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Re: [TCML] understanding DRSSTC

   Hi Steve,


Generally there are 3 frequencies at which this happens: The lower and the
upper pole
and a frequency inbetween. With the most simple feedback circuit, which
switches at zero of
primary current, I believe you will run at the pole, which is closest to the
primary resonant frequency.
With a PLL driver (Steve Conner has done this) you have a choice between the
upper and
the lower pole.
The middle frequency is not stable even with a PLL driver. I have never
heard of a
DRSSTC being run there.


The middle frequency (the Zero) is somewhat interesting for a QCW
design where the TC output voltage is fairly constant.  Basically we
just make a constant voltage source out of the tesla coil, since the
bridge voltage and TC secondary voltage are in phase at this
frequency, there is no "resonant rise". This would mean far less
modulation depth is required for power control to grow straight
sparks.  During my analysis of operation in this mode, i discovered
that ZCS is not always available at this frequency depending on system
loading, so im not sure if it will ever be a good option compared to
the poles.  And as you say, its not stable (at least not until you
build up some energy in that mode), so it requires a smarter
controller to get it to work there.


Yes, system loading makes the center frequency and one pole go away, leaving only
one frequency near the primary resonance. That will happen when Qsec drops
below 1/k. If there is a large frequency split between primary and secondary resonance,
this will happen even earlier.


 If the primary is tuned very low then the voltage gain of the system
could continue to
rise as the streamer grows and puts the secondary in tune.  If the
impedance of the
system is too great then you would actually observe a collapse in primary
current as
the streamer clamps the maximum voltage that the coils can ring up to.




- Has somebody tried this?



Yes, I've seen that and it's a headache, because the collapsed primary
current
leads to a lower power output.


Well, i think most successful DRSSTCs run this way to some degree.  To
restore higher power levels, you have to lower the primary impedance,
or (less favorably) lower the coil coupling coefficient.  The reason
lowering K is less favorable is that it results in more copper losses
(more amps AND more primary coil resistance).


For my particular coil I can't lower primary impedance, since it will
make my OCD kick in before the arc has grown enough to lower
primary current again. I get the best results if I tune my primary as low
as possible so that the OCD doesn't trigger.


Well i think you've touched on an interesting point.  Tuning can greatly
change the behavior of the tesla coil under streamer loading.  If the
primary is tuned very low then the voltage gain of the system could
continue to rise as the streamer grows and puts the secondary in tune.  If
the impedance of the system is too great then you would actually observe a
collapse in primary current as the streamer clamps the maximum voltage
that
the coils can ring up to.  If the system impedance is still low enough
that
driving the spark is not limiting system Q by too much, then the coil will
simply go out of tune and voltage gain will be limited by impedance once
again .



The way I see this is thus:

With a primary ZCS driver you'll have primary voltage and current in phase,
as outlined above. So the primary tank will look just like a resistor,
except
that usually you drive it with a square wave voltage and it responds with a
sine wave
current. But just imagine your bridge would output a sine voltage. Then the
tank would really
look like a resistor. I'm ready to admit, that this idea doesn't describe
the dynamics
of primary current rampup, but I think it is useful during nearly stationary
phases
of coil operation.

This resistance is of twofold interest:

a) It determines the current, the primary will ramp up to and is responsible
for its possible collapse.
b) It also describes the power transferred to the secondary (Ip^2 * R).

The resistance is the sum of copper losses in the primary, which I'm
neglecting here,
and a resistance coupled in from the secondary load, mainly the arc.
This latter resistance gets largest, when the difference between the
operating frequency
and the secondary resonant frequency is lowest. It is thus affected by the
arc capacitance.
If you are way out of tune the resistance will become small and the power
transferred to
the secondary (Ip^2 * R) will also be small. When the arc capacitance drives
the secondary
into tune, the primary current can drop. Whether you'll be seeing this
during actual operation
depends on the dynamics of arc growth. It might well be covered by the
rampup.

The resistance also depends on the resistive part of the arc load. At small
loads loads it will
be small but will peak around some value to drop again when you essentially
shortcut the
secondary, as e.g. during a ground arc. That leads to the well known rise of
primary current
when this happens. This is a impedance match/mismatch issue. The previous
paragraph
is a resonance issue.


I agree with all of this, though id like to point out that coil
resistance is a pretty big part of the system.  I think many of my
DRSSTCs lose around 10-20% of the total power in the coil resistances.
If you get too carried away with detuning your primary coil you will
be adding more and more losses, while at the same time boosting
overall coil power until you reach a point of *less* returns, and
resistive losses start to outweigh the gains from being in tune with a
bigger spark.



Coil resistance is one reason my next QCW DRSSTCs will be running at
the lower pole mode as opposed to my initial designs that used the
upper pole.  There is a very significant difference in the amount of
inductance (and copper) required for the coils, which id assume should
mean a lot less resistive losses.  At this point in time, im not sure
i see any real benefit to upper pole operation since you are
cancelling so much of the inductance that you pay dearly for with
copper losses.  Any ideas to support upper pole operation?


If you've tuned your primary to a lower frequency than your secondary, the upper pole
will be closer to the secondary frequency than the lower pole.
For a given primary current that will result in a larger power transfer to the secondary.
For large system loads, you'll have just one ZCS frequency, so there is no choice between
poles at this point.

Udo


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