Re: novice user: on track or not?

["r_dingo" <rd@xxxxxxxxxxxxxxxx>]

--- In femm@xxxxxxxxxxxxxxx, "David Meeker" <dmeeker@xxxx> wrote:
> This sounds a little odd--it's hard to say what's going on without
> seeing your model. Anyhow, I've uploaded a model of the sort of
> geometry that you'd described that seems to behave in a reasonable
> way. It's in the mailing list's Files section at:
> http://groups.yahoo.com/group/femm/files/2magnets.zip
> 
> Dave.
> 

Yes, this is the model I have used also, since you can't go any 
simpler. I have run your version and find the same result. If you 
run your version and then tweak the view as follows:
Zoom/Keyboard: B -0.5 L -0.3 T 0.5 R 1.11,
View/Density Plot: lower 0, upper 0.05,
then you will see an island of zero flux density centred at r=0.5 
in. Since this is an axisymmetric solution, it represents a toroidal 
region of very low flux density circling around the airgap. Its 
radius of 0.5 inch means that it's fairly close in, as I said.

Alternatively, you can draw a horizontal plot-XY line radially 
outwards from the centre of the airgap, eg, from (r=0, z=0) to 
(r=0.8, z=0), and you will see that:
|B| = 0.61T in the central axis, falling rapidly to ...
|B| = 0 at r=0.5 in, then rising slowly to ...
|B| = 0.015T at the far field maximum at r=0.3 in.

This zero point seems to present in a wide variety of conformations.

Cheers, Ross.