There's also an effect similar to the skin effect that pushes the current
toward the edges of the strip. I don't know how to calculate it, but as a
guess I would make the strip 1.5x or 1.7x wider that a calculation with an
even current distribution would give. The best idea is to use Litz wire,
but that would put you in the rarefied atmosphere of the truly hard core.
---Carl
-----Original Message-----
From: David Rieben
Sent: Thursday, December 18, 2014 3:58 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Ribbon Primary - What size?
Hi John,
Well, let's see. A 10 mil thick copper strip that is 1" wide would have an
effective 0.01" square cross section and on the other end of that spectrum,
a 3" wide, 30 mil thick strip would have a 0.09" square cross section. I've
personally always used copper tubing for my primaries and I'm not sure how
to determine the exact cross section of tubing in a given ID or OD
measurement of copper tubing aside from cutting a short sample of the tubing
wall length-wise and micrometer measuring the thickness of the wall. Let's
assume that 1/2" ID copper tubing has 1/16" thick walls. That would work out
to an average diameter of 17/32" or 0.53125"(pi) x 0.0625" = ~0.1" square
cross section area, if my math is correct. So, that's still a bit more cross
section of conductor than the 3" wide 30 mil strip! Some would say that 1/2"
ID copper tubing is a bit overkill, even for a 10 kVA piggy driven system,
but I have noticed a bit of warming with my 1/2" ID tubing, at
least on the innermost primary turn or so with my Green Monster coil after a
longer run. Of course the majority of the resonant frequency currents do
flow very near the surface of (a good) conductor. My coil is driven with a
10 kVA, 14.4 kV pig and a 0.1 uFd primary cap across an ARSG, at about 350
BPS. Maybe the 1/2" copper tubing would be the more economical route for
you, although you can make a strip primary considerably narrower than an
equivalent tubing primary for a given L, which should help to reduce the
topload to primary strikes. It seems to me that you would be walking a
fairly tight line between sufficiently spacing the turns to prevent
turn/turn flashover while not over spacing them, which would reduce e-field
shielding and possibly allow for possible corona forming on the thin edges
and causing flashovers to or racing sparks on the secondary. I'll let others
who have built copper strip primaries comment further on this.
David Rieben
On Thursday, December 18, 2014 1:25 PM, John Paul Grippa
<pupman@xxxxxxxxxxxx> wrote:
Hello All,
It has been a while since I have posted, but do read all of the emails on a
regular basis. I am in the process of source materials for my first BIG
Coil and plan on using a ribbon primary. Does anyone know how to calculate
what thickness and height the copper stip would need to be when building a
primary of this type? As you know, the price of copper stip goes up
drastically when you move from say 10 mil to 20 mil or from 1" to 2'' in
height. I was hoping to use the cheapest 100' stip that would sustain the
current. I have seen many setups that were using stips of 2" or 3" in
height and 30 mil thickness. Is this really necessary? Currently, I have a
10 kVA piggie (courtesy of the amazing John Freau) just waiting to be put
into action. I am not sure if I will be using all 10 kVA, but would rather
build a primary that can sustain the power I have available in case I decide
to do so. Thanks for all of your help!
Best Regards,JP
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