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Re: [TCML] primary & power input
On 1/13/13 11:54 AM, david baehr wrote:
Hello, On a primary for my little conical sec coil , I want to
go with the banded type. Georgia Copper has .5"
wide , .022" thick copper ground strap . Would this be adequate
for 2 kw ?
Skin depth in copper at 100kHz is 0.008 inches.
What you can do is think about what size tubing/wire you would use
(circumference) and compare that to the width of your strap.
Your 0.022" thick is only 3 skindepths thick, so you could probably
assume it has uniform current and calculate the AC resistance that way
(resistivity * length /cross sectional area) to see if it's comparable
to what you'd use for tubing/wire.
Certainly the strap has good cooling..
But, that's a pretty thin edge, so corona might be an issue. (and, on
general principle, sheet metal is like a knife...cuts right through
skin, gloves, and you don't even feel it at first)
If the edge is radiused (you'll have to do this) and has 0.011 inch
radius, the field at 20kV would be 20/0.011 kV/inch or 1800 kV/inch.
Considering that air breaks down at about 70kV/inch, corona is going to
be a *significant* problem with something that thin.
With, for instance, 1/4" OD tubing, the radius is 0.125, so with 20kV
from the NST on it, you have 20/0.125 = 160 kV/inch. While more than
double the breakdown in air, most people don't have a big problem at
that voltage (probably because having adjacent turns close by with a
voltage that is a few kV different makes the overall field "smoother".
But going to 10 times the field with your thin strap is looking for
excitement.
1/8" thick strap might work fairly well, is still bendable, etc?
A good way to check it is to to try a trial primary with some bare wire
of the same diameter as the thickness, before you buy your strap.. if it
arcs everywhere or sprays corona, you know you've got trouble.
0.022" is going to be about awg 22
SKin depth calculator:
http://www.microwaves101.com/encyclopedia/calsdepth.cfm
(copper is 1.67 conductivity, frequency is 0.0001 GHz)
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