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
[email protected]
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