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Re: Optimal Quenching Tests Subject: Optimal Quenching Tests > > Subject: Optimal Quenching Tests



Tesla List wrote:
> 
> Subscriber: FutureT-at-aol-dot-com Tue Jan  7 22:58:33 1997
> Date: Tue, 7 Jan 1997 20:50:44 -0500 (EST)
> From: FutureT-at-aol-dot-com
> To: tesla-at-pupman-dot-com
> Subject: Re: Optimal Quenching Tests Subject: Optimal Quenching Tests
> 
> In a message dated 97-01-07 03:23:04 EST, you write:
> 
> > <<snip
>  >>  Subscriber: bert.hickman-at-aquila-dot-com Sat Jan  4 21:54:49 1997
>  >> Date: Sat, 04 Jan 1997 19:34:26 -0800
>  > > From: Bert Hickman <bert.hickman-at-aquila-dot-com>
>  > > To: tesla-at-pupman-dot-com
>  > > Subject: Optimal Quenching Tests
>  >
>  >>  Hi all!
>  >
>  >> >I had a few more thoughts on quenching after answering John Freau's post
>  >>>today.
>  >
>  >>  >The bottom line:
>  > >> Existing methods for estimating proper quenching time predict
>  >> > excessively long quench times.
>  >
>  >> snip
>  >> However,  the Corums' theories, as expressed in TC Tutor and elsewhere,
> do
>  > >account for spark-gap losses.  Their TC tutor program shows this nicely;
>  if
>  >>you change the primary resistance value, the 1st beat notch "time of
>  > >occurance" will change accordingly.  In one of my tests, experimental
>  > >observed quench  time was ~ 8 uS.  I  plugged in a value of 10 ohms for
> the
>  > >primary losses, and  the program gave me a ~ 8 uS quench time.  If I
> plugged
>  > >in a 1 ohm resistance, the program gave me a ~ 10 uS quench time.   I of
> have
>  >>no idea what my actual primary resistance is, but it seems reasonable to
> me
>  >> that it is about 10 ohms.  Thus, I find complete agreement with my
>  >> experimental findings and the Corums' quench time theories.
>  >>
>  > John Freau
> 
>  >John,
> 
>  >Thanks for the update. I don't have the TC Tutor program, but do have
>  >their "Vacuum Tube Tesla Coils" book. When I did an indirect measurement
>  >of my primary alone, by measuring Q under high power, then back-figuring
> >the "effective" series resistance of the gap, primary, and associated
> > wiring, I got a value of about 8 ohms. Plugging this into a PSPICE model
>  >for my coil, I get results which are in the same ballpark. Malcolm did
>  >some earlier spark-gap measurements which indicated that, in reality,
>  >the gap's non-linear characteristics could actually result linear,
>  >rather than exponential, decay if primary current.
> 
>  >Safe coilin' to you!
> 
>  -- Bert --
>   >>
> Bert,
> 
> I'm not familiar with methods for measuring the Q under high power, could you
> explain your technique?  Also could you explain how you back figured the
> effective resistance?  I am not familiar with Malcolm's non-linear--linear
> work, could you explain this also?  (many questions today :-)  ).
> 
> Happy and effective coiling,
> 
> John Freau

John,

High Power Q measurement:
The Q of the primary circuit can be estimated by measuring the primary
circuit waveform and doing a little math. Circuit Q is a measure of
loss, and is proportional the the amount of reactive energy stored
versus the amount of energy lost over time, usually over one radian or 1
cycle. Experimentally, the secondary is completely removed, and the
primary voltage or current under power (i.e., with the gaps firing) is
sampled with a storage oscilloscope. By counting the number of RF cycles
required to go from the initial peak to 10% of the initial peak value,
the "average" Q can be estimated. 

Duane Bylund's "Supplement to Modern Tesla Coil Theory" (Page 14)
provides a handy derivation whereby the number of RF cycles (N) are
counted from the 100% to the 10% point in order to calculate Q. This
trick can also be used on the secondaryWhen I performed this measurement
on my isolated primary, it took approximately 8 complete RF cycles to go
from max to 10% of max current.

          N = 0.7329Qp
         Qp = N/0.7329
         Qp = 8/0.7329 = 10.91 

My gaps consist of 18 static and vacuum gaps in series (I know, very
inefficient from a gap Q standpoint!  :^)). The primary is 3/8" Cu
tubing, operating frequency 90 kHz, and the primary inductance is
about 154 uH. Since Q is also the reactance divided by the resistance at
resonance, we can define it as the ratio of primary reactance (Zl), and
total primary gap + interconnect + gap resistance (Rt). MOST of this
comes directly from the gap.

          Qp = Zl/Rt = 2*Pi*Fo*L/Rt
 
Solving for Rt: 

          Rt = 2*Pi*Fo*L/Qp
          Rt = 6.28*90000*151*1e-6/10.91
          Rt = 7.82 Ohms

I rounded this to 8 ohms on the PSPICE model.

Non-Linear/Linear Gap Characteristic:
Malcom made a number of primary gap measurements late last year. He
found that the non-linear, negative resistance characteristic of the gap
seemed to offset the exponential decay of primary current that you'd
normally expect with an RLC circuit. The result was more like a linear
decrease in primary current versus time. In effect, although the gap was
still the lossiest part of the system, it was not as lossy as was
originally feared. The intent of these studies was to use this
information to help make more optimal L, C, and gap design decisions. 

BTW, independent of whether the current decays linearly, exponentially,
or whatever, one can still use the "energy stored vs energy lost"
definition to estimate an "average" Q. Chip's archives probably has most
of these earlier posts if you'd like to dig a little deeper, or you can
ask Malcolm when he returns.

Safe coilin' to you!

-- Bert --