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# Re: [femm] Help Eddy current in three phase induction motor

- To: femm@yahoogroups.com
- Subject: Re: [femm] Help Eddy current in three phase induction motor
- From: dcm3c@aol.com
- Date: Thu, 13 Sep 2001 21:30:23 EDT

In a message dated 9/13/01 8:51:30 AM Eastern Daylight Time, albertoperarnau@yahoo.es writes:

> Hello,
> I'm simulating three phase induction motor.I have
> problems with eddy current. When I define material of
> rotor and stator I put 3 MS/m in the box of electrical
> conductivity , but the results are illogical, the
> field isn't hoped it. If I don`t put electrical
> conductivity the results are logical. What do I make
> bad?

The trick here is that the machine that you are trying to model is most likely built up out of laminations. Just putting in 3 MS/m without changing the defaults for lamination thickness and fill factor means that you are modeling the iron in your machine as a slab of solid iron. In this case, you'd get profound eddy currents in the solid iron at very low frequencies, which is probably what you are seeing.
When you define a zero conductivity, it makes the eddy currents go away, and you see what you'd expect. Try changing the default of 0 in the d_lam box to the thickenss of your laminations in millimeters. Then, you'll get the field distribution that looks like you'd expect, and the eddy currents in the laminations (which tend to make the core's permeability look lower and complex valued) are also modeled.

> I'm watching problem of tube.fem in tutorial, I watch
> that define a current=0 for ring of iron, why?. When I
> don't put current=0, the results are different from
> the previous case. I do the same in my problem, but it
> doesn't change.

The zero current constraint is meant to model an iron tube of finite length. The idea there was that all current has to be conserved within the tube, thuse the total current=0 constraint. In your machine, the sinusoidally distributed fields would cause eddy currents that tend to naturally add up to zero, so the constraint is naturally satisfied without imposing it explicitly.
Dave Meeker
--
http://members.aol.com/_ht_a/dcm3c

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