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Re: Recent s.s.t.c work



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

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-streamer.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