Irreversible demagnetisation of a permanent magnet (PM) at high temperature can be caused by the field distributions because of the stator currents. It can be prevented if the magnet thickness is sufficient. However local irreversible demagnetisation can occur even if the magnet thickness is sufficient. The local irreversible demagnetisation is varied according to the magnetic path. To analyse the cause of local irreversible demagnetisation, the authors introduced an interior PMSM (IPMSM), consisting of a V-shaped magnet and concentrated winding, and performed a parameter analysis of the magnetic field based on the barrier width and magnet position considerably related to the magnetic path using the finite-element method. On the basis of the results, the authors developed a model avoiding local irreversible demagnetisation. A maximum load test was performed after a thermal saturation test to confirm whether irreversible demagnetisation of the manufactured models occurred. Finally, to verify the validity of the finite-element analysis (FEA) results, the torque, efficiency, power factor and induced voltage of the manufactured model, which were obtained by FEA, were compared with the test results.

Local demagnetisation analysis of a permanent magnet motor

A hybrid method, called DBCI (Dirichlet boundary condition iteration), is described for the finite element computation of time-harmonic eddy current problems in axisymmetric openboundary geometry. The procedure couples a differential equation for the interior problem with an integral equation for the exterior one. The global algebraic system is efficiently solved in an iterative way. A validation example is also given. Introduction Various methods have been devised for the finite element method (FEM) computation of static and quasi-static electromagnetic field problems in infinite domains, such as the hybrid FEM/BEM (boundary element method), infinite elements, and co-ordinate transformations (Chen and Konrad, 1997). In this context the authors have developed an iterative method, called Dirichlet boundary condition iteration (DBCI), to solve electrostatic problems in twoor three-dimensional infinite domains (Aiello et al., 1994, 1997) and timeharmonic skin effect problems in two-dimensional or axisymmetric unbounded domains (Aiello et al., 1995, 1996). In this paper DBCI is extended to cover time-harmonic axisymmetric eddy current problems in infinite domains, which are very common in electrical engineering, as for example in eddy current non-destructive testing and induction heating applications. The DBCI method for axisymmetric time-harmonic eddy current problems The field analysis we are dealing with concerns a quasi-static time-harmonic eddy current problem which exhibits material and geometrical axisymmetry. By introducing a cylindrical co-ordinate frame (r, ', z) with the z-axis coincident with the axisymmetric one, the analysis can be conveniently performed in terms of the auxiliary magnetic scalar potential V(r,z) related to the azimuthal component A'(r,z) of magnetic vector potential by V  A' r 1 The current issue and full text archive of this journal is available at http://www.emerald-library.com

Finite element computation of axisymmetric eddy currents in an infinite domain

The influence of temperature on knee point and squareness of the M-H demagnetization characteristic of permanent magnets is important information for the full utilization of the capabilities of samarium-cobalt magnets at high temperature in demagnetization resistent permanent magnet devices. Composite plots of the knee field and the demagnetizing field required to produce a given magnetic induction swing below remanence were obtained for several commercial Sm2Co17 type magnet samples in the temperature range of 23 to 300 C. Using the knee point to define the limits of operation safe against irreversible demagnetization, such plots are shown to provide an effective overview of the useable regions in the space of temperature-induction swing parameters. The observed second quadrant M-H characteristic squareness is shown, by two measures, to increase gradually with temperature, reaching a peak in the interval 200 to 300 C.

The 23 to 300 C demagnetization resistance of samarium-cobalt permanent magnets

A three dimensional magnetic vector potential (m.v.p.) formulation of magnetostatic fields with non-uniform permanent magnet source distribution is given. A right-hand term expression describing the nonuniform permanent magnet source for a first order tetrahedral finite element is derived. The three dimensional finite element m.v.p. formulation presented is rather straightforward and easy to implement. Moreover, it is applicable both to uniformly and non-uniformly magnetized permanent magnet fields and hence has high generality. The field solution for a rectangular parallelepiped magnet stabilised in air is computed by the presented method, and is found to be in good agreement with measured results.

Three dimensional finite element vector potential formulation of magnetostatic field with non-uniform permanent magnet source distribution

For electric machines with their permanent magnet assemblies stabilized in air, a certain amount of irreversible loss of magnetization will occur in permanent magnets when magnet material with knee points in the demagnetization curve is used. To estimate quantitatively the irreversible demagnetization loss in permanent magnet machines, the maximum loss of magnetization, the remanent flux loss and the demagnetization area are defined as their three figures of merits. A series of design curves for predicting the irreversible demagnetization loss for arc-shaped permanent magnets stabilized in air is presented in the paper. The effects of material characteristics, magnet geometry and its magnetic orientation on these merits are also discussed. Design curves are obtained from field computation results by using the multi-hysteresis-loop PM model recently proposed by the first author. These design curves are useful for permanent magnet machine designers.

Irreversible demagnetization loss for electric machines with arc-shaped permanent magnets stabilized in air

Since the demagnetisation curves of permanent magnets with knee points are not single-valued, hysteresis effects below the knee points need to be considered. A field problem with an unknown and nonuniform permanent-magnet source distribution occurs. A hysteresis model for simulating permanent magnets with such knee points is presented in the paper. By using the model and a permanent-magnet source-iteration method, the field problem can be easily solved by the finite-element method. The field solutions for both a single magnet and a magnet assembly, each air-stabilised, are computed by the presented method, and are found to be in good agreement with test results.

http://ieeexplore.ieee.org/iel1/2204/1473/00035427.pdf

C. Lin

Papers

Field computation of permanent magnets with knee points