A Novel Axial-flux Electric Machine for In-wheel Gearless Drive in Plug-in Hybrid Electric Vehicles W. N. Fu, and S. L. Ho The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong A novel low-speed axial-flux-modulated motor is presented. The proposed motor breaks the traditional design rule which stipulates that the pole-pair numbers of the stator and the rotor must be the same. Special iron segments in the airgap are used to modulate the magnetic field. It has the merit of having high power density at low speed. It can be fitted into very limited space such as inside the wheel rim of electric vehicles. Compared with those of radial-flux-modulated motor, its manufacturing and assembling process are simple and easy. Its performance is compared with those of a radial-flux-modulated motor using two-dimensional and threedimensional finite-element methods. Index Terms Axial flux, electric motor, electric vehicle, finite element method, flux modulation, low-speed, magnetic field, power density. C I. INTRODUCTION RUDE OIL price is expected to rise rapidly in the long term. Emissions from gasoline driven automobiles are among the main causes of global warming and environmental pollution. One of the best solutions is to develop hybrid electric vehicle (HEV). For HEV drives, inwheel electric motors mounted in the rear wheel axles have many advantages in that the front wheels and rear wheels can form a series-parallel drive combination without special mechanical coupling. Since the wheels of vehicles run in low speed and the dimension of electric machines is inversely proportional to its running speed, conventional direct drive electric motor is very bulk if the traction motor needs to produce a reasonable torque over a wide speed range. Normally, a mechanical gear is needed to reduce the motor speed. The use of a mechanical gear reduces the motor size, but additional space is needed for the gear. The mechanical gear also reduces the energy transmission efficiency. Recently, magnetic gears (MG) are proposed to compete with mechanical gears in terms of torque transmission capability and efficiency [1]. Compared to their mechanical counterparts, MGs have a highly competitive torque transmission capability with very high efficiency. The MG can be directly combined with a conventional permanent magnet (PM) motor inside one frame [2-4]. The system torque density can be significantly improved. However such system has two rotating parts. Its mechanical structure is complex and it runs noisily. A novel simple magnetic geared motor that integrates MG with a conventional outer-rotor PM brushless motor is presented recently [5-6]. According to the reported system, the MG is integrated with a conventional outer-rotor PM brushless motor and there is only one rotary part. The outerrotor is equipped with sintered NdFeB magnets. Fig. 1 shows such a motor. Its stator has a 3-phase concentrated winding which can produce a rotating magnetic field with 3 pole pairs, and the outer-rotor is equipped with 22 pole pairs. It has stationary iron segments which are made of silicon steel laminations to modulate the airgap field space harmonics, and the rotor can rotate at low speed. The operating principle of the setup is similar to that of MG. However, the high-speed rotary field is created by an armature rather than with magnets. The overall size of the unit is more compact than a motor and gear combination. In this paper the idea of flux-modulation is extended to axial-flux motors. A novel low-speed axial-flux-modulated motor (AFMM) for the in-wheel gearless drive of HEV is proposed. The AFMM has a 3-phase concentrated winding which can produce a rotary magnetic field with 3 pole pairs, and the outer-rotor has 22 pole pairs. Iron segments in the airgap are used to modulate the magnetic field. It can operate with high power density at low speed and hence can be used as direct drives in electric vehicles. Its manufacturing and assembling processes are simple when compared with those of radial-flux-modulated motor (RFMM). With AFMM, the front wheels and rear wheels can operate as a series-parallel drive without special mechanical coupling between them. The advantages of AFMM are: (1) Because of the space constraints in the wheel, the disc shape is well suited for direct coupling of the motor with a wheel. Because the ratio between the airgap diameter and the axial length of iron cores is large, the axial-flux design can further boost the torque density significantly. (2) The manufacture process of AFMM is much simpler than that of the radial-flux-modulated motor (RFMM). Both the iron segments and the stator are made from soft magnetic compound (SMC) materials in modular structures and can be assembled easily [7-9]. (3) The coils on the two sides of the stator core are wound back-to-back toroidally in order to shorten the length of the
end windings which share a common back iron, thereby saving the copper materials and improving the power density. (4) Because of the small number of stator slots, the slot space is used efficiently. (5) It has good heat dissipation because of the naturally formed ventilating ducts between the iron segments. II. BASIC DESIGN RULES OF FLUX-MODULATED MOTORS In flux-modulated motors (FMM), the numbers of pole pairs of the stator and the rotor are different. Usually the rotor has PMs and has large number of pole pairs to allow it to rotate at low speed efficiently. The stator has armature windings and it has small number of pole pairs and thus the number of slots can be small. The difference between FMM and normal motors is that the former has stationary iron pieces between the stator and rotor. These iron pieces modulate the magnetic field produced by the stator windings and the number of pole pairs of the strongest specific high-order harmonic is the same as that of the PM rotor. That means the magnetic field produced by rotor s PMs and the strongest high-order harmonic field produced by the stator winding will rotate at the synchronous speed of the motor. The constant average output torque is produced by the reactions between the magnetic fields of the stator and rotor. The theory of magnetic field modulation is the same as that of magnetic gears [1][5]. The gear ratio of the stationary iron pieces is: k1 pstator + k2niron Gr = (1) k1 pstator where p stator is the number of stator pole pairs and N iron is the number of stationary iron pieces. In (1) k 1 and k 2 can be 1, 3, 5,,, respectively. If k 1 = 1 and k 2 = -1, the stator winding will produce the highest space harmonic field. The number of rotor pole pairs p rotor should be G r p stator. Therefore, the relationship among the number of stator pole pairs p stator, the number of rotor pole pairs p rotor and the number of stationary iron pieces N iron is: N p = p (2) iron stator rotor Fig. 1. An overview of an intersection structure of a radial-flux-modulated PM motor (Motor I). In this paper an AFMM is presented and its performances are compared with that of a RFMM. In both motors the number of pole pairs in the stator p stator is 3 and the number of pole pairs in the rotor p rotor is 22. The number of stationary iron pieces N iron is 25. The gear ratio 3 25 22 G = = and r 3 3 G r pstator = 22 3 = 22 = p. The coil arrangement of the stator rotor 3 windings is the same as that of a conventional 3-phase motor with 3 pole pairs. The phases in the stator slots along the circumferential direction are: A, A, -C, -C, B, B, -A, -A, C, C, -B, -B. For AFMM, in the opposite side of the core, the current in each slot will be the same as that in slots on the other side but with opposite direction. To compare power densities, the RFMM and the AFMM are assumed to have the same outside dimensions. III. PERFORMANCE ANALYSIS OF RADIAL-FLUX-MODULATED MOTOR The original RFMM as shown in Fig. 1 is referred as Motor I. Its design data are listed in Table I. Taking into account of the limitation in current density (assumed to be 9 A/mm 2 in this study) in the conductors of the stator slots, the magnitude of the ampere-conductors IN slot N conductor is 28758 A, where I is the phase current, N slot is the number of stator slots and N conductor is the number of conductors in each slot. The performances of RFMM are analyzed using 2-D timestepping finite element method (FEM) of transient magnetic field electric circuit mechanical motion coupled model [10-11]. The FEM mesh is refined until the solution
differences between the final mesh with 27250 triangle elements and the previously refined mesh with 15448 triangle elements are within a small tolerance. When this problem is also solved in 3-D, the differences in solution accuracy are virtually unnoticeable. The magnetic torque is computed using the virtual work method [12-13]. The core loss is computed in time domain using the method presented in [14]. When the rotor is at full load, the computed torque is 58 Nm and the back emf is 37 V. The core loss is 282.3 W. The magnitude of this motor s cogging torque is 0.03 Nm. TABLE I DESIGN DATA OF THE RADIAL FM MOTOR (MOTOR I) Frequency 220 Hz Axial length of iron core 40 mm Total axial length 64 mm Outside radius of outer-rotor 92 mm Outside radius of PM 88.6 mm Inside radius of PM 80.8 mm Outside radius of stationary ring 80.2 mm Inside radius of stationary ring 67.2 mm Outside radius of stator 66.6 mm Inside radius of stator 17.5 mm Number of outer rotor pole pairs 22 Number of stationary iron pieces 25 Number of stator pole pairs 3 Number of stator slots 36 IV. PERFORMANCE ANALYSIS OF AXIAL-FLUX-MODULATED MOTOR The structure of AFMM is shown in Figs. 2 and 3. It is referred as Motor II. The coils on the two sides of the stator core are wound back-to-back toroidally. The basic structure of the PM and iron core is similar to those of normal axial-flux motors. The difference is that between the PM and the stator iron core, there are iron segments in the AFMM. Its design data are listed in Table II. To compare the performance of the two motors, both machines are assumed to have the same axial length, the same outside frame radius, the same phase number, the same stator and rotor pole number as well as the same total PM thickness. There are 3 phases in the two motors being studied. The supply frequency is 220 Hz and the rotor runs at 600 rpm. The basis for comparison on the power density of different motors is that the temperature rises at full-load are the same for the two motors. For simplicity, the temperature rises are assumed to be proportional to the total losses in the machines. As the core loss is only a small percentage of the total losses in these motors, it suffices to assume the temperature rises in both motors are the same if their copper losses are the same. The values of IN slot N conductor for the AFMM and RFMM are listed in Table III. The performances of AFMM are analyzed using 3-D timestepping FEM of transient magnetic field electric circuit mechanical motion coupled model [15]. The FEM mesh is refined to ensure reliable solutions. The solution differences between the mesh with 338798 tetrahedron elements and the mesh with 513987 elements are almost the same. The plot of magnetic flux density on the surface of rotor iron core is given in Fig. 4. It shows that at the rotor side, one can still see the three pole-pair magnetic field produced by the stator windings. The plot of magnetic flux density on the cross-section of the x-z plane (z is the axial direction of the motor) is shown in Fig. 5. The object in the center is the central piece of the stator iron core. It shows that the arrangement of polarizations of the PMs at the two sides of the stator is south pole to south pole and north pole to north pole. The torque curve and back emf curves versus time at full-load are shown in Figs. 6 and 7, respectively. It is noted that the torque ripple is not very large and the three-phase back emf waveforms are symmetrical. The magnitude of the cogging torque is 1.2 Nm. TABLE II DESIGN DATA OF THE AXIAL FM MOTOR (MOTOR II) Frequency 220 Hz Total axial length 64 mm Outside radius 92 mm Inside radius 60 mm Thickness of PM 3.9 mm Thickness of stationary iron 6.5 mm Airgap between PM and stationary iron 0.6 mm radius of stationary ring Airgap between stationary iron and stator 0.6 mm Number of outer rotor pole pairs 22 Number of stationary iron pieces 25 Number of stator pole pairs 3 Number of stator slots 36 TABLE III ELECTRIC LOADINGS OF THE TWO DIFFERENT MOTORS Motor type Motor I (Radial flux) Motor II (Axial flux) N slot 36 36 N conductor 5 5 I (A) 159.77 124.4
Fig. 2. An axial-flux-modulated-motor (Motor II) (assembled view). Fig. 4. The plot of magnetic flux density on the surface of rotor iron core (Motor II). The performances of the two motors are summarized in Table IV. The FEM simulations show that the AFMM produces about 40% higher torque when compared to that of RFMM. The coreloss of the AFMM is higher than that of the RFMM because the stator volume of the AFMM is smaller than that of the RFMM. However, because the AFMM has shorter end windings, the copper loss is small. Therefore, the total loss of the AFMM is smaller that those of the RFMM. Fig. 5. The plot of magnetic flux density on the cross-section of x-z plane (Motor II). Fig. 3. An axial-flux-modulated-motor (Motor II) (disassembled view). Fig. 6. The torque at full-load operation (Motor II).
Fig. 7. The induced emf at full-load operation (Motor II). TABLE IV COMPARISON OF DIFFERENT MOTORS Motor type Motor I (Radial flux) Motor II (Axial flux) Output torque (Nm) 58.0 81.2 Coreloss (W) 59.0 98.5 Copper loss (W) 282.3 223.3 Total coreloss and copper loss 341.3 321.8 V. CONCLUSION The RFMM can be modeled using 2-D FEM and the AFMM can be modeled using 3-D FEM. For the in-wheel drive of HEV, AFMM has higher power density than that of RFMM. The manufacturing process of AFMM is much simpler than that of RFMM. [6] W. N. Fu and S. L. Ho, A quantitative comparative analysis of a novel flux-modulated permanent magnet motor for low-speed drive, IEEE Trans. Magn., vol. 46, no. 1, pp. 127-134, Jan. 2010. [7] M. A. Khan, P. Pillay, R. Guan, N. R. Batane, and D. J. Morrison, Performance assessment of a PM wind generator with machined SMC cores, IEEE International Electric Machines & Drives Conference, vol. 2, 3-5 May 2007, pp. 1049-1053. [8] F. Marignetti, Colli, V. Delli, Stefano R. Di and A. Cavagnino, Design issues of a fractional-slot windings axial flux PM machine with soft magnetic compound stator, 33rd Annual Conference of the IEEE Industrial Electronics Society, 5-8 Nov. 2007, pp. 187-192. [9] F. Marignetti, G. Tomassi, P. Cancelliere, Colli, V. Delli, Stefano R. Di and M. Scarano, Electromagnetic and mechanical design of a fractional-slot-windings axial-flux PM synchronous machine with soft magnetic compound stator, 41st IAS Annual Meeting, vol. 1, Oct. 2006, pp. 62-69. [10] W. N. Fu, P. Zhou, D. Lin, S. Stanton and Z.J. Cendes, Modeling of solid conductors in two-dimensional transient finite-element analysis and its application to electric machines, IEEE Trans. Magn., vol. 40, no. 2, pp. 426-434, March 2004. [11] W. N. Fu and S.L. Ho, Enhanced nonlinear algorithm for the transient analysis of magnetic field and electric circuit coupled problems, IEEE Trans. Magn., vol. 45, no. 2, pp. 701-706, Feb. 2009. [12] W. N. Fu, P. Zhou, D. Lin, S. Stanton and Z. J. Cendes, Magnetic force computation in permanent magnets using a local energy coordinate derivative method, IEEE Tran. Magn., vol. 40, no. 2, pp. 683-686, March 2004. [13] W. N. Fu and S. L. Ho, Error estimation for the computation of force using the virtual work method on finite element models, IEEE Trans. Magn., vol. 45, no. 3, pp. 1388-1391, March 2009. [14] D. Lin, P. Zhou, W. N. Fu, Z. Badics and Z. J. Cendes, A dynamic core loss model for soft ferromagnetic and power ferrite materials in transient finite element analysis, IEEE Tran. Magn., vol. 40, no. 2, pp. 1318-1321, March 2004. [15] P. Zhou, W. N. Fu, D. Lin, S. Stanton and Z. J. Cendes, Numerical modeling of magnetic devices, IEEE Trans. Magn., vol. 40, no. 4, pp. 1803-1809, July 2004. ACKNOWLEDGMENT This work was supported in part by The Hong Kong Polytechnic University under Grant B-Q18X. REFERENCES [1] M. Aubertin, A. Tounzi and Y. Le Menach, Study of an electromagnetic gearbox involving two permanent magnet synchronous machines using 3-D-FEM, IEEE Trans. Magn., vol. 44, no. 11, Part 2, pp. 4381-4384, Nov. 2008. [2] K. T. Chau, Dong Zhang, J. Z. Jiang, Chunhua Liu and Yuejin Zhang, Design of a magnetic-geared outer-rotor permanent-magnet brushless motor for electric vehicles, IEEE Trans. Magn., vol. 43, no. 6, pp. 2504-2506, June 2007. [3] K. T. Chau, C. C. Chan, Liu Chunhua, Overview of permanent-magnet brushless drives for electric and hybrid electric vehicles, IEEE Trans. Industrial Electronics, vol. 55, no. 6, pp. 2246-2257, June 2008. [4] L. Jian, K. T. Chau and J. Z. Jiang, A magnetic-geared outer-rotor permanent-magnet brushless machine for wind power generation, IEEE Trans. Industry Applications, vol. 45, no. 3, pp. 954-962, May-June 2009. [5] L. L. Wang, J. X. Shen, Y. Wang and K. Wang, A novel magneticgeared outer-rotor permanent-magnet brushless motor, 4th IET Conference on Power Electronics, Machines and Drives, 2-4 April 2008, pp. 33-36.