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From: "Ashlock,William" <William.Ashlock@edwards.boc.com>
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE: Re: [Lowfer] RE: litz wire
Date: Fri, 17 May 2002 10:51:48 -0400
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Hi Tom,

The theory of how Litz has an advantage over standard wire is well
documented. A check of the Web will give one a good evening of reading. On
the practical side I have conducted numerous loop experiments using solid vs
Litz wire and find the AC resistance for Litz wire is ~1/2 the AC resistance
for the same diameter solid copper. These experiments required subtracting
the soil loss (0.22ohms at this location when dry) from the total loss in
order to determine the actual conductor values. I believe one of the reasons
for conflicting statements over various advantages (or lack there of) in
using certain loop conductors is the lack of characterizing the soil loss.
This requires a VERY careful study of the total loop Rac derived from the
installation of a range of solid conductor sizes for which the skin effect,
hence conductor Rac (without soil loss), is known. 
 
Bill A 

PS: I also find that a Litz loading coil has approximately the same 2x
advantage over a coil using std copper wire of the same diameter.


> Hi Bill & Group,
> 
> I believe why Litz wire has such a low AC resistance
> is because each strand is insulated and when soldered
> together you get the parallel resistance of each of
> the strand of insulated wire.  With uninsulated,
> stranded wire you may have the same current carrying
> capacitiy but you more than likely have a higher
> resistance because all the strands are in contact with
> one another & the current path will be through all the
> contacts that offer the least resistance but better
> than solid wire because of the increase perimeter
> length.

I'm a little surprised the measured conductor resistance a loop was 
significantly reduced with Litz wire compared to solid wire with the 
same size and same insulation.

Litz wire *sometimes* offers higher Q in multilayer coils because it 
reduces eddy currents in the wire as flux from adjacent turns cut the 
cross section of the wire.

I have an engineering text that goes into details about Litz wire. It 
states that Litz wire reduces eddy currents when significant flux 
cuts the cross-section of a wire.

Of course eddy currents would be reduced at the expense of increased 
unit resistance for a given physical conductor size, since some 
current carrying conductor area is replaced with insulation. 

Litz wire can also increase dielectric losses, but any change in 
dielectrics where the electric field between conductors is very 
intense can do that. It depends on the insulation used in the non-
Litz wire, and the insulation used in the Litz wire.

I believe the textbook description I have is correct, because I've 
measured Q of hundreds of inductors and have sometimes found Litz 
wire an improvement in ~ 50-500kHz systems using multilayer coils or 
transformers (like switching supplies operating in the 100kHz range, 
using transformers with large high-current conductors). 

On the other hand, I've *never* measured a case where it improved Q 
or reduced losses in solenoid or single conductor applications at any 
frequency. It has actually caused problems above 1MHz when used in 
high current leads (that had to be flexed, so I thought Litz wire was 
a good choice).73, Tom W8JI
W8JI@contesting.com 


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