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Re: [TCML] Magnifier topics
Paul
I would like to interject several additional points from Alex's 20kVA
magnifier:
1. Quenching / dwell time is a critical component. When we first started
operating this large machine, the magnifier was drawing 60A @ 280V
with a 12 point ASRSG set as a 4point series gap; at this operational
point approximately 8' (measured) streamer length could be obtained.
I suggested we remove 1/2 of the electrodes on the rotary wheel and
double the speed of the ASRSG (same BPS, 1/2 dwell time), no change
in bang energy. Spark length increased to ~10' at same Vin and current
demand dropped to ~ 40A. I know that kVA / ft of streamer is a "hokey"
parameter, however it is difficult to ignore the below metrics:
16.8kVA/8' =2.1kVA/ft control
11.2kVA/10' = 1.1kVA/ft at 1/2 dwell time
2. With the machine in its present configuration, Alex would adjust the RSG
for very low BPS (perhaps 40-60), streamer length is 12-18". By
adjusting
BPS _only_ to nearly burst speed of RSG, streamer length would grow to
well beyond 10' and never showed any reduction is growth versus BPS by
visual observation. I've seen similar results on a 1kVA machine, there
the
streamer length would slow due to transformer saturation. With 4 20kV
2.0kVA PT's, saturation occurs in the 20-22kVA range so is much less
of a power input limitation. This strongly suggests that dwell/quench
time
for a given machine is likely a critical parameter.
All experiments that Richard Hull, Alex and I have done with magnifiers
strongly suggest us must "trap" the energy in the driver secondary, and
decouple
the "parasitic" load of the powering transformer/primary tank from the
resonant
secondary/tertiary. This requires super series rotary spark gaps, magnetic
quenching, air-blast assist, and whatever edifice is available to force the
gap
to commutate off as soon as possible. Series gaps have additional loss, my
comment to that is "oh well", I compensate by extremely high input primary
voltage and smaller caps to boost bang energy.
On Sun, Feb 14, 2010 at 5:48 AM, Paul Nicholson <tcml88@xxxxxxxxxxx> wrote:
> So far I think the only documented example of multiple resonant
> tuning of a TC is Antonio's 3:4:5 system described here,
>
>
> http://www.coe.ufrj.br/~acmq/tesla/mag345.html<http://www.coe.ufrj.br/%7Eacmq/tesla/mag345.html>
>
> As a test of modelling software I set up a distributed model
> of this system. Plugging in the dimensions of all the coils,
> the 62pF 'transmission line' capacitance, and a guess at some
> of the topload dimensions, reproduces the behaviour quite well.
> I had to add an extra 2pF of top capacitance and I actually
> needed 70pF on the t-line compared with Antonio's 62pF, in
> order to get a reasonable tuning.
>
> Here is a comparison of lumped and distributed models, showing
> the impedance response seen looking into the gap:-
>
> http://abelian.org/tssp/acmq345a.gif
>
> The red curve is from a lumped model using the component
> values given by Antonio, plus a bit of resistance to simulate
> a plausible loss factor. The green curve is the distributed
> model, adjusted to match reasonably well by tweaking C2 and
> C3 as mentioned above.
>
> From Antonio's web page you can see that the lumped model
> gives a very good account and in this case the distributed
> model doesn't have anything much to add.
>
> Here is an animation of the resonator voltages (800 kbyte,
> 200 frames):
>
> http://www.abelian.netcom.co.uk/tssp/acmq345.anim.gif
>
> The 'resonator' in the top two graphs is the combined secondary
> and tertiary, with the dividing line marked at position 29.
>
> Output voltage reaches a peak at 6.6uS which is frame 132.
> If you can freeze it at that point, you see that there's
> only a little residual current in the secondary and hardly any
> secondary or transmission line voltage at all. Nor is there any
> tertiary current - all the bang energy that survives the losses
> is momentarily in the third coil's E-field. This animation
> has a 10kV firing voltage and the peak output is about 210kV
> (the lossless lumped model predicts 227kV).
>
> The step change in current between the top of the secondary
> and base of the tertiary is due to the current diverted into
> the added C2 - the 'transmission line' capacitance.
>
> You'll see some HF components excited in the secondary at
> the beginning of the bang. These don't couple well into the
> tertiary and are almost completely trapped in the secondary.
>
> It's fascinating to watch how the secondary coil drives the
> tertiary resonance, whipping it up to a peak. Getting the
> timing of that 'whiplash' correct is what multiple resonant
> tuning is all about. You can see from the animation that it
> really 'looks' right, but is not quite perfect because my model
> is still slightly out of tune.
>
> If more overtones where brought in and tuned correctly,
> it really would start to look like a whiplash at the peak,
> with (momentarily) all the resonator voltage rise occurring at
> the top end of the tertiary. Extra overtones can be tuned
> by hanging capacitance onto the resonator at key points,
> and/or splitting the resonator into yet more coils, each with
> suitable reactance. What you end up with in the limit is a
> pulse forming network involving stepped or graduated impedance
> transformation. This could be described as a 'wide band' or
> 'pulse' TC. It wouldn't be of any practical use because it
> would be lossy and the final segment of the resonator has
> to withstand the entire peak output voltage. However, the
> principle probably crops up here and there in pulse forming
> networks in power electronics.
>
> Note that the current distribution in both coils is almost
> uniform, especially so the secondary. This means that the
> effective inductance is very close to Ldc. Consequently the
> voltage distribution in each coil is almost a linear rise
> and this makes the Medhurst C valid for the equivalent shunt
> capacitance. This enables Antonio's lumped model based on Ldc
> and Medhurst to give such a good prediction of the resonant
> frequencies.
>
> Apart from those little HF ripples, the distributed model
> doesn't have a lot to add here, although it gives a nice
> picture of the resonator in action.
>
> There are three resonant modes (almost correctly tuned) in
> operation here. Separate animations of each:-
>
> 1/4 wave, 227kHz (232kHz measured, -0.4% error):
> http://www.abelian.netcom.co.uk/tssp/acmq345-1.anim.gif
>
> 3/4 wave, 303kHz (307kHz measured, -1.3% error):
> http://www.abelian.netcom.co.uk/tssp/acmq345-2.anim.gif
>
> 3/4 wave 383kHz (385kHz measured, -0.5% error):
> http://www.abelian.netcom.co.uk/tssp/acmq345-3.anim.gif
>
> and just for fun, the next higher resonance, which is not
> being tuned here,
>
> 5/4 wave 873kHz:
> http://www.abelian.netcom.co.uk/tssp/acmq345-4.anim.gif
>
> This overtone makes 500V available at the topload at 20k ohms
> reactive impedance.
>
> The errors of the distributed model (-0.4%, -1.3%, -0.5%)
> could be improved if we took the trouble to measure all the
> dimensions accurately and track down all the 'stray' wiring
> capacitances, etc, but the lumped model with errors of (1.8%,
> -1%, -1.3%) is already more than adequate for practical work.
>
> This 3:4:5 system really is a very good demonstration of
> multiple resonant tuning. The added C2 is large compared with
> the secondary coil's own shunt capacitance, which means that
> Ldc and Medhurst C are good to use. The tertiary coil is not
> quite so 'lumped' due to the relatively small topload but is
> still very well described. These conditions are likely to
> be present in most magnifiers and the well thought out set
> of design equations and programs that Antonio provides are
> completely justified for practical work.
>
> Hopefully the animations in this post will be helpful in
> visualising what the multiple resonant tuning is aiming for.
>
> The challenge for the coiler is as follows: you build the
> system according to the design equations and measure the three
> resonant frequencies. They will not initially be correct
> because the reactances can't be designed and built perfectly.
> They need to be adjusted to the desired ratios.
>
> Once the coils are wound, the system has three tuning variables
> to play with: topload height, added C2, and the primary tap.
> Or perhaps four if coupling can also be adjusted.
>
> Actually I think it is quite a problem to know which to adjust
> and by how much, in order to move the three frequencies into
> a correct alignment. Perhaps it is possible to define a
> tuning procedure? For example: C2 affects the 3rd frequency
> more than the others; coupling mostly affects the distance
> between 1st and 2nd (or is that 1st and 3rd); C3 affects the
> 1st frequency more than the other two;
>
> If one can allow that it is only the ratios of the frequencies
> that matter so that there are only two degrees of freedom,
> then in theory only two of the reactances need to be tunable.
> From considerations like these, can a tuning procedure be
> defined?
>
> There is no quenching in these models, so after 6.6uS the
> energy starts to transfer back to the primary. If anyone
> is interested, I can animate the response with quenching,
> and also with simulated discharges to ground from the topload.
>
> I'd like to post some more models of 3-coil systems, ones that
> aren't so lumped. There's some interesting physics to be found
> by looking at how the overtone mix, excited by the bang, changes
> when the secondary is split. I hope to show that it is not
> so much the higher k, but the extent to which the coupling is
> concentrated onto a smaller fraction of the overall resonator,
> that is responsible for higher overtone content.
>
>
> Dex wrote:
> > 1.Overall coupling in many magnifier systems
> > is in 0.2+ range.That means magnifier is
> > more efficient than a typical coil which
> > usually has problems when coupling is that
> > high.
>
> That's surely the simplest explanation for magnifier
> performance. Simply a fast energy transfer being more
> efficient.
>
>
> > 2.Voltage shape of the outputs are not completely
> > same even with the same overall coupling
> > (perhaps some effect to spark is possible due to
> > that difference)
>
> Another great possibility - the wave shape of the 3-coil is
> just better for developing a good breakout.
>
> I think 'accidental' multi-mode tuning must come further down
> the list of possibilities, and HF overtone heating effects at
> the bottom. Having spent much time studying coil overtones,
> I've still not found a use for them but I keep trying.
>
>
> --
> Paul Nicholson
> --
>
>
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>
--
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA USA
Sharpe's Axiom of Murphy's Law
"Physics trumps opinion!"
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