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Re: Power in a TC System

In a message dated 5/17/00 7:01:10 PM Pacific Daylight Time, tesla-at-pupman-dot-com 
writes:

> Original Poster: "Gavin Dingley" <gavin.dingley-at-astra.ukf-dot-net> 

>  My goal in this question is to find a mathematical, but practical rule for
>  calculating the primary capacitor value for a given NST knowing it's max 
> power
>  rating and rms voltage output. I want to get the maximum power out of the 
> NST
>  without it being destroyed/ magnetically saturated etc. One formula states 
> that
>  the reactance of the capacitor should be equal to X = V/I, where V is the 
> NST's
>  output voltage and I is the maximum current that can be drawn from it. It 
> goes
>  on to say that the capacitor value that comes out of this relation can be
>  doubled due to the low duty cycle of the resulting impulsing current.

Gavin,

Using the standard formula to obtain a "matched" size cap for a 
12/30 NST gives about 0.006uF.  This particular size cap does seem
to "neutralize" the NST's leakage reactance and permits a lot of
current to be drawn.  My 12/30 NST drew 620 watts (as measured
with a wattmeter) with that sized cap, and the VA was about the
same when I used PFC.  Although I used a similar sized cap for
a few years in my coil with no trouble, some folks on the list
questioned the wisdom of letting the resonant voltage build up
so high, and they advocated used a larger (LTR) sized cap.  The
LTR sized cap draws about the same input power as far as I can
tell, and gives the same spark length for a given power input,
although I do use a 140V step up type variac.  One test that I did
seemed to show that a NST can draw more than its rated power
only with a matched sized cap, if the step up variac is not used
(I'd have to check my notes to be sure).  In any case, I'd say that
the use of this LTR sized cap is simply to keep the voltage lower to
protect the NST.  Inductive kick charging permits the NST to
deliver a good amount of power to an LTR cap.  If you use a non-
shunted transformer such as a pole pig, these can use a larger
than expected capacitor because a pig can deliver a lot more power
than it's rated for.  In addition the external ballast can be adjusted for
various capacitor sizes.
>  
>  I had a look at the graph in the above mentioned book (fig 14) and found 
the
>  graph to be linearish (big emphasis on the "ish"). From it I deduced a very
>  rough formula:
>  
>  C = POWER (VA) / 45,000      uF
>  
>  This in not perfect and personally I have got away with more capacitance
>  than is
>  lifted from the graph (12nF for a 5kV/125VA NST). Of course, the graph 
> probably
>  represents NST voltages in the range of 10kV and so would not give a good
>  indication for my 5kV set-up.

The voltage and the break rate will of course have a tremendous 
effect on the cap size for a given power input.  But assuming that
those parameters are kept constant, then your formula works well
especially at lower powers.  I've done some studies that suggest
that spark lengths do not exactly follow the square law as the power
in ramped up.  I posted some formulas recently that attempted to 
address the break rate and power ramp up issues.
>  
>  I can see what you mean regarding energy and power, the streamers from the
>  secondary isotropic are more to do with energy, mainly due to the very low
>  currents flowing in them. Put at the primary circuit, we must be talking 
> power.
>  At the end of the day average energy over time equals power and this is
>  fore the
>  whole process from NST to streamer. But it is get the most from your tank 
> cap
>  that matters to me at this time, the rest has too many variables as you 
have
>  pointed out.

Power input is the key of course, and I have found low
( ~ 120 bps) break rates to be most "efficient" in my work.  (I am
using a practical definition of efficiency here, not the true definition
of efficiency).  By "efficiency", I mean spark length vs. power input.

I too have been interested in this issue of what sized cap gives the 
most power output from an NST.  It seems to me that a matched
sized cap gives the best power output, and an LTR cap seems to
give a similar power output.  The LTR cap can be perhaps 2 or more
times larger than the matched size.

Cheers,
John Freau

>  
>  So, how do you calculate the highest value of primary tank cap given NST 
> specs,
>  while taking into account the irregular current wave form drawn from the 
NST?
> 
>  
>  Thanks for your help and look forward to your reply,
>  
>  Regards,
>  
>  Gavin, U.K.
>