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

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

On 1 Feb 2005, at 12:51, Tesla list wrote:

> 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

Forgive me if I'm teaching my "grandmother" to suck eggs - I think
I'm detecting a possible misapprehension in the words above. A (over)
coupled double-tuned system exhibits two frequency poles (described
in the time domain as "beating") regardless of whether the circuits
are tuned to exactly the same frequency or not. In physical/pictorial
terms this is a direct consequence of the amplitude response varying
(i.e. ringup/ringdown). When you change the amplitude of a sinewave
on a continuous basis you are generating a frequency component that
is not obvious to see in the time domain. The slope (dV/dt) of the
waveform is *not* that which is produced by the fundamental as seen
in the time domain - it is the same as that which would be produced
by a higher frequency (if the slope is getting steeper as it does
with ringup). This is a necessarily simple attempt to try and explain
in "pictorial" terms how the pole frequencies arise. If I have
misunderstood what is being said above, please ignore this rant. I do
hope it might help however.

Malcolm