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Author Topic: General coil winding question  (Read 1541 times)

Offline captainpecan

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General coil winding question
« on: May 20, 2022, 04:34:01 PM »
I am trying to wrap my mind around a physics problem. I am deciding the best way to wrap coils for a new motor build. I am not finding the answer I am searching for anywhere and I have had a couple different responses when I've asked this so I am a little unsure on the math. There are some great minds with tons of experience in here. Can someone please try and explain to me the correct way to do the math here?

The coils I am winding are just going into a new axial flux motor design I am trying. The coils will be serpentine wound so there could actually be 5ft wire per turn. But exact numbers here aren't as important because I may change some things after I know the correct way to figure the math. I will be using 30 awg magnet wire that carries a high resistance of 102.3 ohms per 1000 ft. I want to use thin wire to get more turns in a smaller space for more magnetic flux. But the trade off is that the current is severely affected due to the high resistance as i add more turns.

I think I can compensate for this problem by simply winding with parallel strands of wire. The problem I have is that I am getting two different answers for how much it effects it and I am unsure of which is correct.

Let's say I have a coil of 400 turns and is 120 ohms. If I split the coil into 2 strands, the overall resistance is cut in half. So now I have 2 coils of 200 turns each for a TOTAL of 60 ohms. Here is where I am confused. I have a source that says that because the 2 coils are in parallel, I then cut the resistance in half again for a total of only 30 ohms because of how the flux works together. Is this correct? Or do I only cut it in half once? It makes a difference for deciding the turns and the current I want it to draw. I would like to learn this lesson BEFORE I wind all my coils and learn the hard way like I usually do.

The point of this experiment is simply trying to get as much magnetic field from the coils as possible without adding any extra current. If I can cut the resistance in half for the same amount of wire using 2 strands, I also double the current. If I then double the amount of turns, I get back to the same amount of current used, but now I have doubled my ampere turns and doubled my flux output. Thats the theory. Increased efficiency that only costs me copper and space, not current.

Any help with the math here is appreciated. I would like to make a decision this weekend and begin winding. Thank you for any help in this.


Offline skywatcher

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Re: General coil winding question
« Reply #1 on: May 20, 2022, 07:42:23 PM »
It depends on the details... for many applications only the 'amount of copper' counts, which means it doesn't matter if you use more turns of thin wire, or if you use less turns of thick wire. So you will need more voltage, or more current in one case or the other.


The only difference in both models is inductance. Of course a coil with more turns has higher inductance. So it depends in the frequency you want to use, what inductance is necessary or acceptable for your application.

Offline captainpecan

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Re: General coil winding question
« Reply #2 on: May 20, 2022, 08:43:31 PM »
Well, in this case it doesn't really matter to me. Amount of copper OR number of turns. Because both cases would be valid here. It's the resistance I'm questioning. I know the resistance is halved with 2 strands. But is it halved a second time because it's parallel wound? I'm sure it has to do with inductance amd how the fields interact. I'm just not sure how to predict the resistance. I feel like it would only be halved once, but it seems when in other projects I hook two coils in parallel, more than double the current flows. I think anyway. I just want to understand that concept better.

But, the goal is to get more copper and more turns with more flux for the same voltage and current by modifying the resistance. It's what I'm working on anyway.

Offline bistander

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Re: General coil winding question
« Reply #3 on: May 20, 2022, 09:52:20 PM »
...
Let's say I have a coil of 400 turns and is 120 ohms. If I split the coil into 2 strands, the overall resistance is cut in half. So now I have 2 coils of 200 turns each for a TOTAL of 60 ohms. Here is where I am confused. I have a source that says that because the 2 coils are in parallel, I then cut the resistance in half again for a total of only 30 ohms because of how the flux works together. Is this correct? Or do I only cut it in half once? It makes a difference for deciding the turns and the current I want it to draw. I would like to learn this lesson BEFORE I wind all my coils and learn the hard way like I usually do.
...

Hi CP,
For example purposes, say each turn is equal length of wire. Then:

400 turns, 1 wire, 120 ohms.
200 turns, 1 wire, 60 ohms.
200 turns, 2 wires, 30 ohms.
400 turns, 2 wires, 60 ohms.

For a fixed excitation (mmf or AT or Ampere Turns) the amount of copper determines the power dissipated in the coil. Varying # of turns and wire size only changes the voltage and current, not power.
bi

Offline captainpecan

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Re: General coil winding question
« Reply #4 on: May 20, 2022, 10:42:37 PM »
Thank you for the response. Exactly what I thought, except maybe your citation at the end. I was under the impression that if a coil had 200 turns at 12v 100ma it would have half the flux as one with 400 turns at 12v 100ma, even though it is exactly same power draw. Am I incorrect? I thought with the equation, if all variables stay the same except for N, the number of turns, wouldn't just increasing the turns increase the flux? Not arguing by any means, just trying to understand. Like if you ran the same current into 1 turn there should be a way less strong field than 500 turns for the same power. But maybe I am wrong? It sounds like you are saying as long as the voltage amd current is the same, the turns can increase or decrease but the flux will not change?

Offline bistander

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Re: General coil winding question
« Reply #5 on: May 20, 2022, 11:10:35 PM »
Thank you for the response. Exactly what I thought, except maybe your citation at the end. I was under the impression that if a coil had 200 turns at 12v 100ma it would have half the flux as one with 400 turns at 12v 100ma, even though it is exactly same power draw. Am I incorrect? I thought with the equation, if all variables stay the same except for N, the number of turns, wouldn't just increasing the turns increase the flux? Not arguing by any means, just trying to understand. Like if you ran the same current into 1 turn there should be a way less strong field than 500 turns for the same power. But maybe I am wrong? It sounds like you are saying as long as the voltage amd current is the same, the turns can increase or decrease but the flux will not change?

What I said is correct.

Your example:
400 turn coil vs 200 turn coil, same wire size has twice resistance, needs twice voltage for 100mA.
bi

Offline skywatcher

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Re: General coil winding question
« Reply #6 on: May 20, 2022, 11:12:16 PM »
If you use 2 strands of wire (parallel wire) you will get the same result as if you would use only 1 wire with twice the cross section. Maybe there will be some subtle differences if you work with high frequency. But not for DC or low frequency.

Offline captainpecan

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Re: General coil winding question
« Reply #7 on: May 20, 2022, 11:37:00 PM »
What I said is correct.

Your example:
400 turn coil vs 200 turn coil, same wire size has twice resistance, needs twice voltage for 100mA.
bi
Oh, I think I see. I think we are saying the same thing. I am just looking at it different. What you are saying i understand and it's exactly what I was expecting. I was just saying that when you split the coil into 2 strands, you half the resistance. Now I was saying to double the windings so you get back to the original resistance value again except now you have twice the turns amd 2 strands of wire. It should double the original coils flux for the same power the original coil used because now you have double the turns. At least that was my theory.

Offline skywatcher

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Re: General coil winding question
« Reply #8 on: May 20, 2022, 11:46:59 PM »
Oh, I think I see. I think we are saying the same thing. I am just looking at it different. What you are saying i understand and it's exactly what I was expecting. I was just saying that when you split the coil into 2 strands, you half the resistance. Now I was saying to double the windings so you get back to the original resistance value again except now you have twice the turns amd 2 strands of wire. It should double the original coils flux for the same power the original coil used because now you have double the turns. At least that was my theory.
...and we are back to 'same amount of copper'   ???
To make it extreme: you could also use a solid block of copper, wrap it around, and use it as a coil with 0.00001 ohm resistance and 1000 A of current at 0.01 V (or whatever)  ;D

Offline captainpecan

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Re: General coil winding question
« Reply #9 on: May 21, 2022, 12:20:34 AM »
No, not what i am asking at all. I never said same amount of copper. Doubling it is the entire point here. What I am saying is double the copper after we half the resistance. Go from a single strand of 400 turns 120 ohms. To 2 200 turn at 60 ohms..  then double the copper for 2 400 turn coils with 2 strands ending up with 800 turns now, (double the copper), but back to the original 120 ohms. So same power, double the copper. Thats what I am referring to. Double the copper, and double the turns, after we halved the resistance in parallel. That brings resistance back up to original value with original power. Only change at the end is same power, double copper. Does it double the flux from the original, or is it just a wash and it's actually the same no matter how much extra copper we add if we balance the resistance and power.

Offline captainpecan

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Re: General coil winding question
« Reply #10 on: May 21, 2022, 12:39:05 AM »
Let me rephrase it. My original question of if it would divide by 4 instead of just two was answered but then a new one come up from the response.

If we could double the copper, by doubling the turns, without changing the resistance, would we still double the output flux? Then using a parallel wind, we can do just that. Half the coil with 2 strands to cut the resistance in half. Then double all the copper and turns to bring it back to the original resistance except now its double the copper and double the turns, with the same power. I'm not saying it works that way, I am just asking because it seems it would.

Offline Cadman

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Re: General coil winding question
« Reply #11 on: May 21, 2022, 08:22:29 PM »
Thank you for the response. Exactly what I thought, except maybe your citation at the end. I was under the impression that if a coil had 200 turns at 12v 100ma it would have half the flux as one with 400 turns at 12v 100ma, even though it is exactly same power draw. Am I incorrect? I thought with the equation, if all variables stay the same except for N, the number of turns, wouldn't just increasing the turns increase the flux? Not arguing by any means, just trying to understand. Like if you ran the same current into 1 turn there should be a way less strong field than 500 turns for the same power. But maybe I am wrong? It sounds like you are saying as long as the voltage amd current is the same, the turns can increase or decrease but the flux will not change?

Bi told it like it is.
The amount of flux is directly proportional to the amps per turn of wire.
All else being equal, 200 turns @ 100ma is half the flux of 400 turns @ 100ma.
It's the amp turns that matter.


Offline captainpecan

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Re: General coil winding question
« Reply #12 on: May 21, 2022, 11:01:12 PM »
Bi told it like it is.
The amount of flux is directly proportional to the amps per turn of wire.
All else being equal, 200 turns @ 100ma is half the flux of 400 turns @ 100ma.
It's the amp turns that matter.
Thats what I thought. Doubling the copper and the turns for the same current, does double the flux. So I do understand it correctly. Thank you very much for clearing it up.

Offline bistander

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Re: General coil winding question
« Reply #13 on: May 22, 2022, 01:47:40 AM »
Thats what I thought. Doubling the copper and the turns for the same current, does double the flux. So I do understand it correctly. Thank you very much for clearing it up.

Actually it doubles the mmf, not necessarily the flux. That depends on the material BvsH characteristic and the shape of the core.
bi

Offline bistander

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Re: General coil winding question
« Reply #14 on: May 22, 2022, 01:52:38 AM »
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