MAGNETIC TOPOLOGY WITH AXIAL FLUX CONCENTRATION: A TECHNIQUE TO IMPROVE PERMANENT-MAGNET MOTOR PERFORMANCE
Keywords:DC Brushless rotating machines, Magnetic circuits, Permanentmagnets, Flux concentration, Synchronous motors
This article presents a different topology for the magnetic circuit of permanent-magnet motors, using the concept of Axial Flux Concentration. By means of longitudinal extension of the rotor length beyond the stator core, a substantial increase in the flux per pole is enabled, improving the general performance of the machine. Even in low cost motors with ferrite magnets, this improvement is easily achieved utilizing the proposed scheme. With rare-earth magnets this technique permits a higher torque density, contributing to the compactness of the machine. This configuration is suitable for several magnetic topologies, but particularly for small diameter and low pole number permanent-magnet motors of the embedded-magnet type.
The proposed topology is briefly described and the theoretical aspects presented. It is shown that, from the stator point of view, this scheme behaves like a conventional motor with fictitious equivalent magnets with augmented remanent flux density and recoil permeability. A 3D finite element modeling is performed to validate the theoretical analysis permitting some additional conclusions concerning magnetic saturation. The methodology is applied to a prototype brushless motor constructed for research purposes, composed of a stator and two rotors. Various characteristics are compared, concerning flux per pole, torque, power density and efficiency. The improvement in the performance of the axial flux concentration rotor is confirmed by experimental results, in accordance with the theoretical previsions.
 T. J. E Miller, Brushless Permanent-Magnet and Reluctance Motor Drives, Clarendon Press, 1993.
 A.J. Rix and M.J. Kamper, “Radial-Flux Permanent-Magnet Hub Drives: A Comparison Based on Stator and Rotor
Topologies,” IEEE Trans. Ind. Electron., vol. 59, No. 6,pp. 2475-2483, Jun. 2012.
 C. Mi, M. Filippa, W. Liu and R. Ma, “Analytical method for predicting the airgap flux of interior-type permanentmagnet machines,” IEEE Trans. Magnet., Vol. 40, No. 1, 50-58, January 2004.
 M. Aydin, S. Huang and T. A. Lipo, “Design, analysis, and control of a hybrid field-controlled axial-flux permanentmagnet motor,” IEEE Trans. Ind. Electron., vol. 57, No. 1, 78-87, Jan. 2010.
 P.B. Reddy, A.M. El Rafaie, K.K. Huh, J.K. Tangudu and T.M. Jahns, “Comparison of Interior and Surface PM
Machines Equipped With Fractional-Slot Concentrated Windings for Hybrid Traction Applications,” IEEE Trans.
Energy Conv., 2012. Digital Object Identifier 10.1109/TEC.2012.2195316.
 G. Pellegrino, A. Vagati, P. Guglielmi and B. Boazzo, “Performance Comparison Between Surface-Mounted and
Interior PM Motor Drives for Electric Vehicle Application,” IEEE Trans. Ind. Electron., vol. 59, No. 2, 803-811, Feb.
 K.I. Laskaris and A. G. Kladas, “Internal permanent magnet motor design for electric vehicle drive,” IEEE Trans. Ind.
Electron., vol. 57, No. 1, 138-145, Jan. 2010.
 R. S. Colby and D. W. Novotny, “Efficient operation of surface-mounted PM synchronous machines,” IEEE Trans. Ind.
Applicat., IA-23, 1048-1054, 1987.
 V. B. Honsinger, “The fields and parameters of interior type AC permanent magnet machines,” IEEE Trans. Power
Apparat. Syst., PAS-101, 867-875, 1982.
 D. Qin, R. Qu and T. A. Lipo, “A Novel Electric Machine Employing Torque Magnification And Flux Concentration Effects”,
Conference Record of the 1999 IEEE Industry Applications Conference. Thirty-Forth IAS Annual Meeting, vol. 1, 132-139, 1999.
Digital Object Identifier 10.1109/IAS.1999.799943.
 H. Yang et al., “A Flux-Concentrating External-Rotor Switched Flux Hybrid Magnet Memory Machine for Direct-Drive Automotive
Applications”, 2015 IEEE International Conference on Applied Superconductivity and Electromagnetic Devices (ASEMD), 209-201,
2015. Digital Object Identifier 10.1109/ASEMD.2015.7453539.
 A. Christen and V. V. Haerri, “Analysis of a Six- and Three-Phase Interior Permanentmagnet Synchronous Machine with Flux
Concentration for an Electrical Bike”, 2014 International Symposium on Power Electronics, Electrical Drives, Automation and
Motion, 1251-1255, 2014. Digital Object Identifier 10.1109/SPEEDAM.2014.6871995.
 I. E. Chabu, V. C. Silva, S. I. Nabeta, M. A. M. Afonso and J. R. Cardoso, “Axial flux concentration technique applied
to the design of permanent magnet motors: theoretical aspects and their numerical and experimental validation,” in
Conf. Rec. IEMDC’2005 – The IEEE International Electric Machines and Drives Conference, 1988-1994, May 2005.
 P. Viarouge, M. Lajoie-Mazenc and C. Andrieux, “Design and construction of a brushless permanent-magnet
servomotor for direct-drive application,” IEEE Trans. Ind. Applicat., IA-23, 526-531, 1987.
 T. Lipo, M. Aydin, “Field Weakening of Permanent Magnet Machines–Design Approaches”, presented at the 11th EPE
Power Electronics and Motion Control Conf., Riga, Latvia, Sep. 2-4, 2004.
 P. Sergeant, F. De Belie, J. Melkebeek, Senior Member, “Rotor Geometry Design of Interior PMSMs With and Without
Flux Barriers for More Accurate Sensorless Control,” IEEE Trans. Ind. Electron., vol. 59, No. 6, 2457-2465. Jun. 2012.
 R. Wrobel et al., “Rotor Design for Sensorless Position Estimation in Permanent-Magnet Machines,” IEEE Trans. Ind.
Electron., vol. 58, No. 9, 3815-3824, Sep. 2011.
 L. W. Matsch and J. D. Morgan, Electromagnetic and Electromechanical Machines, Harper & Row Publishers, 1986.
 M. Marinescu and N. Marinescu, “New concept of permanent magnet excitation for electrical machines: analytical and
numerical computation,” IEEE Trans. Magnet., Vol. 28, No. 2, 1390-1393, March 1992.
 Z. Q. Zhu and D. Howe, “Halbach permanent magnet machines and applications: a review,” Proc. IEE - Electr. Power
Appl., Vol. 148, No. 4, 299-308, July 2001.
 C. Breton et al., “Influence of machine symmetry on reduction of cogging torque in permanent-magnet brushless
motors,” IEEE Trans. Magnet., vol. 36, 3819 – 3823, 2000.
 A. M. El-Rafaie, T. M. Jahns and D. W. Novotny, “Analysis of surface permanent magnet machines with fractional-slot
concentrated windings,” IEEE Trans. Energy Conv., vol. 21, issue 1, 34-43, 2006.
 J. P. A. Bastos and G. Quichaud, “3D modelling of a nonlinear anisotropic lamination,” IEEE Trans. Magnet., vol. 21,
pp. 2366-2369, 1985.
 W. Fei and P. C. K. Luk, "An Improved Model for the Back-EMF and Cogging Torque Characteristics of a Novel Axial
Flux Permanent Magnet Synchronous Machine With a Segmental Laminated Stator,” IEEE Trans. Magnet., vol. 45, pp.
4609-4612, Oct. 2009.