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Re: [femm] Re: novice user: on track or not?

r_dingo wrote:

--- 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.

Ok--I couldn't visualize what you were talking about before. This behavior makes sense. The region that you are talking about lies along a line of symmetry, where the radially directed component of the field goes zero. In the region between the magnets, the axially-directed flux is traveling upwards between the two magnets. Farther out, the axially-directed flux is returning by travelling downward, which must happen if the flux is to make a complete circuit. At some point, the flux density must transition from pointing upwards to pointing downwards, i.e. the axial part of the flux density must go through zero. Since the radial component is zero along the line of symmetry, you get a point at which the total field is zero. I'd expect this behavior in any similar "open circuit" configuration of magnets, just as you'd noted.

It is interesting to think about what happens as the gap between the magnets gets progressively smaller. The transition point between upwardly directly flux and downwardly directed return flux moves in closer in towards the outer edge of the magnets, and the size of this dimple shrinks. When the magnets actually come into contact, there dimple vanishes all together, and there is a discontinuous "jump" in the field going across the outer radius of the the magnets along radially directed line where the magnets meet.

Dave.
--
David Meeker
email: dmeeker@xxxxxxxx
www: http://femm.berlios.de/dmeeker