France



Interior permanent magnet synchronous motor (IPMSM) are widely used for industrial and traction applications. IPMSM&#39;s are known for high efficiency, reliability, and power density. For certain applications, like traction in hybrid electric vehicles, a high torque and power density with wide range of speed is of high importance. The efficiency of the motors can be increased in two ways: (1) increasing the output torque of the motor or (2) reducing the losses of the motor. The torque of various motors has been improved by introducing dual rotor structure and dual excitation using the unused space in electrical machine [1, 2]. The authors in [3] compares number of electrical motors. A quantitative analysis of three types of dual stator flux modulation permanent magnet (PM) motors including magnetic geared motor, and vernier machine. High torque density has been achieved in [4] for double stator brushless PM motors. The design has also been optimized based on split ratio to achieve maximum torque density. Recently, a lot of research has been carried out on vernier permanent magnet motors (VPMMs). The authors proposed a dual stator VPMMs and performs a comparative analysis of proposed design with regular VPM [5]. The proposed design achieved high efficiency and torque density and lower cogging torque. Similarly, power factor based analytical and FEM analysis have been provided in [6] on dual stator VPMM. A high torque density VPMM with dual stator topology has been proposed in [7]. This draft proposes a drum type configuration of dual stator IPMSM with improvement in the torque density, rotor alignment and inner winding temperature compared to single stator IPMSM by utilizing the unused space in the motor.&lt;br/&gt;The new design of dual stator IPMSM with 16-poles and 72-slots in each stator are presented with the aim of improving the average torque and torque ripple for industrial and traction applications. The presented design is then compared with the commercial design of single stator IPMSM to show the direct and fair comparison. Fig. 1 (a) shows the proposed design (PD) and conventional design (CD). The CD uses tangential type rotor with rare-earth PMs mounted inside the surface of rotor. The conventional IPMSM offers a good torque capability, but this torque capability can be improved by utilizing the unused space of the rotor. For this purpose, another stator with the same number of stator slots has been added to replace the unused space of rotor resulting in a proposed dual stator IPMSM. To increase the torque of the proposed models, the inner stator is shifted by a half slot pitch to aid the flux lines of outer stator with the inner stator. As the slots of inner stator are aligned with the outer stator-teeth, hence the slot area of an inner stator is somewhat smaller compared to the outer stator-slots. The number of turns of the inner stator-slots are double compared to the outer stator-slots to reduce the current density and inner winding temperature.&lt;br/&gt;Due to the addition of an inner stator, the cup rotor is the suitable. However, the cup type rotor configuration decreases the rotor alignment because of only one bearing towards the rotor shaft. Hence, in this draft, the drum type rotor is introduced comprising of two bearing on both sides of stator to support the inner stator shaft and to improve the alignment of the rotor. Furthermore, to reduce the inner winding temperature, the curve-windows are introduced in the drum configuration for the air inlet to reduce the inner winding temperature as presented in Fig. 1 (b).&lt;br/&gt;The analysis is performed on the conventional and proposed designs to show the competency of the proposed model. The Fig. 2 (a), and (b) shows the back EMF of the inner and outer stator. Moreover, the back EMF of the conventional design is identical to the outer stator back EMF of the proposed model due to same geometry of outer stator. Moreover, the Fig. 2 (b) shows the substantial improvement in the average torque and torque ripple of the proposed model compared to the conventional design.

**References:**

[1] A. Toba and T. A. Lipo, &quot;Generic torque-maximizing design methodology of surface permanent-magnet vernier machine,&quot; vol. 36, no. 6, pp. 1539-1546, 2000.&lt;br/&gt;[2] F. Zhao, T. A. Lipo, and B. Kwon, &quot;Magnet flux focusing design of double stator permanent magnet vernier machine,&quot; in &lt;i&gt;Proc. COMPUMAG&lt;/i&gt;, 2013.&lt;br/&gt;[3] C. Liu, K. Chau, J. Zhong, W. Li, and F. J. I. T. o. a. s. Li, &quot;Quantitative comparison of double-stator permanent magnet vernier machines with and without HTS bulks,&quot; vol. 22, no. 3, pp. 5202405-5202405, 2011.&lt;br/&gt;[4] Y. Wang, M. Cheng, M. Chen, Y. Du, and K. J. I. E. p. a. Chau, &quot;Design of high-torque-density double-stator permanent magnet brushless motors,&quot; vol. 5, no. 3, pp. 317-323, 2011.&lt;br/&gt;[5] Y. Gao, R. Qu, D. Li, H. Fang, J. Li, and W. J. I. T. o. M. Kong, &quot;A novel dual-stator Vernier permanent magnet machine,&quot; vol. 53, no. 11, pp. 1-5, 2017.&lt;br/&gt;[6] Y. Shi and T. J. I. T. o. M. Ching, &quot;Power Factor Analysis of Dual-Stator Permanent Magnet Vernier Motor with Consideration on Turn-Number Assignment of Inner and Outer Stator Windings,&quot; 2020.&lt;br/&gt;[7] N. Baloch, B.-i. Kwon, and Y. J. I. T. o. M. Gao, &quot;Low-cost high-torque-density dual-stator consequent-pole permanent magnet Vernier machine,&quot; vol. 54, no. 11, pp. 1-5, 2018.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e08dd9b8ef2a303357bf995cb72dbe36.png)
Fig. 1. (a) Geometry of the proposed and conventional designs; (b) Proposed dual stator drum type rotor; (c) Configuration of the inner and outer stator with the inverter.

![](https://assets.underline.io/uploads/markdown_image/1/image/5d21bfb823abd43232cbdc236793432a.jpg)
Fig. 2. Back EMF of the (a) Inner Stator; (b) Outer Stator; and (c) torque comparison.

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 Dual Stator Drum Rotor Interior Permanent Magnet Synchronous Motor with Improvement in Torque Density, and Rotor Alignment

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Interior permanent magnet synchronous motor (IPMSM) are widely used for industrial and traction applications. IPMSM's are known for high efficiency, reliability, and power density. For certain applications, like traction in hybrid electric vehicles, a high torque and power density with wide range of speed is of high importance. The efficiency of the motors can be increased in two ways: (1) increasing the output torque of the motor or (2) reducing the losses of the motor. The torque of various motors has been improved by introducing dual rotor structure and dual excitation using the unused space in electrical machine [1, 2]. The authors in [3] compares number of electrical motors. A quantitative analysis of three types of dual stator flux modulation permanent magnet (PM) motors including magnetic geared motor, and vernier machine. High torque density has been achieved in [4] for double stator brushless PM motors. The design has also been optimized based on split ratio to achieve maximum torque density. Recently, a lot of research has been carried out on vernier permanent magnet motors (VPMMs). The authors proposed a dual stator VPMMs and performs a comparative analysis of proposed design with regular VPM [5]. The proposed design achieved high efficiency and torque density and lower cogging torque. Similarly, power factor based analytical and FEM analysis have been provided in [6] on dual stator VPMM. A high torque density VPMM with dual stator topology has been proposed in [7]. This draft proposes a drum type configuration of dual stator IPMSM with improvement in the torque density, rotor alignment and inner winding temperature compared to single stator IPMSM by utilizing the unused space in the motor. The new design of dual stator IPMSM with 16-poles and 72-slots in each stator are presented with the aim of improving the average torque and torque ripple for industrial and traction applications. The presented design is then compared with the commercial design of single stator IPMSM to show the direct and fair comparison. Fig. 1 (a) shows the proposed design (PD) and conventional design (CD). The CD uses tangential type rotor with rare-earth PMs mounted inside the surface of rotor. The conventional IPMSM offers a good torque capability, but this torque capability can be improved by utilizing the unused space of the rotor. For this purpose, another stator with the same number of stator slots has been added to replace the unused space of rotor resulting in a proposed dual stator IPMSM. To increase the torque of the proposed models, the inner stator is shifted by a half slot pitch to aid the flux lines of outer stator with the inner stator. As the slots of inner stator are aligned with the outer stator-teeth, hence the slot area of an inner stator is somewhat smaller compared to the outer stator-slots. The number of turns of the inner stator-slots are double compared to the outer stator-slots to reduce the current density and inner winding temperature. Due to the addition of an inner stator, the cup rotor is the suitable. However, the cup type rotor configuration decreases the rotor alignment because of only one bearing towards the rotor shaft. Hence, in this draft, the drum type rotor is introduced comprising of two bearing on both sides of stator to support the inner stator shaft and to improve the alignment of the rotor. Furthermore, to reduce the inner winding temperature, the curve-windows are introduced in the drum configuration for the air inlet to reduce the inner winding temperature as presented in Fig. 1 (b). The analysis is performed on the conventional and proposed designs to show the competency of the proposed model. The Fig. 2 (a), and (b) shows the back EMF of the inner and outer stator. Moreover, the back EMF of the conventional design is identical to the outer stator back EMF of the proposed model due to same geometry of outer stator. Moreover, the Fig. 2 (b) shows the substantial improvement in the average torque and torque ripple of the proposed model compared to the conventional design.

**References:**

[1] A. Toba and T. A. Lipo, "Generic torque-maximizing design methodology of surface permanent-magnet vernier machine," vol. 36, no. 6, pp. 1539-1546, 2000. [2] F. Zhao, T. A. Lipo, and B. Kwon, "Magnet flux focusing design of double stator permanent magnet vernier machine," in Proc. COMPUMAG, 2013. [3] C. Liu, K. Chau, J. Zhong, W. Li, and F. J. I. T. o. a. s. Li, "Quantitative comparison of double-stator permanent magnet vernier machines with and without HTS bulks," vol. 22, no. 3, pp. 5202405-5202405, 2011. [4] Y. Wang, M. Cheng, M. Chen, Y. Du, and K. J. I. E. p. a. Chau, "Design of high-torque-density double-stator permanent magnet brushless motors," vol. 5, no. 3, pp. 317-323, 2011. [5] Y. Gao, R. Qu, D. Li, H. Fang, J. Li, and W. J. I. T. o. M. Kong, "A novel dual-stator Vernier permanent magnet machine," vol. 53, no. 11, pp. 1-5, 2017. [6] Y. Shi and T. J. I. T. o. M. Ching, "Power Factor Analysis of Dual-Stator Permanent Magnet Vernier Motor with Consideration on Turn-Number Assignment of Inner and Outer Stator Windings," 2020. [7] N. Baloch, B.-i. Kwon, and Y. J. I. T. o. M. Gao, "Low-cost high-torque-density dual-stator consequent-pole permanent magnet Vernier machine," vol. 54, no. 11, pp. 1-5, 2018.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e08dd9b8ef2a303357bf995cb72dbe36.png)
Fig. 1. (a) Geometry of the proposed and conventional designs; (b) Proposed dual stator drum type rotor; (c) Configuration of the inner and outer stator with the inverter.

![](https://assets.underline.io/uploads/markdown_image/1/image/5d21bfb823abd43232cbdc236793432a.jpg)
Fig. 2. Back EMF of the (a) Inner Stator; (b) Outer Stator; and (c) torque comparison.

technical paper

I. Introduction Recently, compressed air has been used in many applications and compressors tend to high energy efficiency and high-power density, which becomes a hot issue and draws more and more interest. As a powerful candidate, permanent magnet (PM) machine including PM located on rotor and stator is an excellent candidate for the compressors [1]. However, due to the increase of rotation speed and rotor diameter, the rotor integrity for PM located on rotor becomes a critical issue. Although bandings have been installed in rotor, they inevitably increase the air-gap width and rotor loss and hence reduce the torque capability of the machine. Meanwhile, the electrical machines driven compressors has challenges in machines cooling, sizing and loss. The PM located on the stator, namely the flux-switching PM machines (FSPMMs), have been an intense topic of research for the high-speed compressors. And the torque density is comparable to the conventional rotor-PM machines. By combining the armature winding and excitation source in the stator, its rotor is a robust steel sheet lamination, a passive single material and salient pole rotor, making it inherently suitable for high-speed applications. Hence, there is a feasible solution to integrate the machine with the compressor, which combines the function of motor drive and compressor. In additional, the self-cooling capability of this topology is another important advantage due to its forced convection flow to the stator, rotor, magnet and winding generated by the rotor rotation, especially in high power density applications, which is robust in harsh environment. Even though the conventional topology (12/10 FSPMMs) can achieve more symmetrical phase back-EMF waves, it is not optimistic in terms of core loss under high-frequency excitation. A feasible design with the lowest possible slot-poles (6/4 FSPMMs) for three-phase windings is proposed by Li et al. [2], which possesses the lowest fundamental frequency for the given speed. The purpose of this paper gives a suitable structure for high-speed axial flow compressors based on the topology of FSPM machines. In order to minimize the size of compressor system, a design of dual-stator 6/4 FSPM machine-compressor is proposed. The electromagnetic performance of C-core and E-core topology is calculated and analyzed by the 3-D finite element analysis (FEA). The design is electromagnetically and aerodynamically coupling since the structure of rotor has significant effects on both the machine and compressor. For better fitting high-speed operation and generating compressed air, the rotor with asymmetric pole-width is used and optimized. II. Machines Topology The FSPM machines are attractive in ultra-high-speed application due to its robust rotor. A specification of 10 kW, 30 krpm dual-stator FSPM machines is designed and analyzed by 3-D FEA. For given speed, the dual-stator structure is a promising solution to minimize the fundamental frequency of the source system. Fig. 1 (a) and (b) show the topology of C-core and E-core stator of FSPM. The electromagnetic performance of two stator topology is analyzed and compared such as average torque, torque ripple and over load capability. As shown in Fig. 1 (c), the front rotor is offset-ed by 45° mechanically with respect to the rear rotor and the even harmonics are counteracted in the total flux linkage based on this topology. In order to better air flow, the asymmetric rotor poles are used in the integrated 6/4 FSPM machines. The parameters of the rotor are optimized in terms of electromagnetics and aerodynamics. III. Results The design of integrated machines-compressor based on the lowest slot-pole numbers is proposed. By the FEM analysis, the electromagnetic and aerodynamic performance of the proposed machines is calculated and compared. Fig. 2 (a) and (b) show the 3-D flux density distribution of the C-core and E-core 6/4 FSPM machines at no-load condition. Fig.2 (c) show the torque characteristic of two FSPM machines with different stator cores at on-load condition. The torque ripple of the machine with E-core stator reduce 25.7% than the one with C-core stator. The optimized E-core FSPM machine-compressor is verified to generate better torque characteristic and overload capability. Furthermore, after optimizing rotor topology, the E-core stator FSPM machines is a good candidate for axial-flow compressors, especially in higher rotation speed.

**References:**

[1] Gerada D, Mebarki A, Brown N L, et al. High-speed electrical machines: Technologies, trends, and developments[J]. IEEE Transactions on Industrial Electronics, 2013, 61(6): 2946-2959 [2] Liu M , Li Y , Sixel W , et al. Design and Testing of Low Pole Dual-stator Flux-Switching Permanent Magnet Machine for Electric Vehicle Applications[J]. IEEE Transactions on Vehicular Technology, 2019, PP(99):1-1.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/65a9694b1204d088dfcbc8e8506afb68.jpg)
Fig. 1. Topology of 6/4 FSPM machines. (a) C-core dual-stator. (b) E-core dual-stator. (c) Design parameters of rotor .

![](https://assets.underline.io/uploads/markdown_image/1/image/6a40f9af98ca6d5787928acb0d498802.jpg)
Fig. 2. The performance characterization of the FSPM machines. (a) Flux density distribution of C-core FSPMMs at no-load condition. (b) Flux density distribution of E-core FSPMMs at no load condition. (c) The torque characteristic of the machines with different stator cores.

 Design and Analysis of Dual-Stator Flux-Switching Permanent Magnet Machine-Compressor with Asymmetric Rotor Poles

For many low-speed electrical machine applications it is usually cost effective to employ a high-speed machine together with a mechanical gearbox. This can also minimise system size and weight. The gearbox however brings disadvantages in terms of reliability, acoustic noise, mechanical vibration and the need for lubrication and maintenance. For the case of offshore wind turbines with high service life the disadvantages are compounded by the high cost of gearbox replacements. To eliminate these concerns in offshore wind applications, it is commonplace to delete the gearbox and use a direct drive machine, but care is required to ensure the size, mass and cost of the system do not become unfeasible. The Pseudo Direct Drive (or PDD®) is a magnetic and mechanical integration of a magnetic planetary gear with a permanent magnet generator. A magnetic equivalent to a planetary gear is embedded within a permanent magnet generator. This integrated approach is used to reduce the size and mass of the machine without recourse to a mechanical gearbox. By doing this the disadvantages of using a mechanical gearbox are avoided, but the size of the generator is minimised. This paper discusses the design, build and test results of a 200,000Nm, 500kW PDD machine shown in Fig 1. This is the world’s largest PDD machine and serves as a technology demonstrator for a larger 10MW project, providing validated data for this new class of machine at this size. The discussion presented includes the principle of operation of magnetically geared machines, and specifically how this is applied in the design of very large machines. The magnetic gear embedded in the generator is described and the equivalence of a mechanical planetary system with analogous components (sun, planets and ring) is detailed. The high torque is reacted magnetically and there is no mechanical contact between the shafts. The inclusion of this single stage magnetic gear gives the same uplift in performance to the generator system, i.e. increased rotor speed, reduced airgap torque and increased efficiency whilst allowing the size and mass of the generator to be minimised. The key design features of a highly integrated magnetic gear are discussed:- ● The magnetic components service both the generator and the gear; hence the losses can be assumed to be shared and minimised, leading to very high system efficiencies ● The compliant nature of the gear leads to the attenuation of drive train oscillations and reduces acoustic noise. ● There is no lubrication of the gear element (except bearings) and the service interval can be extended accordingly. ● The integrated magnetic gear is placed in the centre of the machine and does not increase the overall volume envelope. ● Small size means higher power machines may be built with existing facilities designed for direct drive generators. This paper gives quantitative experimental data of a 500kW, 200,000Nm magnetically geared PDD wind turbine generator that was designed, built and tested as part of the DemoWind Programme funded under the European Research Area Network. Key operating data is shown, including verified electromagnetic parameters. Component thermal traces verify the thermal and operation models with an exemplar case presented at 80% load showing a 70°C rise in winding temperature. Dynamic operating data is presented as a measured 10minute recorded trace of speed, torque, current and electrical power whilst the machine is operated with a real-world input profile example at nominal 10m/s wind speed conditions, this verifies the performance of the machine in realistic wind conditions. The results presented confirm both the accuracy of the underlying analysis models and the suitability of the generator system as a wind turbine machine. Finally, the verified data is used to provide Levelized Cost of Energy (LCOE) analysis for a 10MW system (performed independently). The LCOE calculations are based on OPEX, CAPEX and Energy production models comprising 17 sub-categories upon which the influence of the PDD efficiency, mass, CAPEX and operational parameters can be individually evaluated. The results show a reduction in LCOE of 2.8% and 2.5% when the PDD system is compared to the equivalent direct-drive and mid-speed geared systems respectively as detailed in Fig.2

**References:**



**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/2fc5625af5504e957c7e8d3a8a5b8a8a.jpg)
Fig. 1. 200,000Nm, 500kW PDD generator on test

![](https://assets.underline.io/uploads/markdown_image/1/image/53494368d068af346a6f950f995e5d77.jpg)
Fig. 2. Source of LCOE savings of PDD compared to alternative drivetrains

 Design, Build and Test of a Magnetically Geared Generator for Wind Turbine Applications INVITED

Tidal turbine generators are submerged in water, which means reliability is a critical criterion, arguably even more than efficiency. The objective of this paper is to discuss the main electromagnetic design aspects of a flooded permanent magnet (PM) direct-drive generator for tidal turbines. In a flooded generator design, the stator-rotor gap is filled with sea water instead of air, as shown in Fig. 1. The reasons to consider flooded direct-drive generators are to get rid of highly pressurized rotary seals that keep the water outside the nacelle which holds the generator; additionally, a flooded design could also improve cooling. This could result in a higher power density generator and/or potentially better performance. As a ramification of the flooded design, however, protective sleeves (also called cans) are needed to shield the magnets, iron and windings from sea water [1]. The paper mainly focuses on the following three aspects of the generator, which could provide useful guidelines for the design of flooded generators. I. WINDING TYPE SELECTION Fractional slot windings are cheaper and easier to manufacture than distributed windings [2]. In large direct-drive generators, it may also result in lower copper losses, improving the lifetime cost of the generator. However, because of the higher space harmonic content of the fractional slot windings, they could cause high eddy current losses in the rotor magnets and the rotor sleeve if it is conductive [3]. In contrast, a distributed winding is costlier to manufacture, especially in a flooded design due to larger end windings, but contains much lower space harmonics. These aspects need to be carefully considered while selecting a winding layout for the flooded generator. II. SLEEVE MATERIAL SELECTION For the stator, a non-conductive sleeve is strongly preferred. Even at no-load, the losses in a conductive stator sleeve due to rotating magnets would be too high for a reasonable design. On the other hand, for the rotor a conductive sleeve may be acceptable. In this study, we compared the eddy-current losses in the rotor sleeve of a flooded generator with different sleeve materials, ranging from highly conductive metallic materials, such as stainless steel to non-conductive glass fiber. Whereas the glass fiber has no loss, it is not as robust against water ingression as metallic materials [4]. Consequently, a thicker layer of glass fiber would be necessary compared to the metallic sleeve. This would increase the volume of the magnets required, increasing the generator cost. Thus, the selection of sleeve material should be based on multiple aspects, such as losses in the sleeve, material strength, water-proofing properties, thermal performance of the generator, winding layout, etc. rather than just losses. III. THERMAL ANALYSIS The effect of flooding the generator on the temperature distribution is analyzed in this section. For this purpose, a flooded generator was compared with a similarly sized airgap generator (both are 300 kW at 30 rpm and both contain a rotor sleeve). Consequently, we assumed that the loss distribution in the airgap and the flooded generator is the same. Figure 2 shows the influence of flooding the stator-rotor gap on the magnet and the slot temperatures in a flooded and an airgap PM generator with fractional slot windings (non-negligible rotor losses). The figure also illustrates the effect of using materials with different electrical conductivities on the temperature distribution inside the machine. It is clearly seen that the materials with higher conductivity exhibit higher temperature in the magnets and in the stator slots on account of higher eddy current losses in the rotor. For the corresponding decrease in the magnet temperature on flooding, there is an increase in the temperature of the stator slot. This is because of the better heat transfer in the flooded stator-rotor gap compared to the airgap. Whether the net heat transfer is from the rotor-to-stator or vice-versa would depend on the losses in the rotor and the stator, rotor-to-ambient thermal path via the stator iron and rotor-to-ambient thermal via the shaft and bearings, etc [5]. In the case of glass fiber (GRP) sleeve, it is seen that the magnet temperature in the airgap design is more than the flooded design. This is because in this case magnet (or rotor) temperature is less than the slot (or stator) temperature in the airgap design. As a result, on flooding the heat transfer improves enhancing the heat transfer to the rotor, which increases the magnet temperature rather than decreasing it [5].

**References:**

[1] O. Krovel, R. Nilssen and S. Skaar, in Proceedings of ICEM, 2004. [2] A. El-Refaie, IEEE Transactions on Industrial Electronics, vol. 57, no. 1, pp. 107-121, 2009. [3] F. Wani, U. Shipurkar and H. Polinder, in 13th European Wave and Tidal Energy Conference, Napoli, Italy, 2019. [4] L. Bian, J. Xiao and J. Zeng, Journal of Composite Materials, vol. 46, no. 25, pp. 3151-3162, 2012. [5] F. Wani, J. Dong and H. Polinder, in 13th International Conference on Electrical Machines (ICEM), Alexandroupolis, Greece, 2018.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e77fcd839c90ba32859760e499dd9002.jpg)
Fig. 1. A representative diagram of a flooded permanent magnet generator (not drawn to scale).

![](https://assets.underline.io/uploads/markdown_image/1/image/98fb2f2d28af815d02e8628cf2f594ca.jpg)
Fig. 2. Temperature of the magnets and the slots is shown as a function of different rotor sleeve materials. Electrical conductivities of the materials are: Stainless steel 304L—1.30e6 S/m ; Inconel alloy 718—0.80e6 S/m ; Titanium SP700—0.61e6 S/m; Glass reinforced plastic (GRP)—0 S/m [5].

 Flooded Permanent Magnet Direct-drive Generator for Tidal Turbines INVITED

I.Introduction With the development of more electric aircraft (MEA), higher demand for electrical energy is put forward in generation systems. In order to reduce the weight of the aircraft generation system, the requirement on the power density of generator becomes higher [1]. Wound rotor synchronous machines (WRSMs) are most widely used in aircraft applications. However, the reliability of rotating rectifier is low. In addition, the starting processing is complex. Hybrid excitation synchronous machines (HESMs) are recently proposed as the potential candidates for EV drive systems, wind power generation and more electric aircraft, for the air-gap flux can be regulated by field current efficiently. This paper proposes a new hybrid excited starter-generator (HESG) with magnetic field decoupling topology for MEA applications. The predicted electromagnetic performance is validated by the 3-D finite-element analysis (FEA) model. Finally, the results of FEA are confirmed by the experiments on a 60kVA HESG. The contribution of this paper is to propose a promising HESG to replace WRSMs in aircraft starter-generator applications. II.Operation Principle and Machine Specification Table I presents the design requirements of a starter-generator for aircraft generation system. The structure of the HESG with decoupling magnetic field is shown in Fig. 1, it can be seen that the PM side air-gap is larger than EE side air-gap. In order to implement high air-gap flux density, the Halbach-array PM rotor is preferred for the flux concentrating characteristic, low harmonic components and low inductance characteristic. Fig. 2 shows the HESG topology. It can be seen that the PM rotor and the two EE rotors can be installed on the shaft together seamlessly, which can greatly improve the compactness, torque density and power density. As shown in Fig. 3, the magnetic flux path can be divided into axial part and radial part. There is no axial flux in the stator core so that the stator can keep the same with that in WRSM and brings sinusoidal back EMF, which makes it suitable for starter-generator system. Table II provides the main parameters of the designed HESG with decoupling magnetic field. III.Electromagnetic Performance Analysis In order to verify the applicability of the new HESG magnetic field decoupling topology for AC starter-generator system, the electromagnetic performance analysis mainly focus on the power characteristics and flux regulation capability. Fig. 4 shows the distribution of magnetic field with different field currents. As shown in Fig. 5, the back-EMF magnitude increases with field current. Fig. 6 shows the output voltage versus output current at 8000r/min for different field currents. It can be seen that the output characteristics can be regulated by field current. The output power versus speed for different field currents is shown in Fig. 7. A 60kVA prototype is manufactured and measured for the verification of previous analysis. The photos of the motor are shown in Fig. 8. The experimental results will be supplemented in the later formal paper. IV.CONCLUSION In this paper, a new HESG is proposed for aircraft generation system. The topology and operating principle are described in detail. In addition, the electromagnetic performance of HESG is investigated to verify the applicability of the HESM to a starter-generator system for aircraft applications.

**References:**

[1] Y. Liu, Z. Zhang and X. Zhang, "Design and Optimization of Hybrid Excitation Synchronous Machines With Magnetic Shunting Rotor for Electric Vehicle Traction Applications," in IEEE Transactions on Industry Applications, vol. 53, no. 6, pp. 5252-5261, Nov.-Dec. 2017.

**Images:**
![](https://assets.underline.io/uploads/markdown_image/1/image/d1a01027692aa59030478c3a38de0002.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/112fdd56d19723570dd66e8465c54e37.jpg)

 Electromagnetic Performance Analysis of a New Hybrid Excitation Synchronous Generator with Decoupling Magnetic Field

I. Introduction There are many studies on axial flux permanent magnet machines (AFPMMs) due to their compactness [1]. The conventional AFPMMs comprise a stator and two rotors. In general, the stator with toroidal windings or overlapping windings is sandwiched between two rotors [2][3]. Toroidal windings have shorter ends than overlapping windings, which can reduce copper loss and ending effect. However, this structure also wastes the ends of windings, which means that the windings are not utilized sufficiently. To some extent, it reduces the torque density of the machine. This paper proposes a novel axial-radial flux permanent magnet machine (ARFPMM) to solve the problem. In the ARFPMM, the endings of windings are also used to produce torque with little increase of volume, which results in the improvement of torque density. II. Structure of ARFPMM The proposed ARFPMM integrates both axial-flux and radial-flux machines in a compact space, which improves the torque density. As shown in Fig. 1(a), the ARFPMM is composed of one stator, two axial rotors and one radial rotor. Because the end of overlapping windings is useless, the machine adopts toroidal windings. Thus, the stator is equipped with opening slots on three faces. Permanent magnets are mounted on the surface of rotors. The radial rotor is fixed and aligned with two axial rotors, which can avoid the tilt of axial rotors and generation of harmonics in flux density per phase. The inner and outer diameters of permanent magnets on axial rotors are the same as those of stator. Similarly, the height of permanent magnets on radial rotor is equal to that of stator. In order to suppress air-gap flux density harmonics and get more sinusoidal waveform of air-gap flux [4], the permanent magnets are arranged as Halbach-array. It can also increase the flux density in the air gap. The magnetization direction of PMs is related to the number of segments per pole pair. As shown in Fig. 1(b), each pole pair is divided into six segments, so the magnetization direction of each segment varies 60° in turn. III. Design and Analysis Compared with integer-slot concentrated windings, fractional-slot concentrated windings can enhance flux-weakening capability and reduce cogging torque [5]. Therefore, a 20 pole 24 slot fractional-slot concentrated-winding machine is chosen. In the case, the winding factor is 0.966. It is related to the no load back electromotive force (EMF) which can be calculated as equation (1). E0=4.44fkwNΦ10 (1) Where f is the electric frequency of the machine, kw is the winding factor, N is the number of coils per phase in series, Φ10 is the magnetic flux. In the ARFPMM, three rotors are used to increase the magnetic flux in air gap. The result of 3D FEA for back EMF is shown in Fig. 2(a). Fig. 2(b) is the fast Fourier transform of back EMF. It shows that the maximum harmonic is the third order harmonic, which can be neglected due to the star-connection of three phase windings. Because the magnetic flux in air gap is increased under the same operating state, the torque of ARFPMM is also increased. Fig. 2(c) shows the torque of the machine at rated conditions when the current is 7 A and rotor speed is 600 rpm. In the condition, the average torque is 41.63 Nm and the torque density is 28.58 Nm/L. The torque ripple is 3.4 Nm, 8.1% of the rated torque. As shown in Fig. 2(c), the peak-peak value of cogging torque is 3.26 Nm. IV. Conclusion This paper proposes a novel ARFPMM with a unique structure of two axial rotors and one radial rotor, and implements its design and FEA. Due to the Halbach-array PMs, the main harmonics of back EMF is the third order harmonic, which is negligible when the windings are connected in star-connection. Compared with conventional AFPMM, the structure increases the torque through increasing the magnetic flux through windings per phase. Thus, the ARFPMM reaches an extremely high torque density of 28.58 Nm/L. On the other hand, fractional-slot concentrated winding configuration reduces the cogging torque, of which the peak-peak value is only 3.26 Nm. Acknowledge This work is supported in part by NSFC (52077186&51677159), in part by STIC of Shenzhen Municipality (JCYJ20180307123918658), and in part by ITF (ITP/027/19AP), HK.

**References:**

[1] W. Tong, S. Wang, S. Dai and S. Wu, et al, "A Quasi-Three-Dimensional Magnetic Equivalent Circuit Model of a Double-Sided Axial Flux Permanent Magnet Machine Considering Local Saturation," in IEEE Transactions on Energy Conversion, vol. 33, no. 4, pp. 2163-2173, Dec. 2018. [2] M. F. H. Khatab, Z. Q. Zhu, H. Y. Li and Y. Liu, "Comparative study of novel axial flux magnetically geared and conventional axial flux permanent magnet machines," in CES Transactions on Electrical Machines and Systems, vol. 2, no. 4, pp. 392-398, Dec. 2018. [3] M. Aydin and M. Gulec, "Reduction of Cogging Torque in Double-Rotor Axial-Flux Permanent-Magnet Disk Motors: A Review of Cost-Effective Magnet-Skewing Techniques With Experimental Verification," in IEEE Transactions on Industrial Electronics, vol. 61, no. 9, pp. 5025-5034, Sept. 2014. [4] Z. Song, C. Liu, K. Feng, H. Zhao and J. Yu, "Field Prediction and Validation of a Slotless Segmented-Halbach Permanent Magnet Synchronous Machine for More Electric Aircraft," in IEEE Transactions on Transportation Electrification, vol. 6, no. 4, pp. 1577-1591, Dec. 2020. [5] G. De Donato, F. Giulii Capponi, A. Rivellini and F. Caricchi, "Integer-slot vs fractional-slot concentrated-winding axial-flux permanent magnet machines: Comparative design, FEA and experimental tests," 2011 IEEE Energy Conversion Congress and Exposition, Phoenix, AZ, 2011, pp. 3120-3127.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/697cf24946ef7e94bfba61f933056aa6.jpg)
Fig. 1 Structure of ARFPMM. (a) Topology overview, (b) Halbach-array PMs

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Fig. 2 Back EMF and torque analyzed by FEA. (a) Back EMF waveform, (b) Fast Fourier transform of back EMF, (c) Electromagnetic torque and cogging torque

 Design of a Novel Axial-Radial Flux Permanent Magnet Machine with Halbach-array Permanent Magnets

I. Permanent magnet (PM) motors have become great alternatives to traditional motors. Overhang, either on the rotor or the stator, is commonly used is these motors. In this study, various alternatives of stator or rotor overhang in PM synchronous motors are investigated in detail. Both asymmetric and symmetric overhang cases are considered in the paper. Impact of overhang on the torque quality and torque densities are investigated using 3D Finite Element Analysis (3D-FEA). Axial magnetic forces during assembly with and without overhang are also simulated and variation of axial forces for different cases of overhangs are explored. A prototype PM motor with rotor overhang is used to validate the analyses experimentally. II. INTRODUCTION Rotor overhang is commonly used in different types of permanent magnet motors for numerous reasons. There are various studies in literature covering rotor overhang including magnetic modelling, 2D and 3D-FEA mostly for linear permanent magnet and induction motors, axial flux motors and brushless DC motors. Very limited studies exists on radial flux PM synchronous motors. Most studies are focused on analytical modelling of overhang and 2D/3D simulations [1-4]. However, no detailed investigation on both symmetric and asymmetric overhangs on the motor performance are conducted. Influence of torque quality, cogging and back-EMF harmonics are not covered especially for asymmetric overhang cases since some applications do require asymmetric placement of the rotor. In this paper, both asymmetric and symmetric overhang cases will be investigated in detail. Impact of such cases on the motor performance and torque quality are covered. Axial magnetic forces are also be explored for PM motors since it is a crucial issue in assembly of framed and frameless PM motors. III. INVESTIGATED PM MOTOR CASES AND FEA SIMULATIONS AND IV. EXPERIMENTAL VERIFICATION Detailed 3D-FEA is performed for different cases of overhang. The motor is selected as a low speed surface mounted permanent magnet synchronous motor as shown in Fig. 1. The prototype motor in a test system is also shown in Fig. 2. Various cases of overhang is investigated in great detail using 3D-FEA. Cases-1 does not have any overhang. Case-2 has rotor overhang and Case-3 has stator overhang. In each case, rotor stack is kept the same and torque quality as well as motor performance is explored. Axial magnetic forces acting on the rotor are also covered in 3D-FEA as seen in Fig3, 4 and 5. A prototype motor with rotor overhang is also manufactured and tested for different cases and some of the 3D-FEA simulations including axial magnetic forces are verified. The test results and comparison with the FEA simulations will also be given in the final version of the paper.

**References:**

V. References: 1. Yon-Do Chun;Ju Lee;S. Wakao,‘Overhang effect analysis of brushless DC motor by 3-D equivalent magnetic circuit network method’, IEEE Trans. Magn., vol. 39, pp. 1610–1613, May. 2003. 2. P. Upadhayay; K. R. Rajagopal, ‘Permanent magnet overhang effect in PM BLDC motor using 2D & 3D finite element analysis’, 2012 Nirma University International Conference on Engineering , India. 3. Jung-Moo Seo, In-Soung Jung, Hyun-Kyo Jung, and Jong-Suk Ro, ‘Analysis of Overhang Effect for a Surface-Mounted Permanent Magnet Machine Using a Lumped Magnetic Circuit Model’, IEEE Trans. Magn., vol. 50, Issue: 5, May. 2014. 4. Kyu-Yun Hwang;Hai Lin;Se-Hyun Rhyu;Byung-Il Kwon, ‘A Study on the Novel Coefficient Modeling for a Skewed Permanent Magnet and Overhang Structure for Optimal Design of Brushless DC Motor’, IEEE Trans. Magnetics., vol. 48, Issue: 5, May. 2012. 5. Han-Kyeol Yeo;Hyeon-Jeong Park;Jung-Moo Seo;Sang-Yong Jung;Jong-Suk Ro;Hyun-Kyo Jung, ‘Electromagnetic and Thermal Analysis of a Surface-Mounted Permanent-Magnet Motor with Overhang Structure’, IEEE Trans. Magnetics., vol. 53, Issue: 6, June 2017.

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 Impact of Asymmetric and Symmetric Overhangs on Torque Quality and Axial Magnetic Force Computations in Surface Mounted PM Synchronous Motors

1. Introduction The advantage of interior permanent magnet synchronous motor (IPMSM) is known for having high power density by utilizing not only magnetic torque but also reluctance torque. Accordingly, the IPMSM is used in various industrial components such as robots, electric vehicles (EVs), and home appliances to exert high-performance. The performances required for the industrial components are high power density and low torque ripple. The torque ripple is related to noise and vibration characteristics of the IPMSM. Thus, it is necessary to reduce the torque ripple when designing the IPMSM. Many design method has been reported for torque ripple reduction, such as rotor eccentricity, skew, notch, and chamfer. These methods are related to changing the length of the air-gap [1]. However, changing the air-gap length makes the average torque decrease because the effective air-gap length increases [2]. To reduce the torque ripple while maintaining the average torque, a design method of asymmetric rotor shape has been proposed [3]. The symmetric and asymmetric rotor shapes are illustrated in Fig. 1. The significant difference between the symmetric and asymmetric rotors is the effects of the rotating directions on the torque ripple. When applying the symmetric rotor, the torque ripple is constant regardless of the rotating directions. On the other hand, the torque ripple is different depending on the rotating directions when applying the asymmetric rotor. Since some applications such as an EV traction motor require rotating in the forward and reverse directions, it is necessary to consider the rotating directions in the design for the torque ripple reduction of an IPMSM. In this paper, the design of an IPMSM for an EV traction to reduce the torque ripple is performed considering the rotating directions of the asymmetric rotor. Unlike the conventional design method for torque ripple reduction, the average torque is maintained with an asymmetric rotor. In the EV traction motor, the driving time in the forward direction is longer than the driving time in the reverse direction. Therefore, the torque ripple of the forward direction is minimized, and the torque ripple of the reverse direction is designed at the same level as the base model. To verify the result of the design optimization, prototypes of the base and the optimum models are fabricated and the experimental results of the base and optimum models are compared. 2. Design Optimization Process By applying the asymmetric rotor, the air-gap magnetic field distribution at load conditions can be sinusoidal compared with the base model. Considering that the magnetic permeance of the rotor is determined by the shape of the rotor, the three design variables are selected as shown in Fig. 2. In the figure, the lx and ly represent the x-axis and y-axis directional displacements. la represents the positional angle of the permanent magnet (PMs) which implies the degree of the asymmetry of the rotor. For comparison, the same stator of the base model is used in the asymmetric rotor design. Moreover, the PM size, the pole angle, the rib thickness, and the rib angles are the same as those of the base model. The objective function and the constraints for the torque ripple minimization are defined considering the rotational directions: the forward and reverse directions. Since the targeted system mainly rotates in the forward direction, the torque ripple when rotating the forward direction is set as the objective function while the torque ripple when rotating in the reverse direction is considered in the constraint. Besides, the average torque is maintained with the value of the base model. In the optimization process, the objective and constraints values are predicted using kriging surrogate models based on the finite element analysis (FEA). As the design of experiment (DOE) technique for constructing the kriging surrogate models, a combination of optimal Latin hypercube (OLHD) and sequential maximin distance design (SMDD) are adopted [4]. Using the constructed kriging surrogate models, the design optimization is performed and the optimum model of the asymmetric rotor is determined. Using the constructed kriging surrogate models, the design optimization is performed and the optimum model of the asymmetric rotor is determined. 3. Design Result In Fig. 2, the FEA results are compared to the base model with the symmetric rotor and the optimum model with the asymmetric rotor. The average torque in the forward and reverse directions is the same. The torque ripple in the forward direction is reduced in the optimal model compared to the base model. The torque ripple in the reverse direction is similar between the base model and the optimum model. 4. Experiment Verification and Conclusion The test result is used to compare and verify the base model and the optimum model. The test of the base model and the optimum model will be performed under the same load conditions. In conclusion, a design for torque ripple reduction using the asymmetric rotor was performed considering the forward and reverse directions. The torque ripple in the reverse direction and the average torque are similar to that of the base model, while the torque ripple in the forward direction is reduced. Therefore, the design result can be applied to various applications having the forward and reverse rotations.

**References:**

[1] T. Ishikawa, M. Yamada, and N. Kurita, IEEE Trans.Magn., vol. 47, pp. 1290–1293, (2011) [2] Y.H. Jung, M.S. Lim, and J.-P. Hong, IEEE Trans. Energy Convers., vol. 33, pp. 333–340, (2018) [3] W. Ren, Q. Xu, Q. Li, and L. Zhou, IEEE Trans.Magn., vol. 52, Art. ID 8104105, (2016) [4] S.K. Kim, S.G. Lee, and M.S. Lim, IEEE Trans. Energy Convers., vol. 35, pp. 2056–2065, (2020)

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 Design Optimization for Torque Ripple Reduction using Asymmetric Rotor in IPMSM considering Forward and Reverse Directions

Flux Switching Machines (FSMs) are competent applicant for high speed brushless AC applications due to double-salient robust rotor structure, high torque, efficiency, and power density. Hybrid excited FSM (HEFSM) are widely applicable to many industrial applications, Electric Vehicles (EVs), Hybrid Electric Vehicles, electric propulsion, electric craft, electric power-assisted steering, renewable and automotive application due to high power and torque density and flux controllability [1]. However, the compact stator structure of conventional HEFSM (as shown in Fig. 1(a)) [2] suffer from parasitic effects of iron loss, demagnetization due to spatial harmonics in non-overlapped winding. This effect greatly effects electromagnetic performance i.e. average torque (Tavg), torque density (Tden), average power (Pavg), power density (Pden), efficiency (η), flux controllability, flux focusing effects and fault tolerant capability. To overcome the aforesaid demerits, in this research a novel H-type stator core (as shown in Fig. 1(b)) is proposed to improve electromagnetic performance and flux controllability. Furthermore, the proposed model is converted to modular stator by introducing flux gaps in all and alternate stator teeth as shown in Fig 1(c, d), because considering steel lamination in modular topologies with segmented stator, manufacturing process i.e. winding process is made at ease. Moreover, modular structure offers thermal, mechanical, and physically magnetic separation between armatures winding hence, on-site maintenance is reduced, and fault tolerant capability is improved [3]. In mechanical aspect, it is noteworthy that combination of consequent pole rotor with modular stator structure effectively results higher PM utilization and ease manufacturing process, transportation, assembly and on-site maintenance when design in large-dimension for wind power application whereas in electromagnetic analysis, modular structure shows dominant influence on electromagnetic performance. In order to show effectiveness of the proposed modular H-type HEFSM, a comprehensive electromagnetic performance analysis is investigated (as shown in Fig. 2(a-d)) at armature current density (Js) of 15 at constant magnetic loading in both conventional and proposed design. Analysis concludes that in proposed design, Tavg is improved by 42.35% whereas Pavg is enhanced by 48.48%. Moreover, Tden is increased by 42.04% and Pden by 48.68% in comparison with conventional design. In additional, the proposed design offers highest efficiency of 98.68% whereas conventional design has efficiency of 87.20%. thus, improving the efficiency by 13.07%. Despite of electromagnetic performance proposed modular structure improve flux focusing effects by decoupling mutual phase coupled flux linkage which greatly enhanced fault tolerant capability and improve flux weakening capability and overload capability. This analysis is comprehensively investigated under varying flux gap at all stator teeth and alternate stator teeth. Analysis concludes that flux gap (FG) increases self-inductance and reduces mutual inductance thus, mitigating short circuit current and separate healthy and faulty phases during fault and hence improve fault tolerant capability.

**References:**

[1] W. Ullah, F. Khan, E. Sulaiman, M. Umair, N. Ullah and B. Khan, "Analytical validation of novel consequent pole E-core stator permanent magnet flux switching machine," in IET Electric Power Applications, vol. 14, no. 5, pp. 789-796, 5 2020 [2] S. Cai, Z. -Q. Zhu, C. Wang, J. -C. Mipo and S. Personnaz, "A Novel Fractional Slot Non-Overlapping Winding Hybrid Excited Machine With Consequent-Pole PM Rotor," in IEEE Transactions on Energy Conversion, vol. 35, no. 3, pp. 1628-1637, Sept. 2020 [3] R. Zhou, G. -J. Li, K. Zhang, Z. Q. Zhu, M. Foster and D. Stone, "Performance Investigation of Consequent-Pole PM Machines with E-core and C-core Modular Stators," in IEEE Transactions on Energy Conversion, doi: 10.1109/TEC.2020.3027366.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/03bf8c8b62ca7e93acef9a016b0c2808.jpg)
Fig. 1: Cross sectional view (a) conventional design (b) proposed non-modular (c) proposed modular with alternate stator teeth FGs and (d) proposed modular with all stator teeth FGs

![](https://assets.underline.io/uploads/markdown_image/1/image/30ce2522eab9246f0089c98108deb2b5.jpg)
Fig. 2: Electromagnetic performance analysis (a) Tavg and Tden (b) Pavg and Pden (c) η and (d) Torque speed curve

 A Novel Hybrid Excited Machine with H-type Modular Stator and Consequent Pole PM Rotor

I. In this paper, a new two-stage harmonic current injection (TSHCI) technique is proposed for permanent magnet synchronous machines to reduce the cogging torque and torque ripple components at low speeds. The developed algorithm is validated for different AFPM motors experimentally. It has been shown that the proposed technique help reduce the cogging components significantly especially at very low speeds. II. INTRODUCTION The use of permanent magnet (PM) synchronous motors is increasing in industrial applications compared to conventional AC and DC motors due to their high torque density and efficiency. Axial flux permanent magnet (AFPM) motors move forward in many applications due to their geometric benefits, high efficiency and high power density. Such motors may suffer from high torque ripple in some slot-pole combinations. At high speeds, torque ripple can be filtered by the system inertia. However, it would be imperative to reduce the cogging torque components at low speeds in some applications. Harmonic current injection is used to reduce the torque ripple in some PM motors [1-5]. However, it has been observed that such studies are generally carried out at high speeds [6-7]. In this paper, a harmonic current injection algorithm is proposed to reduce both torque ripple and cogging torque at very low speeds. Two AFPM prototype motors is used so as to validate the developed proposed algorithm. The proposed algorithm is validated with and without harmonic current injection for such PM motors at different speed levels. III. EXPERIMENTAL VERIFICATION OF PROPOSED TWO-STAGE HARMONIC CURRENT INJECTION The proposed two-stage harmonic current injection method is used for two different AFPM synchronous motors. In order to adapt the algorithm, shown in Fig. 1, the cogging torque and its harmonics are determined first as shown in Fig. 2. After the harmonic current injection, the variation of cogging component and its harmonics are shown in Fig. 3. It has been observed that the effect of the cogging torque can be reduced from 55.91% to 36.2% with the proposed method. The reduction of the impact of the cogging torque by the harmonic current addition method applied to the q-axis current when the AFPM motor is running at 4.5Nm at 10rpm speed. It has been observed that the impact of the cogging torque can be reduced from 55.91% to 30.9% with the proposed method. IV. CONCLUSIONS In this paper, two-stage harmonic current injection is proposed and the algorithm is validated for AFPM motors at very low speeds. Torque ripple at low speeds are reduced by more than 51% with the developed method. The validity of the proposed technique is shown in 2 different AFPM motor.

**References:**

[1] Hongyun Jia, M. Cheng, W. Hua, Zhengzhuan Yang and Yunqian Zhang, "Compensation of cogging torque for flux-switching permanent magnet motor based on current harmonics injection," 2009 IEEE IEMDC, Miami, FL, 2009, pp. 286-291. [2] A. Ebrahimi, "A Novel Harmonic Current Control Algorithm for Torque Ripple Reduction of Permanent Magnet Synchronous Motors for Traction Application," 2018 IEEE International Conference on Electrical Systems for Aircraft, Railway, Ship Propulsion and Road Vehicles & International Transportation Electrification Conference (ESARS-ITEC), Nottingham, 2018, pp. 1-5. [3] H. Le-Huy, R. Perret and R. Feuillet, "Minimization of Torque Ripple in Brushless DC Motor Drives," in IEEE Transactions on Industry Applications, vol. IA-22, no. 4, pp. 748-755, July 1986. [4] Ma, Y.; Li, Y. Active Disturbance Compensation Based Robust Control for Speed Regulation System of Permanent Magnet Synchronous Motor. Appl. Sci. 2020, 10, 709. [5] Y. A. I. Mohamed and E. F. El-Saadany, "A Current Control Scheme With an Adaptive Internal Model for Torque Ripple Minimization and Robust Current Regulation in PMSM Drive Systems," in IEEE Transactions on Energy Conversion, vol. 23, no. 1, pp. 92-100, March 2008. [6] M. Lukaniszyn, M. JagieLa and R. Wrobel, "Optimization of permanent magnet shape for minimum cogging torque using a genetic algorithm," in IEEE Transactions on Magnetics, vol. 40, no. 2, pp. 1228-1231, March 2004. [7] P. Mattavelli, L. Tubiana and M. Zigliotto, "Torque-ripple reduction in PM synchronous motor drives using repetitive current control," in IEEE Transactions on Power Electronics, vol. 20, no. 6, pp. 1423-1431, Nov. 2005.

**Images:**
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 A New Harmonic Current Injection Technique to Reduce Cogging Torque in Axial Flux Permanent Magnet Motors

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

**References:**

[1] H. Zhao, C. Liu, Z. Song and W. Wang, "Exact Modeling and Multi-Objective Optimization of Vernier Machines," in IEEE Transactions on Industrial Electronics, early access. [2] M. Aydin, Surong Huang and T. A. Lipo, "Torque quality and comparison of internal and external rotor axial flux surface-magnet disc machines," in IEEE Transactions on Industrial Electronics, vol. 53, no. 3, pp. 822-830, June 2006. [3] C. Liu, K. T. Chau, and C. H. T. Lee, "A Critical Review of Advanced Electric Machines and Control Strategies for Electric Vehicles," in Proceedings of the IEEE, early access. [4] M. Wang, C. Tong, Z. Song, J. Liu and P. Zheng, "Performance Analysis of an Axial Magnetic-Field-Modulated Brushless Double-Rotor Machine for Hybrid Electric Vehicles," in IEEE Transactions on Industrial Electronics, vol. 66, no. 1, pp. 806-817, Jan. 2019. [5] A. Dwivedi, S. k. Singh and R. K. Srivastava, "Analysis and Performance Evaluation of Axial Flux Permanent Magnet Motors," in IEEE Transactions on Industry Applications, vol. 54, no. 2, pp. 1765-1772, March-April 2018. [6] Z. Song, C. Liu, K. Feng, H. Zhao and J. Yu, "Field Prediction and Validation of a Slotless Segmented-Halbach Permanent Magnet Synchronous Machine for More Electric Aircraft," in IEEE Transactions on Transportation Electrification, vol. 6, no. 4, pp. 1577-1591, Dec. 2020.

**Images:**

Fig. 1 Topologies of three machines. (a) ARFPMM, (b) AFPMM, (c) RFPMM. 1-Stator, 2-Windings, 3-Radial PMs, 4-Radial back iron, 5-Axial PMs, 6-Axial back iron.

Fig. 2 Back EMF and torque analyzed through 3D FEM. (a) Back EMF, (b) Fast Fourier transform of back EMF, (c) Electromagnetic torque.

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