Soft Iron for an electromagnet

[oxeimon]

Hey, I read somewhere that soft iron was the best to use in the core of an electromagnet.

I found this disk-shaped piece of what seems like iron in the base of an old lamp. Its got a dark-silver metallic color, it's magnetic, and has a density of very roughly 8370 kg/m3(volume was measured by geometry, not liquid displacement).

How can I tell what kind of iron, if it is iron, this piece of metal is?

 

[mc5w]

So called soft iron essentially has a low magnetic retentivity so that is loses most magnetism when deenergized. Transformer steel is vastly superior when making an electromagnet. Nobody has made electromagnets out of soft iron in a very long time.

A direct current electromagnet would still need to use a laminated core. Otherwise, eddy current would prolong buildup and decay of the magnetic field when energized and deenergized respectively.

 

[oxeimon]

If I wanted to make a really strong electromagnet, capable of holding up say, 100 kg, or around 200 lb, what kind of core geometry should I be going for? My piece of iron right now is a disk with radius 4cm and thickness 0.7 cm. Is that any good?

 

[itsmoked]

Not really.. Generally you need a horse shoe. Those big electromagnets that lift things like cars are actually squashed horseshoes. You need to think of the magnetic field running through the iron very easily and everywhere else, NOT! So your standard rod with coil is not very good for picking anything up. Bend that rod in a circle so there is a gap and inside that gap you have a great field. Don't make a circle but instead make a "U" then stick an iron object against that "U" and again you have a closed path that works well. How does a flat disk fit into any of this? It doesn't.

 

[mc5w]

You can also make a very powerful lifting magnet using two different diameter toroidal cores with the windings run between the two cores. The cores would then need to joined on top with a stack of magnetic laminations. This is essentially a horseshoe revolved around a circle.

 

[oxeimon]

I made a magnet, using 6 AA batteries in series and about 8 meters of 26 ga. magnet wire wrapped around a 8 cm diameter iron disc. I understand that you guys have said a horseshoe geometry was best for an electromagnet. However, I can barely pick up even 2 paperclips, which doesn't seem to make much sense, considering a magnet made of about 10 loops of even thinner wire around a nail connected to 1 AA battery can do about as much. So my first question is, why is it so weak, and the second is, if I wanted more current without burning tons of batteries (ie, use a wall socket), how can I build something to convert the ac current into direct current?

 

[israelkk]

oxeimon

From the previous discussion I understand that you have no basic knowledge of magnetic circuit theory. To create a strong magnet you have to design the magnet such that the coil in encapsulated inside an all around iron body this way all the magnetic flux will flow in the iron parts with very limited leakage to the air. The piece of steel that you are planing to pull will be the closing part of the magnetic flux circuit.

If it is a private home project I suggest that you should do some reading on the sujbect. The best is the book Electromagnetic Devices by Roters 1941 which is quite rare but you may find it in the nearby university library.

If it is a work project I would recommend hiring an expert consultant.

 

[UKpete]

oxeimon, there is also some help on the web, e.g.:

http://www.arnoldmagnetics.com/mtc/manuals.htm

If you want a simple electrical analogy:

flux [Φ] [≡] current I
mmf F [≡] voltage V
reluctance S [≡] resistance R

- so from ohms law:

[Φ] = F/S

also [Φ] = BA (flux density x cross sectional area)
mmf F = NI (number of turns x current)
S = l/[μ]A
where l=length of magnetic path and [μ]=permeability
- analogous to the equation for electrical resistivity
R=l/[σ]A
i.e. permeability [≡] electrical conductance, the reciprocal of electrical resistivity.

All the above is in SI units.

The magnetic circuit (as referred to by israelkk above) can be considered analogous to a series electrical circuit with either a magnet or a coil representing the voltage source and resistors representing the iron and air parts.

The problem you have is that for a large pole piece i.e. large A, you also need a lot of magnetizing force F (i.e. a large coil with plenty of current) to get a high flux density B.