Re: [FEMM] How do I model a multiple turn copper wire coil?

[David Meeker <dmeeker@xxxxxxxxxxxxxxxxx>]

Mike Middleton wrote:

> I'm a real new user to FEA and magnetic modeling.  I DO have a lot of 
> experience in designing and winding transformers, via a hand 
> calculator, pencil, and paper.  Most of my work has been in RF and 
> switching power supplies.
>
> I have done Examples 1 and 2 of the FEMM Tutorial and have an idea of 
> what's going on.
>
> I've been asked to re-design/analyze a magnetic sensor.  This sensor 
> is basically a steel cup about 1" diameter by 1" height.  The center 
> post is an Alnico 8 magnet about 0.375" diameter.  A few hundred turn 
> copper wire coil is wound around the magnet.
>
> Of concern, is the electrical output we get from the coil as a piece 
> of steel is passed across the open end of the sensor.  I think I can 
> figure out how to model the steel cup and magnet in FEMM.
>
> How do I model/enter the multiple turn wire coil?  Modeling it turn-by 
> turn will take a LONG time.

You typically don't have to model each individual turn in a coil 
(unless, perhaps, you are trying a brute-force solution for proximity 
effects in a winding).  Multi-turn coils are typically represented as a 
homogeneous area with either an even current distribution assigned to 
the region where the current density is chosen to be the Amp*Turns in 
the coil divided by the bulk area of the coil, or as a homogeneous 
region with a "current property" assigned to the region that forces to 
the total current in the region to be the desired number of amp-turns.  
An example in which a wound coil is represented in this way is 
http://femm.foster-miller.net/examples/induct1/induct1.htm

Dave.
--

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