Electromagnets and spinning objects

[D9 XTC]

Okay, so let's say there is an object with a shaft connected to it. The shaft rotationally spins the object.

On the top and bottom of the object are N magnetic fields. Not touching the object or shaft are magnets attached to a solid surface with N fields. Those are within distance to act on the objects magnetic fields that are N fields as well.

Basically, the object is floating (and kept in place by mono fields on the top and bottom) and can be spun.

My question: if the magnetic fields were just strong enough to make the object float (not a very strong field) would it take as much or more energy to spin the same setup except with extremely strong magnetic fields? It seems to me that since there is no friction between parts then even a crushing strong field would be just as easy to spin...

 

[berkeman]

Okay, so let's say there is an object with a shaft connected to it. The shaft rotationally spins the object.

On the top and bottom of the object are N magnetic fields. Not touching the object or shaft are magnets attached to a solid surface with N fields. Those are within distance to act on the objects magnetic fields that are N fields as well.

Basically, the object is floating (and kept in place by mono fields on the top and bottom) and can be spun.

My question: if the magnetic fields were just strong enough to make the object float (not a very strong field) would it take as much or more energy to spin the same setup except with extremely strong magnetic fields? It seems to me that since there is no friction between parts then even a crushing strong field would be just as easy to spin...

Could you post a sketch of the setup your are asking about? Thanks.

 

[D9 XTC]

Hmm the drawing is done and the picture is on my mobile... I don't see a post picture option though. I tried getting out of the mobile version of this site but it redirects back to the mobile site.

 

[D9 XTC]

[URL]http://www.flickr.com/photos/57509695@N07/5655695264/[/URL]

Hope this works... did it all from the mobile...

 

[D9 XTC]

http://www.flickr.com/photos/57509695@N07/5655695264/"

Maybe this will work if the other didn't because it can't tell.

 

[berkeman]

Ah, got it. BTW, you would probably have N-S magnets on your rotating part (all magnets have N and S ends, and you haven't shown where the S ends are anywhere). But that's not a fundamental part of your question. You can still arrange the outside fixed magnets to all repel the rotating magnetic assembly so it floats against the force of gravity.

I think the main variation when you increase the strenght of the magnets will be that you will get higher eddy currents in the magnetic metal pieces themselves, which will increase the drag on the rotating assembly. If you could make each magnetic assembly out of a hollow cylinder the same radius as the rotating piece, then you might be able to have magnetic force without a changing magnetic field during the rotation, so you wouldn't have any eddy current losses.

This is more of a Physics question rather than EE, so I may move it here in a bit to one of the Physics sub-forums here...

 

[D9 XTC]

I think I somewhat understand what you're saying about the hollow cylinder. I'm not sure what eddy currents are... do those only affect electromagnets or do this also affect permanent magnets?

 

[berkeman]

I think I somewhat understand what you're saying about the hollow cylinder. I'm not sure what eddy currents are... do those only affect electromagnets or do this also affect permanent magnets?

Here is an intro to eddy currents:

http://en.wikipedia.org/wiki/Eddy_currents

Whenever you have a changing magnetic field through a conducting loop (like with wire in a transformer, or just the solid metal of a magnet or other conducting object), a current is generated that itself generates a magnetic field that is in the direction to oppose the original magnetic field. This is what generates the "back EMF" in transformer action, and what is used in eddy current braking systems.

So when your drawing shows individual solid cylindrical magnets, as the rotor spins, the magnetic fields are going to be changing as the magnets align and mis-align during different parts of the rotation. If you could make the magnets always "see" the same magnetic field, then I don't think you'd have eddy current issues that will be generating drag.

So I think one way to do that would be to use a full hollow cylinder for the rotor magnet, instead of individual solid cylinders. The outer radius of the big hollow cylinder matches the outer radius of your rotor, and the inner radius is something like half of that. The magnet is magnetized vertically, as your picture is drawn now.

And opposing that rotating hollow cylinder magnet are two fixed, similar-shaped magnets, one below the rotor and one above. You line up the N-S faces of the two outer magnets to give you repulsion from both to the rotor magnet in the middle.