The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Modeling Demagnetization of Sintered NdFeB Magnet Material in Time-Discretized Finite Element Analysis

Sami Ruoho

Doctoral dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Electronics, Communications and Automation for public examination and debate in Auditorium S4 at the Aalto University School of Electrical Engineering (Espoo, Finland) on the 14th of January 2011 at 12 noon.

Overview in PDF format (ISBN 978-952-60-4001-1)   [1233 KB]
Dissertation is also available in print (ISBN 978-952-60-4000-4)

Abstract

The aim of this work was to develop a tool able to simulate the behavior of a permanent magnet machine after demagnetization. The tool would include a demagnetization model, an eddy current model, and a thermal model. The eddy current calculation accuracy in two-dimensional geometries will also be improved. The other goals were to study how the demagnetization should be modeled in different situations and to evaluate a mixed-grade pole idea, where there can be several magnet grades in a pole of a machine.

A demagnetization model based on an exponential function was developed. The new model can be defined by using only four parameters. The new model can take into account the squareness of the hysteresis curve. The new model also takes into account the demagnetizing field perpendicular to the orientation direction, which is often ignored. The demagnetization model was implemented in an existing finite element method model. The demagnetization model was evaluated by modeling a locked-rotor situation of a permanent magnet machine. The simulation results were compared with the demagnetization of the magnets of a real motor after the same situation. It was discovered that the demagnetization model can accurately predict the demagnetization of the magnets in a permanent magnet machine.

The resistivity of NdFeB permanent magnet material was measured as a function of temperature. The resistivity of rare earth magnet materials was found to be anisotropic. It was shown that the resistivity can be treated as an isotropic scalar property, as long as the resistivity value perpendicular to the magnetization direction of the magnets is used.

An eddy current model was developed. The eddy current model modifies the resistivity of the magnet material as a function of temperature and as a function of the shape of the magnet. The modification as a function of the shape was shown to improve the accuracy of the eddy current calculation in two-dimensional modeling.

The modeling of the demagnetization was studied with simulations using an overheated motor loaded with a constant torque as an example. It was shown that it is important to include a thermal model in the demagnetization calculations.

The mixed-grade pole machine was used as a calculation example in the simulations. It was shown that a slight improvement in the performance of the machine can be achieved with a simultaneous potential for cost savings by using a mixed-grade pole.

This thesis consists of an overview and of the following 9 publications:

  1. Sami Ruoho, Emad Dlala, and Antero Arkkio. 2007. Comparison of demagnetization models for finite-element analysis of permanent-magnet synchronous machines. IEEE Transactions on Magnetics, volume 43, number 11, pages 3964-3968. © 2007 Institute of Electrical and Electronics Engineers (IEEE). By permission.
  2. Sami Ruoho and Antero Arkkio. 2007. Mixed-grade pole design for permanent magnet synchronous machines. In: Proceedings of the 5th International Aegean Conference on Electrical Machines and Power Electronics and 7th International Symposium on Advanced Electromechanical Motion Systems Joint Conference (ACEMP 2007 & Electromotion 2007). Bodrum, Turkey. 10-12 September 2007. Pages 452-457. ISBN 1-4244-0891-1. © 2007 Institute of Electrical and Electronics Engineers (IEEE). By permission.
  3. Sami Ruoho. 2007. A mathematical method to describe recoil behavior of Nd-Fe-B-material. In: Seminar on Advanced Magnetic Materials and their Applications 2007. Pori, Finland. 10-11 October 2007. Seminar presentation. © 2007 Prizztech Ltd, Magnet Technology Centre. By permission.
  4. Sami Ruoho and Antero Arkkio. 2008. Partial demagnetization of permanent magnets in electrical machines caused by an inclined field. IEEE Transactions on Magnetics, volume 44, number 7, pages 1773-1778. © 2008 Institute of Electrical and Electronics Engineers (IEEE). By permission.
  5. Sami Ruoho, Minna Haavisto, Eelis Takala, Timo Santa-Nokki, and Martti Paju. 2010. Temperature dependence of resistivity of sintered rare-earth permanent-magnet materials. IEEE Transactions on Magnetics, volume 46, number 1, pages 15-20. © 2009 Institute of Electrical and Electronics Engineers (IEEE). By permission.
  6. Sami Ruoho, Timo Santa-Nokki, Jere Kolehmainen, and Antero Arkkio. 2009. Modeling magnet length in 2-D finite-element analysis of electric machines. IEEE Transactions on Magnetics, volume 45, number 8, pages 3114-3120. © 2009 Institute of Electrical and Electronics Engineers (IEEE). By permission.
  7. Sami Ruoho, Jere Kolehmainen, and Jouni Ikäheimo. 2008. Anisotropy of resistivity of Nd-Fe-B magnets - Consequences in eddy-current calculations. In: Dimitris Niarchos (editor). Proceedings of the 20th International Workshop on Rare-Earth Permanent Magnets and their Applications (REPM 2008). Crete, Greece. 8-10 September 2008. Pages 87-90. ISBN 978-960-86733-6-6. © 2008 National Centre for Scientific Research (NCSR) "Demokritos". By permission.
  8. Sami Ruoho, Jere Kolehmainen, Jouni Ikäheimo, and Antero Arkkio. 2009. Demagnetization testing for a mixed-grade dovetail permanent-magnet machine. IEEE Transactions on Magnetics, volume 45, number 9, pages 3284-3289. © 2009 Institute of Electrical and Electronics Engineers (IEEE). By permission.
  9. Sami Ruoho, Jere Kolehmainen, Jouni Ikäheimo, and Antero Arkkio. 2010. Interdependence of demagnetization, loading, and temperature rise in a permanent-magnet synchronous motor. IEEE Transactions on Magnetics, volume 46, number 3, pages 949-953. © 2010 Institute of Electrical and Electronics Engineers (IEEE). By permission.

Keywords: permanent magnet, demagnetization, electric machine, finite element method

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Last update 2011-05-26