Re: [TCML] design idea

[Steve Ward <steve.ward@xxxxxxxxx>]

Hi Carlos,

See my responses below.

On Mon, Feb 6, 2012 at 8:09 PM,  <lightningfor@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> Hi Steve,
> Thanks for such a detailed explanation.
> My experience is hands on, largly with SGTC's, so I often lack an accurate
> undestanding of the theory behind things.
>

Well, its taken me years to even understand these things on a really
good instinctual level, and im a degree'd engineer who is supposed to
just know how it all works, right? :-).  Yeah, trust me, its not real
simple stuff.. im just kinda getting good at it finally, but not good
enough to know all the answers yet :-).  And furthermore, this CW
stuff is MUCH different than the transient DR SSTC or SGTC theories!

> So are you saying that a Tesla coil with a low voltage supply and a large
> tank cap (for a given frequency), can give just as big arcs off the
> secondary, as a Tesla coil with a small tank capacitor (with the same
> frequency) and a higher voltage supply?

Yes, look at any SSTC, they will operate with a very low primary
impedance in order to get the voltage transformation from the tesla
coil.  Think of it this way, the voltage gain of the tesla transformer
is related to coupling and the square root of the inductances of the 2
coils (im speaking loosely here since i dont remember exactly the
equation), so if you make the primary coil less inductance, but keep
them coupled the same, then the step up ratio of the tesla coil goes
*up*, right?  Its like you have less primary turns, so the turns ratio
went up.  BUT, this automatically implies a bigger tank capacitor
because its still gotta resonate with less primary inductance, yes? So
then it stands to reason that if you want more power out of your CW
tesla coil, you need more top voltage, and one way to achieve this is
to lower the primary/drive impedance.

You may think, however, that you could simply *add* secondary
inductance and get more volts, well this would be a mistake. More
secondary L would drop the Fres for a constant secondary capacitance.
lowering Fres dramatically raises the streamer impedance (because the
spark is afterall a capacitive load through the air).  This has a
negative effect on streamer power, so it effectively works against
having extra top voltage that was gained with the extra secondary
inductance.

> And this is assuming an impedance match of the supply to the tank
> circuit...?

Well, the impedance match is *really* between your supply and the
streamers!  The tesla transformer is just stuck in the middle there to
boost the voltage in a convenient, resonant fashion.  But, in order
for the tesla transformer to really do its job, you gotta pick the
impedances properly!  So yes, you want to determine how to design the
tesla transformer to meet your specs, which requires knowing a lot
about the streamer's impedance (that is, how many amps of current does
it use at a given voltage, and how resistive is it?).  Anyway, i could
go on and on, but its all sort of guesswork and only at ~350khz regime
of my QCW system.  I ultimately would like to characterize streamer
impedances at various time scales and maybe remove some guesswork as
to how to design the right tesla transformer for a given
power/frequency/whatever.

>
> I never really thought of it that way before...
>
> I know the principle behind the voltage rise in a Tesla coil is due to
> resonant rise, but I guess I always thought that voltage transformation
> between Prim and Sec coils still would play its part...

Its all depending on how you look at it.  The resonant thing is really
just "convenient" because it caused Tesla's transformer to "ring" all
on its own without any fancy switching.  Really, you just want a
transformer!  not all this extra capacitor energy storage!  And
indeed, in CW mode, it boils down to being just a transformer based on
the inductance of the coils, and the coupling that determines the
effective turns ratio and leakage impedance.  The capactive part
serves to make the current and voltage sinusoidal, which is merely
convenient for power switches to obtain efficient switching, and the
fact that some secondary capacitance is un-avoidable so you may as
well make it resonate.  Im not sure if im driving home the issue,
resonance is not a requirement, it is merely a convenience to the
designer!

>
> I.E Higher voltage in means higher voltage out...
> Is this Still the case?
>

Yeah, this should always be the case unless things go wrong!  But, it
doesnt have to be a linear response, the top voltage tends to get
clamped by the streamer load.  For example, i measured the top voltage
on my QCW tesla coil where i ramp the supply voltage.  The input
voltage goes from 50VDC to 300VDC, while the toroid voltage goes from
46kV to 56kV respectively, and the spark grows from a few inches to 5
feet.  I also had an indirect way of determining the streamer current
based on a lumped model, but i dont have the numbers off hand.  I
could effectively work out the impedance curve of the streamer in 1
specific operating mode of my system... and guess what, it changes
when i change my ramp parameters!  So the bottom line seems to be that
there is no easy answer (yet).

> I have tried many ratio's of L to C in my tank, and found a point that
> worked best.
> The arcs were only half the length I needed for the display and was
> drawing only 30kw out of the 50kw it is capable of.
> In saying that, at 20kw the arcs were almost as long as 30kw input, they
> just get fatter... (this is a CW system)

This is not surprising to me.  If you must do CW, then id be
interested in seeing a low frequency CW coil because i think that
there might be a benefit of a higher streamer impedance in that you
might have more electrostatic forces driving long streamers without
pumping so much current which makes the root of the streamer really
fat and power hungry!

What frequency did it operate at?

>
> It is my hope that increasing the voltage input to the tank will cause it
> to draw more power and if I need to I will reduce the duty cycle.

Yeah, this always works if you take the duty cycle trick far enough ;-).

>
> Given what you mentioned about having a single switch (the valve), do you
> think it is worth trying...?
> Or do you think I am going to need to abandon the valve etc, and start
> using silicon etc...?

Well, i would not suggest switching to Si unless you are really
experienced with it... it seems to take most people many months of
hard work to pick it all up and get something reliable.  Id recommend
sticking with tubes if you got them, and focus on modulation and maybe
tesla transformer design.

>
> If I did start from stcratch, what type of switch(s) would you recomend to
> give me multiple 8ft long QCW arcs off the secondary...
> I would be using a rectified 400v 3phase supply...

Well, honestly i considered buying some big vacuum tubes to make a
bigger QCW tesla coil.  Ultimately i found some nice little IGBTs that
i want to gang up 48 of, and make some 10-15 foot "trees" with i hope!
 I go this route because im stubborn!!  You could likely get a tube
big enough to out do my fleet of IGBTs, and not worry about the tube
randomly exploding because you messed up some stray inductance
somewhere that threw off the switching and caused it to overheat 1
IGBT specifically.  But ive got a vision of this working and am just
motivated enough to see it through... not because its the simplest
way!

Of course, driving your tube coil properly might involve some special
techniques depending on what you want.  Can you elaborate more on what
you want the system to do in terms of spark characteristics?  I know
its an art project and this is important, maybe i can help offer
ideas.

And you say "multiple", well what if you just had a breakout point
that move around quickly and let the human eye integrate all the
sparks?  That is, have the spark emitted from a spinner or something
and pump out a bunch of them in fast succession.  Certainly much
cheaper than making all 8 sparks at once!


Steve Ward

>
> Kind regards,
>
> Carlos
>
>
>
>
> On Mon, 6 Feb 2012 17:15:23 -0600, Steve Ward <steve.ward@xxxxxxxxx>
> wrote:
>> Hi Carlos,
>>
>> I see the predicament you are in with the VTTC: simply cant get enough
>> power through it.  I havent modeled this type of oscillator yet, but i
>> suspect that if you do have other options available in terms of tank
>> capacitor that you can increase the Q of the system by lowering the
>> primary impedance (more C, less L) which should increase the total
>> power throughput by driving the secondary to a higher voltage which
>> makes it push more current through the plasma.
>>
>> You *could* try the transformer thing, but this would be particularly
>> tricky with a single-ended drive like most VTTCs are (just 1 switch
>> element).  The issue here is avoiding saturation of the core by
>> applying any DC voltage to it long term.  I personally have not worked
>> out what issues there may be, but it does stray considerably from any
>> type of power supply i have made (yes there are forward and flyback
>> converters, but their output current is DC not AC like a tesla coil,
>> and this messes up the basis on which these designs function).
>>
>> Since you mention you are already starting with 10kV, and want to go
>> higher still, you'd likely need to do an oil submerged transformer in
>> order to keep the coupling high (and stray inductance down).  To me
>> this seems like more work than its worth.  For my scenario i was
>> simply working with hundreds of volts where insulation issues are
>> rather trivial.
>>
>> Personally, i would be investigating what changes could be made to the
>> tesla coil design itself (the primary and secondary circuits) to get
>> more power throughput at 10kV.  The main methods of getting more power
>> through a CW-ish system is to raise the loaded Q of the system so that
>> the secondary voltage is higher.  You can do this on the primary or
>> secondary side, essentially by increasing the total capacitance but
>> keeping the frequency the *same* (so that means less L).  It might
>> sound "strange" that less secondary inductance will result in higher
>> voltage, but ive done it myself and it can work, but usually the more
>> appealing option is to use more tank capacitance and leave the
>> secondary alone, since i dont think more secondary C is particularly
>> useful for CW spark growth.  Another thing to check into is if any
>> tuning or coupling changes might give the power increase you are
>> after.  I know that at for my QCW DRSSTC, i can actually increase my
>> power throughput by *lowering* the coupling, when tuned a specific
>> way.  Just a thought, not sure if i actually would have a chance to
>> look into this much for you.
>>
>> Also, i got the note about the job in chicago, which is funny since i
>> used to live there until i left to work with arcattack.  I dont think
>> we can do that date since we will be across the country in california
>> just a few days before that, but i do know some guys (who i worked
>> with) that can do tesla coils in chicago so perhaps i can pass the
>> information to them.  I appreciate the thought, either way.
>>
>> Steve
>>
>>
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