Re: Problem with FEMM

["David Meeker <dcm3c@xxxxxxxxxxxxx>" <dcm3c@xxxxxxxxxxxxx>]

--- In femm@xxxxxxxxxxxxxxx, "zifera <emiliano.menghi@xxxx>"
<emiliano.menghi@xxxx> wrote:
> Dear David and Femm users, i've a big problem with Femm and i hope 
> that you want help me.
> I use FEMM to computate the shielding effectiveness (low frequency: 
> 50-60 hetz) of plane aluminium screen (tickness 3 mm, lenght 2 m). 
> The source is a triphase cable system with simmtric current (500 A). 
> The distance between the conductor is 25 cm. The position of the 
> screen is simmetric respect the conductors (the conductors are in 
> plane) 
> The problem is that near the screen the magnetic field (calculated 
> along a line parallel to the sceen) is not simmetric!! For distance 
> bigger than 50 cm magnetic field return to be simmetric. Why?
> I've tested the same configuration with a commercial fem (Ansoft) and 
> the result is equal.
> Thanks.
> 
> Emiliano

If I understand your geometry correctly, I don't think that the field
distribution here should be symmetric. Essentially, if you've put
three-phase conductors in a line on top of a plate, you've made
yourself a linear induction motor. A wave of flux travels from one
side of the plate to another, and you can get a funny looking total
field when you include the reaction field from the plate.

When viewed from far away, the combined field of the three wires in a
row begins to look like that of a dipole whose strength varies
sinusoidally with respect to time. The field then looks a lot more
regular.

Dave.