The Ossie motor

[Augen]

Hi Gyula,

You mentioned two electromagnet optimizing software. Do you have one or you just found them on the internet? Maxwell 3D seems to be a complex program. I just want to make a strong electromagnet using a 100m copper wire (0.45mm diameter) to repulse my neodym magnet (diameter=25mmm, thickness=5mm). Maybe you can help me with this, if you have this program and you know how to use it. It would be a great help to me.

Using your advices, I calculated a new coil. (Coil3.png) Do you suggest this coil will be stronger then the previous I posted? (Coil1.png)


Hi Jimboot,

Replying Gyula's last post: he is right. When I'd advised you to make a shorter coil, I'd ment what Gyula sad. I'm sorry if I misleaded you.

Thanks Augen

[gyulasun]

Hi Augen,

Unfortunately I do not have any of the software programs I mentioned. I have read about them in the internet referred to as top rate electromagnetic analysers and optimizers. Maybe at a local university EE faculty near to you they may have such (there are other similar ones like Infolytica, etc) and you could approach them with your problem.

On you new coil Coil3.png: it has the length=8mm while your Coil1.png has only 5mm, this is against your suggestion of using shorter coils?
IT is true Coil3 has less inner diameter 20mm versus the other 25mm.  I would suggest choosing the inner diameter to be 12mm and the length be 5mm. Then you would get about 95m wire length, with 759 number of turns, DC resistance 12.78 Ohm.

rgds,  Gyula

[Augen]

Hi Gyula,

I'm sorry to hear that, but never mind. I'm gonna figure something out.

I know. My new coil has 8mm length. That's because I'm not sure, that the calculated "B" value is really gonna evolve when the coil is that thin. I did some tests: a 4mm and a 2mm thin coils were connected in series (same ID and OD and the thinner coil had half the wire length of the 4mm coil). Using the formula for B, the two coils should have had the same strength. Instead, the 4mm thin coil was a bit stronger. Then I connected them is parallel. In that case, the 2mm thin could should have been twice as strong as the other. Instead it was just a bit stronger than the 4mm thin coil. So I guess, there must be some sort of rule for determining the geometry of the coil, when the usual formulas are correct. Like OD/l has to be below 4 or I don't know, "l" has to be min. 4mm.

The other problem I've been having is that formula: F=B*B*A/u. When you use a smaller ID the area of the electromagnet (A) will be much smaller, decreasing the value of "F". Am I right?

There must be a difference between these types of coils (and maybe the formula for B???):

H
H
H      HHH
H      HHH
H      HHH      HHHHHH

1.      2.      3.

1. thin coil
2. thicker coil
3. a long coil

Thanks Augen.

[gyulasun]

Hi Augen,

When you compare coils,  PLEASE consider their number of turns and their DC resistances too. Meaningful comparisons can only be made if you use the same length of wire for both coils. I hoped I had explained it in the previous page of this thread (Reply #534).
When your coils were in series, the same current caused a higher B for the 4mm coil because it had higher number of turns, right?
When your coils were in parallel, the 2mm coil got stronger current (its DC resistance is half of the 4mm coil), but had less number of turns, so it performed only a little better than the 4mm coil.

Regarding the cross section area of the coils I think you cannot consider ONLY the inner diameter of a coil as the EFFECTIVE area. It would be true for a coil that has a soft iron core which would 'collect' and guide the coils flux. Here we have air core so no collection and guidance.

This means for me that the total effective area for an air core electromagnet coil is its total facing area towards the permanent magnet.
This is why I suggest using for the coil's ID at least half times the OD of the permanent magnet, maybe even less than half times of it would be good, experimentation can answer this or a good EM analyzer software.

I agree with you that the different coil shapes you showed must have different flux strengths etc.  But if you wish to compare them, use the same length of wire for making them.

rgds,  Gyula

PS: Just found, here is a video showing the magnetic poles of a pancake air core coil, the strongest field seems to  be in the center:
http://video.filestube.com/watch,1df15c74c7cb52ab03ea/hydro4f3a-016-Pancake-polarity.html

Also, there is polarity check for pancake here:
http://www.tesla-coil-builder.com/rmfd_experiment.htm

EDIT: pancake coil here is a spiral coil, its length is only the wire diameter used for making it.

[Augen]

Hi Gyula,

You are right, but I used different coil-length for a reason. I wanted to test the formula for calculating the value of "B". The length is proportional to the DC resistance and the number of the turns. The DC resistance is proportional to the current. You know, using shorter wire, you get less turns and less DC resistance, but a higher current. It is important to compare coils with different length of wire, because if the formula for "B" is correct, you will be able to get more power from the same length of wire using thinner coils. For example (using 100m long wire, and the same ID and OD): you can make 1 thicker coil or 4 thinner coils in series. Like:

HHHH or    H   +   H   +   H   +   H
HHHH       H        H        H        H
HHHH       H        H        H        H
HHHH       H        H        H        H
HHHH       H        H        H        H

Using the formula for B, the 4 thinner coils together should be 4 times stronger then the 1 thicker.

Explanation: The sum of the length is the same (100m) so the DC resistance and the current is the same.

Thicker coil:
   Number of turns: 4 times more then the thinner, so 4N
   Thickness: 4 times more then the thinner, so 4l

   B=u*4N*I/4l=u*N*I/l

Thinner coil:
   Number of turns: N
   Thickness: l

   B=u*N*I/l, but you have 4 of it, so B=4*u*N*I/l

What do you think?

Thanks Augen