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# Re: [femm] Calculating Shieldi Effectiveness with Femm

- To: femm@egroups.com
- Subject: Re: [femm] Calculating Shieldi Effectiveness with Femm
- From: David Meeker <dmeeker@foster-miller.com>
- Date: Tue, 23 Jan 2001 17:15:06 -0500
- Organization: Foster-Miller, Inc.
- References: <94kcru+dcgq@eGroups.com>

    emiliano.menghi@inwind.it wrote:

    > Hi,i use Femm to calculate the shielding effecctiveness (at 50 Hz) of
    > various ferromagnetic and non-ferromangetic materials. But i've some
    > problems.
    > My configuration is simple: a copper conductor with 1000 A and a
    > cylindrical shield.
    > But Femm calculate error value for A vector.
    > I used some boundary condition (A=0 and unbounded condition) but the
    > problem is present.
    > Infact when i calculate source impedence at distance of 20 mm (Ez/Hx)
    > the excact value is 140 but Femm tell me about 30.
    > Where is the problem?
    > Thank you and excuse me for my poor enghish.
    > By

    Hmm--It's hard to say exactly what's going on. I cooked up a problem
    that sounds like the kind of thing that you are describing. I was too
    lazy to solve for an analytical expression this afternoon, but I did port
    the same problem to the student version of Quickfield 3.4a for
    comparison. The two programs give essentially the same answer, give or
    take a percent. I've put the example in the "files" section on the femm
    egroups page: http://www.egroups.com/files/femm/sample.zip

    Anyhow, what could be going on is sort of a common source of confusion
    with eddy current problems in femm. The "natural" way to define the
    impressed currents in an eddy current problem is to define a current
    density and apply it to the region of interest. However, this current
    density is really just the "source" current density--the current density
    that would be there at 0 Hz. At non-zero frequencies, eddy currents are
    induced that knock down the total current carried by the conductor, and
    therefore, the field made by the "source" current. An alternative
    approach is to assign a current with a "circuit" property, as I've done
    in the example. This allows you to fix the current flowing in a
    particular part, while allowing eddy currents to distort the distribution
    of current within the part (e.g. so a wire carries a prescribed total
    current but shows the "right" skin effect behavior).

    Dave.

    begin:vcard
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    url:http://members.aol.com/dcm3c
    org:Foster-Miller, Inc.;Electrical and Electronic Systems Group
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    email;internet:dmeeker@foster-miller.com
    title:Senior Engineer
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- References:
  - Calculating Shieldi Effectiveness with Femm
    - From: emiliano.menghi@inwind.it

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