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# Re: [femm] PM modeling (Re:Suggestions)

- To: femm@yahoogroups.com
- Subject: Re: [femm] PM modeling (Re:Suggestions)
- From: Victor Javier Raposo Funcia <victor@gugu.usal.es>
- Date: Wed, 27 Jun 2001 11:51:33 +0200
- In-reply-to: <3B3773B6.C410205@foster-miller.com>
- References: <5.0.2.1.2.20010625132623.00b60900@gugu.usal.es>

The explanation is clear. The femm manual also explains it, I read that apendix and deduced also the origin of the absence of a demagnetizing field as a consequence of considering a magnet as mu=1 + surface currents. But I still think that the solution from that approximation is partial, and the correct field is given by M = B/muo - Hd. Also this result comes in a more natural way with the magnetic charge formulation.
Anyway it is possible to back out the demagnetizing H as you indicate although it is quite long.
Thanks.
At 13.24 25/6/01 -0400, you wrote:

> Victor Javier Raposo Funcia wrote:

> > Just two points:
> > - What about adding a vector plot of H and B in the postprocessor?
> > - When I try to solve a problem consisting on a rectangular magnet (in the
> > air) I found the following problem:
> > B seems to be correct, but H inside the magnet is in the same direction as
> > the magnetization M, and it should be a demagnetizing field (when I solve
> > it with the magnetic charges). It seems that H is calculated as H=B/mu
> > without considering M.

> Well, femm takes the view that permanent magnets are essentially the same as solenoids. The magnet consists of a chunk of low-permeability material (such as air). The "source" of the PM's flux is a current sheet of density Hc*(m cross n) on the edges of the permanet magnet, where Hc is the magnet's coercivity, m is a unit vector in the direction of magnetization, and n is an outward unit normal to the edge of the PM. This is a fairly typical way to model permanent magnets in the context of a finite element formuation (because it is any easy way to include PMs), and it is also equivalent to a magnetic charge-based formulation. An interesting web site that discusses this approach is here, on the Magnequench website. This issue is also discussed in appendix A1 in the femm manual.
> In any case, it is possible to back out the demagnetizing H and M that one would obtain in a formulation that represents M explicitly. To do this, we are just mapping back onto the second quadrant of the demagnetization curve. If Hf is the field intensity reported by femm, and Hd is the demagnetizing field intensity, then Hd = Hf - Hc*m, where Hc is the coercivity of the material, and again, m is a unit vector pointing in the magnetization direction. Magnetization is defined by B=muo(Hd+M), so the magnetization corresponding to the operating point on the demagnetization curve is M = B/muo - Hd, where muo is the permeability of free space and B is the flux density. If the magnetic material has a unit relative permeability (nearly true with most SmCo, NdFeB, and some ceramic magnets), M=Hf-Hd.
> At least, I don't think I've screwed up this explanation....
> Dave Meeker
> Yahoo! Groups Sponsor
> `Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.`

-----------------------------------
Víctor Raposo
Dpto. Física Aplicada
Universidad de Salamanca
Plaza de la Merced s/n
37008 Salamanca. Spain
Tel 923 29 45 00 - 1301
Fax 923 29 45 84
-----------------------------------

- References:
  - Re: [femm] Suggestions
    - From: Victor Javier Raposo Funcia <victor@gugu.usal.es>
  - [femm] PM modeling (Re:Suggestions)
    - From: David Meeker <dmeeker@foster-miller.com>

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