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Re: Recent s.s.t.c work



Original poster: Steve Ward <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