Re: Recent s.s.t.c work

["Tesla list" <tesla@xxxxxxxxxx>]

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