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Re: [TCML] DRSSTC tuning - primary current modes
Hi David,
You're seeing "beating" between the secondary and primary LC circuits.
This is normal behavior that reflects the way that energy transfers
between coupled tuned circuits when operated near their common resonant
frequency (Fo). The resulting frequency splitting causes lower and upper
frequency poles that are functions of Fo, primary and secondary Q's, and
coupling coefficient.
A good general discussion of tuned coupled circuits can be found in
Terman's "Radio Engineers' Handbook", McGraw-Hill, 1943, Chapter 3
(Circuit Theory). [BTW, this title should be part of any serious Tesla
coiler's private library]. When the (isolated/uncoupled) natural
frequency of the primary circuit is equal to the uncoupled secondary
natural frequency, the result is a significant decrease in primary
current when the system is driven at the common resonant frequency (Fo).
The upper and lower peaks (Flower and Fupper) correspond to the lower
and upper poles of the coupled system.
The tighter the coupling, the more pronounced the effect. Following is a
page from the above book which shows how the primary current is
minimized in a tuned coupled system. The particular example uses equal
primary and secondary Q's, but similar behavior holds for the typical
Tesla Coil case (where Qs > Qp).
http://capturedlightning.com/photos/HVStuff/Coupled-LC-Circuits1.gif
Since DRSSTC's use significantly higher coupling and the secondary Q is
much higher, the center notch and degree of frequency splitting becomes
even more pronounced, and the two peaks become asymmetrical. A very nice
analysis of this phenomenon as it applies to DRSSTC's and tuning was
performed by Dan McCauly in 2006. See:
http://www.easternvoltageresearch.com/tuning01.pdf
Dan found that, by setting the primary's natural (uncoupled) frequency
to be significantly lower than that of the secondary/toroid, he could
cleanly transfer power from the primary to the secondary over a wide set
of operating conditions and spark loading.
Another discussion of primary current notching on a DRSSTC can also be
found on Steve Conner's site for a simulated coil using a k of 0.2 - see
the bottom of the following page:
http://scopeboy.com/tesla/drsstc/simulation.html
Many DRSSTC coilers have subsequently found that driving the primary at,
or near, the lower frequency pole seems to be optimal for best
performance, particularly when running at higher power levels. But, YMMV.
Note that, for primary tuning points between the lower pole and Fo,
primary notches will still be there, just spaced wider. As long as the
front half of the first notch waveform peak does not begin to reduce
primary current until well after the interrupter normally turns off you
should be fine.
Note that the last tuning point you used in wide range.gif still showed
evidence of primary current flattening. You may want to add more primary
inductance to permit higher ringup (commensurate with your driver's
maximum current capabilities). For a system that has a k in the range of
0.18 - 0.20, the lower pole would be about 90-91% of Fo, so setting the
primary 10% lower than the uncoupled secondary Fo is indeed in the
ballpark.
Bert
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David Kronstein wrote:
Hi All,
I was working on tuning my DRSSTC (coil details at
http://4hv.org/e107_plugins/forum/forum_viewtopic.php?63284), and I
notice that the primary current goes very suddenly into a "notch" mode
when moving the tap point. I've taken a series of scope shots of this
and made a couple of animated GIFs.
Condtions: 100VDC bus, single shot, some breakout into air, ~1' sparks.
200A/div current, interrupter output shown on scope.
This one shows a relatively wide range of primay turns, ranging from 3
11/12 to 4 9/12 turns, 1/12 turn per step:
http://4hv.org/e107_files/public/1235287411_347_FT63284_wide_range.gif
There's a very sudden change early on. I tried to take more steps, shown
in the shot below, but the transition point is somewhere in the middle
of the primary support, so I can't take extra shots in between. 4 2/36
to 4 7/36 turns in 1/36 turn steps.
http://4hv.org/e107_files/public/1235287411_347_FT63284_narrow_range.gif
Why do these sudden changes occur with changing tap point? And what
would you generally recommend for setting the best tap point? I've heard
people say to tune the primary about 10% lower than the secondary.
Thanks,
David
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