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

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.

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

....

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

Hi Augen,

Well I think the magnetic field, B can indeed be increased by 'sandwiching' several thin coils of the same thickness as a single thicker coil has, made of the same wire length. However I think you have to make the ID of the thin coils as small as practicable because only this way can you fill out the space in the radial direction all the way to the OD. I think this is important, and by choosing a higher ID you eventually lose on the benefits of the smaller ID (radially you introduce more air space if your ID is higher). I think of ID values as small as 5-6mm.

I think the B field increase effect is similar to the one when you attach two identical permanent magnets to each other, NSNS, and B becomes higher in the axial direction, flux lines will be more oval-like than for the same one single magnet.  I am not sure the B doubles in this case but may approach the double value.

Also, I am not sure the 4 thin coils together will increase the magnetic field, B by four times as the formula may predict (you have to consider the air gap between the 4 layers, and possibly some other factors).

rgds,  Gyula

Hi Gyula,

Last time when I wrote about the 4 thinner coils, I didn't mean to use them as a sandwich, but to put them to 4 different places, like the 4 magnets on Jimboot's rotor so you have 4 magnet-coil pairs. This way, you use all the 4 magnets at the same time, not just 1 at the time. I guess, it is like the original Jimboot replica of the Ossie motor (4 magnets, with 4 coils around them). The problem is, if you have a geometrically good coil (like my blue coil seems to be, which has special geometrical rate), cutting it to pieces and use them in series make it worse, because in that way, you ruin the good geometrical rate. The thinner coils will be too thin comparing to their OD-ID rate. I will explain it later. But if you have a geometrically good coil, using 4 of it will decrease their strength because the 4 coils have a 4 times greater resistance, so each coil will get 4 times less current comparing to the setup, when you use only 1 coil. That's why Jimboot got a higher rpm using 1 coil instead of 4, but using that setup, you using greater current. Is that clear? Because it is harder to explain things by writing them instead of telling them.  "See coils.png"

I'm not sure that the sandwich type is better. The outer coil is too far from the magnets. Check this out: http://www.magneticsolutions.com.au/cgi-bin/flux-graphs?page=fluxgraphs
On that graph you can see magnetic flux, and how it decreases by increasing the distance.
The sandwich type is just a longer coil, but it has an air gape between the 2 coils it is made of. If the sandwich type has a better geometrical rate, it can be stronger, I guess.

Returning to my previous problem:
- Gyula, I guess you were right about the smaller ID, but this means when you calculate the force between the magnet and the coil, the 'A' is not the ID, but the OD. But why? I always belived that the effective area of a coil is the inner area, not area of the coil itself. Maybe I was wrong.

- Calculating B, I got that the Green coil should be about twice as strong as the Blue. But, when I tested them by connecting them is series, the result showed me that the Blue coil is stronger then the Green. So I guess, there must be some sort if geometrical rate, when the calculated 'B' is correct. The length of the wire in the 2 coils is almost the same.

- Next I'm going to make a new coil, with a much smaller ID (about 5mm), using the same geometrical rate as the Blue has: (OD-ID)/2 = l

I posted some pics, so you can compare the 2 coils I used, and you can see my setup of the Ossie motor.

Regards Augen.

...
I'm not sure that the sandwich type is better. The outer coil is too far from the magnets. Check this out: http://www.magneticsolutions.com.au/cgi-bin/flux-graphs?page=fluxgraphs
On that graph you can see magnetic flux, and how it decreases by increasing the distance.
The sandwich type is just a longer coil, but it has an air gape between the 2 coils it is made of. If the sandwich type has a better geometrical rate, it can be stronger, I guess.

Returning to my previous problem:
- Gyula, I guess you were right about the smaller ID, but this means when you calculate the force between the magnet and the coil, the 'A' is not the ID, but the OD. But why? I always belived that the effective area of a coil is the inner area, not area of the coil itself. Maybe I was wrong.

- Calculating B, I got that the Green coil should be about twice as strong as the Blue. But, when I tested them by connecting them is series, the result showed me that the Blue coil is stronger then the Green. So I guess, there must be some sort if geometrical rate, when the calculated 'B' is correct. The length of the wire in the 2 coils is almost the same.

- Next I'm going to make a new coil, with a much smaller ID (about 5mm), using the same geometrical rate as the Blue has: (OD-ID)/2 = l
...

Hi Augen,

Ok on your 4 thinner coils you did not mean to sandwich but use them separately with 4 magnets, I do think it is BETTER than sandwiching them.

However, the sandwich arrangement also has merits especially if the coils are very narrow indeed, so that the mechanical length or thichness of the sandwiched coils are kept at a minimum possible. And the minimum possible length is attained if you use a single wire thickness coil for the sandwich layers, the case for the pancake coils. Please read in the Abstract part of this paper for explanation: ..."the turns of the coils are not concentrated at the circumferences but distributed across the diameters."  i.e. the turns of the two (or more) coils can directly and very closely face each other in the total area embedded within the circumference, so the (magnetic) coupling improves between the coils.
 This explains why the small ID is needed: you have to fill the inside space with wire radially up and up, increasing the circumference;  in a conventional solenoid coil (or in a multilayer coil with high ID) there is wire only at or near to the circumference, mainly length-wise, and the inner radial-wise space is 'empty'.
Here is the paper:
http://www.ece.uvic.ca/~btill/papers/neurimp/Zierhofer_Hochmair_1996_00503178.pdf
 
 Now back to your 4 separate coils because I also think they will be more promising.    I would like to draw your attention on an experiment Ian showed with a pulse motor, using only one big coil and he compared it to a pulse motor with 8 coils, using the same input power like the one coil motor:

http://www.overunity.com/index.php/topic,1754.msg31932.html#msg31932
 
(It is worth reading his further posts in the second page of the above link, though not reported ou.)

You have built a nice setup from Lego parts for sure, not to mention your coils.    I think your blue coil is 'stronger' because (even though it has higher ID than the green coil) it fits much better to your magnet cross section area than the huge green one,  this latter 'wastes' its bigger area: if you had magnets with comparable diameter to the green coil (and suppose these magnets would have the same strength than your present magnets), then I think you would have experienced the green coil was better.  I already described in previous posts above what ratio I think may be good for the coil/magnet diameter but of course the tests are to decide.

Re on the geometry of your coil, (OD-ID)/2 = l,   is it the Brooks coil geometry?  IF yes you may have to change a little that ratio to be able to reduce the length of the coils, no?

So I think the bottom line is the more coils and magnets you use in such setups, the better torque you get and if you combine this with Ossie's switching circuits to utilize the collapsing fields energy at the switchoffs, then you can have even better performance.  Will it be ou? I do not know unfortunately, I hope.  Maybe Ossie already knows? 

rgds, Gyula

running 7 days non stop at 460rpm. Voltage drop hard to measure. Tried diff meters but is irregular. May have dropped .01 Time to switch & try more Augen & Gyula ideas.

Hi Augen,

Well, I think the 2cm distance between the coils would be too long, maybe 6-7mm would give much better flux strength in the gap and a 5mm thick strong magnet could still pass easily in the gap.
Re. on the more coils use less energy, this is ok and have you read Ian's suggestion on connecting series coils groups also in parallel with each other?
here it is:
http://www.overunity.com/index.php?topic=1754.msg35080#msg35080

It seems the more magnets you bring in to interact with the coils, the more flux quantity would be involved for the same input power.

rgds,  Gyula