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# Re: [FEMM] Help again

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- Subject: Re: [FEMM] Help again
- From: David Meeker <dmeeker@foster-miller.com>
- Date: Mon, 22 Dec 2003 12:34:39 -0500
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`Bruce G. Kang wrote:`

> `Dear David:`

>     I need your help again. Please see attachment in
>     figures about the question:

> `1. for ferrite, we want to know the leakage field at
> one side region about 3~5 times the ferrite dimension.
> basically it is a 3D problem. You suggested to use BEM
> methods to solve it. But I found BEM is difficult to
> pick up.
> `

`Your request was for a recommendation for a commercial program that would be good for examining the distant leakage field. Although this is certainly possible to calculate this sort of thing with finite elements, boundary element techniques are well-suited to this particular problem since no outer boundary of the solution region need be defined. One example of such a commercial code would be IES Amperes, which is a good and fairly intuitive program. As far as freeware, there is Radia (http://www.esrf.fr/machine/groups/insertion_devices/Codes/Radia/Radia.html), which is actually a volume integral code (rather than a boundary integral code) but has the the same nice properties with respect to evaluating the field at distant locations. However, you have to have Mathematica installed to be able to run Radia.`

>     2. Is this a good idea? To change the vector problem
>     into a potential problem as in "Answer.bmp"? Due to
>     mu_r is much larger than 1, we can claim each side
>     have different potentials. The rest will be a
>     finite-difference problem to get all the potential
>     distribution of the region. B will be the gradient of
>     potential.

> `Please comment and suggest.`
> `Best Wishes,`
> `Bruce
> `

`My original suggestion still stands--at a big enough distance, everything looks like a dipole (or perhaps a collection of a small number of dipoles). You could use a 2D finite element solution to back out a dipole moment for your device as seen from a distance, and then shove that dipole moment into the equation for the field of a dipole in 3D space.`
`If one wanted to get more complicated, one could build up a 3D solution for the leakage field from the contributions of the magnetization in each element of the 2D mesh containing iron (where you'd use the 2D solution to determine the magnetization), plus contributions from the coils. In this approach, the contribution from each element would be represented as a point dipole located at the center of the element, and the resulting field contribution would be that of a point dipole in 3D space. The Biot-Savart law would be used to account for the contributions of the coil.`
`Dave.`
`--
David Meeker
Senior Engineer
Foster-Miller, Inc.
350 Second Avenue
Waltham, MA 02451-1196
781-684-4070
781-890-3489 (fax)
dmeeker@foster-miller.com http://femm.foster-miller.com`

- Follow-Ups:
  - [FEMM] Merry Christmas to David !!!
    - From: "Bruce G. Kang" <xiandaiwuxue@yahoo.com>

- References:
  - [FEMM] Help again
    - From: "Bruce G. Kang" <xiandaiwuxue@yahoo.com>

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