# Winding two U cores electromagnet

Discussion in 'General Electronics Chat' started by ramsey, Nov 12, 2010.

1. ### ramsey Thread Starter New Member

Nov 4, 2010
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0
In trying to make a strong electromagnet the challenge for me was in finding soft ferrite cores but I was able to finally find a couple of U cores and since they're hard to come by I bought two since it didn't cost all that much more. But basically I was wondering what the effect would be of using the same magnet wire to wind one U core and then continue the wire onto a second U core and having both stacked together. What comes to mind is mutual inductance, but as far as field strength/size I have zero clue what the outcome will be other than I am guessing it will be stronger assuming I am winding in the proper directions.

So, assuming I laid out two U cores size by side and winding magnet wire on each leg of each U core (no windings on the bottom) I would start on the left leg of the first U core and arbitrarily wind clockwise applying multiple layers on that leg, then I would proceed to the second leg of the first U my direction of winding would then be counter-clockwise with multiple layers. At that point I would begin with the first leg on the second U core and I would now wind clockwise on that leg and then counter-clockwise on the second leg of the second U core. Does this sound right?

On all legs I would be winding multiple layers of wire so that each leg fills up nearly half the space of the gap between the U, and my idea was that when winding say top to bottom on a given leg I wouldn't send a straight length of wire back to the top and then down, I would simply continue winding back from bottom to top in the same wrap direction as I did from top to bottom.

The final configuration would be taking those two U cores and stacking them (as opposed to side by side during winding). Does my idea make sense or is there a better way in using two U cores like this? This is territory where I have like no experience but I truly appreciate the insights you all have to offer. Thanks very much.

(EDIT: When I say stacked together I mean stacked in such a way so that if you were looking at them directly forward, the other U core would be hidden behind the one in front)

2. ### timrobbins Active Member

Aug 29, 2009
318
16
Maybe you can find a picture on the internet that describes what you are describing.

You also need to describe if you want a constant magnetic field, and what you want the field to do. Remember that a magnetic field is three dimensional, and will fan out into all space when it leaves the 'core' that you are using.

3. ### ramsey Thread Starter New Member

Nov 4, 2010
14
0
This is for an AC electromagnet or transformer.

Some information:

- 14 Gauge magnet wire
- 2 amps drawn from an 18 VAC output wall transformer (fed through a few 50 ohm resistors that are in parallel to meet power dissipation requirements under 10 watts)

I cannot find a picture of this setup, what I'm trying to do is not described anywhere that I've perused. It is just as I described with two U cores stacked together so that looking at it from the front the second U core would be hidden behind the first one. The air gap of one is parallel to the air gap of the other one. I would assume some amount of field strength increase if this setup was done but my winding directions on each leg of each core would have to be done correctly.

Another way to look at it is if the U cores are stood up straight and you are looking directly down at them you'd see the tops of the U's like so:

O = O
O = O

During winding of the cores I'd have them arranged like so:

O = O O = O
Starting on the left leg of the first, then over to the second leg of the first, first leg of the second, second leg of the second, all from the same wire. Afterwards the second one would be rotated so the two are stacked as shown in my first drawing.

I suppose all I could do is try it out but I really just want to know if my idea for winding directions on each leg is correct to begin with so that the field on one leg will not neutralize the field on another thereby reducing the overall field strength.

EDIT: I guess in the end I'm just trying to see how it's possible to maximize the max field strength (it's AC) by using the second core instead of increasing amps because I'd be maximizing the amount of wire turns on a single core to begin with.

Last edited: Nov 12, 2010
4. ### timrobbins Active Member

Aug 29, 2009
318
16
In:
http://en.wikipedia.org/wiki/Electromagnet
there is a cross section of core and winding for a U-I core, with a winding on the U section. You indicate that you want to put two U cores together so that they equal an E core (O=OO=O), but I don't appreciate what you then want to do. Do you want two electromagnets that can be separated (ie. two separate U core/winding assemblies - as per in the link), or do you want the cores to be bound together as a single E core, with windings made to the E core?

5. ### ramsey Thread Starter New Member

Nov 4, 2010
14
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That's the thing, it does appear somewhat like an E core with the exception of two inner legs adjacent to each other. But let's assume this is what I want, I am trying to figure out the winding direction properly. If I were wrapping multiple layers clockwise on the first leg of the first core, I would then wrap layers counter-clockwise on the second leg of the first core? The way I am imagining it is if we virtually have one long cylindrical core we will always wrap in the same direction up and down on that cylinder, and then we virtually bend the lower half of that cylindrical core so that now the second leg now appears to have windings going counter-clockwise relative to the first leg. Am I on the right track here?

Now assuming I am correct, the next leg after that (which is the first leg of the second core) would have wrappings going now clockwise. Basically each leg (for a total of 4) would have windings in the opposite direction to the one before it, is that correct? Only difference is this is spanning across 2 cores with the same wire.

Now the E configuration is a close analog to what I want with the exception that after winding I would rotate the second U core so that it is stacked on top of the first U core - The reason for that is that I am trying to figure out if this configuration would produce a stronger total field at the air gaps than having two U cores side by side. I initially said side-by-side cores initially because it made it easier to describe the winding directions.

Really, the end result is that I want to see how I can concentrate the magnetic fields even stronger by configuring the 2 U core positioning properly. I believe that to begin with, the air gaps cause flux to be concentrated in that region for a given U core, so I am trying to reason from there again considering that both cores share the same wire.

6. ### timrobbins Active Member

Aug 29, 2009
318
16
Your on the right track with winding on a 'long cylinderical core'.

Refering to the link I gave, you can move that winding to any part of the U and get the same effect. You can also put the winding around the two legs that but together - ie. the middle leg of the E core.

I don't appreciate what you mean by "I would rotate the second U core so that it is stacked on top of the first U core". In the link drawing, do you want to replace the 'I' core that has no winding, with another U core with a winding?

7. ### ramsey Thread Starter New Member

Nov 4, 2010
14
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I think the problem is that what I came up with (out of thin air) is just not a standard way of doing things. It is essentially two U cores that when placed side by side sort of represents an E core (with two inner legs). My question is probably something for someone who has probably dedicated much of their work into magnetism. I probably will just stick to a single U core instead of two U cores.

With one U core flux is concentrated at the air gap, but really I'd wonder what happens when you have two air gaps from two cores placed in a sort of box configuration i.e.:

O=O
O=O

which I guess is technically 4 air gaps and how they would interact in practice. Would they neutralize each other, increase flux in some manner, no idea since I have no professional background in this area.

8. ### timrobbins Active Member

Aug 29, 2009
318
16
You are effectively thinking about two 'magnets', each in the shape of a horseshoe. Buy two such magnets, and then place them close to each other and move them around and think about what is happening to the magnetic field lines in space as they move from one 'pole' to the other 'pole'. Google for some pictures of field lines around magnets to get a better appreciation.

9. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Is this for a mass Hard Drive eraser, or picking up heavy items, or ??

Just curious, if you could post a photo of how you intend to have the cores next to each other, it might help.

10. ### ramsey Thread Starter New Member

Nov 4, 2010
14
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This is general purpose, I want to be able to do experiments with induction like using it as a transformer for circuits I'll build and try to see how far away I can induce current at various levels of power and I want to be able to just build one device that I can adjust power as needed. I would probably also use it to zap my old hard drives too when they break. When I build stuff I usually get a bit gear-headed and want to make things as powerful as I safely can. I really don't have any specific power requirements I just want to get it as strong as I can with what I have. A single U core might be more than enough for hobbyist use though but I figure since I got 2 cores I wanted to see if I could make use of the second one other than just having it laying around. No photo available but I guess I could sketch a diagram. I decided that unless someone understands what I conjured up I'll just wire one core and be done with it. I'd hate to wire two together and then end up negating things. The problem I have is that I have no idea how strong is strong enough or too strong. When I read around the net people talk about having hundreds of coil turns and 5+ amps running through an electromagnet but I definitely don't want to go pushing that much current and I am guessing 2 amps through 500 turns of wire is safe enough so I don't go inadvertently ruining devices in my house (I hope).

11. ### timrobbins Active Member

Aug 29, 2009
318
16
Playing with electrical power, no matter how mundane you may think it is, is not a great idea, especially if you know very little and want to push boundaries. Most here would strongly suggest you 'start to walk' with properly designed projects or kits - at least then someone else suitably knowledgable has designed out the risks and provided the cautions and summarised the theory so that you learn in a progressive manner.

12. ### ramsey Thread Starter New Member

Nov 4, 2010
14
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That's probably reason enough to keep it simple then. I definitely have enough knowledge to build something so that I don't create an electrical hazard but I think I'll just build it with one core and then play with it. I want to have it so that I'm not putting out a significant field more than a foot or two around it. I just have not figured out how to calculate the "size of a field" so that I know how much is too much. But I'll experiment, perhaps a gauss/tesla meter would be good.

13. ### retched AAC Fanatic!

Dec 5, 2009
5,201
313
Thats right! And a good interim project would be to build your own gauss/tesla meter.

With hall effect sensors being readily available and cheap, a decent meter will help with these experiments drastically.

That way, you need not say "I think this magnet was stronger than the last."

You can meter it and KNOW for sure that the changes you make are worth while.

14. ### ramsey Thread Starter New Member

Nov 4, 2010
14
0
Yeah, just had a look, it's remarkably cheap and simple to build a circuit with a hall effect sensor since I have a multimeter already.

15. ### retched AAC Fanatic!

Dec 5, 2009
5,201
313
Yessir. Your existing meter will do the "metering" so you basically build a gauss to mV converter.