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Electromagnets?

R

Replicon

Member
Hi,

I have a project which requires some home made (or custom-machined, anyway) electromagnets. Anyway, i need some high quality core material (high quality as in, good permeability, but extremely low magnetic property retention once the power is cut).

The core material will be in the form of little rods (3 or so inches in length, about... 3-4 mm diameter?). So far, the best I found was metal coathangers (the ones consisting of a piece of thick wire, twisted together). Unfortunately, cutting those up and using them is a rather ghetto solution, and I need something a bit more professional-looking. Where can I look for such materials? I've got university resources at my disposal, as well as a generous prof who would fund the project if it gets expensive.

Thanks!
 
Out of curiosity... you're willing to purchase materials, but not willing to just purchase the electromagnets themselves? Do they need to be some specific size or something?
 
Well, I would need them to be somewhat specific. I looked for surface mount pre-made electromagnets but couldn't find any... they were all HUGE coils, pretty much... mine have to be thin, likethe coathanger rods with some extra thickness from the coil... Because I need many of them to be close to one another. If such magnets exist, please point me to a website where I can order them. Thanks,
 
Well, your best bet for the cores is still going to be iron, often known as ferrite, becuase it is cheap and easy to get a hold of plus has a relatively low coercivity and high saturation magnetization. You can get rods of it from McMaster-Carr cheaply, but they only sell it in 6.5 and 9.5mm dia rods.(catalog page 3453 which also links to super thin gauge copper wire to use for the windings) I'm sure you can find it smaller elsewhere.

Now, if you want to go all out, you are going to have to look for something like Supermalloy, Sendust, Permalloy, Hypersil, or Metglas. They are also difficult to find, you can try cartech.com, protolam.com, metglas.com, or goodfellows.com or just try google searching for them. All of these tend to have extremely low coercivities coupled with extremely high permeabilities, but none of them have the high saturation magnetization that Iron has. Metglas is the closest at ~1.3e6 A/m with an alloy called Permendur even higher at 1.9, but I don't know Permendur's coercivity.(Fe is ~1.7) So the question becomes, do you want an extremely powerful magnet, or one that turns off more completely and requires less power for a given field?

If you want to be really elegent, you then need to get a rod of the same size that is something like Fe-35Co or 49Fe-49Co-2V. You then use a small thin piece of that at either end as a pole cap which will greatly increase your saturation magnetization, but should have little effect on the overall coercivity. I'm not sure if this will work for your application, however, as this is normally used in the large NMR type electromagnets where the two poles are nearly touching each other...it could be worth a shot though.
 
I suppose it's best if I give a full description of the project, if you guys are interested. Ever played pinpressions boards? It's basically a board with 2-inch long nails in it, and you can put it on your face (or whatever) and the pins will create the same shape on the other side... Here, lemme find a picture:

pinpressions

Anyway, that was the main inspiration for the project, which is to control something similar. That is, software gives our device some inputs (ie pin heights), and the pins rise to the appropriate height. We've already got a prototype, but it's really ghetto hehe. (it's a university project).

Anyway, the way we do it is as follows:

The board rests on its side (such that the pins are horizontal). That way, there's very little friction, and no gravity to worry about. Another board, alignedto the pin board, can move towards and away from the pin board. THAT board has electromagnets on it, alined with the pins (ie one per pin).

Step 1: Movable board moves right next to the pin board, thus pushing all the pins out to maximum "height".

Step 2: Electromagnets are turned on.

Step 3: Board moves away from pin board (begins to pull all the pins back).

Step 4: Electromagnets are turned off as needed. Then, the corresponding pins stay where they are. When the board reaches the end, all pins are in the right place, and you can see the shape when looking at the other side.

Needless to say, we're not gonna get the kind of resolution as the actual pinpression has, but still, we want to do a nice 10x10 board, where the pins are (say) less than 2cm apart.
Our current prototype is a hackish proof-of-concept prototype. Instead of nice pins, we used pieces of iron coathangers. Instead of nice alignable boards, we used wood (from a plywood floor), and instead of a nice mechanism for keeping it aligned while going into motion, we fastned said wood to a printer we totally destroyed. Here's what it looks like:

picture

As I said, pretty ghetto, but it works.

Anyway, to answer the question, as you can guess from this I'm sure, we don't need an extremely powerful magnet, but rather the second variety you mention ("one that turns off more completely and requires less power for a given field").

In any case, we want to get the second, symposium, version to be a lot nicer for show and tell purposes. We've got most of the mounting stuff figured out (ther's all kinds of linear actuators, etc. out there), but I was wondering if there was an elegant way to do the rods, cause coathanger pieces aren't always straight, which means they don't align very well with the magnets.

Thanks for reading all that =).

have fun
 
Well, I would suggest looking for one of the alloy's I mentioned above other then ferrite then. Most of the companies should be able to provide the metals in the form of small rods that you can take to a bandsaw with a metal cutting blade and slice off, polishing up the ends to look nice. They may even be able to provide you with some samples if you call them up and explain to them what you are doing.

You should also be able to use a much lower winding density than what you have in the picture which will allow you to make the coils look better.(which will also then have a lower power drain) Unfortunately, there is no real way to make "nice looking" coils. Normally in industry they'll coil the wire and then coat it so you can't see the coils. Something like heat shrink tubing would probably work well in this case and you could use it to completely cover the electromagnet assembly.

Hope this helps some. If you want to do the quick and easy route, I would just recommend trying the ferrite rods I mentioned from mcmaster above. They are cheap, easily cut, are still the most widely used electromagnet material, and you can then cover them.

Good Luck. Let us know how it turns out.
 
Definitely consider permalloy, coballoy, molyperm or other high saturation low coercivity material. Reade sells these materials in all sorts of physical forms

http://www.reade.com/Products/.../magnetic_powders.html

The alloys you want to look at the correspond to the above trade names are the FeNi alloys (80% Fe 20% NI is the most common permalloy alloy, it has a saturation magnetization ~1-1.1 Tesla w/ low coercivity), CoFe alloys (sometiome referred to as coballoy, some CoFe alloys have the highest saturation magentization currently known, ~2.45 Tesla, so an electromagnet would be strong as hell. However, the coercivity of these alloys is usually pretty high, so if you want a switchable electomagnet,. you'll have to ask for a CoFe alloy that contains some other non-magentic element which reduces the alloys coercivity).

Oh! One last thing, molyperm is a permalloy alloy that contains Molybdenum. Generally this improves permeability at the expense of the materials soft magentic properties.
 
It seems similar to the concept used in the old PC dot matrix printers. I?d take a look at one of those to see if it could be modified for your purpose.
 
Hello Guys,
Sorry for barging in but since the discussion was about magnets, Thought to ask my question here as well.

The question is,
Why is it that carbon steel gets magnetized on heavy grinding and machining? Any answers?

Thanks in advance.
 
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