France



&lt;br/&gt;&lt;br/&gt;I. Introduction&lt;br/&gt;Permanent-magnet (PM) machines, which exhibit the merits of high torque/power density and high efficiency, have been widely employed in many industrial applications in recent decades, such as wind power generation and electric vehicles [1-5]. As these applications are safe-critical, the development of new machine structures with improved reliability is speeding up [6, 7]. To achieve the feature of fault tolerance, various design techniques of PM machines have been put forward, such as the redundant designs with multi-phase windings [8] or dual three-phase windings [9], and the modular designs with modular windings [10] or modular stator [11], etc. Recently, the stator-slot PM (SSPM) machine with inherent fault-tolerant ability is proposed to achieve high failure-safe capability during uncontrolled generator operation or winding short circuits [12]. However, relatively low torque density puts this structure at a disadvantage, particularly at light load condition. To improve the torque performances, the split-tooth (ST) structure is introduced in the machine [13], where the PMs are placed between the split tooth and tangentially magnetized with opposite polarities in the adjacent PMs, as shown in Fig. 1(a).&lt;br/&gt;&lt;br/&gt;In this paper, three new types of split-tooth stator-slot PM (ST-SSPM) machines are proposed as shown in Fig. 1(b)-(d), which are designated as Type-2, Type-3, and Type-4 machine, respectively. Four types of machines share the same structure with the split-tooth stator and salient-pole rotor but the PMs are arranged with different positions and polarities. The operation principles are introduced while the electromagnetic performances of four types of machines are comprehensively analyzed and compared by using finite-element analysis (FEA). Based on the analyzed results, the merits and demerits of four types of machines with different PM arrangements are concluded.&lt;br/&gt;&lt;br/&gt;II. Methodology&lt;br/&gt;Similar to the other stator-PM machine with salient rotor poles, the operation principle of four types of machines can be explained by the field modulation theory [14]. Then, the slot-pole combinations of Type-1 and Type-3 machines can be derived as &lt;i&gt;P&lt;sub&gt;a&lt;/sub&gt;&lt;/i&gt; = min(|&lt;i&gt;iN&lt;sub&gt;s&lt;/sub&gt;&lt;/i&gt;/2±&lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt;|), (&lt;i&gt;i&lt;/i&gt;=1, 3, 5…) while those of Type-2 and Type-4 machines can be derived as &lt;i&gt;P&lt;sub&gt;a&lt;/sub&gt;&lt;/i&gt; = min(|&lt;i&gt;iN&lt;sub&gt;s&lt;/sub&gt;&lt;/i&gt;±&lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt;|), (&lt;i&gt;i&lt;/i&gt;=1, 2, 3…), where &lt;i&gt;P&lt;sub&gt;a&lt;/sub&gt;&lt;/i&gt; is the pole-pair number of armature winding, &lt;i&gt;N&lt;sub&gt;s&lt;/sub&gt;&lt;/i&gt; is the stator slot number, &lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt; is the rotor pole number, and &lt;i&gt;i&lt;/i&gt; is a positive integer.&lt;br/&gt;&lt;br/&gt;By performing FEA in JMAG-Designer, the torques and torque ripples of machines with various slot-pole combinations are optimized in Fig. 2. It can be seen that Type-1 to Type-4 machines can produce the largest torque with &lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt;=22, 16, 23 and 17 respectively, while the torque of Type-4 machine with &lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt;=17 is the largest among all the machines. However, Type-3 machine with &lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt;=23 and Type-4 machine with &lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt;=17 produce large unbalanced magnetic force due to the odd number of &lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt;, thus undesired electromagnetic vibration and noise will be incurred. To resolve this concern, it is desirable to achieve good torque performances with an even number of &lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt;. One can see that the torques of Type-3 and Type-4 machines with &lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt;=22 are the highest among all the machines with even numbers of &lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt;, despite slightly lower than those of Type-3 machine with &lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt;=23 and Type-4 machine with &lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt;=17, respectively. Therefore, the optimal &lt;i&gt;N&lt;sub&gt;r&lt;/sub&gt;&lt;/i&gt; of Type-1 to Type-4 machines are preliminarily selected as 22, 16, 22, and 22, respectively. Furthermore, the torque ripples of these four machines are all lower than 10 %, and hence are acceptable. Finally, taking into consideration all the above factors, Type-1 to Type-4 machines are globally optimized with optimal &lt;i&gt;Nr&lt;/i&gt;=22, 16, 22, and 22, respectively, which are further investigated, including the cogging torque, back-EMF, steady torque, PM utilization ratio, loss, efficiency, power factor and PM demagnetization.&lt;br/&gt;&lt;br/&gt;III. Conclusion&lt;br/&gt;Based on the analyzed results of four types of machines, some findings are listed as follows.&lt;br/&gt;a) Type-1 machine has the lowest cogging torque due to the circulation of PM flux in the stator at no load condition, whereas Type-2 machine shows the highest cogging torque due to the largest cogging torque factor.&lt;br/&gt;b) Type-1, Type-2 and Type-4 machines have the similar and largest average torque at rated condition. However, Type-4 and Type-2 machines offer the highest and lowest PMs utilization ratio, respectively.&lt;br/&gt;c) Type-4 machine exhibits the best torque performance at rated and light load conditions, while Type-1 machine has the best overload capability. Type-2 machine shows good torque performances both in light and overload conditions, despite not the best,&lt;br/&gt;d) Type-4 machine offers the highest power factor, despite the power factors of four types of machines tend to be low. However, their power factors can be improved by increasing the value of stator inner radius/airgap.&lt;br/&gt;e) Type-1 to Type-3 machines show irreversible PM demagnetization with different degrees at the inner corner of PMs under over load and high temperature condition, whereas this does not happen in Type-4 machine with the best PM demagnetization withstand.&lt;br/&gt;&lt;br/&gt;More detailed analysis and results will be given in the full paper.&lt;br/&gt;&lt;br/&gt;Acknowledgment&lt;br/&gt;This work was supported by a grant (Project No. 17205518) from the Hong Kong Research Grants Council, Hong Kong Special Administrative Region, China.

**References:**

&lt;br/&gt;[1] K. T. Chau, C. C. Chan, and C. Liu, “Overview of permanent-magnet brushless drives for electric and hybrid electric vehicles,” &lt;i&gt;IEEE Trans. Ind. Electron.,&lt;/i&gt; vol. 55, no. 6, pp. 2246-2257, May 2008.&lt;br/&gt;[2] Z. Q. Zhu, and D. Howe, “Electrical machines and drives for electric, hybrid, and fuel cell vehicles,” &lt;i&gt;Proc. IEEE,&lt;/i&gt; vol. 95, no. 4, pp. 746-765, Apr. 2007.&lt;br/&gt;[3] J. Li, K. T. Chau, J. Jiang, C. Liu, and W. Li, “A new efficient permanent-magnet vernier machine for wind power generation,” &lt;i&gt;IEEE Trans. Magn.,&lt;/i&gt; vol. 46, no. 6, pp. 1475-1478, May 2010.&lt;br/&gt;[4] C. Liu, K. T. Chau, J. Z. Jiang, and S. Niu, “Comparison of Stator-Permanent-Magnet Brushless Machines,” &lt;i&gt;IEEE Trans. Magn.&lt;/i&gt;, vol. 44, no. 11, pp. 4405-4408, Nov. 2008.&lt;br/&gt;[5] L. Cao, K. T. Chau, and C. H. T. Lee, “A new parallel-hybrid-excited permanent-magnet machine with harmonic-differential effect for electric vehicles,” &lt;i&gt;IEEE Trans. Veh. Technol.,&lt;/i&gt; vol. 69, no. 11, pp. 12734-12750, Sept. 2020.&lt;br/&gt;[6] A. M. E.-. Refaie, M. R. Shah, and K. Huh, “High-power-density fault-tolerant PM generator for safety-critical applications,” &lt;i&gt;IEEE Trans. Ind. Appl.,&lt;/i&gt; vol. 50, no. 3, pp. 1717-1728, Sept. 2014.&lt;br/&gt;[7] W. Zhao, L. Xu, and G. Liu, “Overview of permanent-magnet fault-tolerant machines: Topology and design,” &lt;i&gt;CES Trans. Elect. Mach. and Syst.,&lt;/i&gt; vol. 2, no. 1, pp. 51-64, Mar. 2018.&lt;br/&gt;[8] F. Barrero, and M. J. Duran, “Recent advances in the design, modeling, and control of multiphase machines—Part I,” &lt;i&gt;IEEE Trans. Ind. Electron.&lt;/i&gt;, vol. 63, no. 1, pp. 449-458, Jun. 2016.&lt;br/&gt;[9] X. Jiang, W. Huang, R. Cao, Z. Hao, and W. Jiang, “Electric drive system of dual-winding fault-tolerant permanent-magnet motor for aerospace applications,” &lt;i&gt;IEEE Trans. Ind. Electron.,&lt;/i&gt; vol. 62, no. 12, pp. 7322-7330, Dec. 2015.&lt;br/&gt;[10] P. Zheng, Y. Sui, J. Zhao, C. Tong, T. A. Lipo, and A. Wang, “Investigation of a novel five-phase modular permanent-magnet in-wheel motor&lt;i&gt;,” IEEE Trans. Magn.&lt;/i&gt;, vol. 47, no. 10, pp. 4084-4087, Oct. 2011.&lt;br/&gt;[11] G. J. Li, Z. Q. Zhu, M. Foster, and D. Stone, “Comparative studies of modular and unequal tooth PM machines either with or without tooth tips,” &lt;i&gt;IEEE Trans. Magn&lt;/i&gt;., vol. 50, no. 7, pp. 1-10, Jul. 2014.&lt;br/&gt;[12] I. A. A. Afinowi, Z. Q. Zhu, Y. Guan, J. Mipo, and P. Farah, “A novel brushless AC doubly salient stator slot permanent magnet machine,” &lt;i&gt;IEEE Trans. Energy Convers&lt;/i&gt;., vol. 31, no. 1, pp. 283-292, Mar. 2016.&lt;br/&gt;[13] H. Qu, and Z. Q. Zhu, “Analysis of split-tooth stator slot PM machine,” &lt;i&gt;IEEE Trans. Ind. Electron.&lt;/i&gt;, 2020, doi: 10.1109/TIE.2020.3031535 (Early Access).&lt;br/&gt;[14] M. Cheng, P. Han, and W. Hua, “General airgap field modulation theory for electrical machines,” &lt;i&gt;IEEE Trans. Ind. Electron&lt;/i&gt;., vol. 64, no. 8, pp. 6063-6074, Mar. 2017.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/3045adcb24c5b43ba666bbd3d248727b.jpg)
Fig. 1. ST-SSPM machines with different PM arrangements. (1) Type-1. (b) Type-2. (c) Type-3. (d) Type-4.

![](https://assets.underline.io/uploads/markdown_image/1/image/a177b936336d0e337ea63e54611f8851.jpg)
Fig. 2. Torques and torque ripples of four types of machines with different numbers of rotor poles. (a) Torques. (b) Torque ripples.

INTERMAG 2021

 Analysis of Split-Tooth Stator-Slot Permanent-Magnet Machines with Different PM Arrangements

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I. Introduction Permanent-magnet (PM) machines, which exhibit the merits of high torque/power density and high efficiency, have been widely employed in many industrial applications in recent decades, such as wind power generation and electric vehicles [1-5]. As these applications are safe-critical, the development of new machine structures with improved reliability is speeding up [6, 7]. To achieve the feature of fault tolerance, various design techniques of PM machines have been put forward, such as the redundant designs with multi-phase windings [8] or dual three-phase windings [9], and the modular designs with modular windings [10] or modular stator [11], etc. Recently, the stator-slot PM (SSPM) machine with inherent fault-tolerant ability is proposed to achieve high failure-safe capability during uncontrolled generator operation or winding short circuits [12]. However, relatively low torque density puts this structure at a disadvantage, particularly at light load condition. To improve the torque performances, the split-tooth (ST) structure is introduced in the machine [13], where the PMs are placed between the split tooth and tangentially magnetized with opposite polarities in the adjacent PMs, as shown in Fig. 1(a). In this paper, three new types of split-tooth stator-slot PM (ST-SSPM) machines are proposed as shown in Fig. 1(b)-(d), which are designated as Type-2, Type-3, and Type-4 machine, respectively. Four types of machines share the same structure with the split-tooth stator and salient-pole rotor but the PMs are arranged with different positions and polarities. The operation principles are introduced while the electromagnetic performances of four types of machines are comprehensively analyzed and compared by using finite-element analysis (FEA). Based on the analyzed results, the merits and demerits of four types of machines with different PM arrangements are concluded. II. Methodology Similar to the other stator-PM machine with salient rotor poles, the operation principle of four types of machines can be explained by the field modulation theory [14]. Then, the slot-pole combinations of Type-1 and Type-3 machines can be derived as Pa = min(|iNs/2±Nr|), (i=1, 3, 5…) while those of Type-2 and Type-4 machines can be derived as Pa = min(|iNs±Nr|), (i=1, 2, 3…), where Pa is the pole-pair number of armature winding, Ns is the stator slot number, Nr is the rotor pole number, and i is a positive integer. By performing FEA in JMAG-Designer, the torques and torque ripples of machines with various slot-pole combinations are optimized in Fig. 2. It can be seen that Type-1 to Type-4 machines can produce the largest torque with Nr=22, 16, 23 and 17 respectively, while the torque of Type-4 machine with Nr=17 is the largest among all the machines. However, Type-3 machine with Nr=23 and Type-4 machine with Nr=17 produce large unbalanced magnetic force due to the odd number of Nr, thus undesired electromagnetic vibration and noise will be incurred. To resolve this concern, it is desirable to achieve good torque performances with an even number of Nr. One can see that the torques of Type-3 and Type-4 machines with Nr=22 are the highest among all the machines with even numbers of Nr, despite slightly lower than those of Type-3 machine with Nr=23 and Type-4 machine with Nr=17, respectively. Therefore, the optimal Nr of Type-1 to Type-4 machines are preliminarily selected as 22, 16, 22, and 22, respectively. Furthermore, the torque ripples of these four machines are all lower than 10 %, and hence are acceptable. Finally, taking into consideration all the above factors, Type-1 to Type-4 machines are globally optimized with optimal Nr=22, 16, 22, and 22, respectively, which are further investigated, including the cogging torque, back-EMF, steady torque, PM utilization ratio, loss, efficiency, power factor and PM demagnetization. III. Conclusion Based on the analyzed results of four types of machines, some findings are listed as follows. a) Type-1 machine has the lowest cogging torque due to the circulation of PM flux in the stator at no load condition, whereas Type-2 machine shows the highest cogging torque due to the largest cogging torque factor. b) Type-1, Type-2 and Type-4 machines have the similar and largest average torque at rated condition. However, Type-4 and Type-2 machines offer the highest and lowest PMs utilization ratio, respectively. c) Type-4 machine exhibits the best torque performance at rated and light load conditions, while Type-1 machine has the best overload capability. Type-2 machine shows good torque performances both in light and overload conditions, despite not the best, d) Type-4 machine offers the highest power factor, despite the power factors of four types of machines tend to be low. However, their power factors can be improved by increasing the value of stator inner radius/airgap. e) Type-1 to Type-3 machines show irreversible PM demagnetization with different degrees at the inner corner of PMs under over load and high temperature condition, whereas this does not happen in Type-4 machine with the best PM demagnetization withstand. More detailed analysis and results will be given in the full paper. Acknowledgment This work was supported by a grant (Project No. 17205518) from the Hong Kong Research Grants Council, Hong Kong Special Administrative Region, China.

**References:**

 [1] K. T. Chau, C. C. Chan, and C. Liu, “Overview of permanent-magnet brushless drives for electric and hybrid electric vehicles,” IEEE Trans. Ind. Electron., vol. 55, no. 6, pp. 2246-2257, May 2008. [2] Z. Q. Zhu, and D. Howe, “Electrical machines and drives for electric, hybrid, and fuel cell vehicles,” Proc. IEEE, vol. 95, no. 4, pp. 746-765, Apr. 2007. [3] J. Li, K. T. Chau, J. Jiang, C. Liu, and W. Li, “A new efficient permanent-magnet vernier machine for wind power generation,” IEEE Trans. Magn., vol. 46, no. 6, pp. 1475-1478, May 2010. [4] C. Liu, K. T. Chau, J. Z. Jiang, and S. Niu, “Comparison of Stator-Permanent-Magnet Brushless Machines,” IEEE Trans. Magn., vol. 44, no. 11, pp. 4405-4408, Nov. 2008. [5] L. Cao, K. T. Chau, and C. H. T. Lee, “A new parallel-hybrid-excited permanent-magnet machine with harmonic-differential effect for electric vehicles,” IEEE Trans. Veh. Technol., vol. 69, no. 11, pp. 12734-12750, Sept. 2020. [6] A. M. E.-. Refaie, M. R. Shah, and K. Huh, “High-power-density fault-tolerant PM generator for safety-critical applications,” IEEE Trans. Ind. Appl., vol. 50, no. 3, pp. 1717-1728, Sept. 2014. [7] W. Zhao, L. Xu, and G. Liu, “Overview of permanent-magnet fault-tolerant machines: Topology and design,” CES Trans. Elect. Mach. and Syst., vol. 2, no. 1, pp. 51-64, Mar. 2018. [8] F. Barrero, and M. J. Duran, “Recent advances in the design, modeling, and control of multiphase machines—Part I,” IEEE Trans. Ind. Electron., vol. 63, no. 1, pp. 449-458, Jun. 2016. [9] X. Jiang, W. Huang, R. Cao, Z. Hao, and W. Jiang, “Electric drive system of dual-winding fault-tolerant permanent-magnet motor for aerospace applications,” IEEE Trans. Ind. Electron., vol. 62, no. 12, pp. 7322-7330, Dec. 2015. [10] P. Zheng, Y. Sui, J. Zhao, C. Tong, T. A. Lipo, and A. Wang, “Investigation of a novel five-phase modular permanent-magnet in-wheel motor,” IEEE Trans. Magn., vol. 47, no. 10, pp. 4084-4087, Oct. 2011. [11] G. J. Li, Z. Q. Zhu, M. Foster, and D. Stone, “Comparative studies of modular and unequal tooth PM machines either with or without tooth tips,” IEEE Trans. Magn., vol. 50, no. 7, pp. 1-10, Jul. 2014. [12] I. A. A. Afinowi, Z. Q. Zhu, Y. Guan, J. Mipo, and P. Farah, “A novel brushless AC doubly salient stator slot permanent magnet machine,” IEEE Trans. Energy Convers., vol. 31, no. 1, pp. 283-292, Mar. 2016. [13] H. Qu, and Z. Q. Zhu, “Analysis of split-tooth stator slot PM machine,” IEEE Trans. Ind. Electron., 2020, doi: 10.1109/TIE.2020.3031535 (Early Access). [14] M. Cheng, P. Han, and W. Hua, “General airgap field modulation theory for electrical machines,” IEEE Trans. Ind. Electron., vol. 64, no. 8, pp. 6063-6074, Mar. 2017.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/3045adcb24c5b43ba666bbd3d248727b.jpg)
Fig. 1. ST-SSPM machines with different PM arrangements. (1) Type-1. (b) Type-2. (c) Type-3. (d) Type-4.

![](https://assets.underline.io/uploads/markdown_image/1/image/a177b936336d0e337ea63e54611f8851.jpg)
Fig. 2. Torques and torque ripples of four types of machines with different numbers of rotor poles. (a) Torques. (b) Torque ripples.

poster

-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

![](https://assets.underline.io/uploads/markdown_image/1/image/b3332c72ac4fe162128c794872159d3a.jpg)

 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.

**Images:**

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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.

**Images:**

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 Design and Comparison Study of a Yokeless Magnetic Gear with Trapezoidal Halbach PM Array for Electric Vehicle Driving

—In this paper, a novel active magnetic gear (AMG) is presented for low-speed large-torque applications. Compared to traditional solutions, the proposed machine has two key advantages: Firstly, by introducing an auxiliary winding, the overload protection, torque output and fault-tolerant capability can be improved significantly. Secondly, the dynamic performance and anti-perturbation capability of the low speed rotor can be enhanced by introducing a zero-variation control strategy applied to auxiliary winding. The validity of the control strategy and structure design is proved by the co-simulation of control program and ANSYS Maxwell. 1. Introduction Magnetic gears (MGs) are an ideal alternative choice for replacing the mechanical gear box in driving system of many low-speed large-torque applications since they can provide inherent overload protection ability, low working noise and physical isolation between rotors [1]. However, for most MGs, since the low speed rotor (LSR) in MG is driven by the high speed rotor (HSR) solely, which means the output torque is determined by the space angle difference between the resultant magnetic field vector (RMFV) generated by PMs on two rotors. Hence, compared to general PI (Proportional-Integral) controller, the conventional MG can be regarded as a PM machine applied the proportional controller to control the output rotor since output torque is determined by relative position of RMFV solely. In such case, the MGs have the drawbacks as the PM machine applied proportional controller, including high steady state error, low anti-interference capability and low controllability. Besides, the overload protection and torque output capability of conventional MGs are limitted, which means the designed power should be increased for the requirement of extreme working condition and it will increase the volume of MG inevitably. Furthermore, conventional MG is only driven by external electric machine, which means it will stop operating if the external motor or transmission shaft is broken, and even result in serious consequences. In this paper, a novel active magnetic gear is proposed for solving those problems mentioned above. 2. Working Principle and Machine Structure The overall structure of the proposed active magnetic gear is shown in Fig. 1(a). An auxiliary winding is palced among modulators. Fig. 1 (b) shows the corresponding coil-emf vectors in electrical degrees for the auxiliary winding, whose number of slots is 27. The total axial length, outer radius, inner radius, rated operation speed for the HSR and LSR are 220 mm, 120 mm, 65.8 mm, 2400 rpm and 300rpm, respectively. After introducing the auxiliary winding, the output torque Toutput can be expressed as: TOutput=TAuxiliary + TMG (1) where TMG and TAuxiliary are the torques generated by HSR and auxiliary winding, respectively. In this paper, the variation control strategy will be applied for enhancing the dynamic performance and anti-perturbation capability, which uses the variation of LSR speed and torque as the control objective. This is because when an output torque or operating speed is stable, the variation of it should be equals to zero nearly. The space vector diagram of AMG is indicated in Fig.1. (c). The ψfL and the ψfH are the flux linkage vectors generated by LSR and HSR, while ψs is that produced by the auxiliary winding. In such case, the torque generated by the auxiliary winding can be described as: TAuxiliary =p[ψfLiq+(Ld-lq)idiq] (2) Since the electromagnetic torque is generated by the interaction between magnetic fields produced by LSR and the summation of auxiliary winding and HSR, hence, in this formula, the flux linkage ψfL in this formula is that of LSR and the p is the pole pairs of LSR. 3. Performance Comparisons and Analysis The output torques with and without the excitation of auxiliary winding are shown in Fig. 2(a). The auxiliary winding can increase the output torque, reaching 1733.2 Nm compared to that without auxiliary winding 1531.9 Nm, all with the current density of 6A/mm2 and slot package factor 0.6. Besides, The AMG torque performances with different values of current density driven by HSR and auxiliary winding, and by auxiliary winding solely are indicated in Fig.2 (b). For the AMG driven by HSR and auxiliary winding, the output torque can increase from 1733 Nm to 2657 Nm with the current density from 6 to 50A/mm2, which shows a strong overload protection capability. As for the fault condition, the AMG will be driven by auxiliary winding solely and the output torque can grow from 242 Nm with the current density 6A/mm2 to 1751 with 50A/ mm2. The load change and given speed change situation with and without the zero-variation control strategy are shown in Fig. 3 (c) and (d) under undamped situation. Simulation results indicate that the speed and torque will become stable within 0.25 second with the control strategy when working condition change, which means the proposed control strategy can improve the dynamic performance of LSR and enhance anti-perturbation capability compared to traditional MGs. 4 Conclusion This paper proposes a novel active MG with an auxiliary winding. First, the structure, working principle and control strategy are introduced. Then, the performance of the proposed structure and control strategy is conducted. Finally, the simulation results verify the efficiency of the novel structure and control strategy.

**References:**

[1] S. Niu, N. Chen, S. L. Ho, and W. N. Fu. "Design optimization of magnetic gears using mesh adjustable finite-element algorithm for improved torque." IEEE Trans. Magnetics, vol.48, no.11, pp. 4156-4159, 2012.

**Images:**

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 Design and Analysis of a Novel Active Magnetic Gear for Low-Speed Large-Torque Applications

— 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)

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![](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.

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

Introduction In recent years, there has been an increase in requirements for motors, as well as its characteristics of high torque transmission, low vibration, low noise, maintenance free and overload protection. In particular, the coaxial magnetic gear (CMG) has become an attractive potential alternative to the traditional mechanical gear boxes [1][2]. Due to its complicated structure and multiple air gaps, however, the conventional radial type CMG is difficult to be manufactured and assembled, as a result its applications are restricted. The axial-radial magnetic gear (ARMG) could be a possible way to simplify such fabrication problems by putting the steel pole pieces around the outside of the two rotors instead of between them, but the longer flux paths between the two rotors lowers the torque and energy transmission. To overcome this drawback while retaining the advantages of the ARMG, this paper proposes a new axial-radial bevel magnetic gear (ARBMG), with new high-performance magnet additive manufacturing (or 3D printing) process to fabricate bevel shape structure. The design of the ARBMG not only shortens the total flux paths, but also increases the efficiency. These results are proved by the FEA simulation and measurement results of a prototype. Model Description and Simulation Fig. 1 shows the comparison between a traditional ARMG as well as an ARBMG with the same gear ratio. The pole pairs of the high-speed rotor (Rotor 1) are 4, while the low-speed rotor’s (Rotor 2) pole pairs are 13, and the number of steel pieces which surround the rotors on the outside are 15. It can be observed that the flux paths between the two rotors of ARBMG are shorter, as a result flux leakage is negligible therefore achieving higher efficiency. Fig. 2 (a) shows the assembly entity of ARBMG, and Fig. 2 (b) shows the ARBMG and ARMG efficiency at different input speeds with the same volume torque density, illustrating that the ARBMG has higher efficiency. Conclusion This paper proposes a new ARBMG with bevel type magnetic gear. With a bevel shaped magnet design, the new ARBMG has shorter flux paths, higher efficiency and much smaller structural volume when compared to the ARMG. Less flux leakage and higher efficiency are assumed, investigated and achieved via analytical as well as Finite Element Analysis (FEA) solutions. A prototype and experimental results based on magnetic 3D printing method are also provided to verify the proposed design.

**References:**

[1] W. Bomela, J. Z. Bird and V. M. Acharya, "The Performance of a Transverse Flux Magnetic Gear," in IEEE Transactions on Magnetics, vol. 50, no. 1, pp. 1-4, Jan. 2014, Art no. 4000104, doi: 10.1109/TMAG.2013.2277431. [2] Y. Li, J. Xing, Y. Lu and Z. Yin, "Torque Analysis of a Novel Non-Contact Permanent Variable Transmission," IEEE Transactions on Magnetics, vol. 47, no. 10, pp. 4465-4468, October 2011.

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 Design and Implementation of New Axial-Radial Bevel Magnetic Gear Based on Magnetic 3D Printing

I. Torque Density Limit of High Gear Ratio Coaxial Magnetic Gears Gearing system is a useful device used to get higher torque. Due to the advantages of non-contact power transmission and easy maintenance of magnetic gears, movement to apply them to gearing systems by replacing mechanical gears is becoming active. Early magnetic gears had a low torque density of 5 kNm/m3 and could not be considered as gearboxes [1]. However, after the coaxial magnetic gear (CMG) proposed by Atallah achieved a high torque density exceeding 100 kNm/m3, the total system torque density coupled with a motor has reached higher than the one motor, indicating the possibility of replacing the mechanical gears [1, 2]. Previous research has shown a high system torque density at a high gear ratio [2], and it is limited to the case where the magnetic gear shows the same torque density (about 100 kNm/m3) regardless of the gear ratio. However, it was observed that the higher the gear ratio, the lower the torque density [1-14]. Fig. 1 tendency shows that gear torque density is decreased when gear ratio is rising and the system torque density is increased up to specific gear ratio and decreased after that. In this paper, a series coupled magnetic gear is proposed to improve the torque density limit of high gear ratio CMGs. In addition, this structure shows reducing-loss effect, its superiority is demonstrated. II. Loss Issue and Series Coupled Coaxial Magnetic Gear In the high gear ratio model, the problem of not only torque density but also efficiency was confirmed. In this paper, a single CMG with a gear ratio of 63:1 was designed and the efficiency of 58.9% was confirmed with a torque density of 21.4kNm/m3. This is because it is accompanied by a rapid change in magnetic flux at once to show a high gear ratio. That is, each change must be magnetically separated while using the stepwise change of magnetic flux. Therefore, in this paper, a series coupled CMG is proposed. Fig. 2 shows a concept of the series coupled coaxial magnetic gear. The gear ratio of 63:1 was divided into 7:1 and 9:1, and each torque density is about 85kNm/m3 and 75 kNm/m3 according to Fig. 1. In addition, losses can theoretically be reduced by 3 and 2.6 times, respectively. In this paper, various series coupling methods were considered, and a model favorable for improving torque density and one for improving loss were finally selected. As a result of the study, the proposed series coupled CMG improves torque density of 3 times and reduces loss of 90% compare to the single high gear ratio CMG. The detailed design of the series coupled CMG will be presented in full paper. Acknowledgment: This research was supported by Korea Electric Power Corporation. (Grant number: R19XO01-34)

**References:**

[1] K. Atallah and D. Howe, “A novel high-performance magnetic gear,” IEEE Trans. Magn., vol. 37, no. 4, pp. 2844-2846, July. 2001. [2] K. Atallah, S. D. Calverley, and D. Howe, “Design, analysis and realisation of a high-performance magnetic gear,” IEE Proceedings - Electric Power Applications, vol. 151, no. 2, pp. 135-143, 2004. [3] Y. Wu, c. Chan, “A novel bicycle rear hub transmission with a magnetic gear mechanism,” Advances in Mechanical Engineering, vol. 10, no. 7, pp. 1-9, 2018. [4] M. Desvaux, B. Traulle, R. Latimier, S. Sire, B. Multon, H. Ahmed, “Computation Time Analysis of the Magnetic Gear Analytical Model,” IEEE Trans. Magn., vol. 53, no. 5, pp. 7000409, 2017. [5] M. Kowol, J. Kolodziej, M. Jagiela, M. Lukaniszyn, “Impact of Modulator Designs and Materials on Efficiency and Losses in Radial Passive Magnetic Gear,” IEEE Trans. Energy Conversion, vol. 34, no. 1, pp. 147-154, 2019. [6] H. Shin, J. Chang, “Comparison of the Characteristics in the Surface Mounted Permanent Magnet and Flux Concentrating Coaxial Magnetic Gears Having the Solid Cores,” J. Electr. Eng. Technol., vol. 13, no. 3, pp. 1275-1284, 2018. [7] Y. Chen, W. Fu, S. Ho, H. Liu, “A Quantitative Comparison Analysis of Radial-Flux, Transverse-Flux, and Axial-Flux Magnetic Gears,” IEEE Trans. Magn., vol. 50, no. 11, pp. 8104604, 2014. [8] D. Jang, J. Chang, “Effect of Stationary Pole Pieces with Bridges on Electromagnetic and Mechanical Performance of a Coaxial Magnetic Gear,” Journal of Magnetics, vol. 18, no. 2, pp. 207-211, 2013. [9] X. Zhang, X. Liu, Z. Chen, “A Novel Coaxial Magnetic Gear and Its Integration With Permanent-Magnet Brushless Motor,” IEEE Trans. Magn., vol. 52, no. 7, pp. 8203304, 2016. [10] X. Liu, Y. Zhao, X. Zhang, J. Gao, S. Huang, “Investigation of the Dynamic Characteristics of a Coaxial Magnetic Gear under Loading Condition Based on Analytical Model,” International conference on electrical machines and systems, 2017. [11] M. Filippini, P, Alotto, “An optimization tool for coaxial magnetic gears,” The international journal for computation and mathematics in electrical and electronic engineering, vol. 36, no. 5, pp. 1526-1539, 2017. [12] O. Molokanov. P. Dergachev, V. Kiruhin, P. Kurbatov, “Analyses and Experimental Validation of Coaxial Magnetic Planetary Gear,” International symposium on electrical apparatus and technologies, 2014. [13] E. Gobl, G. Jungmayr, E. Marth, W. Amrhein, “Optimization and Comparison of Coaxial Magnetic Gears With and Without Back Iron,” IEEE Trans. Magn., vol. 54, no. 11, pp. 8001604, 2018. [14] G. Jungmayr, J. Loeffler, B. Winter, F. Jeske, W. Amrhein, “Magnetic Gear: Radial Force, Cogging Torque, Skewing, and Optimization,” IEEE Trans. Industry Applications, vol. 52, no. 5, pp. 3822-3830, 2016.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/a3ca0046cbb1de078347b44c24d9f9ad.jpg)
Fig. 1. System torque density of preceding researches. (Using same torque density motors)

![](https://assets.underline.io/uploads/markdown_image/1/image/55229a7d699710e3bd380d3723a55ccd.jpg)
Fig. 2. Concept of the series coupled coaxial magnetic gear

 Series Coupled Coaxial Magnetic Gear Design for High Gear Ratio and High Torque Density

This paper shows that by using topology optimization, force density of a reluctance trans-rotary magnetic gear (TROMAG) can be improved significantly. TROMAG is a magnetic device that converts high-force, low-speed linear motion to low-torque, high-speed rotation, and vice versa [1]. The device is also referred to as magnetic screw [2] and magnetic lead screw [3]. As shown in Fig. 1a, a TROMAG consists of an inner and an outer component which are separated by an air gap. One component serves as a rotor and the other as a translator. The linear to rotary motion conversion is achieved via helically-shaped permanent magnet (PM) poles. TROMAG can offer large values of shear stress and gear ratio; resulting in light-weight and compact designs. In addition, similar to other types of magnetic gears, TROMAG offers low noise and high reliability. Advantages of TROMAG have made it a strong contender for a wide range of high force, low speed applications. Helical PM poles can assume different magnetization patterns. Radially-magnetized TROMAG (RMT), axially-magnetized TROMAG (AMT), and Halbach-magnetized TROMAG (HMT) have been proposed. Another configuration, shown in Fig. 1b, is reluctance TROMAG in which one component is furnished by helical PM poles while the other component has a variable reluctance structure. Reluctance TROMAG offers less shear stress compared to configurations having PMs on both components. However, lack of PMs on the variable reluctance component makes it potentially advantageous, for example, in applications with a long stroke. Reluctance TROMAG is investigated in multiple works; with radially-magnetized PMs (R-RMT) in [2], with Halbach-magnetized PMs (R-HMT) in [3], and with axially-magnetized PMs (R-AMT) in [4]. In all these works, teeth in the variable reluctance component are of rectangular shape. This paper aims to find the optimum tooth shape using topology optimization. Noting that 2D approximation of TROMAG introduces no more than a few percent error in force calculation compared to the 3D model, the former is used for topology optimization. Topology optimization has been applied to different electromagnetic devices [5]-[8]. The essence of topology optimization is to divide the design region into small elements and use an optimization algorithm to assign to each element a set of specific materials in order to achieve the optimum topology. In [5] and [6], a genetic algorithm (GA) is used along with the so-called on/off method in which each element can be either air or a pre-defined material. Since elements are triangular or quadrilateral, resultant topologies may have sharp corners. Hence, in [7], topology optimization is performed in two main stages. First, GA and on/off method are used to run a global search and obtain the optimum topology. Then, the level set approach which is gradient based is employed to smooth the corners and create a more practical design. Recently, a topology optimization method based on the normalized Gaussian network (NGnet) has been presented [8]. Here, the on/off method is applied to Gaussian circles mapped to the design region. The method is incorporated in JMAG-Designer software, which is employed in this paper for optimizing the reluctance TROMAG. First, the three reluctance TROMAG configurations, R-RMT, R-AMT, and R-HMT with rectangular teeth are optimized by sweeping teeth dimensions while keeping other design variables fixed. For all cases the results of which are presented here, the pole pitch is 10 mm, PM thickness along radial direction is 10 mm, and air gap length is 1 mm. Optimum values of shear stress for the three designs are 82, 95, and 119 kN/m2 respectively. Then, JMAG is used to apply topology optimization coupled with genetic algorithm to the three configurations. The design region has been divided into quadrilateral elements. Each element area is 0.25 mm2 and the number of circles in the design region is set to 21. The optimization objective is to maximize the shear stress. Fig. 2 shows two cases resulted from topology optimization of R-RMT, offering largest values of shear stress. It was revealed that 1) For R-RMT, trapezoidal tooth shape shown in Fig. 2a offers 10% higher, and elliptical tooth shape with non-magnetic material yoke, shown in Fig. 2b, offers 30% higher shear stress compared to R-RMT with optimized rectangular tooth. 2) Optimum shape for R-AMT is rectangular; i. e., topology optimization algorithm converges to a rectangular shape. 3) For R-HMT, trapezoidal tooth shape offers 5% higher shear stress, and elliptical tooth shape with non-magnetic material yoke offers almost the same shear stress compared to R-HMT with optimized rectangular teeth. Note that although the case of R-HMT with elliptical tooth shape and non-magnetic material yoke does not offer an appreciable increase in shear stress, it has the advantage of replacing iron yoke of the variable reluctance structure with non-magnetic material, resulting in significant weight reduction. The full paper will include detailed topology optimization results for the three configurations.

**References:**

[1] S. Pakdelian, N. W. Frank and H. A. Toliyat, IEEE Trans. Magn., vol. 49, no. 2, pp. 883-889, Feb. 2013. [2] H. M. Mohamed and K. Atallah, IEEE Trans. Magn., vol. 55, no. 7, pp. 1-4, July 2019, Art no. 8001504. [3] N. I. Berg, A. B. Christiansen, R. K. Holm and P. O. Rasmussen, in Proc. Int. Elect. Mach. Drives Conf., 2017, pp. 1–8. [4] M. Cirolini, A. F. Flores Filho, Y. C. Wu and D. G. Dorrell, IEEE Trans. Magn., vol. 55, no. 7, pp. 1-6, July 2019, Art no. 8001906. [5] Chang-Hwan Im, Hyun-Kyo Jung and Yong-Joo Kim, IEEE Trans. Magn., vol. 39, no. 5, pp. 2163-2169, Sep. 2003. [6] N. Takahashi, T. Yamada, S. Shimose and D. Miyagi, IEEE Trans. Magn., vol. 47, no. 5, pp. 1262-1265, May 2011. [7] Y. Hidaka, T. Sato and H. Igarashi, IEEE Trans. Magn., vol. 50, no. 2, pp. 617-620, Feb. 2014, Art no. 7015204. [8] T. Sato, K. Watanabe and H. Igarashi, IEEE Trans. Magn., vol. 51, no. 3, pp. 1-4, March 2015, Art no. 7202604.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/3d37b9293d2c321253fe27683c82b738.jpg)
Fig. 1. 3D views of (a) RMT; and (b) Reluctance RMT.

![](https://assets.underline.io/uploads/markdown_image/1/image/b7f3a050a3b91a9f8e66932038ee6c7c.jpg)
Fig. 2. 2D views of promising R-RM topologies; (a) trapezoidal tooth; (b) elliptical tooth with non-magnetic yoke. Topologies are created by JMAG and reproduced in ANSYS Maxwell.

 Topology Optimization of the Reluctance Trans-Rotary Magnetic Gear

—This paper proposes a novel hybrid excitation alternate stator PM dual stator/rotor armature winding vernier machine with the DC-biased current. Due to the both armature windings and alternate stator PM interact with each other, this proposed machine contain six torque components. Compared with traditional stator permanent magnet machines and flux modulated machines, this machine makes full use of the space in rotor cavity, and giving full play to the flux modulation effect. This digest will introduce the coil-connection, current configuration, and the simulation results, such as flux density, back-EMF, and electromagnetic torque, which are predicted by finite element analysis (FEA). Index Terms—Hybrid excitation, DC-biased current, Vernier machine (VM), Dual stator/rotor armature winding. I. Introduction Recently, stator permanent magnet (PM) machines are gaining more and more research attention for their benefit of a robust rotor structure[1]. In the meantime, the flux modulated machines due to the natural high torque density characteristic are gaining more research interest, too [2]. Combining the advantages of both machines, the DC-biased hybrid excitation stator PM vernier machines (HEPMVMs) are proposed [3]. Compared with the stator PM machines, the DC-biased HEPMVMs improve the shortcomings of the inherent limited magnetic field adjustment ability, meanwhile, improve the torque density. However, these machines don't make full use of the space in the rotor cavity, and the reliability of torque output is poor, due to only contain stator armature winding. In this paper, the topology of DC-biased hybrid excitation alternate stator PM dual stator/rotor armature winding vernier machine (DAWVM) is proposed. As shown In Fig. 1, the stator has two phase DC-biased winding and the teeth located PMs alternately, the rotor has three phase winding. Therefore, high torque will be generated. The drive circuit, operation principle, and the electromagnetic characteristics will be illustrated in the full paper. II.Electromagnetic Performance Evaluation The simulation results of flux density, back-EMF, and electromagnetic torque are analyzed by the finite element analysis (FEA). The no-load flux distribution is shown in Fig. 2. The back-EMF and electromagnetic torque with various excitation modes are shown in Fig. 3. This machine exhibits larger torque due to contain six torque components, which will be illustrated in the full paper.

**References:**

[1] Z. Zhu, H. Hua, D. Wu, J. Shi, and Z. Wu, IEEE Trans. Ind. Appl., vol. 52, no. 1, pp. 199–208, Jan./Feb. 2016. [2] S. Jia, R. Qu, W. Kong, D. Li, and J. Li, IEEE Trans. Ind. App., vol. 54, no.2, Jul./Aug., pp:3187-3196, 2018. [3] S. Jia et al., IEEE Trans. Ind. Appl., vol. 54, no. 2, pp. 1339-1348, March-April 2018.

**Images:**

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Analysis of Split-Tooth Stator-Slot Permanent-Magnet Machines with Different PM Arrangements

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