Eddy current separators...

[Steelforbrains]

Can anyone explain to me how an eddy current separator works to separate aluminum scrap from ferrous and plastic scrap? The separator that I have seen flings the aluminum off of a belt conveyor into a chute. From what I understand the head pulley on the conveyor has a permanent magnet inside of it, that rotates at a much higher speed than the head pulley. This rotating magnet induces an eddy current into the scrap and I guess that sets up an opposing magnetic field in the part which is repelled from the head pulley at great speed. My question is, why is it that the aluminum is thrown so much further than the other materials on the belt? It is simply due to the fact that the ferrous material is heavier and requires more force to throw it as far? I assume that non metallic objects would fall the shortest distance, followed by heavier metal ojects and lighter metals would go the farthest. Please feel free to critique my reasoning and keep in mind that I am not an electrical engineer.

I have also seen another type of separator that is used to separate different alloys/grades of aluminum. This system supplements the preceding system. In this setup there is a long row of what appear to be aluminum panels that hang side by side. The panels pivot at the top and when they detect certain alloys they are actuated to swing forward (in the direction of material flow) to toss the aluminum even further. There is no contact involved between that panels and the aluminum. How does a system like this measure the alloy content, and what causes the attractive force between the alloy and the paddle? This all happens at a very fast pace, I would say several hundred fpm.

Feel free to speculate, discuss, or even guess as to how this works...
Thanks,
Rob Brooker
Conveyor Services Inc.

 

[itsmoked]

Yes it is exactly what you are guessing. The aluminum is about a 1/3 the weight and so gets tossed farther when the same force is applied. The eddy current aspect is as you described.

Now I don't know about those paddle do-hickys.. I can only posit that different alloys have different eddy current "qualities and that this would effect the throwing distance too. ?

you didn't mention any electrical wires etc, going to the panels but an AC electromagnet would induce eddy currents into the aluminum passing by. Other sensors could monitor the resulting eddy fields and make some decisions based on the result.

 

[Steelforbrains]

I don't know if I trust an electrical engineer with a name like itsmoked, but I'll ask anyway... Is it true that when a magnet induces an eddy current, and thus a magnetic field, in a conductor that the magnetic fields will always be repulsive? If so, then maybe what I am seeing is opposite of what I believed. Maybe the panels don't "throw" the parts but instead allow the parts to pass through unobstructed, by flipping out of the way. The problem I see with this theory however is that if the panel system stopped working then the panels would be damaged by the flying scrap metal.

other questions about eddy currents/magnetic fields...
1) Does the magnetic field have to be moving with respect to the conductor in order to induce an eddy current in the conductor?

2) Does the magnetic field have to be alternating or does it just have to be moving with respect to the conductor?

3) Do the poles of an AC electromagnet continuously change direction due to the altenating current? If so, and this may be a stupid question, could you pick up a permanent magnet with an AC electromagnent? Could you heat a piece of steel with an AC electormagnet due to the internal friction caused by continuously rearranging the molecules of the steel?

4) Does the rotational speed of the magnetic rotating drum (mentioned in the original post) affect the amount of inductance in the conductor? Does the speed of the conductor affect the inductance? Does the frequency of an AC electromagnet affect the inductance on the nearby conductor?

I hope that I haven't asked so many questions that nobody wants to reply now...

Thanks

 

[Skogsgurra]

You can trust old Smoked. I had my doubts, too. But I feel comfortable with his attitude. And I trust him.

The aluminum parts have a lot more eddy current induced in them. Mainly because they have a much better conductivity than steel and iron plate has. The lower density also makes Al parts lighter so they accelerate faster and fly longer. The distance they fly is partly reduced by their lower weight, but the positive factors dominate.

Your questions:

1. It has to move (or alternate) to induce eddy currents.

2. Same question, I believe?

3. Yes, they change direction. About picking up a permanent magnet: Probably not as good as with a DC magnet. It depends on what field strengths you have. A very strong AC magnet can pick up anything ironish, magnet or not. Heating steel with an AC magnet is being done all the time. Induction furnaces, for example.

4. Your question is not comprehensible for an EE. Sorry, can you perhaps reformulate it? Inductance is a property in a wire or a coiled wire.

Gunnar Englund

http://www.gke.org

 

[MJR2]

1. Either the magnetic field has to move or the part to induce an eddy current. Of course an AC field will also generate eddy currents.

2. In the eddy current separator you describe above the magnetic field has alternating poles as you move around the circumference.

The reason the aluminum flies further than say stainless is because it is more conductive and lighter. The eddy current effect is stronger and it takes less force to get it moving. Don't get a ferrous object near the separator.

3. Polarity of an AC magnet does change. Such in a magnet shaped like an E with a coil around the center leg. Yes, you can heat the steel. Red hot and melt through shell and quickly. Ref last comment.

4. It is not so much rotational speed but the effective AC field generated by the changing poles. You get that with rotational speed. Higher speeds and stonger permanent magnets mean better separation, in general.

 

[Steelforbrains]

Thank you for your replies. As for question #2 skogsgurra, what I am trying understand is does the magnetic field need to be alternating or can it simply be moving? Simply waving a magnet past a piece of steel is certainly different than setting up an alternating magnetic field, such as in an AC electromagnet or a spinning permanent magnet.

As for question #4 maybe I have worded it wrong. I mean does the rate that the magnetic field alternates affect the amount of charge induced into the scrap metal?

One last question...
We keep discussing the Eddy Currents that are induced into the scrap metal. If there is current then there is a flow of electrons. Where are these electrons flowing to, and where do they come from? Are there ions being formed in the metal by removing electrons from the molecules? Do these electrons return after the magnetic field is removed? If the electrons are stripped from the molecules does that make the molecules more reactive?
Ok that was a bunch of questions... sorry

 

[MJR2]

Either the magnetic field moves or the part. As an example is the magnetic damping of balance beam type scales. I helped my son do a science project where a pendulum would swing between two magnets. Changing the material demonstrated clearly the forces available. (He wants to be a rocket scientist rather than a magnet engineer)

The change in field is what generates the eddy currents. Increasing the rate of change with shorter poles or higher speed increases the forces.

The current cannot flow. The result is a force on the object. If the object doesn't move, the result is heat.

 

[itsmoked]

Thanks for the VoC skogs..

Steelforbrains; Perhaps your #4 should have been stated:
Does the speed of the rotating magnet near the aluminum have an effect on the strength of the induced eddy currents.

Yes it would. This may be a way to control the throw distance too.

 

[Skogsgurra]

Steelforbrains,

Your last(?) question about electrons - whence and whither?

The electrons are free moving electrons, typical of metals. That is what makes a metal a conductor. They are moving in circles, that's why the currents are named "eddy currents" - it looks a little like back water in a creek with its whirls.

Gunnar Englund

http://www.gke.org

 

[Skogsgurra]

For those who "really knows about electricity":

Yes, the electrons do not move much, it is the current that runs in circles. The electrons just move a little bit to deplace charge.

Gunnar Englund

http://www.gke.org

 

[Steelforbrains]

Thank you very much your posts have been very insightful... There was a question that I had that nobody answered. When you set up these eddy currents can you only create repulsive forces?

Does anyone have an opinion on how the system with the aluminum panels, described earlier, works. I am assuming that the system sets up AC electromagnetic field which induces Eddy Currents into the incoming aluminum scrap. There is a sensor that measures the resultant magnetic field from the scrap and determines whether it should pass through or not. This theory goes on the assumption that different aluminum alloys have a significant difference in conductivity.

 

[MJR2]

In general if you should have a ferrous object get to the eddy current rotor it will be strongly attracted to the shell and held in place until a wiper on the belt cleans it off. If this should take more than a few seconds the eddy current heating of the object will cause it to melt through the belt and the shell of the rotor. A very expensive repair follows.

Do you know the manufacturer of the aluminum panel type separator. We might get a better understanding of their device from their website.

 

[itsmoked]

You can have attractive or repulsive forces with eddy currents.

 

[Steelforbrains]

itsmoked,
How do you control the direction of the force?

I have no idea who manufactured the separator.

 

[itsmoked]

No one has ever explained it to me, but I have seen both cases.

And attractive case was just described in this forum recently (but of course cannot be found). If you let a strong magnet slide down a sheet of aluminium the magnet will slide at about a third the speed it would down a sheet of plastic because of the eddy current generated by its motion down the sheet. (Attractive)

Whereas there are a lot of eddy current projector demonstrators that shoot aluminum disks. And your separator appears to 'shoot' aluminum. There are many eddy current levitators.

I would guess that when an eddy current is set up in a material it is always an alternating field. The resulting magnetic field has Some orientation. If the 'causing' field can be made to shift phase in some manner then you can cause repulsion or attraction.

Repulsions:
See figure 2.

http://www.monorails.org/webpix 2/Seraphim101401.pdf

http://www.sfu.ca/physics/ugrad/courses/teaching_resources/demoindex/eandm/em5k/ek2005.html

http://www.classictesla.com/download/ieee_potentials_2000.pdf

Attractions:
The sliding magnet experiment noted above.
Any eddy current clutch or brake system.

 

[fsmyth]

'smoked, you left rail guns out of your examples.
A pretty severe exclusion, IMO. <g>
<als>

 

[itsmoked]

Your absolutely correct fsmyth!!

Here's one by that wacky lab guy.

http://www.powerlabs.org/railgun.htm

Oh, OH, Ohhh!!

I also left out the eddy current beer can crusher!

http://www.powerlabs.org/pssecc.htm

 

[resqcapt19]

itsmoked,

If you let a strong magnet slide down a sheet of aluminium the magnet will slide at about a third the speed it would down a sheet of plastic because of the eddy current generated by its motion down the sheet. (Attractive)

Are you sure? I was reading about Lentz law and this type of experiment. Take a rare earth magnet with an OD that is slightly smaller than the ID of a copper or aluminum pipe and drop the magnet down the pipe. I did it with 1/2" copper plumbing pipe that was 5' long. It takes 28 seconds for the magnet to come out of the bottom of the pipe with the pipe vertical. The magnet moving down the pipe creates electric current that in turn creates a magnetic field. This field repels the dropping magnet, making it appear to violate the law of gravity. That would not be an attracting force.
Don

 

[Steelforbrains]

I thought that it was due to a repulsive force as well, but I'm no expert. Wow it takes 28 seconds to go through a five foot pipe! That is pretty amazing. I guess the opposite would work too, right? If you had a magnetic tube and an aluminum or copper ball.

 

[itsmoked]

resqcapt19; You could be correct there..

I don't particularly see why you would attribute the result of that slow magnet in a pipe as a repulsive effect why not an attractive effect? Glass half full glass half empty kind of situation.

But I still see comments towards attraction.

Here is a standard Columbia University Demonstration of Eddy Currents.

20. Eddy Currents
5K20.1 Eddy Current Pendulum
A copper plate is free to swing between the poles of a large electromagnet. When the field is turned up, the plate will brake and quickly come to rest. The solid plate can be replaced with a comb-shaped plate, which diminishes the braking effect of the eddy currents.
[Set-Up Time: 25 min.]

This seems to support my "assumption of attraction" but I will admit I haven't ever figured out the physics of Eddy currents. When that magnet slides down the Al plate it appears to be attracted to it to me.. I have done this every few days since the subject came up. But I will do the test again today with a change.

Statement: An Eddy Current repulsive or attractive force could retard a falling magnet on a sloped aluminum sheet.

Hypothesis: If the plate is vertical and the Eddy Current force is attractive then the magnet should again fall in a retarded manner staying close to the plate. If the force is repulsive the magnet should fall AWAY from the plate not landing directly below its release point.

I'll report back.