France



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

**References:**

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

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INTERMAG 2021

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

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

**References:**

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

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/a0475924f8f2f552126552b15ab27ef7.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/fd7314d2948f6f81a1feaf16a2df87d4.jpg)

poster

1. Background and Literature Review Since permanent-magnet vernier machines (PMVMs) can offer high torque directly without the aid of intermediate transmission devices such as mechanical gearbox, they are extremely suitable for low-speed large-toque direct-drive (LLDD) applications [1], such as industrial robots, wind power generation, ship propulsion, etc. Generally, PMVMs can be classified into three types according to the location of permanent magnets (PMs), i.e., stator type (ST), rotor type (RT) and stator and rotor type (SRT). The former two types mean that either PMs are only on stator or only on rotor, while the latter type signifies that PMs are on both stator and rotor. Since there are two PM excitation sources, the SRT type of PMVM is also called “dual-permanent-magnet-excited (DPME) machine”[2]. Clearly, DPME machine has one more PM excitation source than the other two types, so it can output higher torque compared with the other two types, which has been demonstrated in [2,3]. Thus, DPME machine has more advantages in the LLDD field, and much research on it is also growing increasingly. However, according to the literature review, research on DPME machine concentrates on the qualitative explanation of the bidirectional field modulated effect (BFME) [2], new structures [4], design and optimization [5], comparative study [6] and control strategies [7]. 2. Motivation Back-EMF is an important physical quantity in electrical machine (EM), which many designers often care about, as nonsinusoidal back-EMF may cause torque ripple, and increase inverter capacity [8]. For DPME machine (see Fig. 1), it has two PM excitation sources with different pole-pair numbers (PPNs), i.e. stator PMs and rotor PMs; it also has double-salient structure. Due to the double-salient structure, the magnetic field generated by stator PMs or rotor PMs can be modulated into abundant airgap field harmonics through rotor teeth or stator teeth. So, the airgap harmonics of DPME machine are more abundant and more complex than those of traditional PM machine. Regarding the back-EMF of DPME machine, two questions (Qs) arise naturally, i.e., Q1: do all airgap flux density harmonics make contribution to back-EMF? Q2: For these harmonics which contribute to the back EMF, do they all produce positive or negative back EMF? According to the previous literature review, only [4] answered Q1 by qualitative analysis, and pointed out that only specific harmonics can contribute to back-EMF. However, as far as we know, there is no literature to answer Q2. So, it is necessary to quantitatively identify the airgap flux density harmonics contributing to back-EMF, thus helping to suppress the harmonics which produce negative back EMF. 3. Method Magneto motive force (MMF) permeance analytical model (MPAM) is used to identify the airgap flux density harmonics contributing to back-EMF. Firstly, establish the MPAM of DPME machine. Next, derive the expressions of airgap flux density harmonics generated by stator and rotor PMs respectively based on the MPAM, and classify the harmonics into six types (see Table I). Then, deduce the expressions of PM flux-linkage by means of winding function and the derived airgap flux density harmonics. Afterwards, get back-EMF expressions by the derived PM flux-linkage. Finally, identify the airgap flux density harmonics contributing to back-EMF. 4. Results As shown in Fig. 1, a 12/10 DPME machine is selected as an example for analysis. Fig. 2(a) compares back-EMFs obtained by theoretical analysis and FEM. As can be seen from Fig. 2(a), the back-EMF obtained by the MPAM basically coincides with that obtained by FEM. The error between the analytical and FEM results is about 8.7%, which indicates that the MPAM is feasible. By means of the MPAM, one can also identify the airgap flux density harmonics contributing to back-EMF. Fig. 2(b) gives the identification results of the airgap flux density harmonics contributing to the fundamental back-EMF. Apparently, S-II and R-II types of harmonics generate negative back-EMFs, which weakens the total back-EMF; S-III, R-I and R-III types contribute to positive back-EMFs, and R-III type contributes nearly half the back-EMF; S-I type makes no contribution to back-EMF. 5. Note Limited by the length of the digest, 1) not all the references are given in the digest body, and the others will add in the full paper; 2) the derivation of the MPAM for DPME machine as well as the derived expressions will be elaborated in the full paper; 3) other results such as waveforms of flux density will be added in the full paper.

**References:**

[1] F. Wu and A. M. El-Refaie, IET Electr. Power Appl., Vol. 13, p. 127–137 (2019) [2] L. Jian, Y. Shi and C. Liu, IEEE Trans. Magn., Vol. 49, p. 2381–2384 (2013) [3] W. Zhao, X. Sun and J. Ji, IEEE Trans. Appl. Supercond., Vol. 26, p. 1-5 (2016) [4] L. Xu, W. Zhao and G. Liu, IEEE Trans. Veh. Technol., Vol. 69, p. 7104-7115 (2020) [5] Q. Lin, S. Niu and W. N. Fu, IEEE Trans. Magn., Vol. 56, p. 1-5 (2020) [6] Q. Wang, S. Niu and L. Yang, IEEE Trans. Ind. Electron., Vol. 64, p. 9924-9933 (2017) [7] S. Niu, S. L. Ho and W. N. Fu, IEEE Trans. Magn., Vol. 50, p. 805-808 (2014) [8] X. Ren, D. Li and R. Qu, IEEE Trans. Ind. Electron., Vol. 67, p. 6248-6258 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/33770a3501a2d61f1b501d622d324902.jpg)
Fig. 1 and Table I

![](https://assets.underline.io/uploads/markdown_image/1/image/b877c649b02d8cc9ff444a314279b6a8.jpg)
Fig. 2. (a) Back-EMFs. (b) Contribution of airgap flux density harmonics to the fundamental back-EMF.

 Quantitative Identification of Airgap Flux Density Harmonics Contributing to Back-EMF in Dual-Permanent-Magnet-Excited Machine

In this paper, a novel winding switching strategy is proposed to extend the speed range of a consequent-pole ferrite-PM hybrid-excited machine (CPFPM-HEM) for electric vehicle application. The key is that in low-speed range, the ac armature winding and dc field winding are connected in series to achieve large starting torque, while in high-speed range, dc field winding are injected with different dc currents to realize constant back-EMF with flux-weakening effect. Furthermore, consequent-pole rotor is used to reduce the dosage of the permanent magnet (PM). In the meantime, ferrite PM is used to replace rare-earth PM to reduce the cost with the shortage of rare earth resources. In this paper, the machine configuration, winding switching concept are illustrated in detail. Then, the electromagnetic performance of CPFPM-HEM in different speed ranges are investigated. Finally, the speed range which greatly extended with the same back-EMF is testified by the Maxwell/Simplorer co-simulation. 1. Introduction Nowadays, permanent magnet synchronous machine (PMSM) has attracted more and more attention in electronic vehicle application with excellent advantages such as high-power density and high efficiency [1-2]. However, the fixed PM flux of PMSM has limited the speed range of electronic vehicle. In order to extend speed range of the electric vehicle, hybrid excited machine, which incorporates PM excitation and dc field excitation, is proposed [3-4]. For hybrid excited machine, the flux regulation range has a limitation with dc field current control. In order to extend the flux regulation range, a novel winding switching strategy is proposed. 2. Machine configuration and winding switching concept A. Machine configuration Fig. 1 shows the configuration of consequent-pole ferrite permanent magnet hybrid-excited machine (CPFPM-HEM). It is composed of 8 pole pair inner rotor and 12-slot outer stator. The rotor consists of consequent-pole ferrite PMs and the magnetization direction of these PMs is radial outward as denoted in Fig. 1. The stator comprises ac armature winding and dc field winding. Both ac armature winding and dc field winding are made of single layer concentrated windings. The ac armature winding and dc field winding are connected in series in low-speed operation, while which are separately excited in high-speed operation. B. Winding switching concept Fig. 1 presents the winding switching manipulation circuit which consists of ac armature winding, dc field winding, rectifier, dc voltage source as well as some switches. There are two speed modes. Mode 1: Low speed mode S1 and SW1 is OFF. The ac armature winding and dc field winding are connected in series when the machine is in low-speed operation. The back-EMF is the sum of the voltage from ac armature winding and dc field winding. Due to the low speed, the back-EMF provided by the powertrain does not need to be high. Mode 2: High speed mode S1 and SW1 is ON. The ac armature winding and dc field winding are separately excited. While ac armature winding is provided by powertrain, the dc field winding is provided by dc volage source. By regulating the dc source voltage value, the back-EMF provided by the powertrain can keep the same. 3. Electromagnetic performance Fig. 2 shows variable speed constant back-EMF with winding switching operation and dc excitation regulation. It can be noticed that in 0 to 20 ms, the machine operates in mode 1 with 300 rpm, the ac armature winding and dc field winding are connected in series to provide large starting torque. At 40 ms, the ac armature winding and dc field winding are separately excited and the machine switches to mode 2. Between 40 ms to 80 ms, the machine speed is 600 rpm and the dc excitation current is zero. Between 80 ms to 120 ms, the machine speed changes to 900 rpm and dc excitation current changes to -3 A. Between 120 ms to 160 ms, the machine speed attains 1200 rpm and the dc excitation current reaches -6 A. It can be concluded that, the amplitude of back-EMF is almost unchanged when the speed changes from 300 rpm to 1200 rpm with resort to winding switching operation and dc excitation regulation. 4. Conclusion This paper proposes a novel winding switching strategy to enhance the flux regulation capability of CPFPM-HEM, thus whose speed range can be greatly extended. The ac armature winding and dc field winding are connected in series to provide large torque in low-speed mode, while which are separately excited to realize variable speed constant back-EMF in high-speed mode. The paper has verified variable speed constant back-EMF through Maxwell/Simplorer co-simulation. In addition, consequent-pole ferrite PMs are used to save the cost and avoid the usage of rare-earth resources. In the full paper, the machine operation principle, flux regulation principle, as well as winding switching concept will be illustrated in detail. In addition, more electromagnetic analysis will be performed and more conclusions will be drawn from the finite element results. Moreover, a prototype of the machine will be manufactured and relative tests will be carried out. Acknowledgement This work was supported in part by grants from the Hong Kong Research Grants Council, Hong Kong, China, under Project PolyU 152058/17E and PolyU 152143/18E.

**References:**

[1] S. Niu, Y. Luo, W. Fu and X. Zhang, "Robust Model Predictive Control for a Three-Phase PMSM Motor With Improved Control Precision," IEEE Trans. Ind. Electron., vol. 68, no. 1, pp. 838-849, Jan. 2021. [2] S. Niu, Y. Luo, W. Fu and X. Zhang, "An Indirect Reference Vector-Based Model Predictive Control for a Three-Phase PMSM Motor," IEEE Access, vol. 8, pp. 29435-29445, 2020. [3] X. Zhao and S. Niu, "A New Slot-PM Vernier Reluctance Machine With Enhanced Zero-Sequence Current Excitation for Electric Vehicle Propulsion," IEEE Trans. Ind. Electron., vol. 67, no. 5, pp. 3528-3539, May 2020. [4] Y. Fan, G. Qu, and C. Tan, "Design and Analysis of a New Five-Phase Dual-Stator Consequent-Pole Brushless Hybrid Excitation Machine," IEEE Trans. Magn., vol. 55, no. 1, pp. 1-10, Jan. 2019.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/4377008ba78f62d7eee68ed50f53ad09.jpg)
Fig. 1. Configuration of CPFPM-HEM and winding switching manipulation circuit.

![](https://assets.underline.io/uploads/markdown_image/1/image/97d0ea6aa35f7fde6662f17fab9efb15.jpg)
Fig. 2. Variable speed constant back-EMF with winding switching operation and dc excitation regulation. (a) Speed variation and given dc excitation current. (b) Variable speed constant back-EMF.

 A Novel Winding Switching Strategy of a Consequent-pole Ferrite-PM Hybrid-excited Machine for Electric Vehicle Application

I.INTRODUCTION The flux reversal permanent magnet (FRPM) machine, as a stator PM machine with a simple rotor configuration, is suitable for high speed electric propulsion applications However, the conventional 6-slot and 8-pole (6s/8p) FRPM machine has some disadvantages, such as low torque density and large torque ripple. An axial modular flux reversal permanent magnet (AM-FRPM) machine is proposed in this paper as shown in Fig. 1. It contains two modular parts including front modular part 1 and rear modular part 2, and the modular part topology evolves from the conventional 6s/8p FRPM machine. It is worth noting that the PMs of two modular parts have opposite magnetization directions at the same stator position. Meanwhile, the rotor poles in the rear part are shifted by 22.5° mechanically relative to those in the front part. In addition, the toroidal windings are wound around the yokes of the AM-FRPM machine, which are different from those of conventional FRPM machines. Besides, the winding factor is set as 1 to obtain a high torque density. II.PRINCIPLES AND PERFORMANCE Based on the magnetic field modulation theory, the operating principle of AM-FRPM machine is analyzed from three perspectives, including PM field modulation, armature reaction field modulation and torque production mechanism. For the PM field, the PM magneto-motive-force (MMF) of each modular part is modulated by the corresponding modular salient rotor teeth [1]. Then the air gap flux density expression Bg can be deduced, which contains the harmonics components (HCs) with |nPs±kPr| order, where Ps is the number of stator slots, Pr is rotor poles number, n=1, 2…, and k=0, 1, 2…. The air-gap flux density harmonics distributions of the PM field are shown in Fig. 2(a). It can be found that harmonics distributions of two modular parts are similar, and the dominant harmonics components are 2nd, 4th, 6th, 12th, 14th and 24th orders, which verifies the HCs results by the field modulation theory. PMs of the two modular parts have opposite magnetization directions at the same stator position, so the phases differences of the flux density harmonics generated by the PM MMF, such as 6th, 12th and 24th, are 180o between two parts. Besides, the PM-MMFs are modulated by two rotor parts with different initial angles, so the phases of modulated harmonics in two parts are also different. In addition, based on the flux density harmonics phases analysis, the PM flux linkages harmonics with even orders are cancelled as shown in Fig. 2(b). The armature reaction MMF mathematic model of the AM-FRPM machine cannot be obtained by three phases overlapping directly because its winding function is different from that of the conventional FRPM machine. The reason is that the armature coils of the AM-FRPM machine are with the same direction. Hence, the armature reaction MMF distributions need to consider three phases synchronously. In addition, the armature reaction MMF in two modular parts is modulated by the front and rear rotors in Fig. 1(a), respectively, then the armature reaction air gap flux density harmonics distributions can be obtained. It can be found from Fig. 2(c) that the number of HCs can be expressed as 2k (k=1, 2…), and the dominant harmonic orders are 2nd, 4th, 6th, 10th and 12th. It is worth noting that the rotors in two modular parts have different initial angle, and then the armature reaction flux density harmonic distributions of two parts obtain different harmonic performances, including amplitudes and phases. The harmonics characteristics analysis will be illustrated in detail in the full paper. Here it is clearly shown in Fig. 2(c) that the amplitudes of 4th harmonic in front and rear modular parts are 0.22T and 0.11T, respectively, which influence the harmonics contribution of electromagnetic torque. Based on the analysis of the PM field and the armature reaction field, it can be concluded that the electromagnetic torque is mainly contributed by the harmonics with 2nd, 4th, 6th and 12th orders. The 6th and 12th harmonics are generated by the PM MMF, while the 2nd and 4th harmonics are produced by the modulation of salient pole rotor on PM MMF. The electromagnetic torque waveforms are shown in the Fig. 2(d). It can be found that the torque ripples of front and rear modular parts are 8.4% and 8.3%, respectively, and the torque ripple of AM-FRPM machine is only 6.4%. Besides, the torque of front and rear modular are 5.23Nm and 5.24Nm, and the torque of AM-FRPM machine is 10.64Nm. The torque is generated by overlapping of two modular parts is 1.6% less than that of the whole machine, so the magnetic flux leakage between modular parts is small and the structure is compact. III.CONSLUSION The proposed AM-FRPM machine contains two modular parts and integrated toroidal windings. In this paper, the operating principle of the proposed machine was explained by the air gap field modulation theory. It can be found that the armature reaction flux density harmonic distributions of two parts obtain different harmonic performances. Besides, the AM-FRPM machine exhibits a high torque density and the torque ripple is suppressed effectively, which is suitable for electric vehicle applications.

**References:**

[1] X. Zhu and W. Hua, "An Improved Configuration for Cogging Torque Reduction in Flux-Reversal Permanent Magnet Machines," in IEEE Transactions on Magnetics, vol. 53, no. 6, pp. 1-4, (2017). [2] X. Zhu, W. Hua and W. Wang, "Analysis of Back-EMF in Flux-Reversal Permanent Magnet Machines by Air Gap Field Modulation Theory," in IEEE Transactions on Industrial Electronics, vol. 66, no. 5, pp. 3344-3355, (2019).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/43d2798fc3d332c572d2e13b7e7dd580.jpg)
Fig.1 (a) The topology of AM-FRPM machine. (b) front modular part (c) rear modular part.

![](https://assets.underline.io/uploads/markdown_image/1/image/bd9cd746d7829d0311ec363aec69dd5e.jpg)
Fig.2 (a) The air-gap flux density harmonics distributions of PM field. (b) The harmonic spectrum of line flux linkage. (c)The air-gap flux density harmonics distributions of armature field. (d) Torque waveforms.

 Analysis of Axial Modular Flux Reversal Permanent Magnet Machine

A novel brushless dual-electrical-port dual-mechanical-port machine with opening stator slot is proposed to improve the torque density. Compared to the regular BLDD machine, which can be regarded as the combination of the magnetic gear machine (MGM) and permanent magnet synchronous machine (PMSM), the proposed BLDD is formed by MGM and vernier PM machine (VPM). The stator teeth are not only used to go through the main flux, but also worked as a modulator to change the pole pairs and construct the VPM part. Due to the flux modulation effect and reduced effective air-gap, the torque density is significant improved. The operation principle is introduced and electromagnetic performances of the regular and proposed BLDD machine are compared by 2-D finite element analysis (FEA). I. Introduction Continuously variable transmission (CVT) is the key component of the hybrid power system, which has the advantages between the electric drive system and engine [1]-[3]. With the development of the flux modulation principle [4], a new type of CVT called brushless dual-electrical-port dual-mechanical-port (BLDD) machine, which can realize the function of a generator, a motor and the planetary gear in the hybrid power system, has been proposed in [5]. This paper proposes a higher torque density BLDD machine with opening stator slot, as shown in Fig. 1. II. Structure and operation principle The proposed BLDD machine has the same outer and inner rotors structure with the regular BLDD machine [6]. The major difference is the stator configuration. A split-teeth stator structure is used and two sets of windings are placed in the auxiliary teeth and main teeth respectively. In the regular BLDD machine, the auxiliary teeth are closed slot structure, and only used to provide flux path. In the proposed BLDD machine, the auxiliary teeth are opening slot structure, and not only used to go through the magnetic path, but also worked as a modulator to change the PM pole-pair to vernier winding pole-pair. As shown in Fig.1 (a2) and (b2), the MGM part is composed of the modulation winding, teeth in the inner rotor and spoke-array PMs in the outer rotor, which is the same for both regular and proposed BLDD machines. The operation principle of the MGM portion is based on the magnetic gear effect, and the modulation winding pole-pair is equal to the difference between the PM pole-pair and inner rotor teeth number. The other portions of regular and proposed BLDD machines are completely different, one is PMSM portion, and the other is VPM portion. The PMSM portion of the regular BLDD machine in Fig.1 (a3) is formed by regular winding, stator with closed slot and spoke-array PM outer rotor. PMSM works on the same operation principle as a conventional PM machine, and the regular winding pole-pair is equal to PM pole-pair. Nevertheless, the VPM portion of the proposed BLDD machine in Fig.1 (b3) consists of vernier winding, stator with opening slot and spoke-array PM outer rotor. The operation of VPM is based on the flux modulation principle, in which stator auxiliary teeth work as a pole pair transformer and the vernier winding pole-pair is equal to the difference between the stator auxiliary teeth number to PM pole-pair. III. Comparison of the Regular and Proposed BLDD Machines The two BLDD machines with the same machine volume and current density are used to compare the electromagnetic performance. The two BLDD machines have been optimized respectively. Compared to the regular BLDD machine, the proposed BLDD machine has 53.3% higher MGM torque due to increased pole ratio (ratio of PM pole-pair to winding pole-pair) and 77.8% higher VPM torque because of flux modulation effect. IV. Conclusion This paper proposes a novel BLDD machine with opening stator slot. The proposed BLDD machine can significantly improve the back EMF and torque density. Because the composite structure of the proposed BLDD machine is changed and the operation principle is different. More details about machine operation principle, operation features and some design considerations of the proposed BLDD will be given in the full paper.

**References:**

[1] Z. Q. Zhu and S. Cai, "Hybrid excited permanent magnet machines for electric and hybrid electric vehicles," CES Trans. on Electrical Machines and Systems, vol. 3, no. 3, pp. 233-247, Sept. 2019. [2] A. Emadi, Y. J. Lee and K. Rajashekara, "Power Electronics and Motor Drives in Electric, Hybrid Electric, and Plug-In Hybrid Electric Vehicles," IEEE Trans. Ind. Electron., vol. 55, no. 6, pp. 2237-2245, June 2008. [3] X. Zhang, C. T. Li, D. Kum etc., "Prius+ and volt- : configuration analysis of power-split hybrid vehicles with a single planetary gear," IEEE Trans. Veh. Technol., vol. 61, no. 8, pp. 3544-3552, Oct. 2012. [4] D. Li, R. Qu and T. A. Lipo, "High-Power-Factor Vernier Permanent-Magnet Machines," IEEE Trans. Ind. Appl., vol. 50, no. 6, pp. 3664-3674, Nov.-Dec. 2014. [5] D. Li, R. Qu, X. Ren etc., “Brushless dual-electrical-port, dual mechanical port machines based on the flux modulation principle,” in Proc. IEEE Energy Convers. Congr. Expo,2016. [6] Z. Liang, X. Ren, D. Li, etc., "Analysis of a Spoke-Array Brushless Dual-Electrical-Port Dual-Mechanical-Port Machine With Reluctance Rotor," IEEE Trans. Ind. Electron. vol. 68, no. 4, pp. 2999-3011, April 2021.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/2227f1374d906f15e29edd7415114e42.jpg)
Fig.1 Configurations. (a1) Regular BLDD machine. (a2) MGM portion of regular BLDD machine. (a3) PMSM portion of regular BLDD machine. (b1) Proposed BLDD machine. (b2) MGM portion of proposed BLDD machine. (b3) VPM portion of proposed BLDD machine.

![](https://assets.underline.io/uploads/markdown_image/1/image/534945c16b478183b1896e4b123720b8.jpg)
Fig.2 Comparison of regular and proposed BLDD machines. (a1) Back EMFs of modulation winding. (a2) Back EMFs of regular or vernier windings. (b1) MGM torque component. (b2) PMSM or VPM torque component.

 A Novel Brushless Dual-Electrical-Port Dual-Mechanical-Port Machine with Opening Stator Slot

This paper proposes a novel hybrid reluctance machine with dual-layer slot PMs and zero-sequence excitation. Dual-layer slot PMs are utilized to evoke DC-saturation-relieving and flux modulation effect respectively. By introducing zero-sequence current excitation which functions as virtual DC field source, extra dc field winding can be removed. Moreover, flux can be flexibly controlled through regulating zero-sequence current, thus speed range can be greatly extended. In this paper, the machine configuration, DC-saturation-relieving principle, flux modulation effect, and zero-sequence current control are introduced. Electromagnetic performance of the proposed machine is evaluated by finite element analysis (FEA), including variation of flux-linkage, back-EMF, as well as torque density with different zero-sequence currents. Furthermore, a prototype is manufactured and relevant experiments are performed to verified the correctness of FEA. 1. Introduction Nowadays, electric vehicles have attracted more and more attention due to fossil fuel depletion and severe environment pollution [1]. It is required that the motor operates at wide speed range [2]. Reluctance machine has come to be a good solution for electric vehicle application, which includes switched reluctance machine (SRM) [3], doubly salient machine (DSM), flux switching machine (FSM) [4], flux reversal machine (FRM), as well as variable flux reluctance machine (VFRM) [5]. In this paper, a novel DC-saturation-relieving flux-modulated hybrid reluctance machine with zero-sequence current excitation is proposed. Zero-sequence current, which acts as dc field excitation, is injected into ac and dc integrated winding, thus dc field winding is removed. 2. Machine configuration and operation principle The structure of flux-modulated DC-saturation-relieving hybrid reluctance machine with integrated ac and dc winding is presented in Fig. 1. Inner rotor consists of only iron core which provides mechanical robustness. Outer stator comprises only one set of winding, namely ac and dc integrated winding. Zero-sequence current, which acts as dc field excitation, is injected into ac and dc integrated winding. There are two layers of PMs on the slot opening. Inner layer slot PMs are radial magnetized to generate effective torque with flux modulation effect. Meantime, outer layer slot PMs are tangential magnetized to enhance torque density through relieving DC saturation. A dual inverter drive circuit is also showed in Fig. 1. Each phase winding is split into two sub-phase windings. Two sub-phase winding groups are driven by two inverters respectively and the neutral points of which are connected. The advantages of proposed machine are listed as following. (1) Since all excitation sources are mounted in the stator side and the rotor consists of only iron, no brush and slip rings are needed, which enhances mechanical robustness. (2) Radial magnetized PMs are mounted in the slot opening, which provides valid torque with flux modulation effect. Tangential magnetized PMs are placed in the slot opening, which improve torque density through relieving DC saturation. (3) Independent dc field winding is eliminated, which will leave more space for ac and dc integrated winding, thus more load current can be reached. (4) Through regulating zero-sequence current, flux can be flexibly regulated, thus extensive speed range can be realized. 3. Electromagnetic performance Fig. 2 shows on-load steady torque and no-load back-EMF with different zero-sequence currents. It can be seen that the on-load average torque is 3.4 Nm without zero-sequence current, 0.8 Nm with -10 A zero-sequence current, and 5.4 Nm with 10 A zero-sequence current. Also, the peak-to-peak value of no-load back-EMF is 12.8 V without zero-sequence current, 2.1 V with -10 A zero-sequence current, 28 V with 10 A zero-sequence current. By regulating zero-sequence current, on-load steady torque and no-load back EMF can be controlled, thus speed range of the proposed machine can be greatly extended. 4. Conclusion In this paper, the flux-modulated DC-saturation-relieving hybrid reluctance machine with zero-sequence current excitation is proposed. Zero-sequence current, acting as dc field excitation source, is injected into ac and dc integrated winding, thus dc field winding can be eliminated. Initial finite element analysis shows that the on-load steady torque and no-load back-EMF can be controlled by regulating zero sequence current, thus speed range of the proposed machine can be greatly extended. Later, DC-saturation-relieving principle and flux regulation principle will be illustrated in detail. In addition, more electromagnetic analysis will be performed and more conclusions will be drawn from the FEA results. Furthermore, a prototype of the proposed machine will be manufactured and relative tests will be performed. This work was supported in part by grants from the Hong Kong Research Grants Council, Hong Kong, China, under Project PolyU 152058/17E and PolyU 152143/18E.

**References:**

[1] X. Zhao, S. Niu, and X. Zhang, "Design of a New Relieving-DC-Saturation Hybrid Reluctance Machine for Fault-Tolerant In-Wheel Direct Drive," IEEE Trans. Ind. Electron., vol. 67, no. 11, pp. 9571-9581, Nov. 2020. [2] S. Hemmati and T. A. Lipo, "Field weakening of a surface mounted permanent magnet motor by winding switching," International Symposium on Power Electronics Power Electronics, Electrical Drives, Automation and Motion, Sorrento, 2012, pp. 736-740. [3] X. D. Xue, K. W. E. Cheng, and T. W. Ng, “Multi-objective optimization design of in-wheel switched reluctance motors in electric vehicles,” IEEE Trans. Ind. Electron., vol. 57, no. 9, pp. 29–29, Sep. 2010. [4] C. Yu and S. Niu, “Development of a magnetless flux switching machine for rooftop wind power generation,” IEEE Trans. Energy Convers., vol. 30, no. 4, pp. 1703–1711, Dec. 2015. [5] J. T. Shi, X. Liu, and D. Wu, “Influence of stator and rotor pole arcs on electromagnetic torque of variable flux reluctance machines,” IEEE Trans. Magn., vol. 50, no. 11, pp. 1–4, Nov. 2014

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/4d0f859ab7e3b8556f78ec13bbbba70b.jpg)
Fig. 1. Structure of flux-modulated DC-saturation-relieving hybrid reluctance machine and dual inverter drive control system.

![](https://assets.underline.io/uploads/markdown_image/1/image/8b10fb9e70ef75e14f27745d5fa53162.jpg)
Fig. 2. On-load steady torque and no-load back-EMF with different zero-sequence currents.

 Flux-Modulated DC-Saturation-Relieving Hybrid Reluctance Machine using Zero-sequence Current Excitation for Electric Vehicle Application

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

**References:**

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

**Images:**

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

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

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

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

**References:**

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

**Images:**

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

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

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

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

**References:**

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

**Images:**

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

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

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

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

**References:**

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

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/4591486958f5f351e30729cab9a67925.jpg)
Fig. 1. Machine structure.

![](https://assets.underline.io/uploads/markdown_image/1/image/6df4037a5e04493e5bc01b1b09a49803.jpg)
Fig. 2. Machine performance under different armature winding ratio. (a) Rated torque. (b) Power factor. (c) Core loss and efficiency. (d) Voltage regulation rate.

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

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

**References:**

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

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/54809cfda9e3e8155660a8a436d7abcb.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/8b15541776e218181629a91f7929f241.jpg)

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