Re: primary solid/ tube

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Jim and All,

At 09:35 AM 7/12/2002 -0700, you wrote:
>
>
>> >
>> > D = 2.602 / SQRT(200000) = 0.00581825 inches
>> >
>> > So even copper tubing is mostly wasted copper in conducting even LF
>> > currents  ;-)
>> 
>> Well, the current doesn't just magically stop at that depth.  The actual
>> skin depth is the depth where the current has dropped off by a factor of
>> 1/e.  It does actually keep going.  Every skin depth more just decreases by
>> another factor of 1/e.  The skin depth also tells you the "equivalent"
>> conductor size to pass the current at DC.  i.e. a 0.00581825" conductor at
>> DC will have the same resistance as a conductor operating at 200 kHz <IF>
>> the depth of the copper is much larger than the depth of the copper, say
>> approximately 10 times, the skin depth equivalence is just an approximation
>> when that condition is true.
>> This is all just for technical correctness, or at least what I remember of
>> it.  The tubing wall is most likely greater than 0.05", and the
>> approximation holds.  (this is all correct, isn't it? at least that's how I
>> remember it all!)
>> 
>> > Basically, you want lots of external surface area and the interior copper
>> > is of no use.  In all cases I can think of, go for copper tubing.
>
>You are correct.. it is the depth at which the current has dropped to 1/e... 
>And, if you were to integrate in an infinite slab, the contribution of the 
>part below the skin depth adds just enough to make it as if you had a finite 
>thickness equal to the skin depth with uniform thickness.
>
>Hey folks, though, something else to bear in mind, particularly for thin 
>wire (where diameter is < 10 skin depths, give or take) is that the formula 
>above is for an infinite FLAT slab.  In a round wire, it's actually worse.  
>If you think of the current as trying to squeeze other nearby out, you can 
>see that in a round wire, the current on the "back side" of the wire is also 
>contributing, as is the wire in the sides, etc.
>
>There are a bunch of tables around that give Rac/Rdc for round and tubular 
>conductors for various ratios of diameter to "infinite flat plate skin depth".
>

You bet!  It can get really "messy and gnarly" as I mentioned ;-)  Imagine
what goes on in Litz wire with proximity effects #:-|  I didn't mean to
over simplify or miss-state anything but I just wanted to cover the
"basics" to answer to original poster's question.  Apparently, there are
some who want the details :o))  Check this paper out and other great stuff
by Charles Sullivan.

http://thayer.dartmouth.edu/other/inductor/litzj.pdf

Here is a ton of great papers!

http://thayer.dartmouth.edu/other/inductor/papers.html#chron

Paul's programs are very sophisticated about taking these effects into
account too.

Forgive me making it "too simple" :o))  But only a few of us, like Paul for
his programs, need to take such effects into account.  Advanced knowledge
in these matters is not needed while wandering the isles of Home Depot ;-))

Cheers,

	Terry