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 Eddy Current Circulation in the Magnets

Reducing the Eddy Current Losses in Permanent Magnets by Segmentation

Motors and Generators with MagNet

Permanent magnet machines are increasingly popular. When dealing with high speed applications, the eddy current losses will often cause deterioration of their magnetic properties and potentially risk thermal demagnetization.

Segmentation of the permanent magnets helps to reduce the eddy current losses and risk of damage. In this example, MagNet is used to investigate the effect of segmentation in a surface mounted permanent magnet machine.

MagNet provides a very simple method to model the insulation layer by using a perfect electric insulator boundary condition. This replaces the need to create very thin insulation layers, which are hard to model and mesh in a FEA environment.

Surface Mounted PM Machine with PM Segmentation

METHODS and RESULTS

PERMANENT MAGNET SEGMENTATION

In order to investigate the effects of permanent magnet segmentation, several models are designed in MagNet with circumferential segmentation, axial segmentation, and a combination of both.

  1. 2 circumferential segments
  2. 4 circumferential segments
  3. 5 axial segments (front half shown due to symmetry)
  4. 4 circumferential & 5 axial segmentations (front half shown due to symmetry)

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INSULATION LAYER

MagNet's perfect electric insulator boundary condition can be used to model the insulation layer between permanent segments.

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EDDY CURRENT LOSSES in the MACHINE

MagNet's Transient solver is used to predict the eddy current losses in the surface mounted motor. The results of the cases (with no segmentation, 2 and 4 circumferential segmentations) are shown in the figure. The shaded plots indicate that the losses decrease significantly with each increase to the number of segments.

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CURRENT DENSITY in the MAGNETS

The current density arrow plots demonstrate the current path variations that occur with each increase in segmentation. As the permanent magnet is segmented into smaller and smaller pieces, the resistance increases, thereby limiting the eddy current losses just as in the principles of lamination.

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EDDY CURRENT CIRCULATION in the MAGNETS

The eddy current circulation in the permanent magnet segments over time, demonstrates the barrier that the perfect electric boundary condition sets up, forcing the eddy currents to stay within each segment.

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COMPARISON of the LOSSES

The eddy current losses for each case are compared in the table. As the chart indicates, the losses decrease significantly as the segment count increases, whether it be by using circumferential, axial or a combination of the two segmentation methods.

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