RE: [femm] SRM Modeling

["Pradeepkumar Ashok" <pradeepkumar@xxxxxxxxxxxxxxx>]

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@xxxxxxxx] 
Sent: Wednesday, November 27, 2002 12:01 AM
To: femm@xxxxxxxxxxxxxxx
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@xxxxxxxx
http://femm.berlios.de/dmeeker



 

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