152058/17E and PolyU 152143/18E.

**References:**

[1] C. H. T. Lee, K. T. Chau and C. Liu, IEEE Trans. Magn., Vol. 49, p.3969-3972 (2013) [2] Q. Wang and S. Niu, Renew. Sust. Energ. Rev., Vol. 68, p.475-491 (2017) [3] X. Zhao, S. Niu and W. Fu, IEEE Trans. Magn., Vol. 55, p.1-5 (2019)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ba1c165a7e100b914ff4f9e684f65ffa.jpg)
Fig. 1. The proposed machine. (a) Machine structure. (b) Armature winding configuration.

![](https://assets.underline.io/uploads/markdown_image/1/image/ccc85cbb6967248ee6a295b8876e1d75.jpg)
Fig. 2. Main flux path and torque production. (a) Flux path of DC current and tangential component of PM. (b) Torque production under rated armature winding.

 A Novel Relieving-DC-Saturation Hybrid-Excited Machine with Skewed Permanent Magnets for Electric Vehicle Application

This paper investigates the influence of DC/AC winding split ratio on electromagnetic performance of hybrid reluctance machine with synthetic slot PM excitation. The machine structure and working principle are introduced briefly at first. Then, the machine performance, including power factor, torque production, loss, efficiency, and voltage regulation rate, are investigated with different DC/AC winding ratio. It is found that the machine has higher torque production and efficiency when the AC windings take up more space. But higher voltage regulation rate can be achieved when the space used for DC windings is close to AC winding. More detailed investigation will be presented in the further version. I. Introduction Recently, the DC-excited reluctance machine has attracted lots of research interests and it is a very promising solution for electric vehicles because of its reliable rotor structure and wide speed range [1]. However, the torque production and efficiency are not satisfying. To address this issue, a novel hybrid reluctance machine with synthetic slot PM is proposed in [2]. But the influence of DC/AC winding split ratio, which is one of key design parameters on machine performance, has yet to be comprehensively studied. To give a design guideline for this hybrid reluctance machine, this paper will fully investigate the power factor, torque production, loss, efficiency and voltage regulation rate with different DC/AC winding ratio. II. Machine structure and working principle The machine structure is presented in Fig. 1(a). The rotor consists of iron core only, so it is reliable and easy to manufacture. There are two groups of winding, one is the 3-phase armature winding and another one is the DC field winding. There are two groups of PMs located at slot openings. One group is radically magnetized PMs with the same magnetization direction. Another group is tangentially magnetized PMs with alternate polarity in every tooth. In the proposed machine, the flux excited by DC current varies as rotor permeance to provide an effective flux. However, the biased flux can cause saturation in stator core. To address this issue, the tangentially magnetized PMs are used to relieve saturation. They produce a constant biased flux with opposite direction with DC current field in the stator core, so that the flux density can be reduced. As for the radically magnetized PMs, they are used to provide an effective flux linked to armature winding to enhance the torque production directly. III. Machine performance In this part, the machine performance under different DC/AC winding split ratio is presented based on FEA result. The maximum diameter is 150 mm, the stack length is 80 mm, the air gap length is 0.5 mm, the slot filling factor is 0.7 and the rated current density is 6A/mm2. The DC/AC winding split ratio is defined as follow. kdc=Ndc/(Nac+Ndc) kac=Nac/(Nac+Ndc) kdc and kac are the winding split ratios of DC and AC winding, respectively. And Ndc and Nac are the number of conductors of DC field winding and armature winding in one slot, respectively. The rated torque, loss, efficiency and voltage regulation rate are presented in Fig. 2. The higher rated torque and can be achieved when less space is used for DC field winding, because the excitation ability of DC field current is poorer than PMs. The higher torque production brings with higher efficiency when armature winding takes up more space in slot, in spite of higher core loss. High power factor can be obtained when DC field winding takes up more space because it brings stronger excitation ability. As for voltage regulation rate (kVRR), it is defined as follows. kVRR=(V2r-V0)/Vr V0, Vr and V2r are the open-circuit back EMF when the DC field current is at zero, rated value and twice of rated value. It reflects the voltage regulation ability of the DC field winding and it is directly related to the speed range. When DC field winding split ratio is very small, the excitation ability of DC current is also very small. When DC field winding ratio is very large, the saturation effect becomes very severe. Hence, the voltage regulation rate is not high when DC field winding ratio is either too high or too low. From Fig. 2(d), it can be found that the highest voltage regulation rate can be achieved when the AC winding ratio is at around 40%. IV. Conclusion This paper presents the machine performance of the hybrid-excited reluctance machine with synthetic slot PM excitation under various DC/AC winding split ratio. It is concluded that when the more slot space is used for armature winding, the higher rated torque, core loss and efficiency can be achieved, but the power factor is lower. The highest voltage regulation rate can be obtained when the armature winding ratio is at around 40%. Acknowledgement This work was supported in part by grants from the Hong Kong Research Grants Council, Hong Kong, China, under Project PolyU 152058/17E and PolyU 152143/18E.

**References:**

[1] E. Trancho, E. Ibarra and A. Arias, IEEE Trans. Ind. Electron. Vol. 65, p.2986-2995 (2018) [2] X. Zhao, S. Niu and X. Zhang, IEEE Trans. Ind. Electron. doi: 10.1109/TIE.2020.2996140 (2020)

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Fig. 1. Machine structure.

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Fig. 2. Machine performance under different armature winding ratio. (a) Rated torque. (b) Power factor. (c) Core loss and efficiency. (d) Voltage regulation rate.

 Influence of DC/AC Winding Split Ratio on Electromagnetic Performance of Hybrid Reluctance Machine with Synthetic Slot PM Excitation

I. INTRODUCTION Axial flux permanent magnet (AFPM) machines with tooth-wound concentrated windings (TCWs) and segmented stator were founded a growing interest for high-performance drive applications such as electric vehicles, aircrafts, ships, etc. [1] Four dual-rotor single-stator (DRSS) axial flux permanent magnet vernier (AFPMV) machines with segmented stator and consequent-pole (CP) embedded rotors are proposed and compared in this paper. The configuration and operating principle of AFPMV machines are introduced first. Then, the critical design parameters and electromagnetic performances of proposed AFPMV machines are analyzed and compared both in no-load and load conditions. Finite element analysis (FEA) results demonstrate that the machine with Halbach arrays employed on the stator split teeth features higher torque capability and lower rotor eddy current loss compared to its counterparts with conventional PMs or stator magnet less machines. In addition, for better flux-weakening capability, the AFPMV machine with hybrid field excitation from both rotor PM and stator DC coils is also discussed. II. MACHINE CONFIGURATIONS AND OPERATION PRICIPLE The proposed AFPMV machine A is shown in Fig.1(a) as the benchmark. The machine has a single stator sandwiched between two-disc rotors which embedded CPs. The stator consists of 12 segments, each with three split teeth on the major tooth. TCWs are employed to shorten the end length and suppress copper loss. With the absent of the stator yoke, such machines take the advantageous performance of lower stator core loss. Furthermore, as shown in the Fig.1(b) and Fig.1(c), the radially magnetized PMs and the Halbach arrays are equipped in the split slots to form machine B and C, respectively. Finally, each stator segment of machine D is wounded with a DC field winding on the middle split tooth, which is illustrated in the Fig.1(d). The flux modulation effect has enhanced the torque producing capability according to the pole ratio of rotor and AC windings [2]. In the vernier machine with modulation poles fixed on the stator, the angular speed of the working flux harmonic component is ωp = -NPMωPM /(Nm-NPM) where NPM and Nm are the pole-pair number of the magnetic fields excited with PM and the number of modulation teeth, respectively, ωp and ωPM are the speed of the working rotation magnetic field produced by the stator windings and the speed of the PMs, respectively. III. PERFORMANCE ANALYSES AND COMPARISONS A. No-Load Flux Linkage and No-Load Back-EMFs The flux distributions of four investigated machines were conducted by the FEA, the simulation results show that the machine C tend has lower flux leakage in the stator split teeth and higher magnetic density compared with the machine B because of the PM with circumferentially magnetization. Lower magnetic density is distributed in machine D when the DC winding produces the reverse flux, that is, the flux adjustment can be realized through the DC winding in the split teeth. The waveforms of no-load back-EMFs of the models are illustrated in Fig.2, in which the DC winding of the machine D operates in the flux-weakening condition. It can be seen that the amplitude of the no-load back-EMFs increase with the introduction of the PM in the split teeth. For the machine C, with the increase of circumferential PM volume, the amplitude of the back-EMF first increases and then decreases, that is, it is necessary to balance the flux leakage and the main flux. B. Output Torque and Losses Fig. 3 shows the output torque of the machines, the average output torque of the machines with PMs in the stators generate an average output torque at least 37% higher than those of the machines without PMs in the stators. The results of comprehensive comparison are shown in table I, which verify that the machine with Halbach arrays employed in the split slots tend to have higher output torque and lower stator core loss and rotor PM loss. And the parametric sensitivity studies and design optimizations will be presented in the final paper. III. CONCLUSION This paper extends the theory of magnetic field modulation into AFPM machines with DRSS and TCWs, making it particularly suitable for low-speed, high-torque, and axial length limited applications. The comparative analyses of four topologies of AFPVM machines are conducted by FEA. The results verify that it is a feasible method to modulate the magnetic field in the air gap with the split teeth in the segmented stator modules of the AFPM machine. By comparing the relative merits and disadvantages of the performances of the proposed machines, it is concluded that the machine with Halbach PMs embedded in the stator split slots shows the characteristics of higher torque density and lower loss compared with other topologies, and machine with DC field winding in the stator split slots shows the adequate flux-weakening performance.

**References:**

 [1] F. Giulii Capponi, G. De Donato and F. Caricchi, "Recent Advances in Axial-Flux Permanent-Magnet Machine Technology," IEEE Trans. Ind. Appl., vol. 48, no. 6, pp. 2190-2205, (2012). [2] R. Qu, D. Li and J. Wang, "Relationship between magnetic gears and vernier machines," 2011 International Conference on Electrical Machines and Systems, Beijing, pp. 1-6, (2011).

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 Electromagnetic Performance Analysis and Comparison of Fractional-Slot Non-Overlapping Winding Dual-rotor Axial Flux Permanent Magnet Vernier Machines with Segmented Stators

I. Introduction Flux-switching permanent magnet (FSPM) motor, possessing the potential merits of high torque and power density, has aroused considerable attention and extensive research in recent years [1]. Electromagnetic torque, which is the most important evaluation index of the FSPM motor, is researched in majority of the existed papers. For the FSPM motor, the magnetic density of the motor has a decisive influence on the electromagnetic torque. As the excitation source, PMs serve to generate the magnetic power, which goes through stator, airgap and rotor to form the effective magnetic path. In other words, the electromagnetic torque relies heavily on the magnetic flux density of the stator, airgap and rotor. Actually, the improvement of the magnetic flux density of the stator, airgap and rotor may contribute to the increasing of electromagnetic torque. In [2], from the perspective of the magnetic flux density of the stator, different types of PM arrangement are investigated, which indicates that the arrangement of V-shaped PMs is superior to that of the I-shaped and Arc-shaped PMs in the stator in terms of torque output capability. On the other hand, from the perspective of the magnetic flux density of airgap, various parameters close to the airgap are selected during the multi-layer design of the FSPM motor for improved torque in [3]. Based on the above research, many attempts are made to study the magnetic flux density of stator and airgap related to airgap for the torque improvement. Yet, few efforts are made to analyzing the effective magnetic flux density of the rotor which contributes to the magnetic energy transmission and torque production. Hence, the concept of flux density harmonic deterioration rate is introduced to analyze the rotor effective magnetic flux in this paper, which eventually reflects the loss of magnetic energy transmission from the airgap. By analyzing the influence of harmonic flux density variation from airgap to rotor yoke on electromagnetic torque, the relevant between electromagnetic torque and flux density harmonic deterioration rate is explored, and the resulting torque represents to be increased, which provides a new perspective for motor performance analysis. II. Motor topology and analysis of the performance The topology of the studied I-shaped FSPM motor is shown in Fig. 1 (a), with the related structure parameters marked in Fig. 1 (b). In order to select the harmonic for research use, the torque contribution of different harmonic is obtained as depicted in Fig. 2, and contributions of the 4th-, 18th-, 28th-, 30th- and 40th-order harmonic are chosen as the basis of the comprehensive deterioration rate. The flux density and harmonic amplitude of airgap and rotor yoke are illustrated in Fig. 3 (a) and (b). It is obvious that the 4th- and 18th-order harmonic remain high amplitude at both airgap and rotor yoke, while the 28th-, 30th- and 40th-order harmonic present a sharp decline. The single harmonic deterioration rate is also calculated in Fig. 3 (b), where the positive value denotes an actual downward trend while the negative one indicates the opposite. To further investigate the relationship between torque capability and harmonic deterioration rate, the parameters are selected from both stator and rotor side, and the partial results are shown in Fig. 4 (a) and (b). With the PM cost keeping constant and the PM width bpm and the rotor tooth width brt increasing, the total torque decreases while the comprehensive deterioration rate mainly represents uptrend as expected. Considering the lowest comprehensive deterioration rate, the final value of bpm and brt are chosen to be 2deg and 3.4deg for comparison. Moreover, the magnetic field and flux distribution of the initial and final motor are shown in Fig. 5, which indicates that the final motor possesses less flux leakage. Meanwhile, the back-EMF and torque of the motors are presented in Fig. 6 and Fig. 7. The rationality of analysis perspective of flux density harmonic deterioration rate is confirmed by the higher back-EMF and torque with lower comprehensive deterioration rate of the final motor. III. Conclusion In conclusion, the above results reveal that during the process of magnetic energy transmission, the proposed flux density harmonic deterioration rate has the certain relevant with the electromagnetic torque. The decrease of the comprehensive deterioration rate contributes to the improvement of the flux modulation effect and electromagnetic torque to some degree, which verifies the effectiveness of deterioration rate concept. Finally, more theoretical analysis, performance simulations and experimental results will be presented in the full paper.

**References:**

[1] X. Zhu, M. Jiang, Z. Xiang, L. Quan, W. Hua and M. Cheng, "Design and Optimization of a Flux-Modulated Permanent Magnet Motor Based on an Airgap-Harmonic-Orientated Design Methodology," IEEE Trans. Ind. Electron., vol. 67, no. 7, pp. 5337-5348, July 2020. [2] T. Fukami, Y. Ueno and K. Shima, "Magnet Arrangement in Novel Flux-Modulating Synchronous Machines with Permanent Magnet Excitation," IEEE Trans. Magn., vol. 51, no. 11, pp. 1-4, Nov. 2015, Art no. 8206104. [3] L. Mo, T. Zhang and Q. Lu, "Design and Analysis of an Outer-Rotor-Permanent-Magnet Flux-Switching Machine for Electric Vehicle Applications," IEEE Trans. Appl. Super., vol. 29, no. 2, pp. 1-5, March 2019, Art no. 3601305.

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 Investigation on electromagnetic torque of a flux-switching permanent magnet motor from perspective of flux density harmonic deterioration rate

-This paper has proposed a 3-airgap magnetic geared vernier machine for low-speed direct-drive applications. By combining dual PM split-tooth vernier machine with magnetic gear, an impressive 150 Nm/L output torque density under 140 mm outer diameter can be achieved. The Halbach consequent pole structure is applied to the dual-layer-magnet rotor, which can largely reduce the thickness of rotor back iron compared to regular magnet arrangement, while increasing the airgap flux density. The performances of magnetic gear machines with different topologies are compared in this paper, it shows that proposed MG-DPSTVM has at least 40% higher torque density than all the previous MGM topologies. I.Introduction The magnetic geared machine (MGM) is attractive in continuous low speed direct-drive applications since it has remarkable torque density under relatively low electrical load. The contactless feature of magnet gear makes it more suitable for occasions when maintenance is difficult, such as wind/tide power generation, marine propulsion, oil pumping. In recent years, many MGM topologies were proposed for high torque density between 60-110 Nm/L[1]-[5]. To further enhance the torque density of MGM to a new level, a magnetic-geared dual PM vernier machine is proposed in this paper, which can exceed 150 Nm/L active torque density under low heat load, which also remains 0.8 power factor under rated load. II.Topologies of MG-DPVM The topology of proposed MG-DPVM is shown in Fig. 1, which can be divided into magnetic gear (MG) part and vernier machine (VM) part radially. The structure of proposed machine has been carefully designed for higher torque density. One of the distinct structures is PM rotor, which has two layers of Halbach consequent pole (HCP) magnets with different pole pairs. Compared to surface mounted magnets in [6], this topology can minimize the magnetic circuits and the thickness of back iron between two layers of magnets, while maximum the airgap flux density in both airgaps. For the VM part, there are consequent pole magnets on the stator. Through dual flux modulation effect [7], it can increase the torque density by introducing additional airgap working flux, and the power factor is also higher than regular vernier machines. III.Performance Comparison of MGM Topologies As shown in Table I, under the same outer radius and gap thickness, several MGM topologies are compared in this paper, including pseudo direct drive machine in [2], vernier pseudo direct drive machine in [5], 3-airgap outer rotor magnetic geared machine in [3], and magnetic geared vernier machine newly proposed in this paper. Considering slip risk and operating point of machine, the output torque is selected as 80% of the MG stall torque, and the heat load as well as copper loss (considering the winding) of each model can then be derived. It is obvious that proposed MG-DPVM has the highest MG stall torque density and output torque density, and the DC copper loss is also the lowest. IV.Conclusions A novel 3-airgaps magnetic geared split-tooth vernier machine is proposed in this paper. Through dual stage of magnet gear effect, high output torque density can be achieved under low current density and copper loss. It is impressive that proposed machine has achieved 150 Nm/L active torque volume density and 23 Nm/kg active mass density. According to the comparison, this value is much higher than the existing topologies of MGM, which makes it competitive in long-term low-speed high-torque industrial fields such as wind generation and marine propulsion. The power factor of proposed machine is lower than other MGMs due to the application of vernier machine, however the copper loss and heat load are largely reduced, therefore higher efficiency can be expected.

**References:**

 [1] K. T. Chau, D. Zhang, J. Z. Jiang, C. Liu, and Y. Zhang, “Design of a Magnetic-Geared Outer-Rotor Permanent-Magnet Brushless Motor for Electric Vehicles,” IEEE Transactions on Magnetics, vol. 43, no. 6, pp. 2504-2506, Jun. 2007. [2] K. Atallah, J. Rens, S. Mezani, and D. Howe, “A Novel ‘Pseudo’ Direct-Drive Brushless Permanent Magnet Machine,” IEEE Transactions on Magnetics, vol. 44, no. 11, pp. 4349-4352, Nov. 2008. [3] S. Gerber and R.-J. Wang, “Design and Evaluation of a Magnetically Geared PM Machine,” IEEE Transactions on Magnetics, vol. 51, no. 8, pp. 1-10, Aug. 2015. [4] L. Jian, W. Gong, G. Xu, J. Liang, and W. Zhao, “Integrated Magnetic-Geared Machine with Sandwiched Armature Stator for Low-Speed Large-Torque Applications,” IEEE Transactions on Magnetics, vol. 48, no. 11, pp. 4184-4187. [5] X. Yin, Y. Fang, X. Huang, and P.-D. Pfister, “Analytical Modeling of a Novel Vernier Pseudo-Direct-Drive Permanent-Magnet Machine,” IEEE Transactions on Magnetics, vol. 53, no. 6, pp. 1-4, Jun. 2017. [6] M. Johnson, M. C. Gardner, H. A. Toliyat, S. Englebretson, W. Ouyang, and C. Tschida, “Design, Construction, and Analysis of a Large-Scale Inner Stator Radial Flux Magnetically Geared Generator for Wave Energy Conversion,” IEEE Transactions on Industry Applications, vol. 54, no. 4, pp. 3305–3314, Jul. 2018. [7] S. Niu, S. L. Ho, and W. N. Fu, “A Novel Stator and Rotor Dual PM Vernier Motor with Space Vector Pulse Width Modulation,” IEEE Transactions on Magnetics, vol. 50, no. 2, pp. 805–808, Feb. 2014.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/9d773001e780f823a2203c7317816374.jpg)
Fig. 1 The topology of proposed MG-DPVM

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 A High Torque Density Magnetic-geared Dual-PM Split-tooth Vernier Machine with Halbach Consequent Poles

I. Introduction: Magnetic gears are good candidates to replace mechanical gearboxes, since it has higher efficiency, lower noise and self-protection characteristic [1], [2]. Radial-type magnetic gears (RMGs) are sometimes not suitable for special application scenarios where there is strict requirement for space. Axial-type magnetic gears (AMGs) have an even higher torque density than RMGs and a shorter axial length, [3]. Therefore, AMGs can be applied in scenarios where the weight is a crucial concern. However, one drawback of AMGs is the large amount usage of permanent magnets (PMs), which significantly increases manufacture cost. In this paper, an iron-made flux-enhancing plate is added to the outside of AMGs to increase the output torque. Then, the flux-enhancing principle is elaborated by analyzing the flux path variation before and after the installation of the auxiliary flux-enhancing slice. Finally, the performances of three AMGs are compared, and it is proved that the proposed structure has the highest torque density. II. Machine Configuration The proposed auxiliary flux-enhancing structural axial-type magnetic gear (AFS-AMG) contains four slices, and its structure together with the magnetic flux density distribution are depicted in Fig. 1 (a), (b). Two rotors are those with surface-mounted PMs on them, and the one with less PM pieces is the high-speed rotor, while the one with more PM pieces is the low-speed rotor. The first and third slices from the left to the right are steady slices, and they formed as one stator component. The PMs on the high-speed rotor are magnetized on the axial direction, while the PMs on the low-speed rotor are magnetized on the circumferential direction. To produce a steady electromagnetic torque, the pole-pair number of high-speed and low-speed rotor PMs Phigh and Plow and the number of modulator pieces Qmodulator must satisfy Eq. (1) [4]. Additionally, since the auxiliary flux-enhancing slice serves as a complementary structure for the magnetic flux to form a shorter loop, its tooth number Qteeth should be equal to the number of modulator pieces, as given in Eq. (2). Then, based on the operating principle of magnetic gear, the rotating speed and output torque relations between the high-speed and low-speed rotor satisfy Eq.(3) and (4), where Thigh and Tlow are the electromagnetic torques on the high-speed and low-speed rotor, respectively. Ωhigh and Ωlow are the rotating speeds of the high-speed and low-speed rotor, respectively. III. Flux Enhancing and AMG Evolution The flux-enhancing effect of the auxiliary slice can be illustrated by the flux line path, as shown in Fig. 1 (c). It can be observed that without the auxiliary slice, the magnetic flux path passes through the air region several times to form a close loop. Since the permeability of air is much lower than that of the iron, the magnetic reluctance of the whole loop is very large, leading to a small magnetic flux density and a low torque. When the auxiliary slice is added to the outside of the spoke-type PMs, the corresponding flux line changes to pass through a path with smaller reluctance. Hence, the magnetic flux density on the air gap increases, leading to a larger electromagnetic torque. In fact, the structures of AMGs can be derived from its RMG counterparts, as illustrated in Fig. 2 (a)-(c) [5]. The operating principle of AMGs and RMGs are similar, and their differences lie in two aspects. First, the magnetic flux density direction of the two types of magnetic gears are different: the main flux paths in AMGs are in axial direction, and in radial direction for RMGs. Second, the different rotors are distributed on axial direction for AMGs, making it more suitable to have multi-slice structure. However, adding an auxiliary structure in RMG will lead to a very large size. Therefore, the flux-enhancing slice can maximize its effect for AMGs. IV. Validation and Comparison: To obtain a better electromagnetic performance, three AMGs with the same inner and outer radii are constructed in JMAG to compare their performances, whose parameters are provided in TABLE I. The corresponding simulation results are provided in TABLE II and Fig. 2 (d). It can be observed that AMG (c), namely the proposed AMG, has the highest efficiency and the highest torque density per PM usage. The axial component magnetic flux density distributions and the corresponding Fast Fourier Transform of the AMG (b) and (c) outside the low-speed rotor with the same radius are depicted in Fig. 3 (e), (f). It can be observed that for AMG (c), the amplitude of working harmonic (14th) is much larger than AMG (b), leading to a torque increase. IV: Conclusion In this paper, a novel AFS-AMG is proposed. By introducing an auxiliary flux-enhancing slide to shorten the flux path, the proposed AFS-AMG has a higher torque density per PM usage and a higher efficiency compared to conventional AMG structure. This work was supported by a grant (Project No. 52077186 and 51677159) from NSFC of China; a grant (Project No. JCYJ20180307123918658) from STIC of Shenzhen Municipality, China; a grant (Project No. SGDX2019081623101559) from Shenzhen – Hong Kong Innovation Circle Category D Project from the Science Technology and Innovation Committee of Shenzhen Municipality, China.

**References:**

 [1] H. Zhao, C. Liu, Z. Song and J. Yu, "A Fast Optimization Scheme of Coaxial Magnetic Gears Based on Exact Analytical Model Considering Magnetic Saturation," in IEEE Transactions on Industry Applications (Early Access). [2] P. O. Rasmussen, T. O. Andersen, F. T. Jorgensen and O. Nielsen, "Development of a high-performance magnetic gear," in IEEE Transactions on Industry Applications, vol. 41, no. 3, pp. 764-770, May-June 2005. [3] Y. Chen, W. N. Fu, S. L. Ho and H. Liu, "A Quantitative Comparison Analysis of Radial-Flux, Transverse-Flux, and Axial-Flux Magnetic Gears," IEEE Trans. Magnetics, vol. 50, no. 11, pp. 1-4, Nov. 2014. [4] K. Atallah and D. Howe, "A novel high-performance magnetic gear," IEEE Trans. Magnetics, vol. 37, no. 4, pp. 2844-2846, July 2001. [5] X. Zhang, X. Liu and Z. Chen, "A Novel Dual-Flux-Modulator Coaxial Magnetic Gear for High Torque Capability," in IEEE Transactions on Energy Conversion, vol. 33, no. 2, pp. 682-691, June 2018.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/aadd04789f60af98e91ba09d679eaa4f.jpg)
 
Fig.1 The proposed AFS-AMG: (a) Explosive view; (b) magnetic flux density distribution, (c) illustration of flux path.

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Fig.2 (a) Conventional AMG, (b) Circumferentionally magnetized AMG, (c) AFS-AMG, (d) torque waveform comparisons, (e) magnetic flux density distribution, (f) the corresponding FFT.

 Design of an Axial-type Magnetic Gear with an Auxiliary Flux-enhancing Structure

—In this paper, a novel dual-winding axial-flux magnetic-geared machine (DAMGM) is presented for in-wheel electric vehicle driving. By using armature windings for replacing the high speed rotor and utilizing the space between the modulators with sets of auxiliary winding, the torque density and the fault-tolerant ability can be enhanced significantly since it can operate normally with the excitation of either armature winding or auxiliary winding. Compared with dual-winding radial-flux magnetic-geared machine (DRMGM), axial-flux magnetic-geared machine (AMGM) and conventional axial-flux PM motor, the proposed motor has huge improvement in torque density, efficiency and fault-tolerant capacity. The pole-pair combination and geometric parameters are carefully compared and optimized by using three-dimensional finite-element method for getting the best performance of the proposed structure and verifying the validity of the proposed machine design. 1. Introduction Magnetic gear (MG) and magnetic-geared machine provide the solutions for many high-torque-density applications with the advantages of easy maintenance, inherent overload protection, and low working noise. For electric vehicle, two general solutions have been adopted: the first type consists of a high-speed motor and a mechanical gear box while the second only employs a direct-drive motor. However, the former has the disadvantages of high transmission loss, overload damage, heavy weight and high noise. The latter is limited by efficiency issue and bulky size since the efficiency of the machine is lower when it works at a low level power and the size of the machine will increase with the reduction of nominal speed. Magnetic-geared machine (MGM) can overcome the shortages by integrating a magnetic gear and an electric machine together, hence, it becomes the hotspot among researchers. Nevertheless, most of researches focus on radial-flux magnetically geared machine (RMGM), including traditional MGM , dual-flux-modulated MGM and vernier MGM [8]. For the space constrains in some specific applications, such as electric vehicles, the traditional RMGM is not suitable for direct coupling of the motor with a wheel. Hence an axial-flux magnetically geared machine (AMGM) [1] was proposed to solve these problems. However, none of the existing solutions for axial-flux MGM can fully utilize the space between the modulators, which can actually be used for placing sets of auxiliary winding and strengthening the magnetic field generated by the armature winding. In such case, for getting a higher torque density and fault-tolerance capacity, this paper presents a novel dual-winding axial-flux magnetic-geared machine (DAMGM) to solve those problems. 2.Working Principle and Motor Structure The overall structure of the proposed machine is shown in Fig.1 (a), which consists of two rotors, two sets of modulators and a shared stator. The total axial length, outer radius of the outer-rotor and the inner radius of the stator with a modulator are 60mm, 92mm and 64mm, respectively. Fig. 1 (b) and (c) shows the corresponding coil-emf vectors in electrical degrees for the armature winding and auxiliary winding, whose slot numbers are 45 and 27, respectively. The DAMGM complies with the fundamental rule of magnetic gears, which is expressed as: P=NL±S (1) where NL is the number of pole pairs of rotor, P is the number of modulator and S is the number of pole pairs of armature winding. For improving the torque density, P is equal to S+NL [2]. In this paper, the P, NL and S are 27, 24 and 3, respectively. Besides, the speed of rotor is 300 r/min, which is close to the rated operation speed of electric vehicle driving system. 3. Performance Comparisons and Analysis The performance comparisons are shown in Fig. 2, where Motor 1, 2, 3 and 4 are the proposed machine, the axial-flux PM machine , DRMGM and axial-flux magnetic-geared machine. For the torque performance, as shown in Fig.2 (a), the proposed machine has the best torque output capacity, whose output torque is increased by 71.13% compared to Motor 2, 104.93% compared to Motor 3 and 110.12% compared to AMGM, reaching 166.13 Nm with the ripple of 7.38% under the current density 6A/mm2 and slot package factor 0.6. Besides, the Fig. 2 (c) shows the fault-tolerance capacity of the proposed machine, which can operate with either auxiliary winding or armature winding. Simulation result shows that it can operates normally with the sole excitation of armature winding under the current density reaching about 18A/mm2 or excited by auxiliary winding solely under the current density nearly 10A/mm2. Furthermore, Fig. 2(b) shows the distributions of radial flux density Brδin the air gap under no load condition while the back EMF of those machines can be found in Fig. 2 (d). Finally, the proposed machine has the highest efficiency compared among motor 2, 3, and 4, reaching 93.78% with the copper loss of 226 W and the iron loss of 75.3 W. 4. Conclusion This paper proposes an effective fault-tolerant dual-winding axial-flux magnetic-geared machine. The structure, corresponding coil-emf vectors and working principle are introduced. Then, the performance comparison among the proposed machine, axial-flux PM machine, DRMGM and axial-flux magnetic-geared machine are conducted to verify the efficiency of the design.

**References:**

[1] M. Johnson, M. C. Gardner and H. A. Toliyat, "Design and Analysis of an Axial Flux Magnetically Geared Generator." IEEE Trans. Industry Applications, vol. 53, no. 1, pp. 97-105, Jan.-Feb. 2017. [2] S. L. Ho, S. Niu and W. N. Fu. "Design and Analysis of a Novel Axial-Flux Electric Machine." IEEE Trans. Magnetics, vol. 47, no. 10, pp. 4368-4371, Oct, 2011.

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 Design and Analysis of an Effective Fault-Tolerant Dual-winding Axial-flux Magnetic-geared Machine for In-wheel Electric Vehicle

-In this paper, a novel yokeless magnetic gear (YMG) with trapezoidal Halbach PM array is presented for direct drive of electric vehicles. Compared to traditional structures, the proposed machine has huge performance improvement in the following two aspects: Firstly, by using the aluminum to replace the silicon steel as the rotor iron core, the iron loss and motor weight can be reduced and the torque density can be increased significantly; Secondly, the generated torque and ripple can be optimized by adopting a trapezoidal Halbach PM array, whose leakage flux is far more lower than YMG with Halbach PM array and YMG with conventional PMs. The performance of the proposed motor is compared and analyzed by using three-dimensional finite-element method and the best parameter combination of the proposed machine is found using optimization method. 1. Introduction Magnetic gears provide solutions for replacing the mechanical gearbox in electric vehicle driving system since it inherently has the advantages of high transmission efficiency, overload protection, low working noise and physical isolation. In order to tap the potentials of industrial application, more researchers focus on the magnetic gears, including radial-flux, axial-flux magnetic gear and magnetic-geared machines, for finding high efficiency, high torque density and better performance solutions. Papers [1] proposed the types of a magnetic gear with Halbach PM array and discussed how the arrangement of Halbach PM affects the performance of the machine. However, due to the limit of machine structures, all of these magnetic gears have the rotor iron core, which is used to form the magnetic circuit of PM and reduce the magnetic resistance. However, the performance of the machine is weakened severely with adopting the yokeless structure since it introduces of great leakage flux. Hence, although the design and research in this topology have made some progress, in terms of its own design thought, the improvement level is very limited. In this paper, a novel yokeless magnetic gear (YMG) with trapezoidal Halbach PM array is proposed to solve the drawbacks mentioned above. Compared to the designs of the magnetic gears with Halbach PM array, the proposed machine has a better performance in the torque output capacity, torque density and efficiency. 2. Working Principle and Motor Structure The overall structure of the proposed yokeless magnetic gear with trapezoidal Halbach PM array is shown in Fig. 1. As shown in Fig. 1, the proposed machine contains four kinds of magnetized PM and aluminum supporter for reducing the leakage flux and weight. The total axial length, outer radius of outer-rotor and inner radius of stator are 220 mm, 120 mm and 65.8 mm, respectively. All magnetic gears, including the proposed one, comply with the fundamental rule, which is expressed as: P=NL±NH (1) where NL , NH and P are the number of pole pairs of low speed rotor (LSR), high speed rotor (HSR) and modulator, respectively. For getting a better performance, P should be equal to NH+NL. Besides, similar to the above equation, the operation speed of MG should also follow the following relationship: ωH /ωL= -NL/NH =- Gr (2) where ωL and ωH are the mechanical speed of LSR and HSR. The proportion of the two speed Gr is the gear ratio, which is used to indicate that the speed ratio of MG equals the inverse ratio of the PM pole-pair numbers in two rotors. In this paper, the rated operation speed for the HSR and LSR are 2400 r/min and 300r/min, respectively, which is close to the rated operation speed of direct-drive electric vehicle system. 3. Performance Comparisons and Analysis The output torque comparison between the proposed machine, conventional magnetic gear (MG), yokeless magnetic gear (YMG) and YMG with conventional Halbach PM array are shown in Fig. 2 (a). Simulation results show that the output torque of the proposed machine increases by 169.1%, 97.2% and 21.0% compared to YMG, YMG with conventional Halbach PM array and conventional MG, reaching 1840.5 Nm with the ripple of 1.28%. Besides, the proposed machine has a minimum iron loss and the highest efficiency compared to the other types of MG, reaching 110 W and 99.38%, respectively. Furthermore, the torque density of proposed machine reaches 51.18 Nm per kilogram and 88.1 Nm per PM weight (Kg), which increased by 96.8% and reduced by 7.3% compared to conventional MG. Fig. 2(b) shows the distributions of radial flux density Brδ in the air gap under no load condition. The comparison of leakage flux between YMG with conventional Halbach PM array and proposed machine, YMG and proposed machine are indicated in Fig.4 (c) and (d), respectively. Compared to those two types of YMG, the proposed machine has the lowest density of magnetic flux line in both the outer region of outer PMs and inner region of inner PMs, which means the leakage flux of the proposed machine is the lowest and that is the reason for the high torque density of the proposed machine. 4. Conclusion This paper proposes a yokeless magnetic gear with trapezoidal Halbach PM array for electric vehicle driving. First, the structure, PM arrangement and working principle are introduced. Then, the performance comparison among the proposed machine, conventional MG, YMG and YMG with Halbach array are conducted. Finally, the simulation results verify the efficiency of the novel structure.

**References:**

[1] Yiduan Chen; W. N. Fu; Wei Li. "Performance analysis of a novel triple-permanent-magnet-excited magnetic gear and its design method". IEEE Trans. Magnetics, vol. 52, no.7, pp. 1-4, 2016.

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