Modeling demagnetization of sintered NdFeB magnet material in time-discretized finite element analysis

摘要

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.