Re: NST Resonant Charging?

To: tesla-at-pupman-dot-com
Subject: Re: NST Resonant Charging?
From: Bert Hickman <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Thu, 15 Oct 1998 00:49:36 -0600
Approved: terryf-at-verinet-dot-com
Tesla List wrote:
> 
> Original Poster: Greg Leyh <lod-at-pacbell-dot-net>
> 
> Hi All,
> 
> I still don't understand how resonant charging could possibly
> work in a practical NST-powered TC.
> 
> Assume the cap is set to Xl of the NST's leakage inductance,
> which gives 0.011uF for a 15kV 60mA NST (this seems to be the
> most popular method of choosing Cpri).
> Then the cap resonates with the NST at 60Hz, and the voltage
> will build with each cycle -- and if the gap is set to not fire,
> the voltage buildup will ultimately be limited by the Q (and the
> breakdown strength) of the charging circuit.
> 
> But what if the gap fires at each peak, at 120BPS?
> Then the energy is stolen from the cap every 8.3 mS, preventing
> any significant resonant rise!  Remember, this charging circuit
> resonates at 60Hz, and therefore requires a significant portion
> of the 60Hz cycle to build past even the normal open ckt voltage!
> 
> The charging slope could of course be speeded up by using a smaller
> capacitor, but at the expense of ultimately limiting the available
> primary energy.  The other important point here is that the firing
> rate *must* be 60BPS or a multiple thereof, in order to get any
> decent performance out of a 60Hz-fed resonant charger.
> 
> What is really going on?  Does anyone have voltage waveforms
> of a NST resonant charge cycle in action?
> --
> 
> -GL
> www.lod-dot-org

Greg and all, 

I suspect the most important factor is not that we'll see an increase in
peak capacitor voltage (Q*vac), but that the tank capacitor's recharge
current will be substantially greater than the NST's faceplate value.
The tank cap partially counteracts the leakage inductance of the NST.
Now the maximum cap voltage will be clamped by the main gap, and most
coilers tend to use NST's with static series gaps or synchronous gaps.
Either style gap has a fairly consistent breakdown voltage, and this
voltage is normally adjusted so that its significantly less than the
peak voltage from the NST. 

The main gap will tend to fire "early" in each half-cycle. With heavier
recharge current, the tank cap will recharge more rapidly, resulting in
significantly more gap firings per mains half-cycle than if we were not
resonantly charging. However, because of beating effects with the mains
frequency, AC resonant charging can be considerably different and more
complex than DC resonant charging. In the case of NST-driven coils, it
does not rely on the 2X voltage multiplying effect seen in your DC
resonant charging system. For our systems, it simply relies on getting
greater cap recharge currents. Unlike DC resonant charging, timing
variations between main gap firings and mains phasing can generate
significant chaos in the breakrates. From the sounds of the NST's there
probably some core saturation at work as well, implying significant
non-linearity in the effective leakage current. 

I've noticed that the combination of these effects can cause fairly wild
swings in mains current draw. These swings are particularly noticeable
when the variac is adjusted to the point where the gaps just begin to
fire. However, these current swings also persist to some degree at all
settings. From a theory of operation standpoint (per Glasoe, "Pulse
Generators"), it actually may be more accurate to call this mode of
operation AC "non-resonant charging". In any regard, there's little
doubt that AC resonant charging does improve the performance of a system
by permitting one's NST's to deliver significantly more than the
faceplate current...  

Hope this makes sense, and safe coilin' to you!

-- Bert --