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Re: caps
Hello Robert, all
My comments below.
You wrote:
"I'm rolling my first poly caps. I've followed Mike Hammers advice making
3 - .03 uF caps in series using 4 layers 6 mil poly and 2 layers heavy
duty aluminum foil.
My question is this: I just put my first two on the meter and they read
60 nf! I have not filled the pvc with oil yet, so this is a dry
measurement. Have I done something wrong? Or does the dry measurement
not coincide with oil filled/vacumn pumped one?
I laid two layers of 6mil poly/foil/2 layers of poly/foil and rolled
them up tightly. Total area was 828 sq. inches. Using the calcs this
should give me .03 uF - HELP!?
I don't want to waste oil filling these up if they're wrong. any advice
today is GREATLY appreciated."
No, your measurements are totally correct. If you are lucky (depending on how
you might view it) you might even get higher values, once they are under oil.
I plugged your values into my program and it told me 61.27nF per cap. Here are
some thoughts about where you might have made a mistake:
a.) Your calculations: Donīt forget your rolled cap acts like a flat cap with
3 plates because the backsides of the plates act capacitively on each other in
a rolled cap.
b.) You are using 24 mil TOTAL dielectric thickness. This means you only have
12 mils (I am sure THIS IS your problem: measured vs. calculated) EFFECTIVE
dielectric between the two plates. Mikeīs caps (as far as I remember) have 24
mils BETWEEN the plates and 48 mils of total dielectric thickness. So, if you
add 4 sheets (2 per side) you will get 24 mils per side (48 total) and your
capacitance will be reduced to 30.5nF per cap. As you are running three in
series, this will give you a total capacitance of ~10nF. The total EFFECTIVE
dielectric thickness across all caps will be 72 mils (24mils * 3 caps). This
should be bulletproof up to 16kVac. If you are running a 12 or 15kV NST, you
might want to think about leaving out one or two sheets per cap (one per side
for each cap).
Hope this helped clear up the clouds.
Coiler greets from germany,
Reinhard