Magnet attraction force increases with air gap

    • FixIT

      To clarify:

      • ANSYS AIM 19.1 Academic version

      • Windows 10 x64 Home

      • All dimensions are in millimeters

      • In Russia decimal separator is "," so 0,01 mm means 0.01 mm

      • I am not saying Ansys is wrong, but the result is very strange

      I notices this strange behavior in my computer experiment and I remade it to the simpliest form to show you.

      A NdFe35 disk magnet 70 mm in diameter with the thickness 10 mm that is axially magnetized (Y axis) on a steel 1010 plate 5x80x80 mm (on the picture).

      While the air gap increases from 0 to 0.001 mm the magnet attraction force decreases but at 0.01 mm the force becomes higher than at 0 mm air gap, after that with increasing the air gap the force decreases again.

      You can see it on the picture:

      • Magnet attraction force on the Z axis is the Force Z 1

      • Plate attraction force on the Z axis - Force Z 2

      • Force Z 1 + Force Z 2 = Force Z 3

      • The columns F-H are the magnitudes of the same forces

      So the trend is on the picture.

      I thought this was happening because of the steel plate saturation. I ran another test with the variable thickness of the plate t (column C), but the trend was the same even with a 35 mm steel plate (on the picture).

      Ok you can say that +10 N is a very small amount of difference but in my original more complex experiment with a custom more powerfull magnet the difference between 0 mm and 0.01 mm air gap can be +150 N and the original experiment's calculations were done until hitting the maximum precision that can be achieved with the academic version.

      I spent a lot of time trying to figure out what is wrong. I hope you can give detailed answer.

      PS. Couldn't upload the experiment files on the forum, used another site instead https://www86.zippyshare.com/v/y6as8aAg/file.html

    • mgardner

      My guess is that this is a meshing issue.  With the air gap being so much smaller than the other dimensions in your model, it is very difficult for ANSYS to place mesh elements in the air gap because it requires a large number of extremely small mesh elements.  Additionally, this means that with a small air gap, a lot of tiny mesh elements must also be used near the air gap in the steel and the magnet.  As the air gap size changes, the minimum size of all these mesh elements also changes.  I expect that this change in the mesh is what is causing the strange force behavior.

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