The Permeability of Aluminum

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
So is Aluminum (AKA Aluminium) anti-magnetic or something? Does it have a Permeability of less then 1? It's not supposed to.

But here is an odd thing. After chatting with BudP about transformers and inductors, I was doing some quick experiments on the effect of a metal core on a coil. Took a small 0.2mH air core inductor and placed various metal things in the gap. Was able to raise the inductance to over 0.4mH with large mass of steel, like a socket from a ratchet.

Copper, brass, tin, wood, plastic, flesh and blood had no effect. But aluminum? It caused the measured inductance to drop. The more massive the aluminum piece, the bigger the drop. Why? Is this just an artifact of my inductance meter, or is the aluminum actually causing a drop in inductance?
 

jcx

Member
2003-02-17 7:38 pm
..
solid copper likely will show even more effect

high electrical conductivity metal's conduction electrons get shoved arround by time varying magnetic field - and their resulting motion (Eddy currents) causes them to generate their own magnetic field that opposes the external one

another veiwpoint is that a closely fitting conductive ring or tube forms a shorted turn of a transformer with the coil - the open and shorted turn inductance measurements may be used to calculate the mutual inductance/coupling constant
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
I can see the shorted coil thing. all the aluminum I have is tubes of some sort. Nothing solid. That may be it.
So the steel tube should also be a shorted turn, but that effect is overwhelmed by the mass of the steel.

Set up is simple. Just a little air core coil like this:
[IMGDEAD]http://www.parts-express.com/images/item_standard/266-806_s.jpg[/IMGDEAD]

Metal inserted in the center hole.
 
It's certainly strange that the copper and brass showed a different effect. Ideally for shorting a field you'd like solid. Depending on how you're measuring you could be getting some resonance that is affecting it. Looking at the current or voltage waveforms might give a clue. Incidentally, for very low inductance coil tuning often threaded aluminum tuning slugs are used. It's light and cheap and of course has much better stability and response than any permeable material, so they make the coil a little bigger and then tune down.
 
So is Aluminum (AKA Aluminium) anti-magnetic or something? Does it have a Permeability of less then 1? It's not supposed to.

But here is an odd thing. After chatting with BudP about transformers and inductors, I was doing some quick experiments on the effect of a metal core on a coil. Took a small 0.2mH air core inductor and placed various metal things in the gap. Was able to raise the inductance to over 0.4mH with large mass of steel, like a socket from a ratchet.

Copper, brass, tin, wood, plastic, flesh and blood had no effect. But aluminum? It caused the measured inductance to drop. The more massive the aluminum piece, the bigger the drop. Why? Is this just an artifact of my inductance meter, or is the aluminum actually causing a drop in inductance?

The measured inductance is indeed being lowered by the aluminum, it is not an artifact of the meter.

I can see the shorted coil thing. all the aluminum I have is tubes of some sort. Nothing solid. That may be it.
So the steel tube should also be a shorted turn, but that effect is overwhelmed by the mass of the steel.

Set up is simple. Just a little air core coil like this:
[IMGDEAD]http://www.parts-express.com/images/item_standard/266-806_s.jpg[/IMGDEAD]

Metal inserted in the center hole.

The mass of the steel is not the player here.

The conductivity of the material will dominate the effect. The principal involved is called Lenz's law. It is the nature of the conductive objects to try to exclude the time varying fields from the metal. (loosely stated of course, don't get me started with equations..:eek:..

Here's a link:
HyperPhysics
Look under E/M, then Faraday's law...lenz is the second group.

For your specific results, you have to consider the geometry of the objects you are placing within the field. The biggest effect will occur when the field lines penetrate the metal perpendicular to the best conductivity. How well the geometry of the metal can setup it's internal currents is the dominant factor.

This happens all the time for me, the inductance vs frequency for coils I work with and test will be altered by the surrounding metal objects..typically the inductance will fall off with frequency as a result, and the magnetic field of the coil will really start to phase lag. Also in general, the meter will pick up a higher series resistance if you use it's Ls/Rs function. The meters are not able to distinguish the difference between the coil electrical resistance and the dissipation that results from the external metal taking the field.

But, that's just my guess...y'all's gonna need to speak to a real magnetic field guru if y'all needs ta know the truth..me, I'm just a country doctor...Jim..

Cheers, John
 
Just a small data point... I started my electronics career in the tube era. The RF and IF tuned circuits in radio & TV receivers usually used threaded ferrite cores ("slugs") that could be screwed in and out of the coil formers to alter the tuning. These had a bad habit of seizing, making them liable to breakage if you tried to force them. So rather than try to alter a slug's setting to see if it was correctly tuned, we used a tuning wand - a plastic rod with a small ferrite slug on one end and a small brass slug on the other. Inserting the ferrite slug would lower the resonant frequency, and inserting the brass end would raise it. If both actions caused the performance to get worse, the coil was already correctly tuned and the coil's slug could be left alone.
 

Elvee

Member
2006-09-08 2:04 pm
Aluminium is in fact a paramagnetic material, which means it is very slightly magnetic:
Paramagnetism - Wikipedia, the free encyclopedia
Its relative permeability is 1.000022, and in theory, it should increase the inductance of a coil.
But in its massive form, the shorted-turn effect resulting from its high conductivity is overwhelmingly dominant, making the paramagnetic effect unnoticable.
Aluminium - Wikipedia, the free encyclopedia
The effect might be detected with a very finely powdered material, at very low frequencies to minimize the effect of eddy currents.
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
Don: What a clever troubleshooting idea. So simple, so cool.
Elvee. Yeah, after looking up the permeability of Alu, I figured it was so close to air that it shouldn't make a difference - but it did. So it's the shorted turn, not the permeability that counts.

Maybe I need to make an AF oscillator with the coil. Then I could have fun listening to the charges different metals and shapes make.
 
Great stuff John, thanks!

You are welcome. When I saw the word permeability, I just couldn't resist.

Magnetic circuits are fun. I'll continue to play with this, if only for my own amusement. :D

I get to play with some really neat magnets. What I am happy about is, others call it "work".

I'm working on a 700 lb magnet at the moment. It has to create a time varying field that goes through two half inch thick layers of aluminum, I've been measuring amplitude and phase of the field between the two layers. What was interesting was the amplitude vs frequency with and without the aluminum in place..

Cheers, John
 
Gee John, with magnets like that you could do the levitation tricks.

Um, these are the smallest iron dominated magnets we use.

A few years ago, some guy contacted me about the levitation stuff. He wanted to do some kind of work-together, to make stronger levitators that could hold peoples "product" floating in the air, some kind of visual display to catch the eye..like a case of beer or a shoe floating in the air.. But he wanted to use repulsion instead of attraction. It gets really difficult to try to simply repel some object indefinitely with nothing on the sides or above to lend stability.

Those toy things that hold up the globe with magfields is just so cool, I love em. They use active circuitry to reduce the field when the object gets too high, and increases field when it gets too low.. Wish I had thought of that..

Frogs beware!

I have spent four days trying to figure that comment out. Alas, I have struck out.

The only thing that comes to mind is that singing frog, from a loony tunes cartoon. He only sings when nobody else is around besides the guy who found him..

DO NOT ask me why that comes to me..it scares even me...:eek:

Cheers, John
 
JCX got it! The same guys also do grasshoppers, water and other stuff. About 20T inside the tube, IIRC.

Sorry John, thought you were aware of the floating frogs.
High Field Magnet Laboratory - The Frog That Learned to Fly

No, I've never heard of it..guess I lead a sheltered live, living only to watch looney tune cartoons..

The design of the Bitter magnet is interesting. Looks like the cooling holes are also used to control the current density and therefore the field quality within the bore.

The writeup mentioned gradient...not absolute, so I assume the frog is floating near the top of the bore, where the field diverges..

I've been asked to assist in the initial design of a 50 tesla widgit to store power..my first comment...Holy S##T. My second: Copper becomes malleable at 20 tesla (forces exceed copper's strength), so what insulation are we using???

=Sy:]
"Hello my honey
Hello my baby
Hello my ragtime gal!"
Sy, I will hunt you down..there is no place on this earth that will be safe harbor for you....;)

frog2.gif


Cheers, John