Re: ScanTesla program - Lowering the coupling may be better...

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Original poster: Daniel Hess <dhess1@xxxxxxxxxx>


Hi Gerry;

A clarification; When I mentioned changing the variac setting, I meant the 
main power variac. I'm not using the John Freau variable induction method 
to vary the timing (which I believe is what you're thinking of, correct?) 
Instead, I'm using a stepper motor to mechanically rotate the sync motor 
against a pair of stationary electrodes. So any reference to variac 
settings means ramping voltage up or down to the NST primary.

As far as references to 'advanced' or 'retarded;' The sync motor turns CW 
so if I rotate the motor in a CW direction, the electrodes present sooner 
and I consider this to be advancing the firing point, in relation to where 
the thing fires on the sine wave. Likewise, if I turn the motor CCW, I'm 
retarding this point or angle.

With the stepper motor controller now functional, I can rotate the RSG 
motor under power, while the coil is operating. When I retarded the RSG 
motor (CCW) the performance of the coil increased dramatically but alas, 
eventually came the racing sparks and the cap safety gap began firing as I 
continued to retard. The only time I've had problems with racing sparks in 
the past with this coil is because I had it coupled too tight. Terry's note 
gave me the idea to try a lower K, perhaps much, much lower and then 
attempt to find the 'sweet spot' on the RSG. Without the racing sparks of 
course ;-)

To answer your last question, the operation sounded rough, very rough to 
me. I attribute this to the fact that the cap safety gap was firing 'all 
over the place.' But then again, the safety gap firing is akin to a 
misfire; You would expect rough operation under those circumstances, yes?

Thanks,

Daniel Hess



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06/14/2005 01:01 PM
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Re: ScanTesla program - Lowering the coupling may be better...




Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Daniel,

One comment about setting the firing angle:  as you change the variac
setting, the optimum firing point will change because the variac setting
will change the phase of the charging waveform.

Does your definition of  "too advanced" mean too late in time or too early
in time (not to be confused with automotive timing)???   Also, when your
sparks got louder, longer, and racing sparks started to occur, did the
operation sound smooth or rough???

Gerry R.






>Original poster: Daniel Hess <dhess1@xxxxxxxxxx>
>
>
>Terry;
>
>Re; your comment,
>
>"But it suggests that "very low" coupling may be a good thing....  Like a
>coupling of 0.03!!!"
>
>I noticed something just this past weekend which seems to support the idea
>that lower coupling might be better. About a year ago I built a SRSG for
>my coil and designed the motor to fit in to a cradle so that it could be
>rotated to provide a means to advance or retard the firing angle. I used a
>stepper motor assembly to rotate the motor in its cradle but never got
>around to building the stepper motor controller. So in the meantime I just
>adjusted the motor manually in a trial-and-error fashion until I got (what
>I believed was) the best output. This past weekend I finally completed the
>controller and for the first time was able to adjust the SRSG on the fly
>and some things became readily apparent.
>
>For one thing it turns out that my original static setting was too
>advanced and as I began to retard the setting and it came in to tune, the
>sparks got longer, brighter, it got louder (goodness) but I also began to
>develop racing sparks along the secondary and for the first time, the cap
>safety gap started to fire. I tried to gently push it a few times and the
>racing sparks and cap S-G firing were consistent and proportional to the
>degree that I retarded the SRSG. Even at lower power settings on the
>variac where I wasn't getting the racing sparks the cap safety gap went
>ballistic! Could the firing angle of the RSG have any bearing on resonance
>between C1 and the NST secondary? (my C1 is an mmc @ .049 uf and the NST
>is a 15,000 x .120) Or put another way; perhaps the angle of the RSG has
>no bearing on the resonance between C1 and the NST but running it so far
>advanced attenuated any naturally occurring resonance?
>
>Years ago, I set the coupling of this coil to one of the magic K values of
>.18 using the suicide cord method. (applying 120 VAC to the secondary and
>measuring the induced voltage in the primary, then computing the value of
>K and adjusting the coil height to the desired value. .18 was as tight as
>I could go since any setting higher introduced severe racing sparks in
>this coil.  I was disappointed that I could not achieve a higher coupling
>but wrote it off to the physical dimensions/parameters of the secondary,
>that had provided a ceiling which prevented me from going to a tighter
>coupling. But it never occurred to me that perhaps a LOWER  K value might
>allow me to retard the SRSG to a close-to-max position sans the racing 
sparks!
>
>I'll whup out my suicide cord later this week and readjust to the next
>lower magic K value (or maybe several steps down) and post the results
>soon. BTW, I attached a scale and pointer to the body of the gap motor so
>I can reference how far I've rotated it and in which direction. The scale
>is really handy because, if I ever get totally out of tune the scale
>allows me to quickly return to my starting point.
>
>Daniel Hess