Analytical Calculation of Air Gap Magnetic Field Distribution in Vernier Motor

IEEE PEDS 017, Honolulu, USA 1-15 June 015 Analytical Calculation of Air Gap Magnetic Field Distribution in Vernier Motor Hyoseok Shi, Noboru Niguchi, and Katsuhiro Hirata Department of Adaptive Machine Systems, Graduate School of Engineering, Osaka University shi.hyoseok@ams.eng.osaka-u.ac.jp Abstract This Paper aims to analyze the characteristics of a surface permanent magnet vernier motor (SPMVM) according to its design parameters such as a magnet thickness on its rotor and stator slot opening ratio based on its operational principle. The vernier motor is a kind of electric motors with a magnetic gear effect and has high torque characteristics at low speeds without mechanical gears. The vernier motor also has various advantages such as low noises and maintenance-free operations. The SPMVM is operated using space harmonics of the magnetic flux density distribution in the air gap which is created by the magnetomotive force (MMF) due to the permanent magnet and air gap permeance. This paper presents an analytical calculation method of the vernier motors, and their results are compared with those of -D finite elements method (FEM). Firstly, the operational principle of the SPMVM and the magnetic flux density in the air gap are described in detail. Secondly, the harmonics of the magnetic flux density in the air gap are analyzed according to the design parameters. Finally, the best design parameters are determined, and the -D FEM results verify the analytical calculation method. Fig. 1. 17-pole pairs 18-slot surface permanent magnet vernier motor I. INTRODUCTION Recently, vernier motors have been an issue of growing importance in industrial electrical machine field [1-3]. The SPMVM has a simple structure like a conventional surface permanent magnet synchronous motor (SPMSM) but has the characteristics of a high torque at low speeds. Since the SPMSM rotates at high speeds, it uses a mechanical gear to obtain a desired speed and torque. These mechanical gears have various drawbacks such as noises, vibrations and efficiency decreases. As a way to overcome these disadvantages, a non-contact magnetic gear using magnetic forces has been proposed [4-5]. Since the appearance of the magnetic gears, various types of electrical machines using a magnetic geared effect have been proposed and studied [6-10]. The vernier motor is a kind of motors that uses the operational principle of the magnetic gear and has the feature of a high torque at low speeds similarly to the magnetic gear [11]. Like the magnetic gear, the vernier motor is driven by the harmonics created by the coils and those of the air gap magnetic flux density which is the product of the magnetomotive force (MMF) due to the permanent magnet on the rotor and the air gap permeance of the stator teeth [1-15]. Therefore, it is necessary to examine the design parameters such as a slot opening ratio and permanent magnet thickness which determine the amplitudes and the orders of the harmonics of the magnetic flux density. In this paper, first of all, the Fig.. 978-1-5090-364-6/17/$31.00 c 017 IEEE Stator structure with slotting effect permeance, the MMF and the air gap magnetic flux density are described in detail. Secondly, the magnetic flux density and its harmonics calculated by an analytical calculation method described above are verified by the -D FEM results. Finally, a model designed with the best design parameters due to the analytical calculation is verified by -D FEM. II. ANALYTICAL CALCULATION OF AIR GAP MAGNETIC FLUX DENSITY OF SPMVMS An SPMVM has the same operational principle as a magnetic gear and is operated using space harmonics of the magnetic flux density in the air gap. The magnetic flux in the air gap can be represented by the product of the air gap permeance due to the stator teeth and the MMF due to the 47

TABLE I MAIN DESIGN PARAMETER Item unit value Motor Type Vernier Motor Operation type 40AC 3Phase Stator Outside Radius [mm] R85 Stator Inside Radius [mm] R50 Stator Yoke Thickness [mm] 8 Stator Teeth Depth [mm] 7 air gap length [mm] 0.6 Rotor Outside Radius [mm] R49.4 Rotor Inside Radius [mm] R0 Stack Length [mm] 130 Magnet Material NMX-S5 [1.45T] Steel sheet 35JN10 permanent magnet in the rotor. An SPMVM with 17 pole pairs and 18 stator slots shown in Fig. 1 is used in an analytical calculation, and its main design parameters are listed in Table I. A. Airgap permeance distribution function An analytical method for modeling the effect of stator slotting of a radial field brushless permanent magnet dc motor was presented in [16]. The air gap permeance of the stator teeth with the stator slotting effect shown Fig. can be described by Fourier decomposition with a unit magnetic potential between the stator teeth and rotor permanent magnet. Therefore, the permeance function P (θ) of the stator teeth can be expressed in the following equation: P (θ) = P 0 + P i cos (in s θ) (1) i=1 where N s is the number of stator teeth. The coefficients P 0, P i in the permeance function can be obtained as follow: [ P i = 4 µ 0 iπ g β 0.5 + P 0 = µ 0 K c g (1 1.6βr open) () g = g + h m µ r (3) (ir open ) 0.7815 (ir open ) K c = [ 1 ( ) [ π r open tan 1 b0 g g ln 1 + 1 b 0 4 ] sin (1.6πir open ) ( b0 g )]}] 1 (4) (5) β = 1 + 1 ( ) (6) tsr 1 + open g where r open is the slot opening ratio of the slot opening length bs to the slot pitch ts, µ r and µ 0 are the relative Fig. 3. Variation of air gap permeance waveform and spectra by analytical calculation according to slot opening ratio (a) Waveform. (b)spectra permeability of the stator and rotor yokes and the permeability of vacuum, respectively, g is the air gap length and hm is the permanent magnet thickness as shown in Fig.. In (5), the Carters coefficient Kc has been included to account for the increase of the effective air gap due to the stator slotting [17] and g is the replaced air gap length for the computation of Carters coefficient. Fig. 3 shows the variation of the air gap permeance waveform and harmonic spectra. Fig. 3(a) shows that as the slot opening ratio increases, the average value and the shape of each waveform decreased and changes from a square to a sine, respectively. In (1), P0 in the first term, and Pi in the second term mean the average value and the amplitude of each waveform in Fig. 3(a). The harmonics spectra shown in Fig. 3(b) will be verified below. B. Magnetomotive force function The Fourier series expansion of the MMF due to the rotor permanent magnet is described by F (θ + α) = A j cos (j 1) (θ + α)} (7) j=1 48

where is the number of pole pairs on the rotor. The coefficient A j of the MMF function is the magnetization, which is assumed to be uniform throughout the cross-section of the permanent magnets and is given by [19] A j = B r sin( jπαp α p h m µ 0 j jπα p ) where B r is the permanent magnet remanence, α p is the ratio of the permanent magnet pole arc bp to the pole pitch tp as shown in Fig.. C. Air gap magnetic flux density The air gap magnetic flux density distribution can be obtained by the product of the air gap permeance function P (θ) and the magnetomotive force function F (θ + α) as shown in the following equation: (8) B(θ, α) = P (θ)f (θ + α) = P 0 A j cos (j 1) (θ + α)} + j=1 j=1 i=1 P i A j cos in s (j 1) } θ + (j 1) P } rα in s (j 1) cos in s + (j 1) } θ + (j 1) P } rα in s + (j 1) (9) In (9), the magnetic flux distribution contains three primary harmonic components: 1 fundamental harmonic component (j 1), and two harmonic components in s (j 1). The two harmonic components are created by the modulation of the permeance and MMF. Since a vernier motor is operated with a magnetically geared effect, the number of winding pole pairs of a vernier motor should be equal to either of the two harmonic components and can be obtained by Fig. 4. Comparisons of air gap magnetic density waveforms and spectra by -D FEM and analytical calculation (a) Waveform. (b)spectra P ω = in s (j 1) (10) The amplitude and rotation angle of harmonic components are P i A j and ( j 1) α/(in s ± (j 1), respectively. P j is one of the parameters determining the amplitude of the harmonic order of the flux density. Therefore, as shown in Fig. 3(b), the amplitude of the 17th component corresponding to Pi has a great influence on the harmonics of the permanent magnet flux density. D. Back EMF and Torque In the case of a three-phase surface permanent synchronous motor, the general torque equation is as follows: T e = P ω m = e ui u + e v i v + e w i w ω m (11) where P is the power, ω m is the rotor speed in rad/s, e and i are the back EMF and current of each phase, respectively. For Fig. 5. Comparison of Back EMF according to the slot opening ratio and magnet thickness a conventional surface permanent synchronous motor, the back EMF is the derivative of the flux linkage waveform. However, since the vernier motor has a different operational principle from a conventional surface permanent synchronous motor, the phase EMF can be obtained as follow: 49

e ph = dλ dt = dθ ɛ dλ dλ = k r ω ɛ (1) dt dθ ɛ dθ ɛ where e ph is the phase EMF, λ is the flux linkage, θ ɛ is the electrical angle and k r is the ratio of the number of pole pairs of the permanent magnet to winding pole pairs. Substituting (9) into (1), the back EMF can be redefined by e ph = dλ dt = dθ e dλ dλ = k r k w ω e dt dθ e dθ e = k r k w ω dλ m = k r k w dθ e ω Nφ g m π = k r k w ω m N π ( πrro L stk B g1 = k r k w w m NR ro L stk P i A j j=1 i=1 cos in s (j 1) } θ + (j 1) P } rα in s (j 1) cos in s + (j 1) } θ + (j 1) P } rα in s + (j 1) (13) where θ m is the mechanical angle, k w is the winding factor, L s tk is the stack length, R r o is the outer radius of the rotor, φ g and B g1 are the harmonic flux and flux density, respectively. The torque equation can be obtained by dividing (13) by the rotational speed and multiplying the current as follows, ) Fig. 6. Variation of output torque by FEM and analytical calculation T e = 3 e phi ph w m = 3 k rk w NR ro L stk B g1 i ph (14) III. VERIFICATION OF ANALYTICAL CALCULATION EMPLOYING -D FEM Assuming that i and j in (10) are 1, the 17-pole-pairs-18- slot vernier motor is driven using either of the first or 35th harmonics. In this study, the number of the winding pole pairs is one because the first order harmonics are used. Based on the main design parameters listed in Table I, the SPMVM was designed as shown in Fig. 1. FEM analysis was conducted to compare the analytical calculation result. A. Air gap magnetic flux density Fig. 4 (a) and (b) show the comparisons of the air gap magnetic flux density distribution and its harmonics spectra at t=0 sec. It can be seen that the shapes of the waveforms by the FEM and the analytical calculation look quite similar to each other in Fig. 4 (a). The fundamental component 17th harmonic has the highest value, and the first and 35th harmonic components have the next highest value as shown in Fig. 4 (b). As shown in Fig. 4 (b), the error rate between FEM and analytical calculation method is within ±7.1%. B. Back EMF Fig. 7. FEM model of the designed SPMVM Fig. 5 illustrates the distribution of the back EMF root mean square (RMS) value according to the slot opening ratio and the permanent magnet thickness. Both graphs show a similar pattern, and when the opening ratio and permanent magnet thickness are 60% and mm, respectively, the highest induced voltages are observed in each graph. C. Torque As mentioned above, the opening ratio was fixed 60% and the output torque according to the permanent magnet thickness was analyzed when a three-phase current of 10 Arms was input, and the rotor was rotated at 500 rpm. Fig. 6 shows the trend line of the torque with the permanent magnet thickness change. Two lines showed an almost similar pattern and the model with -mm permanent magnet had the best output torque. IV. DESIGN AND ANALYSIS OF THE SPMVM WITH THE BEST DESIGN PARAMETER We designed an SPMVM with 17 pole pairs 18 slots based on the previously verified design parameters. The N-T, T-I and 50

a good agreement with the FEM results. Finally, the characteristics of an SPMVM designed with the calculated design variables were verified using -D FEM analysis. Fig. 8. Fig. 9. N-T and T-I characteristics T- η characteristic the T-η characteristics of the SPMVM are computed using - D FEM. The -D FEM model of the SPMVM is shown in Fig. 7. Fig. 8 shows the N-T and T-I characteristics when a sinusoidal voltage was supplied from 10 rpm to 500 rpm. The T-η characteristics are shown in Fig. 9 and the efficiency η is given by P out η = (15) P out + W iron + W copper where P out is the output power, W iron is the iron loss of the laminated cores calculated after FEM and W copper is the copper loss in the coils. As shown in Figs 8 and 9, when the load torque is higher than 50 Nm, the efficiency is lower than 80 V. CONCLUSION This paper expressed an analytical calculation of an SP- MVM with a stator slotting effect and designed a model based on a theoretical approach. We described how to calculate the air gap magnetic flux density and harmonics using an SPMVM operating principle. The analytical calculation method showed REFERENCES [1] L. Xu, G. Liu, W. Zhao, X. Yang, and R. Cheng, Hybrid Stator Design of Fault-Tolerant Permanent-Magnet Vernier Machines for Direct-Drive Applications, IEEE Transactions on Industrial Electronics, vol. 64, pp. 179-190, 017. [] F. Zhao, M. s. Kim, B. i. Kwon, and J. h. Baek, A Small Axial-Flux Vernier Machine with Ring-Type Magnets for the Auto-Focusing Lens Drive System, IEEE Transactions on Magnetics, vol. PP, pp. 1-1, 016. [3] Y. Kokubo and S. Shimomura, Design of dual rotor - Axial gap PMVM for hybrid electric vehicle, in Electrical Machines and Systems (ICEMS), 014 17th International Conference on, 014, pp. 573-578. [4] K. Atallah and D. Howe, A novel high-performance magnetic gear, Magnetics, IEEE Transactions on, vol. 37, pp. 844-846, 001. [5] N. Niguchi and K. Hirata, Transmission Torque Analysis of a Novel Magnetic Planetary Gear Employing 3-D FEM, Magnetics, IEEE Transactions on, vol. 48, pp. 1043-1046, 01. [6] A. Zaini, N. Niguchi, and K. Hirata, Continuously Variable Speed Vernier Magnetic Gear, Magnetics, IEEE Transactions on, vol. 48, pp. 3104-3107, 01. [7] N. Niguchi, K. Hirata, A. Zaini, and S. Nagai, Proposal of an axial-type magnetic-geared motor, in Electrical Machines (ICEM), 01 XXth International Conference on, 01, pp. 738-743. [8] N. Niguchi and K. Hirata, Magnetic-geared motors with high transmission torque density, COMPEL - The international journal for computation and mathematics in electrical and electronic engineering, vol. 34, pp. 48-438, 015. [9] K. Atallah, W. Jiabin, S. D. Calverley, and S. Duggan, Design and Operation of a Magnetic Continuously Variable Transmission, Industry Applications, IEEE Transactions on, vol. 48, pp. 188-195, 01. [10] N. Shuangxia, S. L. Ho, and W. N. Fu, Design of a Novel Electrical Continuously Variable Transmission System Based on Harmonic Spectra Analysis of Magnetic Field, Magnetics, IEEE Transactions on, vol. 49, pp. 161-164, 013. [11] Q. Ronghai, L. Dawei, and W. Jin, Relationship between magnetic gears and vernier machines, in Electrical Machines and Systems (ICEMS), 011 International Conference on, 011, pp. 1-6. [1] K. Okada, N. Niguchi, and K. Hirata, Analysis of a Vernier Motor with Concentrated Windings, Magnetics, IEEE Transactions on, vol. 49, pp. 41-44, 013. [13] D. Li, R. Qu, W. Xu, J. Li, and T. Lipo, Design Procedure of Dual-stator, Spoke-array Vernier Permanent Magnet Machines, Industry Applications, IEEE Transactions on, vol. PP, pp. 1-1, 015. [14] L. Dawei, Q. Ronghai, L. Jian, X. Linyuan, W. Leilei, and X. Wei, Analysis of Torque Capability and Quality in Vernier Permanent- Magnet Machines, IEEE Transactions on Industry Applications, vol. 5, pp. 15-135, 016. [15] S. Hyoseok, N. Niguchi, and K. Hirata, Characteristic Analysis of Surface Permanent Magnet Vernier Motor according to Pole Ratio and Winding Pole Number, IEEE Transactions on Magnetics, vol. PP, pp. 1-1, 017. [16] Z. Q. Zhu and D. Howe, Instantaneous magnetic field distribution in brushless permanent magnet DC motors. III. Effect of stator slotting, IEEE Transactions on Magnetics, vol. 9, pp. 143-151, 1993. [17] F. W. Carter, The magnetic field of the dynamo-electric machine, Electrical Engineers, Journal of the Institution of, vol. 64, pp. 1115-1138, 196. [18] Z. Q. Zhu, D. Howe, E. Bolte, and B. Ackermann, Instantaneous magnetic field distribution in brushless permanent magnet DC motors. I. Open-circuit field, IEEE Transactions on Magnetics, vol. 9, pp. 14-135, 1993. 51