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RE: Terry's DRSSTC actually hooked to a coil now >:-))



Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Steve,

At 03:40 AM 2/1/2005, you wrote:
>by following primary current, you just
>"ride along" with the frequency...

Yup, this is exactly what happens. But you have to be careful because there
are two possible "rides" the coil can go on. If you design the system to
give the right voltages and currents on one of these paths, they may be all
wrong if it switches to the other one.

I guess I am not understanding what the "two paths" are here. The two coil system has primary and secondary resonances which beat against each other. One small and the next larger "notches" due to this beating can be seen here:


http://drsstc.com/~terrell/pictures/FullSystem-01.gif

I should have let it go longer and you could see lots of beats. Between the two "notches" you can see where the current (yellow) shifts some. The frequency certainly is not a stable value but bouncing around. But these are perfectly normal for a primary current driven system. The secondary current driven system like Dan's is much different in that the beating is in the drive signals where I think it "beats up" the IGBTs more since the zero current crossing miss in that case. The primary current is a powerful fourth order system but it seems very "stable" as it beat between the two frequencies. If there is other some weird mode, "I" am not aware of it. Perhaps those that have studied the system's transforms have found something there?


Primary current feedback can't select between the two paths, it will just go
to the one whose gain is highest.

Yes. Is that possibly bad?

If they are about equal, then streamer
loading may tip the balance and make it switch mid-burst. You can prevent
that by deliberately mistuning- to ensure lower pole operation, tune the
primary lower than the secondary, and for upper pole, tune it higher.

Right now primary and secondary are tuned to 85.1kHz and tuned to within 0.1kHz of each other. It is trivial to mistune, but I just don't see "why"? If the frequency want's to shift, the driver does not care. I have no local oscillators or anything controlling the H-bridge. It is totally timed from the primary current. It could go from 1Hz to 100kHz and then to 200hZ and the driver would not care (1Hz would probably drain the filter caps or hit current limits). Perhaps in the case of an oscillator controller like your PLL controller, such shifts are far more of an issue?


If the secondary took a heavy hit, it's oscillations could certainly be shifted out of sync with the primary or totally disrupted. But i would think once that load is gone it would all snap right back in place. But the primary would not care much... I can imagine if the primary were driven from a local oscillator, such disruptions could do odd things in that six pole feedback system case.


If you reverse the phase of the primary feedback you get a third possibility
which is the one described by Antonio de Queiroz and Jimmy Hynes. The system
oscillates at the "zero" midway between the two "poles". This oscillation is
not stable in the steady state but it might persist long enough to get
breakout.

It will just shut off. The logic will not allow the IGBTs to turn on out of phase. If you reverse the CT, the driver will just counter the start pulse current and massively damp the oscillation off. In the case of a secondary current driven coil, you can reverse the CT however.



It's hard to tell from the trace which mode you're on but I would guess the
lower pole. The upper pole never shows notches, and the "Antonio" mode won't
oscillate at all if the feedback phasing is set for driving a single
resonant circuit.

I "think" it is riding both poles and the resulting primary current regardless of frequency or poles and zeros. The driver just does not care what the frequency is or how much it shifts from cycle to cycle.




"Two modes diverged in a Tesla burst,
And sorry I could not travel both
with just one driver, loud I cursed
And messed around with PSpice first
To see them pulled by streamer growth"

If you have like a MicroSim model that would help me to understand I would love to study it. I really don't understand the "two modes" thing...


Cheers,

        Terry


etc



Steve C.