The core became a magnet

Thread Starter

atferrari

Joined Jan 6, 2004
3,930
To experiment in repelling a magnet I winded a 3-layers inductor around a piece of a common nail (soft-iron?)

Initially it seemed to work OK but later I found that, after cutting the current, the magnet was atracted by the core which started to behave as a magnet. Not what I wanted, precisely.

Just to be sure I verified that a paper clip is attracted by the core.

I did not foreseen that. How to avoid it? Not using a core? Or is this the wrong material?

Now that I think of it this would be good if I wanted to attract the magnet. But I want just the opposite.
 

praondevou

Joined Jul 9, 2011
2,942
I guess there is no way to use an iron core for an inductor and NOT magnetize it, at least a little bit. In order not to magnetize a ferromagnetic material you will need to avoid aligning the magnetic domains, i.e. the magnetic field applied to the core would have to be very weak. (see also Barkhausen effect)

I don't know if there is any material that concentrates flux but cannot be magnetized. Isn't that a contradiction?

If you want to be absolutly sure that the core is demagnetized after the the inductor is deenergized you could apply a decaying AC current to it.


Does that make sense for your application?

What about an air core?
 

Thread Starter

atferrari

Joined Jan 6, 2004
3,930
Air core. I actually know I have to resort to it but resisted the idea. To get the equivalent effect I will have to make the coil bigger.
 

Ron H

Joined Apr 14, 2005
7,014
From Wikipedia:
Soft iron
"Soft" iron is used in magnetic assemblies, electromagnets and in some electric motors; and it can create a concentrated field that is as much as 50,000 times more intense than an air core.[1]
Iron is desirable to make magnetic cores, as it can withstand high levels of magnetic field without saturating (up to 2.16 teslas at ambient temperature.[2])
It is also used because, unlike "hard" iron, it does not remain magnetised when the field is removed, which is often important in applications where the magnetic field is required to be repeatedly switched.
Unfortunately, due to the electrical conductivity of the metal, at AC frequencies a bulk block or rod of soft iron can often suffer from large eddy currents circulating within it that waste energy and cause undesirable heating of the iron.
Apparently, "soft" refers to its magnetic properties, not its physical hardness.
 
Last edited:

crutschow

Joined Mar 14, 2008
24,750
The core material of transformers is magnetically soft and retain a minimum of magnetism (small hysteresis loop) thus you might try using part of an old transformer core for your electromagnet.
 

praondevou

Joined Jul 9, 2011
2,942
The core material of transformers is magnetically soft and retain a minimum of magnetism (small hysteresis loop) thus you might try using part of an old transformer core for your electromagnet.
It (soft iron) is also used because, unlike "hard" iron, it does not remain magnetised when the field is removed
That's the problem with internet sources. contradicting informations.
IMO even if the material has very low coercivity it can still remain slightly magnetized. Apparently the OP wants zero rest-magnetism of the core after the power to the electromagnet is switched off.
 

Ron H

Joined Apr 14, 2005
7,014
Junk relays, solenoids, motors, transformers, etc., are probably the best sources of "free" soft iron. As praondevou pointed out, they won't have zero residual magnetism, but they should be pretty good.
 

crutschow

Joined Mar 14, 2008
24,750
That's the problem with internet sources. contradicting informations.
IMO even if the material has very low coercivity it can still remain slightly magnetized. Apparently the OP wants zero rest-magnetism of the core after the power to the electromagnet is switched off.
"Zero" is relative to the task. I believe he just wants it low enough so it doesn't noticeably attract magnetic materials. That may be obtainable with typical transformer/motor/solenoid magnetic material.
 
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