High strand count wire?

[BIG BEAM]

I don't know where I remember this from(prolly my old R/C days)but doesn't electric travel on the outside of a wire?So if this is true wouldn't a very high strand count silver coated wire be the best for our application?Or is this just theory?
DON

 

[SilverSurfer]

Thats the way I understand it too. Im not the EE though ............ one should be along here pretty quick Id think ................. might have to shake a tree .....................

 

[DAND214]

I thought it was d/c on the surface, making stranded better than solid and a/c runs the core of the wire or as you said it's that myth? I do like the high count silicon wire, but it has gotten a little over priced.
Dan

 

[dnmun]

nope, current flows through the wire, not on the surface. the high strand count means the wire is more flexible, that's all.

 

[itchynackers]

uhhh, I believe for high voltage AC, the skin effect is pronounced. With low voltage DC, not so much.

 

[BIG BEAM]

Seems to be lotsa confusion .Anyone have a linky to their theory.
DON

 

[liveforphysics]

uhhh, I believe for high voltage AC, the skin effect is pronounced. With low voltage DC, not so much.

Voltage makes no difference, it's frequency.

 

[liveforphysics]

For DC, the surface area of the conductor makes no difference, only the cross section. This means for any given volume available for conductor to fill, going solid yields the best performance.

For low frequency AC this is also true (like the 60hz power in your home for example).

When you start to get into much much higher frequency, then skin-effect starts to play a roll.

For example, if you have a solid piece of 10awg wire, 2,600hz is the limit for being able to use the entire thickness of the wire completely. When the frequency exceeds this point, you start to have a tiny part running down the center of the conductor that isn't being used completely.

Here is a chart to reference, the column on the far right shows the frequency to not be exceeded if you're wanting to make full use of the conductor. Also, this is critical to remember, it's not the frequency of the voltage, it's the frequency of the current that matters for skin effect. So, if your controller is using a 14khz PWM carrier freq, you might think you're limited to 17-18awg strands, but you're actually only needing to worry about the current frequency, which matches the commutation frequency, which will be much much lower, and never a concern for hubmotor users.

http://www.powerstream.com/Wire_Size.htm

 

[BatteryMooch]

High frequency AC can have a portion of the current running on the "skin" of the conductor. This can happen at low or high voltages.

But, stranded conductors all pressed together are just one large conductor that's more flexible. There cannot be any "skin" effect where there is no skin. And at any point there are any strands pressed together, there is no separate skin just waiting to do something differently than the rest of the strands.

As pointed out earlier, the stranding just helps you with flexibility. Considering the same AWG-rated wire, stranding can affect resistance a tiny bit, so go for stranded over solid if the wires are regularly being moved. But don't go paying tons of $$$ for extra-high strand count wire unless you really need that flexibility.

 

[liveforphysics]

All great advise John. It's also worth noting that the amount of copper fit into a given diameter makes no difference between having 7 strands, or 700 strands. But, the lower surface area of the 7 strand wire will have a lower percentage of copper oxidation by volume, but it will be awful to bend, and moving connection points will be likely failure points etc.

If you run across a situation where you need to handle high currents at high frequency, litz wire is your friend. It's a special wind pattern of bundles of very fine strands, and each strand is insulated from the others, so it behaves as a bunch of little wires, where as the same bundle of wires without insulation behaves roughly identical to 1 large strand.

For a given outside diameter, when you go from solid to stranded, you lose about 30% of the DC current handling. When you go from multi-stranded to Litz, you lose another 25-35% or so due to space being occupied with insulation that could have been copper.

However, in very high frequency applications, a little 10awg strand of Litz would have less resistive loss than something like a giant 1" solid copper bar. However, for our e-bike uses, no need to worry about that stuf. Just focus on getting the cross section of copper as high as possible, and go for supple flexible wire for the places you will need to be bending regularly.

 

[dnmun]

Seems to be lotsa confusion .Anyone have a linky to their theory.
DON

you can look for the textbook i used, by Jackson, titled 'Electricity and Magnetism'. if you never took physics, it might be dense.

 

[BatteryMooch]

All great advise John. It's also worth noting that the amount of copper fit into a given diameter makes no difference between having 7 strands, or 700 strands. But, the lower surface area of the 7 strand wire will have a lower percentage of copper oxidation by volume, but it will be awful to bend, and moving connection points will be likely failure points etc.

Great point about oxidation!

 

[BIG BEAM]

So once again" life is a compromise" and the debate goes on.
DON

 

[dnmun]

no debate.

the current is carried inside the conductor, not on the surface. there is no high frequency AC involved.

if you need flexibility, then use stranded wire.

the more strands, the better able it is to withstand the constant flexing before it fails from metal fatigue. period.

 

[BatteryMooch]

I agree...zero debate.

The strand count has no effect on any "skin effect" since it's all one conductor electrically.
Any skin effect that might exist depends on the frequency and you'll have to do the math to see if the benefits are worth using multiple conductors.
Strand count has a big effect on flexibility but can make the conductor larger.

 

[whatever]

litz wire is used in the csiro solar car motor, it plays a role to help reduce eddy currents,

 

[liveforphysics]

litz wire is used in the csiro solar car motor, it plays a role to help reduce eddy currents,

It should be used in any ironless motor.
You also find it in antennae, and many high freq components for obvious reasons.

 

[Kingfish]

Luke, do you have a good reference or two on Litz wire? I had trouble sourcing 1 to 3 mm wide flat Litz. For the AF motor my alternate is just plain flat wire :|

Best, KF

 

[liveforphysics]

Luke, do you have a good reference or two on Litz wire? I had trouble sourcing 1 to 3 mm wide flat Litz. For the AF motor my alternate is just plain flat wire :|

Best, KF

I generally just hunt on flea-bay:

http://shop.ebay.com/?_from=R40&_trksid=p3907.m570.l1313&_nkw=litz&_sacat=See-All-Categories


If your motor has not iron in it, you actually MUST use litz if you plan on running at any decent coil/magnet speeds. Otherwise you end up with a motor that has a built-in eddy brake. As an example, my original plan for the ES motor challenge motor would not have worked worth a damn wound with the solid strips I was planning, it definitely needs to be litz, or you need to have some iron to be a flux guide.