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Re: [femm] Permeance (B/H) Variation vs. Magnet Grade?

shanteserve wrote:

Hello. I am relatively new to magnetics and have a question about
circuit permeance in permanent magnet designs.

As I understand it, the circuit permeance(Bd/Hd) is mainly a function
of the model geometry. I seem to get significantly different
results, however, depending on what magnet grade I choose in my FEMM
models (I obtain these values by using "point properties" and
selecting a specific, pre-determined point inside the magnet). I get
similar results if I model something as simple as a bar magnet, even
though published charts indicate that B/H is only a function of
geometry and the effective spacing of the magnet poles.

The FEMM results suggest that the slope of the load line is very
dependent on the B-H curve of the magnet material. This is contrary
to all of the published theory that I have reviewed....

Any insight into what is causing this phenomenon? Any guidance would
be appreciated.

Thanks,
Scott

FEMM uses an approach where the second quadrant demagnetization curve is shifted to the right by Hc, conceptually representing the magnet as an equivalent thin coil wrapped around a volume of air (or possible weakly ferromagnetic material). There's a blurb about this in Appendix A1--this is a fairly common way to represent permanent magnets in the context of a finite element analysis. The reason that I use this representation is my bias of liking to analyze permanent magnets by representing them as a reluctance and an MMF source in series, rather than a flux source and reluctance in parallel. My rationale is that then I can treat any system, with PM's or coils, in an interchangeable way, via an analogy to a network of resistors and batteries.

Proselytizing aside, you need to put Hd, the demagnetizing field intensity, into get your formula to get it to work right. To get the right point on the second quadrant of the demagnetization curve, you just have to shift back to the left by Hc, i.e.: Hd = H - Hc where H is the value that femm reports for field intensity at points inside the magnet. The Bd is the same as B as reported. Then, things ought to act more like you'd expect, as long as the load part of your magnetic circuit is more or less linear.

Dave.

--
David Meeker
dmeeker@xxxxxxxx
http://femm.berlios.de/dmeeker