Fun with FEMM

[dinges]

Dear Diary,

Last weekend I've been playing around with FEMM some more, this time with LUA scripts. For a while I wanted to create animations showing the flux lines moving and simulate cogging of the rotor. It turned out that FEMM can do this using LUA scripts. I haven't yet tried out to simulate cogging but will soon, as it would be nice to be able to simulate and verify beforehand that a conversion doesn't cog. It would take some of the excitement out of building a genny - will it cog or not ? Did I make an error in the calculations ?

Below is a link to an animated gif image of the 500 W 'Blue boy' conversion that was finished a few weeks ago. The animation clearly shows how the flux lines in the stator move. One can also easily see that, according to this animation, it would cog terribly. But, as I've skewed, it doesn't cog in real life. Since FEMM is 2D software it's hard to model the skewed rotor in 3D though (but not impossible).

(Editors: I know this is technically a violation of the image filesize rule (250 kB). Tried to compress it as much as possible, low resolution and skipping frames. Nevertheless I've added it to this post as preview because it gives an idea of what clicking on the link below with the real, large (9 MB) animation would show. If it's unacceptable feel free to remove it)

The full, hi-resolution (and much more slowly moving) animation can be found here:

http://www.anotherpower.com/gallery/dinges/Blue_boy_animatie_10ms?full=1

I hope it is of use to others in understanding motorconversions.

Peter.

[blueyonder]

 i think this is a great tool for understand just how it works.

  i love it so much it put out five amp hours as i just watched it go round.

  all i need do now is make one.

[PHinker]

I'm not familiar with the software that you're using.  How can you tell from the animation that it should cog (assuming the 2d representation follows through into the 3rd dimension)?  Is it because there are high and low flux areas in the lams as the rotor turns?  Would a non-cogging animation show consistent flux all around the perimeter?

Nice animiation!

Paul

[tecker]

De cogging is not hard .I got this tidbit off of IRC in Zubb's files .That's why I tried some squares ( but the rounds fit better ) . The north and south poles have to just touch the same core segment . That's pretty much it then the pole cancellation starts the current run.

[dinges]

Hello Paul,

In short: ignore that remark. It was incorrect. If you want to read the full explanation of my train of thought, read on.

Have a look at this picture, of the rotor in its initial condition (at 0 deg). Notice that the flux of the top (12 o'clock position) magnet is divided over 2 teeth. Also notice the color of the two teeth, which is an indication of flux density.

Now, look at this picture as the rotor has rotated 22 deg clockwise. Notice that there now is only one tooth with the same colour as in the previous image. That is, only one tooth has the flux density that previously was in two teeth. Hence my initial idea that that was an illustration of cogging. But it was wrong.

One shouldn't just look at flux density but at total flux, and the direction in which this flux tries to pull the rotor. It can be seen that sometimes it's clockwise (CW) and other times counter-clockwise (CCW).

Both the above situations are stable w.r.t. cogging, as there's neither a CW or CCW torque present. So my initial reasoning was flawed. The image below, however, shows the in-between situation from the above two images (only 11 deg. CW rotation):

In the above image it can be clearly seen that the rotor tries to turn CW so that the magnets 'embrace' most of the stator teeth (or, in physics terms, the rotor and stator try to achieve a situation of least energy). So, in conclusion, there is cogging, but it's not as obvious from the animation as I initially thought.

Actually, as I had just made the animation and was looking at it for a few minutes, I noticed a sort of 'swelling' action, where the red areas were appearing and disappearing rhythmically. Anyway, I think my statement you questioned was premature and I shouldn't have included it in the post. My excuse is that I'm still on the learning curve myself

There's only one sure way of telling whether there's cogging and that's to let FEMM give the torque numbers. I've been making another LUA script to make a graph of the cogging torque. At the moment I get the torque figures out but I can't write them to a file. And I don't fancy manually writing down the 90 numbers and typing them in a spreadsheet. The numbers as they appear on screen clearly show that there are CW and CCW moments at various positions of the rotor, so that part at least is working. Just have to try to export the numbers to a file and/or graph.

Peter.

[PHinker]

Thanks for the explanation, Peter.

[OzPete51]

Hi Tecker,

Excellent graphic!

I am trying to do the exact opposite of what you are describing - I am trying to maximise cogging!

I am trying to get Eddy currents to heat water at low speeds but have had little success. One method has been to run a bundle of aluminium strips (5mm) through the center of pairs of neo magnets with alternate polarities

NSNS

<====> (aluminum strips pushed forward and backward by hand for 5 mins.)

SNSN

The neos I have used are round 19mm diam by 25mm high and the heat produced is almost negligible. I am bundling strips of 25mm x5mm aluminum up to 8 at a time (40mm of aluminum x 25mm)

The resistance felt (cogging) is noticeable but minimal.

The hope is to have cogging that is extreme - so more flux energy converts to heat.

Any suggestions to improve the eddy currents cogging effects would be appreciated.

thanks

Ozpete51

[OzPete51]

Sorry Dinges,

I replied to Tecker by mistake (no edit?) - but hopefully no one will take offense

cheers

Ozpete51

[dinges]

Ozpete,

Cogging and eddy current losses aren't the same. In your case you probably still want to minimise cogging (I can't see any reason for maximising them) whilst making eddy current losses as large as possible.

The easiest way to get an idea of what cogging is is by grabbing a stepper motor and turn the shaft. You'll notice that the shaft turns in discrete stepe. That is cogging, the variable torque needed to rotate the shaft, the discrete steps as it turns.

For an idea of what eddy current losses are, take one of your magnets and rub it over a piece of aluminium. You'll notice that it feels as if you're moving a spoon in honey or syrup. That's how the eddy current losses would feel in a generator.

For any significant water heating you probably need much more and larger magnets, with as small an airgap as technically possible. There have been plenty discussions on water heating using magnets in the past. As far as I recall it is technically doable but not the best/easiest way to go about heating water. Have a look at WindstuffEd's experiments too:

http://www.fieldlines.com/story/2007/10/3/195821/664

Peter.

[OzPete51]

Thanks Peter,

I was under the impression that cogging produces heat.

This would explain the poor heat performance using the neos and aluminum.

A bit of a brain bender, as the kinetic energy cogging effect must be extremely efficient (no heat or sound losses) except possibly minute eddy current losses.

Back to the drawing board!

Cheers

OzPete