Re: [FEMM] modeling coils???

[David Meeker <dmeeker@xxxxxxxx>]

Jeff Peery wrote:

> Hello, I am new to using FEMM and to electro magnetism - I'm a 
> mechanical engineer fresh out of Univ. or Washington!  I have a new 
> job and I'm jumping right into this stuff.  I'm looking at modeling 
> the field that results from a magnetric flow meter.  I am pretty much 
> following example 2 from the FEMM website; however the example shows a 
> transverse slice of the inductor and I want to model a axial slice so 
> I am confused about how to do this.  I attached my current FEMM file.
>  
> basicallly I have a simple inductor and I want to know what the mag 
> lines look like.  I setup the geometry, meshed it, applied material 
> properties, and a boundary condition and ran the solution.  the 
> results don't look right. My confusion is that the flux lines should 
> parrallel the inductor, but instead they radiate from the inductor ( 
> in a circular fashion with the inductor at the center). so how does 
> FEMM know how my coils are oriented? From the results it looks as 
> thought FEMM thinks I have a transverse slice of my inductor, however 
> what I want to model is an axial slice. 
>  
> Can anyone offer some advice? thanks!
>  
> Jeff

FEMM is designed to analyze 2D planar or axisymmetric geometries.  Both 
of these types of geometries are characterized by the current flowing 
only in the into-the-screen or out-of-the-screent directions, and by 
flux flowing in the plane of the screen.  You have to take a cut of your 
geometry where the currents and flux have this form to model your 
problem with FEMM.

If you're just learning about eletromagnetism, FEMM isn't the place to 
start.  The place to start is with trying to understand magnetic circuit 
theory (which is sort of an analogy to networks of electrical 
resistors).  The reference that I like is Martin Plonus's "Applied 
Electromagnetics," which is typically available as a used book for about 
$30.  Nasar's "Schaum's Outline of Electric Machines & Electromechanics" 
is also good and is even cheaper. The point to this is that what FEMM is 
doing is essentially building a fairly elaborate network magnetic 
circuits through which flux can flow.  Magnetic circuits will get you a 
long way in understanding and designing magnetic devices, and you can 
then understand what FEMM as an analogy to or extension of the simple 
magnetic circuit representation.

Dave.