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Re: Transformer Questions

Hello "becyn comunication", all 
 
 It was written:
  >From: Tesla List <tesla-at-pupman-dot-com>
 >Original Poster: Chris Tominkson <internetinbox-at-yahoo-dot-com> 

 >What would be better, a 9KV-120mA NST, or a 15KV-72mA NST. So what is
 >more important, Amps or Volts?

Your comment was: 
"A: In resonant charging primary systems, volts. Capcitors store energy
 in the amount of p=.5CV^2. The equation tells us that stored energy
 is a function of the voltage squared. Double the voltage and you have
 quadrupled the energy!"
 
While you are correct in your statement (2xV = 4xJ), have a look at it from a
more practical standpoint. To make things a little easier we will assume Chris
had the following transformers:

a.) Let me "change" his 9kv-at-120mA xformer into a 7.5kV-at-144mA (same 1080VA
rating)
b.) His 15kV-at-72mA xformer (1080VA)

Now lets further assume he will be using two 10nF 7.5kVac (in parallel) RF
rated cap in his tank circuit. The main gap is adjusted to fire at 7500V.

J(a) = 0.5*20*10^-9*7500^2 = 0.5625 Joules

Same as above, except he now needs the two 10nF caps in series and the gap is
set to fire at 15kV.

CSeries = 5nF (!)

J(b) = 0.5*5*10^-9*15000^2 = 0.5625 Joules

So, for equal material he will get the same amount of Joules. I did not go for
an impedance matched cap in the above equations. I used the 7.5kVac rating for
the caps because you can easily build a single cap for 7.5kV (although two is
more preferable). Doing the same for a (single) 15kVac cap isnīt quite as easy
(see down below). 

Now comes the next interesting aspect. His lower (!) voltage xformers will be
able to recharge the caps much faster (because of the higher charge current
they can supply). This means he can go for a higher break rate. 

Plus anyway you look at it, the 7.5kV (or in Chrisīs real case the 9kV setup)
will be cheaper and easier to build. Low voltage setups are just as good.
Saying this, I know a lot of people may tend to disagree with me here (low
voltage/high current vs. high voltage/low current), but:

Although not a record breaker for an 8" coil: I am presently getting 41"
continues sparks from an 8" coil using a single 7.5kV-at-75mA NST and a 25nF cap
(which is only 80% of the needed value for resonant charging) and a very small
topload. I am stuck with the 7.5kV because of damn european VDE and CE laws.
After seeing my results, however, I DO NOT think that a low voltage/high
current setup will really lose out against a high voltage/low current setup. 

My end setup will be using 4.5kVA worth of NSTīs and a 100nF cap. 

Chris wrote:
 >Has anyone heard of or used an air capacitor? It is a bunch of sheet
 >metal squares stacked on top of each other, insulated from each other,
 >and then every other one is wired to in/out, +/-. Like this, 2-4-6-8
 >would be wired to live, 1-3-5-7 would be nuetral. All sheets are
 >spaced apart .75".

 Your comment:
 "A: Yes, and I can smell the ozone and see the corona already. This will
work, but will be physically huge if you want any performance.  Using air as a
dielectric really is not practical in Tesla stuff for  many reasons, not to
mention that capacitance *DECREASES* by the square of the distance between the
plates."

While I agree with you, that air caps are very unpractical for TC usage, etc,
looking at it again from a low voltage vs. high voltage standpoint: Chris will
get better results from an air cap built for 9kV, then a similar air cap built
for 15kV. Plus you talk about corona losses (I totally agree), yet you tell
him to go for the 15kV setup (which means MORE corona) ??!?? 

The main problem with the higher voltages is that you will have to fight more
corona problems. It doesnīt make a difference if you are running an air, PE or
other cap. Oil doesnīt help much either because corona can and does form under
oil. Fact is, that if you use the higher input voltage you will also have to
increase your cap building efforts^2. Have a look around as to how many
coilers (using higher voltage) have blown their caps, even though they have
seriesed them, used oil, etc.

You said the capacitance decreases by the square of the distance between the
plates. I CANNOT agree with you here. A rolled PE cap with 100" x 100" plates
with a 1 mil sheet in between will result in a 8.96nF cap. The same 100" x
100" plates with a 6 mil sheet in between will result in a 1.49nF cap. This is
1/6 of the value of the 1 mil sheet cap and not 1/36 of the value (as your
square law states).

Coiler greets from germany,
Reinhard
(who is stuck with 7.5kV, but this will not prevent him from making bigger
sparks)