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Author Topic: a strange finding  (Read 9548 times)

Cairun

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a strange finding
« on: July 09, 2011, 05:54:16 PM »
Hi all,

I found something interesting lately, something that has intrigued me.
I had two rectangular magnets in repulsion; and I placed a bar of steel on top of the two magnets.
I found that the attraction between the bar and the magnets would be stronger when you forced the magnets together than when they are apart.  I do not understand what is happening there.  I would appreciate it if anyone can provide answers or opinions.

Thanks,
Alex

futuristic

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Re: a strange finding
« Reply #1 on: July 09, 2011, 07:09:13 PM »
If I have time I will model this in FEMM, but out of the head I would say that when magnets are close in repulsion the path of magnetic lines is shorter and the density of magnetic field on the opposite end is therefore greater.

futuristic

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Re: a strange finding
« Reply #2 on: July 09, 2011, 07:32:25 PM »
Here it is...
Huh the sim shows that force is greater when there is no magnet near in repulsion mode:

Only one magnet (iron block is the one on the right):
http://file.si/pfiles/183617/no-repulsion1.png

Two magnets in repulsion:
http://file.si/pfiles/183616/repulsion1.png

Perhaps I did not do the simulation correct? FEMM file is in the attachement (change the extension to .fem)...

MrMag

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Re: a strange finding
« Reply #3 on: July 09, 2011, 08:30:11 PM »
My guess is that since the magnets are opposing, the flux lines are fighting each other. When the bar of steel approached the magnets, the flux will tend to be attracted the bar instead of fighting the other magnet. I hope this makes sense.

Cairun

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Re: a strange finding
« Reply #4 on: July 10, 2011, 06:15:02 AM »
Here it is...
Huh the sim shows that force is greater when there is no magnet near in repulsion mode:

Only one magnet (iron block is the one on the right):
http://file.si/pfiles/183617/no-repulsion1.png

Two magnets in repulsion:
http://file.si/pfiles/183616/repulsion1.png

Perhaps I did not do the simulation correct? FEMM file is in the attachement (change the extension to .fem)...

Thank you for answering my question.  However, the set up I had was different from yours.
I had the iron bar centered and on top of the two magnets.  Would you run the test again with this set up, please?
I downloaded the attached program, but I was not able to change the file extension to .FEM.  Do I need a special software to run this program?

Thanks,
Alex

Cairun

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Re: a strange finding
« Reply #5 on: July 10, 2011, 06:20:51 AM »
My guess is that since the magnets are opposing, the flux lines are fighting each other. When the bar of steel approached the magnets, the flux will tend to be attracted the bar instead of fighting the other magnet. I hope this makes sense.

Thank you for offering answers to my question. 
I had similar thoughts about this. 

Thanks,
Alex

futuristic

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Re: a strange finding
« Reply #6 on: July 10, 2011, 07:26:23 AM »
You have to rename file before you can run it in the FEMM.
Try this: select the file with mouse -> press F2 key, replace text "txt" with "fem", press Enter.

I think now I understand where is the steel bar, will do another sim.

futuristic

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Re: a strange finding
« Reply #7 on: July 10, 2011, 07:40:55 AM »
Is this how you have it?

Magnets close together:
http://file.si/pfiles/183688/bar-on-top1.png

Magnets apart:
http://file.si/pfiles/183687/apart.png

If this is not the correct setup then please draw it...

Cairun

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Re: a strange finding
« Reply #8 on: July 11, 2011, 05:37:52 AM »
You have to rename file before you can run it in the FEMM.
Try this: select the file with mouse -> press F2 key, replace text "txt" with "fem", press Enter.

I think now I understand where is the steel bar, will do another sim.

I can rename the file, but I cannot seem to change the file extension.

Thanks,
Alex

Cairun

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Re: a strange finding
« Reply #9 on: July 11, 2011, 05:48:10 AM »
Is this how you have it?

Magnets close together:
http://file.si/pfiles/183688/bar-on-top1.png

Magnets apart:
http://file.si/pfiles/183687/apart.png

If this is not the correct setup then please draw it...

This is close enough to the set up I had. 
However, if I am reading the results right the simulation is showing the opposite of what I found.

Thanks,
Alex