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Re: AC Resistance (formerly Spice Simulation Pictures)
Tesla List wrote:
>
> >> Subject: Re: Spice simulation pictures
> >Subject: Re: AC Resistance (formerly Spice Simulation Pictures)
> >> Subject: Re: Spice simulation pictures
>
> >From couturejh-at-worldnet.att-dot-netSun Nov 3 22:17:33 1996
> Date: Sun, 3 Nov 1996 05:22:54 +0000
> From: "John H. Couture" <couturejh-at-worldnet.att-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Re: AC Resistance (formerly Spice Simulation Pictures)
>
> At 05:25 AM 10/31/96 +0000, you wrote:
> >> Subject: Re: Spice simulation pictures
> >
> >From bert.hickman-at-aquila-dot-comWed Oct 30 22:04:11 1996
> >Date: Wed, 30 Oct 1996 18:14:25 -0800
> >From: Bert Hickman <bert.hickman-at-aquila-dot-com>
> >To: tesla-at-pupman-dot-com
> >Subject: Re: AC Resistance (formerly Spice Simulation Pictures)
> >
> >Tesla List wrote:
> ><SNIP>
> >>
> >> >From couturejh-at-worldnet.att-dot-netTue Oct 29 22:49:50 1996
> >> Date: Tue, 29 Oct 1996 19:34:48 +0000
> >> From: "John H. Couture" <couturejh-at-worldnet.att-dot-net>
> >> To: tesla-at-pupman-dot-com
> >> Subject: Re: Spice simulation pictures
> >>
> >
> > Big snip ------------------------------------
> >
> >
> >Given the above parameters, what does your graph predict for Rac?
> >
> >Safe computin' to ya, Jack!
> >
> >-- Bert --
> >
> >-------------------------------------------------
>
> Bert -
>
> Your data did not include the Q factor of your coil. The 82.6 ohms you give
> for the Rac value would give a Q factor of 495 which is unrealistic for your
> coil.
> Q factor = 6.283 LF/R = 6.283 (.07113)(91000)/(82.6) = 492
>
> The program gives 2040 ohms for the Reff of your coil and this would give 20
> for the Q factor which is more realistic.
> Q factor = 6.283(.07113)(91000)/(2040) = 20
>
> It is obvious that Rac can not be used to find the Q factor. What is needed
> is an effective resistance Reff (something that works). I have found Reff
> values for various Tesla coils to develop a graph that appears to work as
> shown above. This information was obtained by using data on Q factors from
> Henney's and other books and back-figuring for the Reff. The graph, Fig 6,
> is shown in the Tesla Coil Notebook. I am sending you a copy by snail mail.
>
> This graph shows 290 ohms for an 1800 watt coil compared to 2040 ohms in the
> printout for your coil of 1800 watts. If you can explain this difference in
> ohmic values between the graph and program for the same input wattage you
> have a thorough knowledge of how the program works and of Tesla coil theory.
>
> In the equation R = 6.283 LF/Q note that the R varies directly as the
> inductance L. Your L (using 980 turns) is much greater than the typical coil
> L (using 400 turns) that the the graph is based upon so the Reff resistance
> is greater and the JHCTES program allows for this increase. However, this is
> not the complete story. The Reff is also affected by other parameters and
> these must all be coordinated in any program and to make a workable graph.
>
> It was important to have a proper Q factor and Reff in the JHCTES program
> because other important program parameters could be found using the proper Q
> and Reff. The graph was made by calculating Reff values for typical coils
> with known chacteristics. Only the use of the graph and time will tell us
> about its accuracy.
>
> For example, with the graph it is possible to find the critical (Rp = Rs) or
> other coupling even before the coil is built. As I mention in one of my
> other posts the coupling must be coordinated with the gap break 'ON' time to
> obtain maximum output and spark length.
>
> An important consideration is to assure that any combinations of typical
> coils using these Reff values do not end up with values for Reff that are
> less than Rdc or Rac which would be unrealistic. To avoid this condition the
> program modifies the information from the graph so an unrealistic Reff
> resistance for a realistic coil will not occur in the program.
>
> The graph needs to be verified by much more coil testing but so far coilers
> have not taken the time to study these parameters to improve their Tesla
> coils. To my knowledge this in the only graph that has ever been made for
> this elusive Reff parameter. As the Reff is a rsistance and determines the
> output, spark length, and overall efficiency of the Tesla coil system it is
> important that this parameter be given extra attention and optimized during
> the design stage.
>
> Understanding and studying the Reff parameter would give the advanced
> coilers a means of increasing the spark length and overall efficiency of
> their coils. It is apparent that as we learn more about the Reff resistance
> of Tesla coils we can design the coils to produce more output and be more
> efficient.
>
> Jack C.
Jack,
Well, you're certainly right about the Q being too high! My previously
estimate of 95 ohms implied a Q of 440, which is unreasonably high. Upon
closer examination of the Rac calculation, I discovered that, although
the measurements and equations were right, the numerical calculation was
not! Digging a little deeper, I found that I needed one more set of
parentheses in the spreadsheet calculation to make the calculation order
correct. Sorry about that!! This stuff is really stress-testing my
limited math skills!
For L=73455uH and Tau=670 uS, and Rground = 15 Ohms:
Rac = 2*L/Tau - Rground
Rac = (2*73455*1e-6)/(670*1e-6)-15 = (146910/670)-15 = 219-15
Rac = 204 Ohms
Q = ZL/Rac = 2*pi*L*Fo/Rac = 41994/204
Q = 206 (with no breakout)
This is a rather high, but not unreasonable, value for Q _IF_ there's no
breakout.
I'm now wondering about the value for Rac that I computed from Terman's
tables! However, after re-examining these calculations, I cannot find a
problem, other than the predicted value does not seem to match reality!
However, I think a Q of 20 is much too low for the quenched-gap,
no-breakout, secondary ringdown case, but it _is_ about right if we're
getting secondary breakout. When I measured the secondary Q under
breakout, my secondary Q's _were_ in the 19-20 range. The lower Q is
undoubtedly directly related to energy losses due through the streamers.
However, I'd expect that these losses would tend to look like a
non-linear resistance from the toroid and coil capacitance to ground,
and not as an increased "effective" coil resistance. If your model for
Reff inserts these losses as "effective" coil resistance, then I guess
Reff would be in the right ballpark, but I really am not too sure
exactly how you compute and use this parameter. Maybe when I get the
snail-mail info this will be clearer.
Safe (and more accurate than me...) computin' to ya, Jack!
-- Bert --