Re: [femm] Re: Incorrect force direction

["Steven Stretz" <stretzsj@xxxxxxxxxx>]

Comparison is a very good idea however I have had such terrible luck with
IES software I would not trust it any farther than I could throw the CD.
Boundary elements are great for far field effects but historically their
attempts at handling near fields, nonlinearities, induced currents, etc. has
been at best questionable. I'll trust your judgement and hope the software
has improved but be careful using it. I leased the software for a year and
returned in after several months and asked for my money back, which I never
received.
----- Original Message -----
From: "Greg Watson" <gowatson@xxxxxxxxxxxxxxx>
To: <femm@xxxxxxxxxxx>
Sent: Wednesday, January 03, 2001 3:48 PM
Subject: Re: [femm] Re: Incorrect force direction


> ----- 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
>
>
>
>