Re: [FEMM] Impedance of Voicecoil

[David Meeker <dmeeker@xxxxxxxxxxxxxxxxx>]

Scott Diehl wrote:

> I'm trying to model the so-called "blocked" impedance of a loudspeaker 
> voice coil at various frequencies.  Up to now, I've always modeled a 
> wound coil as a single conductor/region and set the current equal to 
> the number of Amp-turns.  I also set the conductivity of the voice 
> coil material to zero to eliminate eddy current effects in the coil 
> (that would be eliminated by the layers of insulation).  Doing this, 
> however, effectively invalidates the impedance results from FEMM.
>  
> I never worked with bulk laminations in FEMM, but I thought that I 
> might be able to fool FEMM into eliminating the eddy currents while 
> maintaining the conductivity of the coil.  After attempting this, I 
> found that horizontal laminations are not supported in axisymmetric AC 
> models.
>  
> The only other alternatives I can think of are to model each turn as a 
> separate region or try to take the zero conductivity results and 
> include DC resistance.  Has anyone found a better way to model blocked 
> impedance in FEMM?
>  
> Any insight appreciated,
> Scott

The 4.0 development version supports models of wound coils with the 
intention of computing just this sort of result.  You define a series 
"circuit" carrying a particular current, apply it to the coil, specify 
the number of turns in the coil, and analyze.  In the postprocessor, hit 
the button that looks like an inductor to get the impedance (among other 
results).  I've attached an example.

The only "trick" here is to get the right DC resistance, you have to 
scale the conductivity of the coil material by the fill factor--I'm 
planning to have wire gauge be a material property so that it will 
figure out the fill for you, but I haven't gotten around to that yet.

I suppose that another "trick" is that one would ideally like the 
pemeabilities in the problem to be the incremental permeabilities about 
the DC operating point established by the magnet.  I haven't gotten 
around to that one yet, either.

Dave.

--
David Meeker
Senior Engineer
Foster-Miller, Inc.
350 Second Avenue
Waltham, MA 02451-1196
781-684-4070
781-890-3489 (fax)
dmeeker@xxxxxxxxxxxxxxxxx 
http://femm.foster-miller.com

Attachment: example.zip
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