Re: [femm] Calculating Shieldi Effectiveness with Femm

[David Meeker <dmeeker@xxxxxxxxxxxxxxxxx>]

emiliano.menghi@xxxxxxxxx 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.


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