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Soft Magnetic Composite Axial Flux Seven-Phase Machine F. Locment, E. Semail, and F. Piriou

Abstract—A fault-tolerant seven-phase axial-flux permanent magnet (AFPM) machine with a soft magnetic composite (SMC) stator has been designed thanks to analytical and 3D finite element methods. In this paper, we present its characteristics and show that a high number of phases allows to get a good quality of the torque with a design simpler than this one of the three-phase AFPM machines. Experimental back-electromotive forces (backEMF) and the cogging torque are then compared with predeterminations. Finally, experimental torque of a drive composed of a seven-leg Voltage Source Inverter (VSI) supplying the machine in a vector control is presented. Index Terms—multiphase machine, axial flux, soft composite material.

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I. INTRODUCTION

ARE-earth magnets with high flux density reduce the importance of a high permeability for electromagnetic devices. When the use of laminated steel is not easy, other materials such as SMC can become interesting [1]-[3]. The AFPM machines are an example since one of their critical issues is the manufacturing of the stator with laminated steel. If we suppose that the core is made of one sheet, the axial direction of the flux imposes to wound this steel sheet in a spiral fashion to form a ring. Moreover if the stator must be slotted, it is still more difficult because for example it is not easy to preserve the insulation between the steel sheets during the manufacturing. Compared with solid steel which is also easy to work, the SMC presents a higher resistivity and prevents therefore eddy currents. Consequently, various slotted AFPM machines with SMC have been developed [4][6]. For the AFPM machine, another issue is the winding arrangement. Compared with the radial flux machines, there is an asymmetry for the windings since there is less space at inner radius than at the outer radius. So, simple windings are used with usually one conductor per pole and per phase. The consequence is that the back-EMF are not sinusoidal [6], [7] unless using special shapes of magnets [3], [8], [9]. If the Manuscript received June 30, 2006. This work is part of the project ’Futurelec2’ within the ’Centre National de Recherche Technologique (CNRT) de Lille’ with EDF and AREVA/Jeumont. F. Locment and E.Semail are with the L2EP, ENSAM, 8 Bd Louis XIV 59046 FRANCE, (e-mail: [email protected]). F. Piriou is with the L2EP, USTL, Bât P2 59655 FRANCE, (e-mail: [email protected]).

machine has only three phases this implies a difficulty to get a torque without ripples since there are interactions between the first harmonic of current with the 5th and 7th harmonics of back-EMF [10]. That is a problem when the AFPM machine must be used in low speed drives without reduction gear. For multiphase machines it has been shown that in steady state the first harmonic of the pulsating torque due to interaction between back-EMF and currents increases with the number of phases. Moreover the torque ripple can be theoretically equal to zero even with non-sinusoidal backEMF due to the fact that interactions between harmonics are different for multi-phase machines [11], [12]. For multiphase machines it is not necessary to achieve complex shapes of magnets in order to get low torque ripple. The active surface of the rotor magnets is then more important. The last issue widely studied for AFPM machine is the cogging torque. We will discuss only of the case of AFPM machine with two external rotors and one stator since the machine presented in the paper is of this type. In order to reduce the cogging torque, various techniques are proposed for 3-phase machines such as shifting between the two rotor discs [7], magnet skewing [7], [9], [10], various pole pitches [12] or stator side displacement [15]. For our multiphase machine shifting between the two rotor discs has been chosen. This simple technique is more interesting with a 7-phase machine than with a 3-phase one. Besides, multi-phase machines offer also another way to reduce the cogging torque by using fractional winding. A 9-phase AFPM machine is studied in [16] and a 7-phase radial-flux machine in [17]. Moreover, multiphase machines are intrinsically more reliable than 3-phase ones [18]. This property that could be interesting for off-shore wind generator and more generally for embedded systems is studied in [19]. In the paper, the experimental machine is presented and main features are justified. Then experimental results such as back-EMF and cogging torque are compared with results obtained by 3D finite element method (3D-FEM) software1. Finally, we examine in generator mode the drive composed of the AFPM machine and a seven-leg VSI: efficiency and torque in steady state are presented.

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CARMEL developed in our laboratory

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2 II. PRESENTATION OF THE PROTOTYPE

Among the various existing structures of AFPM machine, the studied prototype (Fig. 1) is a slotted TORUS AFPM machine (called TORUS-S in [9] and [10]). It possesses one torus-shape stator (Fig. 2) sandwiched between two external rotors (Fig. 1 and Fig. 3). Even if slotless TORUS AFPM machine [19] have not cogging torque and are easier to manufacture than AFPM machine with slots, the weak values of inductances and consequently of the time constants imply that the carrier frequency of the Pulse Width Modulation (PWM) VSI must be high. It is the reason why the TORUS-S configuration has been chosen. This kind of AFPM machine has still two different topologies depending on the flux paths (see Fig. 4). They are described in [9] and called NN type TORUS-S and NS type TORUS-S. The studied prototype is an NN type TORUS-S AFPM machine. This kind of topology allows a Gramme-Ring winding which presents fewer constraints for end-winding at inner radius than the NS type TORUS-S AFPM machine. On the contrary the thickness of the stator is more important.

Fig. 3 One of the two rotors with shape of one magnet.

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Fig. 4 Two different topologies for TORUS-S AFPM machine. TABLE I PARAMETERS AND MACHINE DIMENSIONS Number of poles (2p) Number of phases Number of slots Pole-arc-ratio Core outer diameter Do (mm) Core inner diameter Di (mm) Axial length of stator core (mm) Axial length of one rotor core (mm) Axial length of one magnet (mm) Length of one airgap (mm) Slot dimensions (mm)

Fig. 1 One sixth of the machine.

Fig. 2 Torus stator of the AFPM machine.

The characteristics of the prototype are resumed in table I. Designed in our laboratory, the prototype has been manufactured by Selem Electrotechnologies ([email protected]).

Number of wires in a slot Slot fill factor (%) Wire type Core soft composite material type (QMP) Magnet type Mean flux density in airgap (T) Cogging torque peak (Nm) Cogging torque p.u. Electrical loading at (Do+Di)/4 (kA/m) Nominal Speed (rpm) Nominal Torque with 50°C in winding (Nm) Nominal power at 750 rpm (kW) Nominal current at 65 Nm (A)

6 7 42 0.8 310 154 78.3 18.4 2.8 1 11.5 × 6.3 40 44 AWG18 ATOMET EM1 NdFeB N48 0.7 4 0.06 11,5 750 65 5.1 5

A 4/5 arc pole for the magnet repartition (Fig. 3) allows the cancellation of the fifth harmonic. The reason of this choice is linked with considerations of vector control of the machine. A wye-coupled 7-phase machine can be considered effectively as the association of three 2-phase fictitious machines M1, M2 and M3 magnetically independent [12]. Each machine is characterized by a harmonic family given in

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pulsating torque. Experimental results confirm the predeterminations obtained by 3D-FEM for the sensitive quantities which are the harmonics of back-EMF and the cogging torque. Finally, a vector control confirms that the mechanical torque has not significant pulsations. Moreover, this machine, simple to manufacture, has also an aptitude to fault mode operation. This feature is developed in [19]. REFERENCES [1]

[2] [3] [4]

[5] [6] [7]

[8] [9]

A.G. Jack, B.C. Mecrow, P. G. Dickinson, D. Stephenson, J. S. Burdess, N. Fawcett, and J. T. Evans, “Permanent-Magnet Machines with Powdered Iron Cores and Prepressed Windings”, IEEE Trans. on Industry Appl., Vol. 36, issue 4, pp. 1077-1084, July-Aug 2000. J. Cros, P. Viarouge, and A. Halila, “Brush DC motors with concentrated windings and soft magnetic composites armatures”, Proc. of IEEEIAS’01, Vol.4, pp 2549-2556, Chicago (USA), Oct. 2001. S.M. Madani, T.A. Lipo, C.E. Nino, and D. Lugo, “A New Trapezoidal Shaped-Pole Permanent Magnet Machine”, Proc. of IEEE-IEMDC’05, pp. 1715-1719, San Antonio (USA), May 2005. A.G. Jack, B.C. Mecrow, G. Nord, and P.G. Dickinson, “Axial Flux Motors Using Compacted Insulated Iron Powder and Laminations – Design and Test Results”, Proc. of IEEE-IEMDC'05, pp. 378-385, San Antonio (USA), May 2005. P. Anpalahan, A. Walker, S. Meister, S. Tsakok, and M. Lampérth, “Axial Flux Machine Stator Construction with Concentrated Windings”, Proc. of ICEM’04, Sept 2004, Cracow (Poland), CD-ROM. Y. Chen, and P. Pillay, “Axial-flux PM wind generator with a soft magnetic composite core”, Proc. of IEEE-IAS’05, Hong-Kong, Vol. 1, pp. 231-237, Oct. 2005. F. Caricchi, F. G. Capponi, F. Crescimbini, and L. Solero, “Experimental study on reducing cogging torque and no-load power loss in axial-flux permanent-magnet machines with slotted winding”, IEEE Trans. on Industry Appl., Vol. 40, issue 4, pp. 1066-1075, July/Aug. 2004. A. Parvianen, M. Niemela and J. Pyrhonen, “Modeling of Axial Flux Permanent-Magnet Machines”, IEEE Trans. on Industry Appl., Vol. 40, issue 5, pp. 1333-1340, Sept/Oct 2004. S. Huang, M. Aydin and T.A. Lipo, “TORUS Concept Machines: PrePrototyping Assessment for Two Major Topologies”, Proc. of IEEEIAS’01, pp. 1619-1625, Chicago (USA), Oct. 2001.

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[10] S. Huang, M. Aydin, and T.A. Lipo, “Torque Quality Assessment and Sizing Optimization for Surface Mounted Permanent Magnet Machines”, Proc. of IEEE-IAS’01, pp. 1603-1610, Chicago (USA), Oct. 2001. [11] H.A. Toliyat, T.A. Lipo, and J. C. White, “Analysis of a Concentrated Winding Induction Machine for Adjustable Speed Drive Application Part 2 (Motor design and Performance)”, IEEE Trans. on Energy Conversion, Vol. 6 no. 4, pp. 684-692, Dec. 1991. [12] E. Semail, X. Kestelyn, and A. Bouscayrol, “Right Harmonic Spectrum for the back-electromotive force of a n-phase synchronous motor”, Proc. of IEEE-IAS’04, Vol. 1, pp.71-78, Seattle (USA), Oct. 2004. [13] H-M Ryu, J-W Kim and S-K Sul, “Synchronous Frame Current Control of Multi-Phase Synchronous Motor, Part I. Modeling and Current Control Based on Multiple d-q Spaces Concept Under Balanced Condition”, Proc. of IEEE-IAS’04, Vol. 1, pp. 56-63, Seattle (USA), Oct. 2004. [14] M. Aydin, R. Qu, and T. A. Lipo, “Cogging torque minimization technique for multiple-rotor, axial-flux, surface-mounted-PM motors: alternating magnet pole-arcs in facing rotors”, Proc. of IEEE-IAS’03, Vol. 1, pp. 555-561, Salt Lake City (USA), Oct. 2003. [15] A. Letelier, J. A. Tapia, R. Wallace, and A. Valenzuela, “Cogging Torque Reduction in an Axial Flux PM Machine with Extended Speed Range”, Proc. of IEEE-IEMDC'05, pp. 1261-1267, San Antonio (USA), May 2005. [16] D. Vizireanu, X. Kestelyn, S. Brisset, P. Brochet, and E. Semail, “Experimental tests on a 9-phase Direct Drive PM Axial-Flux Synchronous Generator”, Proc. of ICEM’06, Chania, (Greece), Sept. 2006, CD-ROM. [17] F. Scuiller, J.F. Charpentier, E. Semail, and S. Clenet, “A global design strategy for multiphase machine applied to the design of a 7-phase fractional slot concentrated winding PM machine”, Proc. of ICEM’06, Chania, (Greece), Sept. 2006, CD-ROM. [18] T.M. Jahns, “Improved reliability in solid state ac drives by means of multiple independent phase-drive units”, IEEE Trans. on Industry Appl., vol. IA-16, pp. 321-331, May-June 1980. [19] F. Locment, E. Semail, and X. Kestelyn, “A vector controlled axial flux seven-phase machine in fault operation”, Proc. of ICEM’06, Chania, (Greece), Sept. 2006, CD-ROM. [20] B.J. Chalmers, W. Wu, and E. Spooner, “An axial-flux permanentmagnet generator for a gearless wind energy system”, IEEE Trans. on Energy Conversion, Vol. 14, n°2, pp. 251-257, June 1999.

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