Eddy current heating for the bored pickup maker

[David King]

But the rare earth magnets don't lose their charge, they don't get weaker.
Also I'm confused by the superconducting statements. Eddy current breaking seems to be much more noticeable on metals that conduct well like copper and aluminum. Why would a superconducting metal suddenly be exempt? It seems to me more current flow just means more heating in a disk anyway. A transformer is a completely different sort of gizmo because the magnetic field energy can be stored (in a steady state) in the permeable core (rather than the electric energy going one-way into heat, never to return). No?

[Steve Conner]

Well, if you think superconductors should be affected by eddy current drag, you have to explain why the toy train in the video doesn't slow down.

[David Schwab]

You can make the experiment a lot simpler. Just drop a strong magnet through a copper pipe. It takes extra time for it to fall because of the distortion of the magnetic field.
Some of the heat is coming from your magnets and they are getting weaker. The rest is from the motor distorting the magnetic field.

The magnets are falling slower because the eddy currents are producing their own magnetic field which opposes the magnet's field. Copper is also diamagnetic.

The disk gets warmer because of the eddy currents. When the current flows on the surface of the disk, the moving current combined with the resistance of the material generates heat. The magnets are not loosing their energy and getting warm.

The disk would also heat up if it were made from steel due to hysteresis.

The motor was not distorting the field, as shown in the example of the pedal powered system.

[Mike Sulzer]

Well, if you think superconductors should be affected by eddy current drag, you have to explain why the toy train in the video doesn't slow down.

I think the analogy between the train and the rotating disk is not perfect but it is interesting.

For example: The train contains a superconducting magnet. It moves along the track, changing the flux through the track. Why does it not induce eddy currents in the track, which should slow the train. Is the track non-conducting? Laminated?

[Mike Sulzer]

But the rare earth magnets don't lose their charge, they don't get weaker.
Also I'm confused by the superconducting statements. Eddy current breaking seems to be much more noticeable on metals that conduct well like copper and aluminum. Why would a superconducting metal suddenly be exempt? It seems to me more current flow just means more heating in a disk anyway. A transformer is a completely different sort of gizmo because the magnetic field energy can be stored (in a steady state) in the permeable core (rather than the electric energy going one-way into heat, never to return). No?

Superconducting really is different from just high conductivity. The resistance really is zero, and so there is no dissipation.

But the eddy currents do make a magnetic field and that magnetic field has energy associated with it. There is no way around that, and it happens in the disk as it does in an inductor.

[rockin roy]

We are discussing several things here, David has a good point about the Diamagnetic properties of materials. Lets just call them non-magnetic though. The difference is copper and air try to exclude (push) a magnetic field. The iron disc (magnetic) provides a better path path then air (non magnetic) and attracts the field. Super conductors (non magnetic) Push (real good) they also conduct electricity very well which creates a magnetic field but that is external to them. So that confusingly makes them super magnetic non magnetic materials by virtue of their lack of resistance.

I think the magnet moves down my copper pipe slowly because the magnetic flux lines make it bounce back and forth a lot increasing the distance on the way down.

Vacuum cleaners use a brush type motor which rather them trying to sync to a line frequency use repulsion so their speed is limited by internal friction and load.

[Kazooman]

An interesting thread! I'm not an EE guy, I'm a chemist. This discussion reminded me of a trick we used to use in the lab. Chemists often stir reaction solutions with a teflon-coated magnet in the flask and a magnetic stirrer under the flask. If you had a stirrer that went too fast, even at the lowest setting you could slip a 1/4" piece of aluminum between and the stirrer would slow right down due to the eddy currents. I never checked the temperature of the aluminum, usually the stirrer was also a hot plate and we would be heating.

So this would be another way of isolating the heat of the motor from the copper disk. Keep the disk still and rotate the magnets.

Not to hijack the thread, but here's another interesting heating/cooling phenomenon for you to ponder: Have you ever seen vortex heating/cooling tubes like these?



You blow compressed air in the side tube and air comes out the ends. One end is hot (as much as 100 degrees C) and the other end is cold (as low as -46 degrees C). There are NO moving parts. I believe that Maxwell's Demon is sorting out the molecules in the air.

[David Schwab]

I think the magnet moves down my copper pipe slowly because the magnetic flux lines make it bounce back and forth a lot increasing the distance on the way down.

No, it actually is falling slower. You can do the same thing with a thick aluminum plate. The magnet will slide slowly down the plate. Just think of the eddy currents as being little magnets pulling on the falling magnet.

[David Schwab]

Have you ever seen vortex heating/cooling tubes like these?



You blow compressed air in the side tube and air comes out the ends. One end is hot (as much as 100 degrees C) and the other end is cold (as low as -46 degrees C). There are NO moving parts. I believe that Maxwell's Demon is sorting out the molecules in the air.

Vortex tube - Wikipedia, the free encyclopedia

Interestingly they can't seem to agree on what causes the effect, but it seems to do with higher and lower air pressure. Pretty cool!

[Kazooman]

Interestingly they can't seem to agree on what causes the effect, but it seems to do with higher and lower air pressure. Pretty cool!

Or.... pretty hot, depending on which end of the tube you are facing.

It is pretty amazing that there is no agreed upon explanation for the effect. These things are used all over the place in industry. A Google search pulls up hundreds of companies that sell them.

[David Schwab]

It is pretty amazing that there is no agreed upon explanation for the effect.

Makes you wonder how it was invented. Was it an accident, like so many other inventions and discoveries?

[Steve Conner]

In the train demo, the track is made of NIB magnets and the train has a lump of superconductor on it. Surely if the superconductor were just a regular conductor, that would be a recipe for massive drag.

You can also have fun dropping a neo magnet between the fins of a large heatsink. It can take a good few seconds to fall to the bottom.

[Mike Sulzer]

In the train demo, the track is made of NIB magnets and the train has a lump of superconductor on it. Surely if the superconductor were just a regular conductor, that would be a recipe for massive drag.

Yes. And considering currents induced in the track, as far as I can tell NIB (neo) magnets have a resistivity that is about 100 times higher than copper (Neodymium magnet), although there is something funny about the units, so I am not sure.

In this case, as long as the magnets are small and electrically isolated from each other, eddy current loss in the track should be small.

[Joe Gwinn]

Levitating Magnets and the Meissner Effect

If the resistance is zero, the eddy currents are at least in theory infinite.

What happens is that the eddy currents grow until they generate an equal but opposing magnetic field, so that none of the external field is able to enter the superconductor. Alternately, the two fields cancel perfectly within the body, but not in the space outside the body.

Superconductivity - Wikipedia


This is related to the Meissner Effect, which is that magnetic fields are expelled when a body of material transitions to the superconducting state.

Meissner effect - Wikipedia

[David King]

Is superconductivity only skin deep?