Re: [TCML] understanding DRSSTC

[Steve Ward <steve.ward@xxxxxxxxx>]

Hi Udo,

>
> 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.

>
>
>>>  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).

>
> @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.

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?

Steve

>
> Udo
>
>
>
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