How to Perform Load Line Analysis for Loudspeaker Drivers in FEMM 4.2?

AI Thread Summary
The discussion focuses on performing load-line analysis for loudspeaker driver magnetic circuits using FEMM 4.2. Participants seek clarity on calculating the permeance coefficient (pc) and interpreting magnetic field results, particularly the relationship between B and H vectors. There is confusion regarding whether to use vector magnitudes or components, especially when considering the magnetization direction. A suggested formula for pc is provided, emphasizing the need for the dot product of B, H, and magnetization vectors. The conversation highlights the complexities of modeling magnetic materials and the intersection of load lines with the BH curve.
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Dear Forumers,

This question is about load-line analysis for loudspeaker driver magnetic circuits in FEMM 4.2 (Finite Element Method for Magnetics), I can't seem to get an answer from the mailing lists.

I would like to get the permeance coefficient for random points inside the magnet.
I have problems interpreting the results reported by femm.
Most information sources tell me that pc = Bd/Hd, but B and H are vector quantities,
and I'm not sure which parts to use (either |B|,|H| or the components).

According to a JMAG newsletter I've read, Pc = - Bd / (µ0 Hd), where
"Bd [T], Hd [A/m]: The projection components in the magnetizing direction M of the magnetic flux density B and magnetic field H"

So does that mean that pc = Bz / Hz, if the magnetization direction is 90 degrees?I saw old mails about the permeance coefficient as well: http://www.femm.info/list/msg01827.html
where it was stated that Hd = H - Hc (if the "Demag. H in PMs" option is not set)

But subtracting Hc from |H| does not result in the |H| that is reported when "Demag. H in PMs" is setAll these confuses me, could I get some help about which values to use?

Also do Bd and Hd necessarily have to fall on the BH curve, or is the load-line given by
the intersection of a line with steepness of pc, where Bd and Hd are not necessarily on the BH curve?Any help would by appreciated.

Akos
 
Are the magnetic materials isotropic (same in every direction)? If so the B and H vectors are in the same direction and I would think the magnitudes would be the natural choice. About the load line problem, I'd like to know more about it as I have similar modeling issues with piezoelectric materials. I too find it confusing.
 
Hello Paul,
I've received an answer from David Meeker:
"For your case, on a point-by-point basis, you'd want to compute:
Pc = (B.M)/(µ0*H.M)
where B is the reported B for a point inside a PM, H is the reported H for a point in the PM (with the default (checked) "Demag H in PMs" setting), and M is the magnetization in the block. Note that B, H, and M are all vectors, and . represents the dot product."

He pointed me to http://www.femm.info/Archives/misc/BarMagnet.pdf
 
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Likes Paul Colby
Very helpful, thanks. I once analyzed a variable reluctance motor using magnetic circuit theory. The parallel with piezoelectric materials should be understandable.
 

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