Re: [TCML] DRSSTC tuning at high power

[Steve Ward <steve.ward@xxxxxxxxx>]

Hi Mike,

On Tue, May 17, 2011 at 12:28 PM, Michael Twieg <mdt24@xxxxxxxx> wrote:

> Hello, for the last year or so I've been working with a large group on a
> pair of large (>20KW) musical DRSSTCs.  So far we've been pretty
> successful,
> but I'm starting to think that our tuning is far from optimal, resulting in
> poor spark lengths for our given power draw.  At low DC bus voltages (like
> under 100V), I'm seeing nice quenching of the primary current, but as the
> voltage increases, the dip in the current envelope starts to disappear and
> peak current rises very quickly.  At around 400V, the quench is completely
> gone, and the current envelope doesn't dip at all (instead it just kind of
> plateaus before increasing again).


This behavior is very much like what ive seen in many of my coils,
particularly the big ones, but at the same time ive also managed to find
tunings that, while maybe not having a primary notch, did not have the
primary current blow up at the end (so just plateau for long enough).  So
the basic phenomenon is that before breakout the secondary back-drives the
primary (as the current phase shifts from zero to 180*, or is that -90 to
90?) and so the current goes back to zero (notch).  Once streamers provide
an exit for the secondary energy, less and less gets "reflected" back to the
primary after 1/k cycles, and instead you end up pumping the primary even
harder.  The thing i havent quite come to grips with yet is why the
performance can often be so pathetic despite the rather huge reactive power
in the system.

I think a critical measurement to make is secondary base current.  Id
suggest watching that along with primary current as you play with tunings.
I should take my own advice and do this next time i get a chance to run
large DRSSTCs.  I think the base current is going to be a strong indicator
of spark performance, though i realize the spark growth is not an
instantaneous thing so talking purely about peak secondary amps is not good
enough, you have to look at how long the current is sustained to feed the
spark.  I havent got a good grasp on how this works yet, but at least with
my small coils ive done some various tuning studies to look at the
relationship between primary current, secondary current and spark
performance, and there does seem to be some subtle "sweet spot" that
maximizes I(s)/I(p), whether or not I(s) is at its maximum seems to be
another issue.  Basically, the most efficient tuning may not be the one to
make the biggest sparks, so there can exist less efficient tunings that
consequently use more power and make bigger sparks.  Im using "efficient"
loosely as the ratio of I(s)/I(p).


>  Also of interest is that as repetition
> rate goes up, the waveform becomes even worse (presumably because the
> plasma
> becomes denser and gives the topload a lower resistance load).  But
> overall,
> putting the bus voltage at 450V, and using a tone frequency of about 440Hz
> gives consistent 7ft arcs, but the thing draws over 10KW and the peak Ip is
> over 1500A, which seems like way too much.  Somehow we're not efficiently
> transferring energy to the topload.
>

I think what you mean to say is "not efficiently transferring energy to the
sparks"?

How is the 10kW measured?  Whats the power supply input?  Is power factor
considered?


>
> I've got several theories on what could be causing the issue:
>
>   1. Simply bad tuning of k and Lp.  I don't think this is really the case
>   because we've played with the tuning a ton and nothing seems to improve
> the
>   Ip waveform or the power draw.  Even if the topload capacitance is
> changing
>   with arc size, we should have been able to compensate with tuning
> changes.
>

You have changed k?


>   2. Losses due to coil resistance, eddy currents, etc.
>

Ive wondered about this, i think this can eat up >10% of the total power, id
estimate up to 20% on some of my systems is lost in components (including
the silicon).


>   3. Bad topload design.  Right now we have a shperical topload
>   approximated with five aluminum rings (about 54" diameter).  We seem to
> get
>   a lot of corona at low power levels, but once we draw a solid arc most of
>   the corona goes away.
>

Seems unlikely to be the problem.


>   4. Bad breakout point.  We're using a 12" long breakout point on the side
>   of the topload.  Maybe it's breaking out too early?
>

Also seems unlikely, though i do find that i always prefer to use the
shortest breakout point that reliably does what its supposed to.  Im not
sure that the performance difference is due to lowering breakout voltage,
because the hot streamer channel seems to persist between firings and
re-ignite fairly early anyway.  I do have some suspicion that the spark
starting in a higher electric field (closer to the toroid) might help, but
this is just a guess.

The main issue I can't figure out is where most of the power is going.  Our
> silicon barely heats up, the primary coil and capacitor get warm but
> probably dissipate no more than 1KW total.  I have trouble believing that
> we're actually putting ~10,000W into arcs, otherwise we should be getting
> at
> least ten feet.
>

Have you done much testing without a secondary coil in place?  This should
allow for better observation of where the power is going.  I found my 7 foot
tall coils (about 8kW max input, 10 foot sparks) happily eat up 1500-2000W
(real power measured from PFC to DC bus) with no secondary installed.  I was
quite surprised by this result and i think most of that power is primary
coil resistance.


>
> Any ideas?  Or maybe I'm expecting too much?
>

Well, i'll at least say that I can understand where you are coming from...
my "Gigantor" coil appeared to be using 6000A in the primary and like 55kVA
from the line just to make ~20 foot sparks, which is greatly out of line
from what I hoped.  The thing that gets me is that some DRSSTCs seem to
claim extraordinary efficiencies, and others just gobble up power and work
so-so.  I suspect this is not largely an issue of losses in copper or
silicon, but rather an issue of proper drive to the spark.  When we measure
sparks only by their length, we might be making some un-fair comparisons,
but other characteristics are hard to measure.  In any case, one
optimization is certainly to make the longest sparks for a given amount of
power consumed, and i think there is probably some issues to look at such as
primary and secondary impedance and operating frequency, and definitely how
quickly the spark energy is delivered.

In my experiments with quasi-CW spark production, i found that my 360khz
setup was *far* superior in converting primary amps to spark length than was
a 280khz or 125khz resonator.  And not just some 10-15%, but it was better
easily by a factor of 2!  So basically, same amount of amp-turns in the
primary, but huge difference in spark output.  At the same time, these QCW
sparks appear to have significantly different behavior than "transient mode"
coils.  Some day i need to do similar testing with a regular DRSSTC and see
if winding low frequency secondaries (to keep the silicon switching slower)
is not shooting ourselves in the foot in order to make long sparks.  In the
QCW case it certainly appeared that way.

Oh, could you give some rough specs on your coil?

Steve


>
> -Mike
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