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2010-08-31
*Demagnetization Analysis of an SPM Motor
When a motor is rotating, the permanent magnet in the motor can be demagnetized due to the temperature rise caused by the eddy current loss and the reverse magnetic field generated by the coil.
As the demagnetization of the permanent magnet reduces the motor performance, it is important that the demagnetization can be evaluated with magnetic field analysis.
This note presents the use of a magnetic field analysis of an SPM motor to evaluate how the demagnetization caused by the reverse magnetic field change with varying current flow.
· Flux Density Distribution
Fig. 1 shows the flux density distribution at the rotation angle of 20 degrees with current flow of 15 A. It is expected that the demagnetization occurs at the end of the magnet, because the orientation of the magnet and the direction of the flux generated by the rotating magnetic field are almost opposed.
· Operating Point
Fig. 2 shows the positions where the operating points are evaluated, and Fig. 3 shows the operating points at the selected positions when the current flow is 15 A. The graph shows the irreversible demagnetization in Position A, because the operating point cannot track back to the original BH curve. The size of the reverse magnetic field imposed upon the positions is also clear from the horizontal axis.
· Permeance Coefficient Distribution
Fig. 4 shows the permeance coefficient distribution of the magnet when the motor is stationary with no load. The contour plot is shown in two colors, that are red and purple. The knee point of the magnet used for the analysis is 0.699. The figure shows that the demagnetization hardly occurs when the motor is stationary with no load.
· Demagnetizing Ratio Distribution
The demagnetizing ratio* is evaluated by the setting the criterion as the demagnetizing state when the motor is stationary with no load. Fig. 5 shows the demagnetizing ratio for a motor that is driven for one period of the electric angle. As the current flow increases, both the demagnetizing area also increases, and the value of the demagnetization increases as well.
*The demagnetizing ratio shows if the magnet has been demagnetized from a certain state which is used as the criterion.
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