Concentrated Winding Axial Flux Permanent Magnet Motor with Plastic Bonded Magnets and Sintered Segmented Magnets

Proceedings of the 28 International Conference on Electrical Machines Paper ID 1113 Concentrated Winding Axial Flux Permanent Magnet Motor with Plastic Bonded Magnets and Sintered Segmented Magnets Hanne Jussila, Pia Salminen, Asko Parviainen*, Janne Nerg and Juha Pyrhönen Elec. Eng. Dept., Technical University of Lappeenranta; PL 2, 53851 Lappeenranta, Finland Tel: (+358)-5-621-6788, fax: (+358)-5-621-6799 e-mail: hanne.jussila@lut.fi *AXCO-Motors Oy, Laserkatu 6, FIN-5385 Lappeenranta, Finland. Email: asko.parviainen@axcomotors.com Abstract- Direct drive axial flux permanent magnet (PM) motors are a cost effective and an energy saving choice for industrial use. Open slots make concentrated winding machines a favourable configuration with respect to manufacturing. However, open slots expose rotor surface magnets to large flux pulsations and the losses of sintered magnets may not be neglected. Plastic bonded magnets have very low eddy current losses but other magnetic properties of such magnet materials are not satisfactory at the moment. Divided sintered neodymium iron boron (NdFeB) may be used instead but the magnet configuration must be carefully analyzed to attain an acceptable eddy current losses level in the magnets. This paper addresses permanent magnet rotor constructions, which eliminate or remarkably reduce eddy-current losses in the magnets of a 25 min -1 / 3 min -1, 37 kw permanent magnet synchronous motor with concentrated windings. Different magnet materials, such as plastic-bonded Neo-magnets and sintered segmented NdFeBmagnets are evaluated. Also a prototype motor with plastic bonded magnets has been built. Analytical Matlab - and finiteelement-method-based (Flux2D/3D by Cedrat) programs are used in the calculations. The permanent magnets should be segmented into many small sections in the Machine which the flux pulsations in the magnets are high. I. INTRODUCTION An axial flux permanent magnet motor with two stators and one rotor is a favourable choice for applications with high torque rating. The benefits of the construction are the high torque density of the motor and balanced axial forces. In a concentrated wound machine utilizing open stator slots and a two layer winding scheme, each prefabricated coil is inserted around one tooth. This makes the machine winding manufacturing process easy and inexpensive, and consequently, the machine type is an interesting option in mass production. Furthermore, the open stator slot configuration is favourable when the minimizations of the iron losses are considered [1-3]. The electromagnetic performance of the concentrated wound permanent magnet machine differs from the conventional serial wound machine. In the serial wound machine the electromagnetic torque is due to the fundamental component of the stator magneto motive force and the permanent magnets. The fundamental component of the stator magneto motive force (mmf) has fewer poles than the rotor, and in which the electromagnetic torque is developed by the interaction of a higher order stator mmf harmonic with the permanent magnets. Therefore, the eddy current losses in permanent magnets could not be negligible. For example, the electromagnetic torque is developed by the interaction of the 5 th space harmonic 12-- slots-1-poles machine [4-6]. Sintered NdFeB magnets have a relatively high conductivity, that is, their resistivity is about 1.5 μωm at room temperature [7]. For comparison traditional construction steel Fe 52 (S355J / EN 125) has a resistivity of about.25 μωm which corresponds one sixth of the resistivity of NdFeB. Furthermore, the thermal conductivity of NdFeB is relatively poor, only about 9 W/mK which may cause problems with surface-magnet structures, since eddy currents caused by the spatial and current harmonics occur in the permanent magnet material. Because the heat conductivity is fairly poor and the power loss density may reach high values in the permanent magnet material, a fatal temperature rise in sintered bulky permanent magnets is possible. Fig. 1 a) illustrates the rotor pole equipped with only one piece of surface mounted sintered NdFeB magnet and b) the rotor pole segmented 2 pieces of NdFeB magnet. a) b) Fig. 1. a) The rotor pole is equipped with only one piece of surface mounted sintered NdFeB magnet and b) the rotor pole is segmented 2 pieces of NdFeB magnet. 978-1-4244-1736-/8/$25. 28 IEEE 1

Proceedings of the 28 International Conference on Electrical Machines Increase in temperature caused by eddy current losses can demagnetize the magnets and in any case reduce the efficiency of the machine. Plastic bonded magnets should offer a practically lossless alternative for the magnets, but the magnetic properties of the plastic bonded magnets available at the moment are still quite poor. For example, the maximum remanent flux density is usually well below 1 T, especially for the plastic bonded magnets tolerating temperatures above 1 C [8]. This paper presents a comparison of a permanent magnet motor equipped with different magnet materials and rotor constructions. The motor is realized with concentrated windings, in which the number of slots per pole and per phase is equal to.4. The machine is an axial flux PM machine with a 12-slot-1-pole configuration. The motor comprises of two stators connected electrically in series and one rotor between the stators. The rotor core material is non-conductive. Two different magnet materials are used. First material is the plastic bonded NdFeB, the remanent flux density being B r =.65 T and the coercive force is H C = 43 ka m -1. The second material is sintered NdFeB magnets are by Neorem 495a1 with the remanent flux density of B r = 1.5 T and the coercivity of H C = 8 ka m -1 (2 ºC) [7]. II. MOTOR DIMENSION The eddy current losses due the permanent magnets are not at an acceptable level with the open-slot 25 rpm / 3 rpm concentrated winding machine, if the rotor pole is equipped with only one piece of surface mounted sintered NdFeB magnet. The magnet eddy-current loss can be minimized by using segmented magnets as illustrated for example in [5, 9-11]. The main dimensions of the prototype machine are listed in table I. The length of the magnet material is kept as a constant value at each rotor design, because the shortening of the rotor would have led to mechanical problems [1]. TABLE I MOTOR MAIN DIMESION Number of stator slots 12 Number of rotor poles 1 The number of winding turns in phase per stator 64 Stator core outer diameter 274 mm Stator core inner diameter Stator apparent length Magnet thickness Average magnet width Relative slot opening 154 mm 6 mm 16 mm 53 mm.33 τ s One could imagine that the magnet eddy-current losses can be minimized by utilizing plastic-bonded magnets, because the eddy-current resistance is maximized. However, the use of plastic-bonded magnets leads to an increase in the volume of the machine, because the energy product of the plastic-bonded magnets is notably lower (4 96 kj/m 3 [12]) than that of sintered NdFeB magnets (19 38 kj/m 3 [12]). Thus the size of the plastic bonded magnets has to be larger and also more winding turns are needed but the permanent magnet losses should be negligible. III. DIFFERENT ANALYTICAL MODELS FOR EDDY CURRENT LOSSES IN PERMANENT MAGNETS A concentrated wound stator produces a large amount of current linkage harmonics travelling across the permanent magnets causing eddy currents. Also, the large slot openings cause flux density variations that generate eddy currents in the permanent magnets. In inverter operation the eddy-current losses are created by the time harmonics of the phase currents. Inverter-caused losses are ignored in this paper. However, they are analysed in [12]. Atallah, Ede and Toda [4-5, 11] presented analytical method that could be utilized to calculate eddy current losses in radial flux permanent magnets caused by stator magnetomotive force space harmonics. However, the effect of the stator slot openings was not considered. The results from the analytical calculations were compared with finite element analysis (FEA) calculations with unmagnetized permanent magnets. Polinder and Hoieijmakers [14] assumed that the magnet losses in radial flux permanent magnet machine caused by the space harmonics of the stator windings and the stator slotting are negligible. The approximation in this equation is based on the assumption that the magnet width is small. The results were compared with FEA calculations with unmagnetized permanent magnets. The analytical calculation method for eddy current losses in permanent magnets with axial flux machine is presented by Gieras [5]. The method take into account also stator slot openings using Heller and Hamata s equation. Number of other papers has been published in the field of analytical eddy current losses calculation in radial flux permanent magnets machines [16-18]. IV. FINITE ELEMENT METHODS Eddy-current losses in permanent magnets of 25/3 rpm and 37 kw permanent magnet synchronous motors with concentrated windings are calculated by using two-dimensional (2D) finite element analysis. Magneto-static and magnetodynamic time stepping method is used. The Joule losses in the magnets caused by the eddy currents are calculated in the FEA as [19] 2

Proceedings of the 28 International Conference on Electrical Machines P m = ρ J dv, (1) V m 2 where ρ m is the material resistivity, V the volume and J the current density. V. RESULTS OF FEA AND ANALYTICAL CALCULATIONS Fig. 2 shows a comparison of the air gap flux density waveforms and Fig. 3 shows a comparison of the phase backelectromagnetic force voltage waveforms. The rotor pole equipped with only one piece of surface mounted sintered NdFeB magnet and the rotor pole which is segmented to 2 pieces of NdFeB magnet. NdFeB magnet remanent flux density used is Br =.9 T @ 12 ºC (Br = 1.5 T @ 2ºC). The FEA calculation is done by using a 2D solution. Fig. 2 and Fig. 3 show the effect of the segmented magnets. It can be seen in Fig. 2, that there is a small ripple in the airgap flux density which is caused by air between segmented magnets. It is shown in Fig.3, that the amplitude of the backemf is smaller with segmented magnets than the one magnet per pole version. Fig. 4 shows the eddy current losses in permanent magnets calculated using 2D-FEA. FEA calculation is done for three different methods. FEA results utilizing no load method take into account only eddy current losses caused by the stator slot openings. Magneto-dynamic calculation using unmagnetized magnets takes into account only the eddy-current losses caused by stator magneto-motive force space harmonics. Load method takes into account both the aforementioned loss components. The comparison of three different analytical methods and FE results is presented in Figs. 5 and 6. In Fig. 4 it is shown that the slot openings have significant effect on eddy current losses in permanent magnet. Those losses are bigger than the eddy-current losses caused by stator magneto-motive force space harmonics. Those losses could be reduced using semi-closed slot, but then the machine winding manufacturing process is not so easy. Fig. 5 shows the comparison of the different analytical methods by [4] and [14] and FEA 2D. Calculation with unmagnetized magnets takes into account only the eddy-current losses caused by stator magneto-motive force space harmonics. 6 Flux density (T) 1.5 1..5. 4. 6. 8. 1. 12. 14. 16. -.5-1. One magnet pole 2 segments in one magnet pole 5 4 3 2 1 2D load 2D unmagnetized 2D no load -1.5 Airgap (mm) Fig. 2. Comparison of the air gap flux density distribution only one piece of surface mounted sintered NdFeB magnet and the rotor pole is segmented 2 pieces of NdFeB magnet. Rotational speed 25 rpm and magnetic flux density.9 T. Back EMF (V) 2 15 1 5.13.14.15.16.17.18-5 -1-15 -2 Time (s) One magnet pole 2 segments in one magnet pole Fig. 3. Comparison of the induced phase-emf voltage waveforms only one piece of surface mounted sintered NdFeB magnet and the rotor pole is segmented 2 pieces of NdFeB magnet. Rotational speed 25 rpm and magnetic flux density.9 T. 5 1 15 2 25 Fig. 4. Eddy current losses in magnets using 2D using three different FEA methods, no load, unmagnetized and load situations. 5 45 4 35 3 25 2 15 1 5 2D unmagnetized Toda unmagnetized Polinder 5 1 15 2 25 Fig. 5. Eddy current losses in magnets unmagnetized situation for three different methods: Atallah, Polinder and 2D. 3

Proceedings of the 28 International Conference on Electrical Machines 1 9 8 2D load Gieras 7 6 5 4 3 2 1 5 1 15 2 25 Fig. 6. Eddy current losses in magnets for two different method: load situation (FEA 2D) and Gieras. Fig. 6 shows the comparison of the different analytical method by [15] and FEA 2D. Calculated magnet remanent load situation takes into account the eddy-current losses caused by stator magneto-motive force space harmonics and stator slot openings caused eddy current losses. In Figs. 5 and 6 it can be seen that the analytical methods shown are in good agreement with FEA results especially with segmented magnets. The different analytical methods by [4] and [14] can be used as an estimation of the eddy current losses in the segmented magnets without slot openings. The eddycurrent losses caused by stator magneto-motive force space harmonics should be quite small if open slot structure is used, because stator slot openings caused significant amount of eddy current losses. The analytical method by [15] can be used as an estimation the total eddy current losses in permanent magnets. The influence of skin effect and saturation are neglected in the analytical models [4, 14-15]. VI. RESULTS OF PROTOTYPE Fig. 7 and 8 show two different views of the 37 kw plastic bonded magnet prototype machine. The machine is the double stator axial flux machine with the 12-slots-1-poles. Magnets are assembled on the rotor surface and open stator slots structure is used. The motor is air cooled. The motor main cooling air flow of the driven machinery and the fan and is also used (Fig. 8). The machine is now under measurement. Fig. 8. Prototype machine assembled. Figs. 9 and 1 shows the measured phase back-emf voltages with plastic bonded magnets compared to the FEA results. The FEA calculation is done by using 2D (Fig. 9) and 3D solutions (Fig. 1). Back EMF (V) 3 2 1-1 -2-3 Phase 1 (m) Phase 1 (FEA).1.2.3.4.5.6 Time (s) Fig. 9. Comparison of calculated (FEA 2D) and measured phase back-emf voltages at 3 rpm with plastic bonded magnets. 4 3 2 Phase 1 Phase 2 Phase 3 3D_1 3D_2 3D_3 1-1.5.1.15.2-2 -3 Fig. 7. The stator of the prototype machine. Fig. 1. Comparison of calculated (FEA 3D) and measured induced phase back-emf voltage waveforms at 3 rpm with plastic bonded magnets. 4

Proceedings of the 28 International Conference on Electrical Machines Figs. 9 and 1 it can be seen that the measured and FEA (2D and 3D) calculated back-emf waveforms are quite similar. Using 3D FEA the time step should be thicker than Fig. 1 is used. Some of the measurement results of the prototype machine are listed in table II. The half power was used in the first measurement. TABLE II MEASUREMENT RESULTS WITH PLASTIC BONDED MACHINE WITH HALF POWER Power 18.5 kw Torque Current 58.9 Nm 35.5 A The results have good agreement with analytical calculations with half power. VII. CONCLUSION Finite element analysis was used in analyzing the eddycurrent losses in PM motors with rotor surface mounted magnets and open stator slots. The eddy-current losses depend considerably on the magnet material and the geometry of the machine. The value of the eddy-current losses in bulky sintered NdFeB magnets is not acceptable. Such magnets cannot be used at elevated speeds in machines, in which the flux pulsations in the magnets are high. Because of this the permanent magnets should be segmented into many small sections. Also, the use of the ferrite or plastic-bonded magnets should take into account. machines, Prog. of SPEEDAM 28, June. 28, Ischia Italy, CD- ROM. [1] C. Deak, A. Binder and K. Magyari, Magnet loss analysis of permanent magnet synchronous motors with concentrated windings, Prog. of ICEM 26, Sept. 26, Chania Creece, CD-ROM. [11] H. Toda, Z. Xia, J. Wang, K. Atallah and D. Howe, Rotor eddy-current loss in permanent magnet brushless machines, IEEE transaction on magnetics, vol. 4, no. 4, July 24. [12] P. Campell, Permanent Magnet Materials and their Application. Great Britain: Cambridge University Press. 1994. [13] J. Nerg, M. Niemelä, J. Pyrhönen and J. Partanen, FEM Calculation of Rotor Losses in a Medium Speed Direct Torque Controlled PM Synchronous Motor at Different Load Conditions, IEEE transaction on magnetics, vol. 38, no. 5, pp. 3255 3257, September 22. [14] H. Polinder and M.J. Hoieijmakers, Eddy-current losses in the segmented surface-mounted magnets of a PM machine, In IEEE Proc.- Electr. Power Appl., Vol. 146, No. 3, May 1999, pp. 261-266 [15] F. Gieras, R. Wang, M. Kamper, Axial Flux Permanent Magnet Machines. Springer Science + Business Media B.V. 28. [16] E. Nipp, Permanent magnet motor drives with switched stator windings. Dissertation. Royal institute of technology, Stocholm, Sweden. ISSN- 112-172. 32p. 1999. [17] F. Sahin, A.M. Tuckey and A.J.A. Vandenput, Design, development and testing of a high-speed axial-flux permanent-magnet machine, Proc. 36th IEEE-Industry- Applications Society Conf. Vol. 3, pp. 164-1647. 21. [18] A.M. El-Rafaie and T.M. Jahns, Impact of winding layer number and magnet type on synchronous surface PM machines designed for wide constant-power speed range operation, IEEE transaction on energy conversion, vol. 23, no. 1, March 28. [19] Cedrat 27. Software solutions: Flux. [Online] Available from http://www.cedrat.com/ [Date accessed 27 June 27] REFERENCES [1] A. Parviainen, Design of axial-flux permanent-magnet low-speed machines and performance comparison between radial and axial-flux machines. Dissertation. Acta Universitatis Lappeenrantaensis 28. Lappeenranta University of Technology, Finland, 25. [2] P. Salminen, J. Pyrhönen, A. Parviainen, H. Jussila, and M. Niemelä, Concentrated Wound PM Motors with Semiclosed Slots and with Open Slots, unpublished. [3] P. Salminen, Fractional slot permanent magnet synchronous motor for low speed applications. Dissertation. Acta Universitatis Lappeenrantaensis 198. Lappeenranta University of Technology, Finland, 24. [4] K. Atallah, D. Howe, P.H. Mellor, and D.A. Stone, Rotor Loss in Permanent Magnet Brushless AC Machines, IEEE Trans. Ind. Appl., vol. 36, no. 6 pp. 1612 1618, Nov./Dec. 2. [5] J.D. Ede, K. Atallah and G.W. Jewell, Effect of axial segmentation of permanent magnets on rotor loss in modular permanent-magnet brushless machines, IEEE transactions on industry applications,, vol. 43, no. 5, Sep./Oct. 27. [6] Z. Q. Zhu and D. Howe, Analytical prediction of the air-gap field distribution in permanent magnet motors accounting for the effect of slotting, In ISEF 1991 Proc., Southampton, UK, 1991. pp. 181 184 [7] Neorem 27. Neorem Magnets. [Online] Available from http://www.neorem.fi / [Date accessed 27 June 27] [8] M. Kume, M. Hayashi, M. Yamamoto, k. Kawamura and k. Ihara, Heat resistant plastic magnets, IEEE transaction on magnetics, vol. 41, no. 1, October 25. [9] C. Deak, l. Petrovic, A. Binder, M. Mirzaei, D. Irimie and B. Funieru, Calculation of eddy current losses in permanent magnets of synchronous 5