Re: Recent s.s.t.c work

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Original poster: Steve Ward <steve.ward@xxxxxxxxx>

Ken

On 10/27/05, Tesla list <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx> wrote:
Original poster: "K. C. Herrick" <<mailto:kchdlh@xxxxxxx>kchdlh@xxxxxxx>

Frequency splitting...  Yes, occurring when both primary and
secondary are resonant (equal Fr's or no) and k is greater than 0.01
or so.  So...I hark back to my previous s.s.t.c. that had a
non-resonant primary.  No frequency splitting there.  So why should
one use a resonant primary in a s.s.t.c?


Frequency splitting isnt something "bad"!  Of course its necessary 
for a double tuned circuit to transfer all of its energy into the 
load.  But you can tune the coil so that frequency splitting occurs 
to a lesser degree, until the secondary is so heavily loaded that the 
Q drops and all the energy is quickly.  Tuning the primary extra low 
will cause the beating to slow down and also lesser in 
amplitude.  This is how i tune smaller DRSSTCs so i can fit more 
drive cycles in before a "notch" occures.

  I measure 15 uH inductance
in my present one, which yields about 9 ohms of inductive reactance
at 100 KHz.  With ~300 V rms applied from the H-bridge and with
relatively negligible primary resistance, the current would be n.g.t.
~330 A.  Thirty amps of maximum mains current would require a duty
cycle of n.g.t. 9% which is OK.


Well, perhaps.  The problem is that your primary voltage is still 
relatively low (no resonant rise).  I think because of the 
limitations of coupling, that you might have much more reactive power 
than real power.  In any case, i dont expect it to make super long 
sparks... sure it could work, though.  My ideals for DRSSTC operation 
are maximum energy in minimum time, within reason of course.

So once again...why use a resonant primary?  It would seem that the
answer is, to get more current in the primary which will yield more
flux cutting the secondary which will yield greater secondary
voltage.  Except that you have to deal with that pesky frequency splitting.


Perhaps someone else can convince you of the exact reasons for a 
tuned primary.  As i understand it, its the only way to deliver a 
great amount of energy in a very short time.  I dont think an untuned 
primary can achieve the results of a tuned circuit.

I wonder if a better feedback scheme would be to feed back from the
secondary rather than from the primary.  That way, the operating
frequency of the system would always be at the Fr of the secondary,
with the Fr of the primary to be adjusted (presumably lower) so as to
reach the maximum primary current that could be accomodated as a
function of duty cycle and allowable mains-current.  Does that sound
reasonable?


Well, as mentioned before, i used to use secondary feedback, i lost 
plenty of IGBTs in those days.  Since using primary feedback, the 
IGBT death-toll is still zero.  Not to say that simply using primary 
feedback fixed everything, but it was a key advancement at least for me.

As to the 15 ft of "antenna" connecting my delay-adjusting pot to the
apparatus:  I plan to put the full 17 V of dc that I have available
across the pot.  Then, at the coil end, I'll capacitively bypass that
to ground at the a/d converter and then resistively-divide it down to
the 0-5 V the a/d requires.  That will yield a) the proper 0-5 V of
signal and b) less than 1/3 of the EMI that would otherwise be present.


Hopefully that is acceptable.

Steve, you're right about the frequency-shifting.  But it is pretty
small: likely no more than 1-3%, I'd think.  At, say, a 6 us delay,
3% would yield a 180 ns error.  That, of course, might well fully add
to the 200 ns resolution-limit that I will see in my 32-stage/5
MHz-clock shift register.  And if I forget primary resonance and go
with only a single-resonant system, there will still be that slight
shift--but then, in the Fr of the secondary.  But still, I should be
able to tweak the pot for best operation at the full power
level.  Also, I'd be operating at ~100 KHz and not ~70 so the
required delay would be less.  And in any case, a significant part of
the delay will be the turn-on time of the IGBTs which, because of
Miller effect, will, I think, vary with applied h.v. but not with Fr.


Well, i still suggest using the tuned primary circuit, if long sparks 
are your goal.

I'm going to see if I can add a secondary in my simulation; but maybe
not because of element-limitations in the freebie v. of the program.


Its definately worth doing if you can.  It can teach much about 
various tunings and their results.  Its still tricky to determine 
what an acceptable load for the secondary is, i believe im currently 
using 180K + 1pf/foot of anticipated spark length... though often 
just using 180K straight to ground made the simulation better match 
my real-world results.  Part of the problem is that the streamer isnt 
present for maybe 60-80% of the actual "bang" time.  Ive experimented 
with switching in a load at various points in time as well... seemed 
to work "ok" there too.  With no load at all, the primary currents 
are simply much higher than reality.

Steve

KCH

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