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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Just out of curiosity, if you were to create the longest sparks for a given peak power and burst length, what would you do? -The lower the frequency the better, or is there a point where decreasing the frequency actually shortens the sparks? -Would a higher tank circuit surge impedance help? (longer "ring up" time) -Use the lower or the upper pole of resonance?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Good question, I wish I knew the answer! all of the parameters you mention are somewhat related, as I'll try to explain.
I believe that the optimum surge impedance for a Tesla coil secondary is about 50k ohms, no matter what size it is. If you keep the surge impedance constant, then bigger coils will end up with lower resonant frequencies.
The lower pole of resonance has a property where the power transfer increases as the streamer load gets heavier. The upper pole does the opposite. So, lower pole operation can help make big sparks, but only in so far as it allows higher peak power. On the other hand, lower pole operation requires more clearance between primary and secondary to avoid flashovers, which implies looser coupling. And loose coupling hurts performance.
Another consideration is that streamer growth gets less efficient as the streamers are made longer compared to the resonator. This is because a short resonator limits the output voltage by flashing over. If you want sparks 3, 4 or even 5x the size of the coil, you have to use long bursts and waste lots of energy expanding them thermally. I don't understand the plasma physics, but I have a mental image of the streamers being "inflated" with hot plasma heated up by displacement current, rather than being driven outwards by a strong E-field.
So, for efficient operation I would keep this ratio below 2:1 (in other words build a bigger coil for the desired spark length) but that doesn't look nearly as impressive.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hmm, interesting Where does the 50k figure come from, what about two coils with the same size but different wire? How does it relate to primary surge impedance?
I really haven't done much work with pulsed DRs, just the quasi-CW ones. I believe the classic DR burst looks like this - few to several cycles of ring up, the streamer grows and current reaches a peak, then drops as the streamer loads down the resonator. Would that current peak exist also on the upper pole? The streamers detune the resonator even more, so I guess the current would rise the whole burst, does this sound right? You could then lower the surge impedance with the same peak power, but the average power would be higher I guess.
Another thing I wondered about, does lower frequency increase the probability of arc overs between the coils, for the same output spark length and coil sizes/geometry?
-Would a higher tank circuit surge impedance help? (longer "ring up" time)
I suppose you refer to the primary tank. A higher surge impedance there does indeed lengthen ring up time. The speed of voltage rise in the secondary depends also on coupling and tuning. I've run some simulations with Conners hungry streamer model and Terry Fritz's model from scantesla, where I made the streamer capacitance proportional to the secondary voltage. Neither could reproduce the primary and secondary current envelopes I measured unless I put in some delay between the secondary voltage and the streamer capacitance. Basically that means, that it takes some time for the streamer load to build up. So I believe, that a longer ring up time will keep the primary peak current lower, since the streamer load will have more time to develop during ring up.
I believe the classic DR burst looks like this - few to several cycles of ring up, the streamer grows and current reaches a peak, then drops as the streamer loads down the resonator. Would that current peak exist also on the upper pole? The streamers detune the resonator even more, so I guess the current would rise the whole burst, does this sound right?
Precisely. But be aware, that the detuning at the upper pole will also reduce the efficiency of the coil which may overcompensate the higher primary current you run on. I've experimented with tuning to the upper pole for exactly the reason you mentioned but the results were quite strange. Sparks were feeble and my primary current envelopes were periodically going up and down. Possibly this is an artefact of my electronics. But it could also be, that the streamers detune the system until the voltage goes down. Then the streamer would die, bringing the coil back in tune, which then would increase the voltage. Then the whole scenario would repeat. That is _very_ speculative. I should make some experiments to confirm that.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I've always tuned my DRSSTC to the upper pole, and I've achieved sparks about 4.4 times the secondary length, which was a record until Steve's QCW beat it.
Here is a picture of one of my bursts:
It includes a ground strike at about 125us. The current falls after the ground strike, but I don't think that is what normally happens. In this case the PLL driver was mistuned and the ground strike shifted the frequency out of its lock range.
Note that the current rises to 300Apk in the first 50us and stays there. It always did that of its own accord, and as far as I know it was some kind of equilibrium state. At least I hope it was, because that was how I planned it to work! I believe the value of current at the balance point can be adjusted to suit the capability of your inverter, just by detuning the primary, and the math posted by Uspring a while ago supported this.
The lower pole has no balance point (at least not that I know of, maybe Uspring can help) because streamer loading pulls the secondary frequency closer to the primary's, increasing power transfer. Once the streamer starts to grow, the energy already stored in the primary will tend to dump violently into it. I think many DRSSTCs are deliberately tuned to operate like this, as it can give spectacular performance.
That's not how I like to do things though, my philosophy is that the coil system is just a matching network to connect the inverter to the streamer load, and should store no more energy than it has to. I originally referred to this as my "Quasi-CW" philosophy, which Steve Ward took to an awesome conclusion.
When considering the two poles, remember that it's not easy to make a regular feedback driver stay on the pole you want, because the relative loop gains of the two poles change during the course of the burst as the streamer develops. My driver is the only one that lets you select the pole you want to use and stick to it all the time. However, the distinction is less important with loose coupling and short bursts: under these conditions no driver can avoid driving both poles to some extent, as you can prove with a bunch of Fourier stuff.
I've since rebuilt the coil with looser coupling (to stop flashovers that burnt up the original secondary) and a lower primary tank impedance. In this configuration the equilibrium point seems to be above the current limit, so the primary current is controlled by the pulse-skipping limiter most of the time. The active current control greatly simplifies the analysis (IMO) because the primary can be considered as a constant source of amp-turns driving the secondary.
The looser coupling (at least according to the constant amp-turn theory) means I need to run about 450A peak to get the same sparks that I could get with 300 before, but apart from that it works just as well.
I'd like to run my coil like you do but as described above it doesn't want to. If I choose the lower pole, my coil will eventually settle in some equilibrium state after the primary current shoots up and then drops. That equilibrium state is one of low primary current and low power, though.
When considering the two poles, remember that it's not easy to make a regular feedback driver stay on the pole you want, because the relative loop gains of the two poles change during the course of the burst as the streamer develops. My driver is the only one that lets you select the pole you want to use and stick to it all the time.
I don't think it is difficult to choose a pole. The pole you end up with depends on whether your (uncoupled) primary tank frequency is below or above the secondary frequency. Initially my coil (primary zero current switching) starts out at its primary frequency and then wanders during rampup to the nearest pole. When tuned to the lower pole, it might happen, that the secondary frequency will drop below the primary one due to the streamer capacitance. In this case the coil might switch poles. During heavy streamer loads the poles will coalesce into a single one, though. If tuned to the higher pole, the coil should stay there, since streamer load pulls primary and secondary frequency even closer. I suspect, that the real difference between tuning high or low is that in the lower case, streamer load will pull the coil into tune while in the upper case it is pulled out of tune. The latter case avoids the problem of the reduction of primary current.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
wrote ... I don't think it is difficult to choose a pole. The pole you end up with depends on whether your (uncoupled) primary tank frequency is below or above the secondary frequency.
That is kind of my point, there are actually 4 possible combinations, but two of them can only be accessed by a PLL driver.
I drive my coils at the upper pole frequency with the primary tuned below the secondary, which is one of the "impossible" combinations.
Maybe we can help figure out why your coil won't do the business, can you post some details of the resonator system? I can't help thinking that I've seen that "shoot up and drop" behaviour in the primary current envelope before.
I drive my coils at the upper pole frequency with the primary tuned below the secondary, which is one of the "impossible" combinations.
That's a fabulous idea. It reduces the likelyhood of flashovers but there is more than that: Whether a coil is in tune depends on the difference of its operating frequency to the resonant frequency of the secondary. In my coil, e.g., I tune the primary about 20% below the secondary and its operating frequency will be initially 20% below the secondary resonance. So it is initially quite out of tune and therefore primary currents will rise until the secondary frequency is lowered by streamer load which brings it into better tune.
In your PLL coil you start out with an operating frequency much closer to the secondary resonance, so your coil is better in tune and you won't see the high peak in the primary current. When the streamer reduces the secondary frequency close to the primary one, the way of functioning of our coils converge.
Maybe we can help figure out why your coil won't do the business, can you post some details of the resonator system? I can't help thinking that I've seen that "shoot up and drop" behaviour in the primary current envelope before.
Thank you for your offer. My coil is a somewhat modified minibrute, where I'm using a secondary with a lower inductance than the one McCauley recommends. The tank parameters are Cp=75nF, Lp=8.9uH, Cs=16.9pF (lumped), Ls=24.6mH, k=0.14. I cannot tell much about the electronics, since it is a commercial design, but I think I can disclose that it has a half bridge supplied with +-200V with IXGN60N60 IGBTs. I run the coil with a 650A limit.
But before we go on trying to improve my coil: Did you ever try tuning your primary higher than your secondary?
The idea is to start off with the coil only slightly out of tune and later avoid the drop of primary current due to getting too much into tune.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
That's a fabulous idea.
Every DRSSTC builder has his own recipe and this is the one that works for me
I cannot tell much about the electronics, since it is a commercial design, but I think I can disclose that it has a half bridge supplied with +-200V with IXGN60N60 IGBTs. I run the coil with a 650A limit.
The Minibrute is just a slightly tweaked version of one of Steve Ward's old DRSSTC designs, so I know how it works. (Allegedly, don't sue me Dan! ) The only relevant thing to this discussion is whether you're taking your feedback from the primary or the secondary.
But before we go on trying to improve my coil: Did you ever try tuning your primary higher than your secondary?
Here's how I tune my coils: I set the current limiter to the maximum current I want to run my IGBTs at. Then I move the primary tapping point around until I get the biggest sparks, every so often rechecking that I am still locked to the upper pole. For all I know, the primary might actually end up higher than the secondary! I just assumed it was lower, because that was what my simulations predicted. I really ought to check that.
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DR spark length vs frequency
all of the parameters you mention are somewhat related, as I'll try to explain.
