Multiple strands, more efficient?

[latrans]

I read something interesting this weekend and I thought that I might bounce it off you guys. What I learned was that for x amount of copper multiple strands of wire will have less resistance than one larger single strand. While thinking about this it occured to me that many commercial motor manufactures wind their motors with multiple strands of wire rather than one single large wire, as an example eflight outrunners are like this. I had been led to believe that this was done simply because it was easier to wind several small wires than one larger one, and my own experience does bear this out (22 gauge wire=sore fingers). Now it seems to me that ease in manufacturing may not be the only reason. So my question is what do you guys think? Couold we improve our motors? Is this old news?

Latrans

[EddieB]

yes it increases efficiency by decreasing winding resistance and by giving better/tighter copper fill.
In transformer land at least, two wires in parallel is called bifilar and 3 is called trifilar so when you see someone using those terms you'll know what they are referring to. :)
I think the other terminology used is the number of winds being the number of parallel wires (as opposed to the number of turns being the number of times the wire is wrapped around the stator.) I had never heard it described that way prior to discovering the home built motor thing.

[LBMiller5]

The cross sectional area of wire decreases at a constant rate with wire gauge size. Every time you step up 3 gauge sizes, the cross sectional area of the wire drops in half, and the resistance increases by a factor of 2. For example, 23ga wire has twice the resistance of 20ga wire, and can carry half as much current. So in a DC current configuration, 2 strands of 23ga wire would be equal to a single strand of 20ga wire.

The other reason that multiple wires gives better performance is a phenomonon called "Skin Effect" This comes into play mostly at higher frequencies, but the motors we build benefit from it to a small degree.

When large numbers of electrons in a copper wire are trying to move from one end of a wire to the other, the ones nearest the outer surface of the wire can move more easily than the ones buried in the middle part of the wire. Because of this fact, more of the current gets passed near the surface or "Skin" of the wire.

If you take a look at 2 pieces of wire, we can illustrate this effect. Let's assume that one of them has a diameter of 1mm, and the other has a diameter of 2mm. In conventional DC circuits, the current carrying capability of the wire is directly proportional to its cross sectional area. The area of the wire is equal to the radius squared times Pi.

In the case of the 1mm wire this would be .5 x .5 x 3.14159 = .785 square mm. In the case of the 2mm wire, this would be 1 x 1 x 3.14159 = 3.14159. This value is 4 times the area of the 1mm wire, which is exactly what you would expect. So in a DC circuit, if you had 4 of the 1mm wires together, they would be able to carry the same amount of current as a single 2mm wire.

When we talk about skin effect, at very high frequencies, almost all of the current travels at the surface of the wire. In fact, in many high frequency applications, you can do away with the wire, and replace it with a piece of tubing, and you get virtually the same current flow. These are called waveguides, and if any of you have done anything with wave guides before, you probably already knew this.

Anyhow, back to our wire. Since the skin effect happens near the surface of the wire, we need to think of the wire more like a piece of tubing. When we do this, we get an entirely different ratio than we calculated earlier. In the 1mm diameter wire, the skin, or distance around the wire is equal to the diameter times Pi, or in this case 1 x 3.14159 = 3.14159. For the 2mm wire the distance around the wire is 2 x 3.14159 = 6.28318, which is exactly 2 times the surface area of the 1mm wire.

Now if we take 4 of the 1mm wires and put them together we have a total surface area of 12.56637 which is double that of a single 2mm wire. In DC circuits, the current flows equally through all portions of the wire, so 4 wires that are 1mm in diameter carries the same amout of current as a single 2mm wire. At the other extreme, if you are running signals in the Gigahertz range, and most of the current is being carried through the skin effect, 4 wires that are 1mm in diameter will carry up to twice the current that a single 2mm wire will.

Those are the absolute extremes, and since out brushless controllers typically work in the 10-20 KHz range, the skin effect is very small, but it does exist. Because of this the multiple wire configurations are slightly more efficient than single wire winds, but it is on the order of 1-2% if that much. But, as you know, every little bit helps!

[latrans]

A fine explanation Lucien, thank you.

Latrans

[DadHav]

Hey Guys, While you're on the subjest of multiple strands, I'd like to ask a question. I keep hearing about adding one strand at a time over each phase. I've been winding all at one time as you see six strands of #30 going on in the pic below. I've been having real good results, but is there something wrong with the idea? I guide the six wires flatley around the leg as I revolve the entire stator and bushing.
Jihn

[EddieB]

Fairly certain you are supposed to wind multiple strands at the same time, otherwise you wind up with wires of significantly different length. They make bonded pairs (bifilar) and triples (trifilar) of wire specifically for this reason (or at least in power transformer land they do.)

:mrgreen:

[DadHav]

Hmmmmm, I figured I'd be closer to the same length winding all at once. If you wind seperatly, each following layer has to be longer because you're winding over a bigger diameter every time you add a winding, No?
Don't forget who you're talking to. I have a hard time getting it right even if I only do it once. I'd never make it through six different layers. :shock:
Thanks.
John

[EddieB]

That is what I said... wind multiple strands at the same time.

:?:

[DadHav]

Sorry Eddie, I'm not paying attention. I think I read several times though that the winds should go over one another. Well anyway, I'll keep winding all at one time for now.
Thanks.
C YA

[Jay C]

The other reason that multiple wires gives better performance is a phenomonon called "Skin Effect" This comes into play mostly at higher frequencies, but the motors we build benefit from it to a small degree.

When large numbers of electrons in a copper wire are trying to move from one end of a wire to the other, the ones nearest the outer surface of the wire can move more easily than the ones buried in the middle part of the wire. Because of this fact, more of the current gets passed near the surface or "Skin" of the wire.

Anyhow, back to our wire. Since the skin effect happens near the surface of the wire, we need to think of the wire more like a piece of tubing. When we do this, we get an entirely different ratio than we calculated earlier. In the 1mm diameter wire, the skin, or distance around the wire is equal to the diameter times Pi, or in this case 1 x 3.14159 = 3.14159. For the 2mm wire the distance around the wire is 2 x 3.14159 = 6.28318, which is exactly 2 times the surface area of the 1mm wire.

Those are the absolute extremes, and since out brushless controllers typically work in the 10-20 KHz range, the skin effect is very small, but it does exist. Because of this the multiple wire configurations are slightly more efficient than single wire winds, but it is on the order of 1-2% if that much. But, as you know, every little bit helps!
Lucien, I have to disagree. The Skin depth of a frequency is 2837/sqrt(freq) in mils. For a wire diameter of 1mm (18AWG is 1.02mm or 40.3mils) you would have to have a switching frequency of over 1MHz to even reach that depth ... thus no effect. There is no skin affect for our motors at these frequencies. The Skin depth of 12KHz is 25.8mils. The radius if a 40.3mil wire is 20.15mils ... no affect and no difference from DC.

Jay