Dissertation: On the System Optimization of Magnetic Circuit with Alternative Permanent Magnets and its Demagnetization

  • Date:
  • Location: Zoom
  • Doctoral student: Jonathan Sjölund
  • Organiser: Department of Electrical Engineering
  • Contact person: Jonathan Sjölund
  • Disputation

Jonathan Sjölund defends his doctoral thesis: On the System Optimization of Magnetic Circuit with Alternative Permanent Magnets and its Demagnetization.

Opponent: Dr. Sami Ruoho

Main supervisor: Professor Sandra Eriksson


Permanent magnet (PM) machines are often associated with the usage of rare earth magnets, due to their high energy density. One such rare earth magnet is the neodymium-iron-boron (NdFeB), which is mainly produced in China. Due to the global scarcity of the rare earth magnets, much interest is put into utilizing other permanent magnet materials. Among those materials is the category of ferrite permanent magnets, known for having lower magnetic properties than NdFeB. Ferrites share some of the properties with NdFeB that makes simulations simpler, namely that they have, at least, partly linear behavior in the demagnetization curve. The lower coercive properties of ferrites can, however, force them more easily into the non-linear regions of the demagnetization curves, resulting in a gradual irreversible demagnetization that lowers the performance of the ferrites. 

In this thesis, the magnetic circuits of electrical machines with ferrites are investigated. The implications of the reduced coercive properties are studied and means to account for the irreversible demagnetization when designing the magnetic circuit. An optimization methodology for the magnetic circuit in a linear generator is developed and presented. It is found that the coercive properties may influence the PM geometry, for the given penalty for demagnetization. By proper pole shoe design, one can reduce the inclination angle of the magnetic fields inside the PMs. The difference in topology between the surface mounted NdFeB and the buried ferrites is studied regarding the inherent longitudinal end forces of linear machines. It is found that the end forces can be reduced under both no-load and load by alterations of the stator ends.  Electrical machine simulations in finite element software are often done in a two-dimensional cross section of the machine. The difference between the two-dimensional cross section and the more accurate three-dimensional model is investigated, showing that the magnetic end leakage flux in the end regions can cause a discrepancy between the two models. 

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Link to the dissertation in Diva