How do eddy currents affect magnet movement through non-magnetic materials?

[Bern123]

Hi, what differences are if i use a pvc tube to build an actuator(solenoid with plunger) or alluminium? Alluminium can intensify magnetic field? but the plunger will stick with alluminium? or solenoid will have less force?

 

[sophiecentaur]

Why not use steel? The permeability of steel will increase the force available. Aluminium is non-magnetic.

 

[Bern123]

The plunger is in steel. I don't know how tube may influence in positive or in negative. I always see actuator in pvc tube.

 

[nasu]

The aluminum may increase the dissipation. Eddy currents will be induced every time the magnetic field changes. This change will happen when the plunger moves in the tube, for example.
Think about the experiment with a magnet falling in a copper or aluminum tube. The steel core will be magnetized, so it is a magnet moving in a conducting tube.
The damping may be a desirable effect, after all. It depends on the application.

See this, for example:

Your tube has much thinner walls, I suppose. So the effect will be much weaker.
But some dissipation will be there so why not avoid it, if you have no special reason for aluminum?

 

[sophiecentaur]

The aluminum may increase the dissipation. Eddy currents will be induced every time the magnetic field changes. This change will happen when the plunger moves in the tube, for example.

I wondered about that but would it be a significant effect unless the actuator was being driven in-out-in-out continuously (or fed with AC, of course)?

 

[nasu]

It will be an effect (don't know how strong) only when the actuator moves. Or actuates.:)
Isn't this when it matters?

Oh, you mean there will be no power dissipation when it does not move.
You are right. I was more concerned about a damping of the motion itself rather than the power lost in the tube.

 

[Bern123]

So, if i need high speed applications, alluminium will not be good?

 

[nasu]

I have no idea how strong the effect will be for your specific tube.
And high-speed is a relative concept.
Only trying you can find out.

 

[UltrafastPED]

So, if i need high speed applications, alluminium will not be good?

A nice demonstration of non-magnetic materials and eddy currents:

1. Use a strong magnet - the modern neodymium magnets works well; for a flat magnet use a flat piece of aluminum - wide enough and long enough to work as a slide.

2. Show that the magnet is not attracted to the aluminum

3. Place the aluminum slide flat on the table, with the magnet at one end

4. Now slowly lift the magnet end of the slide; the magnet will begin to move down the slide ...

5. But as the angle of elevation increases, the magnet will slow down - due to the induced eddy currents. This is "magnetic braking".

A thicker aluminum slide makes for a more dramatic effect.

So for your solenoid I would avoid a metallic enclosure if the magnet is strong.

This site describes some other demonstrations; but the video didn't work for me:
https://www.kjmagnetics.com/blog.asp?p=eddy-currents

This video shows the demonstration of a magnet falling through a copper tube: