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# Re: [femm] Force calculation on an axisymmetric model

- To: femm@egroups.com
- Subject: Re: [femm] Force calculation on an axisymmetric model
- From: David Meeker <dmeeker@foster-miller.com>
- Date: Fri, 29 Sep 2000 16:24:31 -0400
- Organization: Foster-Miller, Inc.
- References: <3.0.5.32.20000929182911.00881400@sun.isec.pt>

    Paulo José Gameiro Pereirinha wrote:

    > Hello!
    >
    > I'm in Portugal and trying to compute the atraction magnetic force on an
    > plunger electromagnet with axisymmetric geometrie. I'm using the Femm 3.0.
    > To compute the force exerced on the iron can I just define a contour in the
    > air around the iron (which means that one side of the contour will be on
    > the line of coordinate z=0) and integrate the Maxwell Stress Tensor? (i.e.
    > is this command valid for axisymmetric models?). If I do so, I obtain
    > -2.783e-3 N, but the experimental result is about 1.35 N. I'm sending the
    > model in attachement.
    >
    > Thanks in advance.

    Well, I looked at your data file, and I think that the problem is the material
    property definition for your Steel material. The relative permeability in the
    radial direction is defined as 1384, but the relative permeability in the axial
    direction is defined as 1. I'd guess that you would want to define the
    permeability to be 1384 in both directions.

    Anyhow, stress tensor is valid in axisymmetric problems as well. When I run
    the problem with the above change in permeability in the z-direction and
    integrate force along the box you have defined around your iron part through
    the air, I get a force of 0.5685 N. As a sanity check, I ran the same geometry
    on the IES Amperes 3-D boundary element solver, which I happen to have laying
    around the office. The force result from Amperes was 0.5456 N. The
    implication is that the femm force result seems reasonable. I can't say what
    the discrepancy is that causes the difference with the experimental result of
    1.35 N.

    Dave.

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- References:
  - Force calculation on an axisymmetric model
    - From: Paulo José Gameiro Pereirinha <ppereiri@sun.isec.pt>

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