Re: Recent s.s.t.c work

["Tesla list" <tesla@xxxxxxxxxx>]

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

On 15 Oct 2005, at 12:26, Tesla list wrote:

> Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>
>
> No, I think that my differential setup is working OK.  At least,
> unless & until I start making serious EMI.
>
> On further reflection, I can now understand the bottom waveform in my
> photo.  What I ideally want is to see half-sine waves.  That would be
> the case if the H-bridge IGBTs were switching at zero-current.  In
> that case they, in effect, act like full wave rectifiers for the
> current in the primary, causing the current in the supply capacitor(s)
> to look just like that in the primary but rectified.  So, the waveform
> tells me that I am not switching at zero-current.  Otherwise, it's a
> reasonable replication of the primary-current waveform.  I think the
> noise-bursts on the upper waveform signify the switching points;
> ideally they should occur at the zero level.  (It would seem that the
> sync in the scope was not fully tracking the upper waveform, so the
> switching points in the two waveforms do not perfectly line up.)
>
> Clearly, then, I have to shift that phase before I fire it up with
> much more power.
>
> I had been starting to wonder why I did not just utilize feedback from
> the secondary rather than from the primary, as I did in my 1st t.c.
> But now I can see that one can't do that if one wants to keep the
> switching at zero-current.  Zero current in the secondary is almost
> never zero current in the primary.
>
> If anyone has some ideas as to how to maintain zero-current switching
> in a simple feedback-system, I'd like to hear them!  In the interim, I
> study the problem.
>
> Steve W., to answer your other questions:  1.  The max. input voltage
> will be ~twice the peak out of the variac, which is connected to go to
> ~140 V rms.  That yields ~390 V across the H-bridge.  I do incorporate
> a cycle-by-cycle over-current protection circuit that seems to work
> OK, but I have not set it yet (via a pot).
>
> 2.  The feedback input signal is the output of my 2nd current
> transformer (following the 1 ohm resistor I've mentioned) clamped by 4
> diodes in series--parallel.  In other words, a ~2.8 V p-p square wave.
>  Paralleling those diodes is a 50 ohm resistor and that whole network
> refers to signal ground.  The capacitor of a Schmitt-trigger-gate
> oscillator connects to the "top" of that network rather than to
> ground, so that the oscillator provides my "pilot oscillator" signal
> with no spark but the feedback-signal takes over as soon as the
> primary starts to draw current, turning the oscillator into merely an
> amplifier for the duration of the spark event.  That seems to work
> seemlessly.
>
> There necessarily exist phase-shifts within the overall feedback
> circuit:  In the 1:100 primary-current transformer, in the 1:1
> transformer following that, in the Schmitt-oscillator/amplifier
> circuit, in the capacitive coupling to the IGBT driver-transformer's
> primary, and within that transformer itself.  All of them add up, of
> course.  I had earlier thought that an automatic shift in operation of
> the feedback loop from dead-on resonance would take care of that, but
> apparently not.  So...I need a means to keep that switching spot-on.
> Suggestions, anyone...?
>
> Ken Herrick

Lead compensation is often used in analogue circuitry. Any use? It is
difficult to eliminate time delays which is what a lag represents.

Malcolm

> P.S.  If I may, I will piggy-back the following invitation, addressed
> to the "2 Steves" and to any others who, in any language, bear the
> name of Steve (Steven, Stephen, Steppen, Stephanie, Steffen, etc.):
> If your profession is that of scientist, in contrast to "merely"
> engineer (like me) or technician, >and< if you accept the theory of
> evolution, go to ncseweb.org and add your names to the roster of those
> subscribing to the...Steve-o-Meter!  Currently, 644 such "Steves" are
> on the list, thereby firmly (and with humour) endorsing the theory of
> evolution.
>
> KCH
>
> Tesla list wrote:
> >Original poster: Steve Ward
> ><mailto:steve.ward@xxxxxxxxx><steve.ward@xxxxxxxxx>
> >
> >Hi Ken,
> >
> >On 10/14/05, Tesla list <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
> >wrote: Original poster: "K. C. Herrick"
> ><<mailto:kchdlh@xxxxxxx>kchdlh@xxxxxxx>
> >
> >2 Steves (& others)-
> >
> >I >am< measuring (differentially) across just the 1-ohm.
> >
> >
> >Steve Conner mentions that maybe a differential measurement is not
> >the best?  Maybe double check with a standard probe connection (one
> >end of CT grounded, probe on other end).  Im not too familiar with
> >doing differential mode on my scope, so i just do it the normal way.
> >
> >   Since the
> >1:1 xfmr is in series, I discount it as affecting the current--which
> >is a pretty good sine wave.  See
> ><http://www.hot-streamer.com/temp/KCH_TCH4.jpg>http://www.hot-streame
> >r.com/temp/KCH_TCH4.jpg
> >
> >
> >
> >Yes, that looks OK, but it seems you are getting some serious
> >switching delays.  I see 2 spots of noise on the sine wave
> >half-cycle.  Im guessing the first noise is the IGBTs shutting off,
> >and then the second (larger) burst of noise is due to the other IGBTs
> >turning ON?  Dont suppose there is any way to check primary current
> >vs maybe gate voltage or the bridge output V?  Would require an
> >isolated DC supply i suppose, but it wouldnt have to supply much
> >power (100W at 100VDC should be adequate).
> >
> >top waveform (5 V/cm via 10:1 probes).  The 4 m-ohm capacitor-common
> >shunt wave is also shown (at 5 V/cm, directly connected); I connect
> >to the shunt via a ~15 foot tw-pair which I've now (arbitrarily)
> >terminated with 100 ohms at the scope.  I set that up differentially
> >also, and since that shunt is referenced to mains common, there's a
> >bit of 60 Hz c.m. voltage there, which the scope seems not to mind.
> >
> >
> >I dont understand what is going on in the bottom waveform?  Is that
> >the current supplied by the DC caps?
> >
> >The 1-ohm wave is about 4 V peak, implying 400 A peak primary current
> >(at a relatively low mains-input from the variac).  The other wave is
> >across the (measured) 4 m-ohms; its zero-line is at the 2nd cm from
> >the bottom so the voltage existing at the peaks of the upper wave
> >appears alternately as ~1.8 and ~3.2.  That implies peak currents of
> >450 and 800 A, altho the negative peaks (when the primary drive
> >reverses) are somewhat higher.  So...that's a lot better than the
> >wide disparity I seemed to see yesterday but still, somewhat at odds.
> > So Steve Conner, you're no doubt right re current shunts.  But this
> >shunt is a commercial Janco item: 2 quite-short flat bars between a
> >pair of substantial brass blocks.  Not a whole lot of L there, it
> >would seem, & the waveform seems not too different from what one
> >might expect...right?
> >
> >Steve Ward, I measure 12 uH and 400 nF for the primary L & C,
> >yielding a calculated Fr of 72 KHz, I believe.
> >
> >
> >  I did a quick sim in pspice using the 12uH and 400nF and 32
> > cycles.  With an extremely low primary resistance, you could get up
> > near 7000A pk with only 400VDC input!  With a realistic primary
> > resistance of .1 ohms, the peak current hits 3.5kA at 32 cycles!  So
> > your 1000A could be *very* real.  The current soars this high
> > because there is no secondary in place to pull that energy out.
> > Also, this is why i suggest running some 5 to maybe 15 cycles (now
> > that i realize how low your tank impedance really is!).  32 cycles
> > is almost definately too much, lets not speak of going higher!  What
> > is the planned input voltage?  Even with 350VDC and running maybe
> > running 10 cycles would probably put you at 1000A pk and probably
> > 6-7 foot streamers.  Running up to 700VDC, you could probably back
> > down to 5-7 cycles and achieve 10' streamers with maybe 1000-1300A
> > pk.
> >
> >So if your driver has survived at what we speculate is 1000A pk, and
> >running 32 cycles (a rather long duration in my opinion) then if you
> >had the secondary installed for that kind of use, id guess you would
> >be seeing at least 6 foot streamers (probably more).  First thing i
> >would do is re-program your counter to operate from maybe 4-16
> >cycles.  When you start pushing high power levels, the difference of
> >just 1 cycle might mean a 15% increase in spark length, so you want a
> >fine control over the cycles.
> >
> >
> >   Not too far from the
> >(eyeballed) 77.  And I do use feedback from the primary's current
> >xfmr to essentially set up an oscillator, during the spark event,
> >with the primary as its resonant circuit.  That seems to be working
> >well.
> >
> >
> >What does the feedback input look like (waveforms)?  Are you getting
> >a nice square wave back?  Or are you amplifying a low level sine
> >wave?  I lost track of your feedback scheme.  I mainly ask because i
> >have a feeling that somehow its adding in more delay than necessary.
> >
> >   I'm presently operating with a 32-cycle gate-duration; when I
> >actually start to make some sparks, I'll consider changing the
> >current selection range from 32/64/128/256/512 cycles to a lower one,
> >as you have suggested.
> >
> >
> >Well, i wouldnt put more than 100VDC into the system as it stands
> >now, running that many cycles and especially without the loading you
> >could be running serious currents.
> >
> >
> >As to operating w/out the secondary...I seem to keep putting you
> >(Steve W.) in the position of reminding me of things I learned 50 yrs
> >ago & should still attend to--e.g. what limits the primary current.
> >Only problem is, I have to put the whole apparatus on the floor when
> >the secondary is on it, & my old bones really creak when & if I have
> >to crouch down over it to check on this or that.  Unhappily, my shop
> >has only an 8 1/2 ft ceiling rather than the 20 ft one I really need
> >these days.  And outside, being somewhat on a hill, I have no handy
> >flat area anywhere nearby.
> >
> >
> >Well, best of luck.  I'd say if you can somehow reduce your
> >switching delay, and if the 1000A you measured is real, then this
> >thing is definately ready for serious spark production!
> >
> >Steve
> >
> >KCH
> >
> >
> >
> >
> >
> >
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