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Multi stranded non-enamelled wire...skin effect?

wwfeldman

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the whole point of using multi-stranded Litz wire is to conduct in all of the copper
because of skin depth at high frequency
if the strands are not enameled (insulated), you may as well have a solid conductor
the enamel forces the current to stay in their strands
without the enamel, the current can move to the outside and the Litz wire accomplishes nothing
 

The Electrician

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Real litz is best but plain old stranded wire can be useful; see: https://engineering.dartmouth.edu/inductor/papers/stranded.pdf
--- Updated ---

Post #15 of this...
..suggests that Litz wire is best done with multiple enamelled strands, and isn't so good with just multi-stranded , non-insulated copper wire.
What are your thoughts?
Have a look at Prof. Sullivan's papers: https://engineering.dartmouth.edu/inductor/bytopic.shtml#arbarb
 

c_mitra

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What are your thoughts?
I have taken a quick look at the references cited. Very briefly:

When you have uninsulated (unvarnished) bunch of copper wires (just like a multistrand core wire), the individual strands are not in perfect contact with each other. For one, there is some air gap and second the max contact area is along a line (between two cylinders). Thus, a multi-strand wire is not the same as its effective cross section for a solid core (or a true litz bundle)- it is somewhere in between.

But why you want to do any serious business with unvarnished thin copper wires? Cost? Cutting pennies?

Copper corrodes rapidly in air and air pollution hastens it. If the wires are fine then some will break in a short time if there is some access to air. The oxide film formed on the copper surface is semiconducting but not protective enough. There is about 30% open space that is hard to fill. The wires are twisted for mechanical reasons and that adds to volume.

Litz wires can be a mess to solder (and good soldering is a key point) but I do not see any other reason - if you need it get it and use it.

My personal thoughts anyway.
 

The Electrician

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I have taken a quick look at the references cited. Very briefly:

When you have uninsulated (unvarnished) bunch of copper wires (just like a multistrand core wire), the individual strands are not in perfect contact with each other. For one, there is some air gap and second the max contact area is along a line (between two cylinders). Thus, a multi-strand wire is not the same as its effective cross section for a solid core (or a true litz bundle)- it is somewhere in between.

But why you want to do any serious business with unvarnished thin copper wires? Cost? Cutting pennies?
The title of the first reference should give you a clue: "Stranded Wire With Uninsulated Strands as a Low-Cost Alternative to Litz Wire"
Copper corrodes rapidly in air and air pollution hastens it. If the wires are fine then some will break in a short time if there is some access to air. The oxide film formed on the copper surface is semiconducting but not protective enough. There is about 30% open space that is hard to fill. The wires are twisted for mechanical reasons and that adds to volume.

Litz wires can be a mess to solder (and good soldering is a key point) but I do not see any other reason - if you need it get it and use it.

My personal thoughts anyway.
--- Updated ---

The title of the first reference should give you a clue: "Stranded Wire With Uninsulated Strands as a Low-Cost Alternative to Litz Wire"
 

c_mitra

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The title of the first reference should give you a clue: "Stranded Wire With Uninsulated Strands as a Low-Cost Alternative to Litz Wire"
I did notice but was surprised that there were no cost-benefit analysis presented. I did look for that.

For the present purpose (relevant for this discussion), figure 6 is both necessary and sufficient. I am talking about the first ref you cited.

Because there was no comparison with a regular Litz wire, I "assumed" that it must be considerably better- why that was excluded from the comparison?
 

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Thanks, i must admit i wish theyd give a quantification in terms of eg 7/0.2mm done with enamelled and then non-enamelled strands....and shown the difference in resistance for say 100khz.
Its interesting why that micrometer thin film of enamel would help so much against the evils of skin effect and proximity loss.

Skin effect and proximity loss is all about the fields, and those fields wont even "see" the enamel.
 
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c_mitra

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Skin effect and proximity loss is all about the fields, and those fields wont even "see" the enamel.
You are right; a static electric field or a static magnetic field will just ignore the enamel.

For DC, the magnetic field is static at constant current. But the magnetic force causes the current carriers (charges) to move further from the center.

Somewhat like a Roget's vibrating coil you read in school.

The charges cannot move away further than the enamel layer and so the story goes.
--- Updated ---

I note that the high voltage high current transmission lines are also multistranded- 2-3 are common but 4 is also seen. No, they are not using enamels to insulate but they are spaced with simple clamps.
 
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crutschow

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Skin effect and proximity loss is all about the fields, and those fields wont even "see" the enamel.
No, but the AC fields push the current towards the outside of the wire (the skin effect) raising the effective wire resistance.
The smaller the wire, the less the effect, so LItz wire reduces the effect for larger currents by putting the current through a number of smaller wires.
If the wires are not insulated, then the multiple strands will tend to act like one larger wire wherever they make contact, increasing the skin effect.
 

treez

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Thnaks, though as C_Mitra already said, non-enamelled wires barely make contact with each other at all, even if they are in the same bundle.
 

c_mitra

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The smaller the wire, the less the effect, so LItz wire reduces the effect for larger currents by putting the current through a number of smaller wires.
This is not accurate. The charge carriers are pushed to the periphery by the magnetic field. Therefore if you have a given cross section of a conductor, the one with the larger periphery shall win (lower resistance).

For a given cross section, a bunch of fine wires will have the effective lower resistance (because of the larger periphery). In the same manner, a foil will be better than a round conductor.

For a given area, the circle has the lowest perimeter. But the distribution pattern of the current density remains the same: but the functions are complex and difficult to visualize.
 
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Easy peasy

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insulated wires that are not stranded, woven, like litz, behave the same as solid, simple twisting introduces a slight improvement in Rac, but the MLT is slightly longer ...
 
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Thanks, by "stranded , woven", you mean like in the following cross section picture of TEX-ELZ..?

I dont know why they dont call it "Quad insulated Wire", because each individual strand is indeed enamel coated, giving 4 overall layers of insulation.

I once saw the cross section of a power cable for a 50kHz lighting bus (!) (you couple to it inductively). It carried a constant 1.9A and is approx. 30 metres max in length. The inner conductors were simply seven 1.2mm diameter bare copper wires. Are we saying that this is just the same as a single copper strand of the same overall cross sectional area? (in terms of AC resistance)
 
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Yes we are - and you can get away with a lot at 50kHz sine .... most of the current will flow in the "öutside" of the bundle, where, at 7 x 1.2 dia there is plenty of ""skin ...

p.s. for safety standards, the PUR doesn't count - as it is easily scratched or nicked, for Litz of course there is very little potential difference between any two strands at a particular point, so they use very thin PUR insulation, and nylon serve over top to give a distance for the varnish to fill - for mains type transformers PEI is used - the chances of two nicks being side by side on the bobbin is quite low - and even if a shorted turn is generated this is a functional issue, not necessarily a safety issue - although hopefully a fuse will blow due to over-current and before things get too hot.
 
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c_mitra

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The inner conductors were simply seven 1.2mm diameter bare copper wires. Are we saying that this is just the same as a single copper strand of the same overall cross sectional area? (in terms of AC resistance)
No, not quite so. Please see post #3 and the reference cited therein: https://engineering.dartmouth.edu/inductor/papers/stranded.pdf

For equal cross section, the stranded (but non-insulated) wire has larger surface (perimeter length in the cross section view) and lower AC resistance.

Just like a thin sheet; for the same cross section, the foil conductor has lower AC resistance.
 
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c_mitra

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My personal understanding:
The TEX-ELZ cable is simply stranded (not woven); the central core conductor stays always in the center. At sufficiently high frequency, the center conductor will drop out from conducting current.

Woven is a process (you see the outer conductor of the coaxial cables?) in which all the individual conductors are placed symmetrically. The core conductor is missing (because it is not symmetrically placed).
 

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wow thanks, it looks as if the problem is proximity effect, and these different Litzs are all about getting spacing between the individual strands.
I actually wonder what type of Litz is most common in SMPS transformers for eg 1kW Boost PFC inductors etc. Surely not type 6? It looks like it would cost a fortune.

Pg 18 of the following doc from Power Integrations specifies Litz for a 300W Boost PFC inductor......it simply specifies 60 strands of #38AWG.
...So would you agree that this is insufficient info?...they need to spec how those 60 strands are wound?
I actually believe that they just intend to have 60 strands "Junked" together willy nilly.....which means, according to our discussion, that
most of the inner strands wouldnt even conduct?
 
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...So would you agree that this is insufficient info?...they need to spec how those 60 strands are wound?
I actually believe that they just intend to have 60 strands "Junked" together willy nilly.....which means, according to our discussion, that
most of the inner strands wouldnt even conduct?
The assumption is almost wrong. All Litz wires on the market (starting with said type 1) achieve uniform current distribution in typical winding geometries, presumed the wire gauge is chosen appropriately. In special cases, e.g. inhomogenous field near air gaps, the construction type and strand length matters more.

Funny, how the thread topic shifts from "is stranded wire better for high frequency than solid wire" to "does litz wire work at all"?
 

The Electrician

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My personal understanding:
The TEX-ELZ cable is simply stranded (not woven); the central core conductor stays always in the center. At sufficiently high frequency, the center conductor will drop out from conducting current.

Woven is a process (you see the outer conductor of the coaxial cables?) in which all the individual conductors are placed symmetrically. The core conductor is missing (because it is not symmetrically placed).
The TEX-ELZ is like type 1 in this: http://litzwire.com/litz_types.htm

However, the type 2 (bundle of bundles) does cause individual strands to spend some time on the inside and some time on the outside.
 

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