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

[Victor Javier Raposo Funcia <victor@xxxxxxxxxxxx>]

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 
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-----------------------------------
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
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