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# Re: [femm] (unknown)

- To: femm@yahoogroups.com
- Subject: Re: [femm] (unknown)
- From: David Meeker <dmeeker@foster-miller.com>
- Date: Wed, 03 Oct 2001 09:57:49 -0400
- Organization: Foster-Miller, Inc.
- References: <20011003112802.49849.qmail@web12603.mail.yahoo.com>

Alberto Perez wrote:

> Hello

> Thanks for your answers about induction motor. I have
> another doubt about histeresis and laminate eddy
> currents loses. I have to calculate loses in stator
> and rotor, for calculation stator loses I use a
> frecuency of 50 Hz and for rotor I use a frecuency of
> 0,5 Hz for slip,

> is it correct to calculate total loses with the sum of
> two loses?

Well, you sort of have to be careful about how you do this. It sounds like what you are after is the core losses when the machine is plugged into the wall and running under load. However, femm will only model static configurations--that is, you can't directly model the motion. However, the motion is important in determining how much flux is actually in the stator at 50 Hz, and therefore how big the stator core losses are. If you just took your motor and analyzed it at 50 Hz, the currents that you'd induce in the rotor that would cancel most all of the flux in the machine, leading to a low core loss estimate.

The trick is to do two runs and add the losses, but to do the runs in a slightly tricky way that gives you the right flux densities for computing core losses:

1) For the rotor losses, analyze the model at 0.5 Hz and evaluate the core losses in the rotor.

2) For the stator losses, modify all the materials in the rotor (both rotor conductor and rotor core material) by multiplying the resistivity of all rotor materials by (nominal frequency)/(slip frequency), specifically in your case by 100. This way, the flux in the machine in a locked rotor, fixed frequency test will be the same as in a 50 Hz machine with the rotor running at a 0.5 Hz slip frequency.

When the machine is spinning, there are some high harmonics than the fundamental present, and they will cause some additional losses--you'll be missing those, but hopefully, these contributions are small. Also, it is my experience that you have to be pretty careful in the choice of material properties if you want this sort of eddy current model to give really accurate loss estimates. This might mean obtaining a test ring of the material and doing some tests on it to regress a good permeability, resistivity, and hysteresis lag so that the agreement between the predections and experiment is acceptable.

Dave.

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- References:
  - [no subject]
    - From: Alberto Perez <albertoperarnau@yahoo.es>

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