Re: Can anyone help ?

To: tesla-at-pupman-dot-com
Subject: Re: Can anyone help ?
From: Bert Hickman <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Thu, 10 Sep 1998 12:12:38 -0600
Approved: terryf-at-verinet-dot-com
Tesla List wrote:
> 
<SNIP>
> 
> Let me continue on with my ramble: If I now use a rotary gap instead of a
> staic gap I can exceed the line BPS (you guys:120 // me: 100 BPS), right?
This
> would, however mean the cap HAS to be smaller (because of less time to
charge
> it) than on a static gap, right?
> 
> Now comes my question:
> 
> What is the relationship between xformer, cap size and BPS? Is there a
formula
> out there that would tell me what cap I need for a given xformer and BPS? If
> so, lemme have it.
> 
> Thanks for all,
> Coiler greets from germany,
> Reinhard

Reinhard,

I've got some good news, and some bad news! 

First, the good news: 
It's quite possible to charge the tank cap to a value much higher than
1.41X the faceplate RMS value of the transformer! This can occur
whenever the effective inductance "seen" in the secondary circuit of
your HV transformer combines with the tank capacitance to form a
relatively low Q series LC cicruit which resonates at (or near) the
mains frequency. Most Tesla Coil design-aid programs will tend to create
this situation by "sizing" the ideal tank cap to the Neon Sign
Transformer's output current, voltage, and mains frequency (i.e., about
0.005 uF for every 30 MA at 15 KV at 60 Hz). In effect, the reactance of
the high voltage source and tank cap are set to be equal:

        E/I = Zeff = 1/(2*Pi*F*C)

 where:
   E = faceplate V (Volts RMS)
   I = Faceplace I (amps RMS)
   F = Mains frequency (Hertz)
   C = Tank cap (Farads)

 Or, solving for C:

       C = (I*1,000,000)/(2*Pi*F*E)  where C is in uF

Now as long as your main gap (or safety gap) voltage consistently breaks
down BEFORE you exceed the breakdown voltage of your tank cap or the
NST, this approach can yield the maximum possible breakrate for your
system. This is because the capacitor recharge current is also maximized
under these conditions, and may be as much as 3X - 5X the "faceplate"
rating of your NST. A series static gap or a synchronous RSG can work
very well in this type of system. 

Now the bad news:
If you attempt to use an asynchronous rotary sparkgap (RSG) with no
safety or bypass gap, the voltage on the NST OR the tank cap can climb
to 3X-5X+ the expected level, destroying whichever is the weakest link -
usually the NST! This same condition can occur with an inductively
ballasted pole pig as well. But since the breakdown voltage of the pig
is normally quite high (90 - 120 kV+) this situation typically results
in overvolting/shorting out the tank cap. Some coilers on this List have
encountered this problem in their pig-driven systems. 

The bottom line:
A system using resonant charging will see "optimum" performance at
between 3-4 "bangs" per half cycle of the AC mains with a properly
chosen static or synchronous RSG (360 - 480 BPS at 60 Hz). Just make
sure that you always break down either the main gap or a safety gap
before smoking your NST or tank cap. By adjusting the gap spacing
closer, higher breakrates can also be achieved, but with poorer overall
performance due to the reduced bang size (as you surmised above).

Safe coilin' to you!

-- Bert --