Re: [femm] Re: Incorrect force direction

["Greg Watson" <gowatson@xxxxxxxxxxxxxxx>]

----- Original Message -----
From: "David Meeker" <dmeeker@xxxxxxxxxxxxxxxxx>
To: <femm@xxxxxxxxxxx>
Sent: Thursday, January 04, 2001 1:47 AM
Subject: Re: [femm] Re: Incorrect force direction


> Anyhow, I modelled your problem on Amperes by IES, which is a rather expensive
> commercial 3-D magnetostatic solver based on boundary elements, rather than
finite
> elements. This ought to give a good idea of how things would run in a real
3-D test
> rig, assuming everything is carefully aligned. I arbitrarily made the machine
1"
> deep in the "into the page" direction, for the purposes of making a 3-D
geometry. I
> chose a relatively fine density for the surface elements, and then set it to
> evaluate the forces on the magnet at a bunch of different axial positions.
What it
> predicts is that the centring force peaks when the front edge of the magnet is
just
> past the iron, with the centring force dropping to near zero when the magnet
is
> completely between the iron bars. femm predicts the same sort of behaviour,
except
> that femm predicts the peak force to be more like 15 Newton/inch, rather than
9 N.
> The force from the 3-D geometry is smaller than in the 2-D model because some
of the
> magnet's flux leaks into the sides of the bars--this is an intrinsically 3-D
effect
> that can't be modelled by a 2-D solver.

Hi David,

Thanks for the effort.

My concern was not that FEMM should report a large centring force but that a
program which I normally have found to be very realistic would report a
repulsive & not a centring force, no matter how fine the mesh or how much care I
took in making sure the geometry of the mesh and / or of the model didn't effect
the result.

My duplication involved a round Neo, 4 on point right angle plastic guides hot
glued to the vertical faces of 2 right angle steel pieces (two guides per face,
1 top, 1 bottom). Alignment isn't too critical. Although the plastic guides
aren't ideal, you can get a feel for the forces by the amount of effort needed
to roll the magnet back & forth between the guides.

Greg