Re: [femm] Alnico 8 relative permeability

[David Meeker <dmeeker@xxxxxxxx>]

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@xxxxxxxx
www: http://femm.berlios.de/dmeeker