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# Re: [femm] Alnico 8 relative permeability

- To: femm@yahoogroups.com
- Subject: Re: [femm] Alnico 8 relative permeability
- From: David Meeker <dmeeker@ieee.org>
- Date: Fri, 10 Jan 2003 16:00:21 -0500
- References: <avn680+1sbr@eGroups.com>
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`sloothrog wrote:`

>     The FEMM materials library lists A8 relative permeability
>     as 6.678, or about four times that of Alnico 5: mu == 1.5.

>     My empirical testing puts the A8 relative permeability close
>     to 1.3 in the absence of any other ferromagnetic work load.

>     This is an important consideration in design of variable
>     reluctance transducers.

>     Can someone suggest any reference documents for Alnico 8
>     relative permeability values?

>     -drh
>     --

`
The entry for Alnico 8 in the materials library has a nonlinear BH curve. The linear permeability is used for the first iteration in the solution of nonlinear magnetostatic problems, and as the permeability for time-harmonic problems in v3.2, which only supports linear time-harmonic problems. Anyhow, the BH curve for Alinco 8 used in the materials library was obtained by taking points off of Fig 4.2(a) of "Permanent Magnets and Their Applications", by Rollin J. Parker and Robert J. Studders, John Wiley and Sons, Inc., New York, 1962. The "linearized" value of 6.678 was obtained via a least squares fit of a line to the points that I picked off of the demagnetization curve.`
`However, for your application, you're proabably be more interested in the "recoil pemeability," which is the average-ized permeability for excursions about some nominal operating point. This is sort of a simple approximation of the rather subtle phenomenon of minor loop hysteresis. There's a little blurb about this on page 8 of http://www.duramag.com/engmach/MMPA%20PMG-88.pdf Typically, the recoil permeability is a lot lower than the slope of the demagnetization curve at the point at which the reversal starts, which is probably the effect that you are seeing.`
`It is non-trivial to do a finite element analysis that captures both the steady-state operating point as well as perturbations about that operating point. As a transducer designer, you'd probably like a nonlinear analysis to determine the nominal operating point in combination with a linearized, small-signal analysis that runs on top of this operating point that you can use for the purposes of dynamics and controls analyses. FEMM doesn't do combined DC + harmonic analyses just yet, although this is something that I've been thinking about. (It seems like I'm always analyzing devices with a DC offset like magnetic bearings or voice coil actuators--this combined analysis capability would be pretty useful to me, too.) Anyhow, it might be reasonable to perform AC analyses that are supposed to represent excursions about the nominal operating point by representing the magnet as a non-magnetic material with a permeability that is the same as the recoil permeability. The results of this analysis would then be superimposed upon the nominal operating point. However, mileage may vary with this approach.... Another alternative would be to do a full time-stepping analysis with a code that implements some flavor of a time-stepping hysteresis model for the alnico.`
`At any rate, the type of modeling that you should perform sort of depends upon what you are trying to get out of the analysis.`

    Dave.
    --
    David Meeker
    email: dmeeker@ieee.org
    www: http://femm.berlios.de/dmeeker

``

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