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

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Discussion Overview

This discussion focuses on performing load-line analysis for loudspeaker driver magnetic circuits using FEMM 4.2. Participants explore the interpretation of permeance coefficients and the relationship between magnetic flux density and magnetic field strength in the context of finite element modeling.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion over calculating the permeance coefficient (pc) and the appropriate values to use for magnetic flux density (B) and magnetic field strength (H), questioning whether to use their magnitudes or components.
  • Another participant suggests that if the magnetic materials are isotropic, the magnitudes of B and H would be the natural choice for calculations.
  • A third participant shares a response from David Meeker, indicating that the permeance coefficient should be computed using the dot product of B and M divided by the product of µ0 and H and emphasizes that B, H, and M are vector quantities.
  • There is a query regarding whether B and H must lie on the BH curve or if they can be determined independently based on the steepness of the load line.
  • One participant mentions their experience with magnetic circuit theory and draws parallels with modeling piezoelectric materials, indicating a shared confusion about similar issues.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and confusion regarding the calculations and interpretations involved in load-line analysis, indicating that multiple competing views remain without a clear consensus.

Contextual Notes

Participants highlight potential limitations in understanding due to the complexity of vector quantities and the specific settings in FEMM 4.2, such as the "Demag H in PMs" option, which may affect the reported values of H.

flowwolf
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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   Reactions: Paul Colby
Very helpful, thanks. I once analyzed a variable reluctance motor using magnetic circuit theory. The parallel with piezoelectric materials should be understandable.