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# RE: [femm] SRM Modeling

- To: <femm@yahoogroups.com>
- Subject: RE: [femm] SRM Modeling
- From: "Pradeepkumar Ashok" <pradeepkumar@mail.utexas.edu>
- Date: Wed, 27 Nov 2002 16:53:22 -0600
- Importance: Normal
- In-reply-to: <3DE45F94.8020404@ieee.org>
- Organization: Robotics Research Group
- Reply-to: <pradeepkumar@mail.utexas.edu>

    Hi Dave,

    Yes, I forgot to change Imin in the Lua script. Will do that first.
    Thanks for the pointer. With regards to centering the contour in the
    airgap, I had it centered when I drew it in Autocad. When I imported it
    somehow got changed. Don’t know why or how ? My first guess is that when
    I exported the autocad dwg to dxf format, the circle got a lot
    linearized ??

    Prem actually modeled my SRM in Maxwell and got results pretty close to
    the analytical results 1200 Nm/m (which was basically double the one
    that I got when I used FEMM stress Tensor torque 600 Nm/m). He used the
    coenergy method to calculate torque. I might try to use that (get the
    co-energy is the airgap and differentiate it with regards to position,
    assuming linearity between two very close positions)

    Peter suggested a different approach. He suggested that I use the flux
    linkage results from FEMM to calculate the torque. This also seems to be
    a good idea since it does not require us to mess around with the airgap
    in the postprocessor phase.

    Well ! I guess it is going to take me a couple of days to use and try
    all these ideas. Thanks to Prem, Peter and Dave for responding. Will let
    Ya all know what I end up with.

    Regards,
    Pradeep

    *********************************************************************
    Pradeepkumar Ashok
    Robotics Research Group-PRC
    Campus Mail Code: R9925
    University of Texas
    Austin, TX 78712
    www.robotics.utexas.edu
    *********************************************************************

    -----Original Message-----
    From: David Meeker [mailto:dmeeker@ieee.org]
    Sent: Wednesday, November 27, 2002 12:01 AM
    To: femm@yahoogroups.com
    Subject: Re: [femm] SRM Modeling

    Pradeepkumar Ashok wrote:

    > Hi all,
    >
    > Has anyone used FEMM to model and evaluate the torque capacity of a
    > switched reluctance motor? I need help
    >
    > My FEM model only gives me half the torque that my analytical
    > calculations predict. For those interested, the plots and the machine
    > dimensions are in the file called comparisons.doc . The analytical
    > method I used was based on a paper by Radun Analytically Computing
    the
    > Flux linked by a Switched reluctance motor phase when the stator and
    > rotor pole overlap. This paper uses the magnetic co-energy method to
    > calculate torque, while FEMM uses the stress tensor method. Could that
    > be a reason for the difference? I dont think so&but I am not sure&
    >
    > In the models, I used triangle to mesh the air gap between the stator
    > and the rotor poles (remodel.fem) . I also used a slightly different
    > model with a finer mesh (.0005) between the stator and rotor poles
    > (remodel2.fem) to see if this problem was due to my mesh being coarse.
    > The airgap is .0125 inch and so I felt that .0005 was more than
    > sufficient. I found the torque output from these two cases to be not
    too
    > different. So I went ahead a created a Lua script to plot the static
    > torque Vs Rotor position Vs Current. (Files inside the folder
    > torque_sim),and that is how I got the plot shown in comparisons.doc
    >
    > Simply put my question is Have I modeled the machine correctly& If
    so,
    > I can safely attribute this difference to errors in the analytical
    > model. Can somebody help ?

    I didn't run the script all the way through, but there were a few of
    things that I noticed:

    -- Your script only changes the current density in the Imax material. I
    think that you also need to change the Imin material in an analogous way

    to get correct results.

    -- It's best to draw the contour along which the stress tensor is taken
    through the center of the air gap, rather than to one side.

    -- Make the arcs that compose the pole faces and integration contour
    have a finer discretization (like 1 degree or less). This will get rid
    of the cogging in your graphs of torque versus rotational position.

    Dave.

    --
    David Meeker
    dmeeker@ieee.org
    http://femm.berlios.de/dmeeker

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- References:
  - Re: [femm] SRM Modeling
    - From: David Meeker <dmeeker@ieee.org>

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