Re: [TCML] understanding DRSSTC

["Udo Lenz" <udo_lenz@xxxxxxxxxxxxxx>]

@Herwig,

> - If the natural frequency of the primary circuit is tuned lower than the natural
> frequency of the secondary circuit, *both* poles move downward. So far so good.
> In my understanding the feedback of DRSSTCs causes the system to oscillate. Which
> component(s) influence the frequency of oscillation/the pole which is used? And what
> means "...DRSSTCs are *run"..."? Which means except tuning the primary circuit do you
> have to influence the operating frequency?

In principle you can run your DRSSTC at any frequency. Just connect a signal generator to
your bridge. Particularly interesting are those frequencies, where the bridges output voltage
is exactly in phase with the primary current. That means, that you will switch at zero
current, which avoids voltage spikes.

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.

> >  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.

@Steve,

> 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.

Udo


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