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# Re: Problem with FEMM

- To: femm@yahoogroups.com
- Subject: Re: Problem with FEMM
- From: "David Meeker <dcm3c@earthlink.net>" <dcm3c@earthlink.net>
- Date: Tue, 07 Jan 2003 22:11:18 -0000
- In-reply-to: <avd073+51ip@eGroups.com>
- User-agent: eGroups-EW/0.82

    --- In femm@yahoogroups.com, "zifera <emiliano.menghi@i...>"
    <emiliano.menghi@i...> wrote:
    > Dear David and Femm users, i've a big problem with Femm and i hope
    > that you want help me.
    > I use FEMM to computate the shielding effectiveness (low frequency:
    > 50-60 hetz) of plane aluminium screen (tickness 3 mm, lenght 2 m).
    > The source is a triphase cable system with simmtric current (500 A).
    > The distance between the conductor is 25 cm. The position of the
    > screen is simmetric respect the conductors (the conductors are in
    > plane)
    > The problem is that near the screen the magnetic field (calculated
    > along a line parallel to the sceen) is not simmetric!! For distance
    > bigger than 50 cm magnetic field return to be simmetric. Why?
    > I've tested the same configuration with a commercial fem (Ansoft) and
    > the result is equal.
    > Thanks.
    >
    > Emiliano

    If I understand your geometry correctly, I don't think that the field
    distribution here should be symmetric. Essentially, if you've put
    three-phase conductors in a line on top of a plate, you've made
    yourself a linear induction motor. A wave of flux travels from one
    side of the plate to another, and you can get a funny looking total
    field when you include the reaction field from the plate.

    When viewed from far away, the combined field of the three wires in a
    row begins to look like that of a dipole whose strength varies
    sinusoidally with respect to time. The field then looks a lot more
    regular.

    Dave.

- References:
  - Problem with FEMM
    - From: "zifera <emiliano.menghi@inwind.it>" <emiliano.menghi@inwind.it>

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