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# Re: [FEMM] A little paradox with anti-periodic boundary condition and torque computation

- To: Finite Element Method Magnetics mailing list <femm@list.foster-miller.net>
- Subject: Re: [FEMM] A little paradox with anti-periodic boundary condition and torque computation
- From: Tsarafidy RAMINOSOA <tsarafidy@yahoo.fr>
- Date: Mon, 20 Oct 2003 11:46:54 +0200 (CEST)
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Thank you Very much for your explanation.
I looked at the set of contours the program use to calculate the Torque by weighted stress tensor
in showing the stress tensor mask, as you said, when the group number2 is selected.
For the part of the arc segment limiting this area for which no longer material is present in
the exterior (the arcsegment having the antiperiodic boundary condition), the program just didn't
put any countour around it cause there's no meshed area in this region.
So for these two positions of the rotor, it computed the torque with quite different countours.
That'why Torques were not equal and both values are false.
Effectively, When I put another arcsegment interior of the previous one - but still including the rotor block - in order to put a meshed area at the exterior of the area of integration, I get the
same Torque result and opposite Ht plot for both positions.
My meshsize has also to be refined.
Again Many thanks,
Best regards
Tsarafidy.
David Meeker <dmeeker@foster-miller.com> wrote:

> It's fairly hard to confuse the "weighted stress tensor" algorithm, but
> not impossible. In this case, by selecting the part of interest as well
> as all the air around it, you haven't really left the program anywhere
> to put contours around the rotor in a programmatic way. You can
> visualize the contours along which the integration takes place by
> selecting the "Show stress tensor mask" block in the View|Contour Plot
> menu in the postprocessor. If you had selected either the windings and
> iron of the stator or the iron of the rotor by themselves (i.e. with no
> additional air), you'd come up with reasonable results in both cases.
> As far as Ht, the smoothing scheme that femm uses yields values of B and
> H that are of lower order accuracy in the direction tangential to the
> boundary. This is exacerbated by a relatively coarse mesh in these
> examples. Note that it is possible, if desired, to turn off the
> smoothing via View|Smoothing selection on the postprocessor main menu.
> Dave.
> Tsarafidy RAMINOSOA wrote:
> > Hello all,
> > I've attached two models of synchronous reluctance motor using
> > anti-periodic boundary conditions
> > in order to reduce the area to be computed. So only a half part of the
> > motor is drawn.
> > The second model is obtained from the first by rotating the rotor of
> > 180 degree angle.
> > Since the rotor is symmetric, for the two models, the Bn and Ht-plot
> > along the airgap arcsegment
> > in front of the rotor should be equal but with opposite sign.
> > For Bn, it seems to be verified, but not for Ht.
> > AJ integrals are the same for the two models but the Torques obtained
> > from weighted stress tensor are not.
> > These torques was computed in taking the whole group n°2 (in the
> > models) as area of integration.
> > So I'm wondering if there is any limitation in using anti-periodic
> > boundary condition or if it's in
> > the torque computation that I should take some care.
> >
> > I got the following result :
> > Model_1
> > AJ integral = 1.390088e+002 Henry Amp^2 / m
> > Torque = -3.040467e-002 N*m /m
> > Model_2
> > AJ integral = 1.389757e+002 Henry Amp^2 /m
> > Torque = 6.750423e+001 N*m /m
> >
> > Could anyone explain me why Ht are not the same ?
> > Any help would be appreciated.
> > Best regards
> >
> > Tsarafidy
> >
> --
> David Meeker
> Senior Engineer
> Foster-Miller, Inc.
> 350 Second Avenue
> Waltham, MA 02451-1196
> 781-684-4070
> 781-890-3489 (fax)
> dmeeker@foster-miller.com
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  - Re: [FEMM] A little paradox with anti-periodic boundary condition and torque computation
    - From: David Meeker <dmeeker@foster-miller.com>

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