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A dynamical arc model

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Steve Ward

Sat Aug 10 2013, 05:38AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

For what its worth, i push the knee well below 100hz on my coils now by operating with pulse durations of 500-1000uS. I can achieve full length arcs (10 feet) at 50hz in this manner. its pretty awesome

. I need about 500uS to get long arcs at these very low rep rates, it seems to be a matter of pumping enough heat into the arc. I wouldnt recommend this type of operation unless you know your system can handle it, the longer pulses can heat up the IGBT chips pretty significantly in just 1mS.

Anyway, nice spice model! I wish i was that clever

. The 20uS figure is familiar, ive read about similar time constants for ion recovery time in plasmas before.

I think your model might do better if you put branches in it to simulate "bushy" versus uhhh... "non bushy" sparks. BIG tesla coils will often make huge arcs with not a whole lot of branches.

Do you suppose any of the high-rate wiggling of power levels in the high-power measurement is due to the low coupling of tesla coils in general? Id expect there to be some back and forth exchange over 1/k cycles or so, which may modulate the streamers power.

Goodchild

Sat Aug 10 2013, 07:09AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Steve Ward wrote ...

For what its worth, i push the knee well below 100hz on my coils now by operating with pulse durations of 500-1000uS. I can achieve full length arcs (10 feet) at 50hz in this manner. its pretty awesome

Finally took the high impedance DR path 'wink' 'wink'. My big DR run in this manner. In MIDI operation we run well into 500uS down near 30Hz to 50Hz on our big CM600 coils.

What coil did you get up to 1mS? That's crazy indeed!

Steve Ward

Sat Aug 10 2013, 08:19AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Im using my new digital drivers so i can do tuning tricks* and primary current regulation. The coils are 30" tall secondaries, running ~50khz. They have made 11.5 foot arcs to the ceiling so far, with 350V-560V bus and 600A peak in the primary with a 0.3uF primary cap. They have a generous 78pF of secondary capacitance (they use a secondary-side MMC at 300kV). These coils managed to essentially "drill a hole" in a concrete floor when it found some rebar just 2" below the surface. There was a 1/4" diameter hole after the ground spewed orange sparks. These coils make incredibly bright arcs to ground. A pair of them will throw 10 foot arcs (each) with a 10kW power supply thats shared. Might be some of the best coils ive ever worked with, mainly because they maintain better tuning with big spark loads. The low primary amps is mainly due to this good tuning effect, the streamer resistance is reflected to the primary side as a significant resistance which keeps the primary amps reasonable with very long pulses. And if that doesnt work out, the driver knows how to step on and off the gas to keep it going at whatever primary amps we want.

I think my latest driver is the most like Steve Conners PLL drivers, except it doesnt use a PLL and uses a PSoC instead. But, in terms of how it drives a coil, they are very similar. So i recon his All-Fragger ought to be able to kick out some fat bassy sparks just fine

.

* the tuning trick is something you can only do if your driver has a start-up oscillator or some other frequency forcing mechanism. You tune the coil however necessary to achieve good tuning with a streamer present. The key is how the driver starts excitation of the coil. The self-oscillating drivers (like the UD2) will ring up both resonant frequencies, which might not be the best thing to do. Rather, its better to drive it at the desired pole frequency with the oscillator for several cycles to establish operation at that frequency only.

For QCW coils: If your coil has a high enough coupling coefficient, you can tune the primary lower than Secondary Fres but force the thing to oscillate at the higher pole frequency, which will has some benefits for growing arcs. It provides a positive feedback mechanism between the TC loading and tuning that causes the coil to draw more power even in the CW state as the arc capacitance looks larger and larger. In this way, it aids the driver at ramping up the coil power while driving the arc. You can observe a similar behavior in Udo's measurements as his coil comes in tune with the streamer capacitance, however he continues driving even though the tuning seems to have degraded, as evident by reduced output current. In the QCW coil, the output needs to constantly rise, so going out of tune makes this become excessively difficult, hence the "tuning trick". My original QCW design ran up to 270A primary current to create a 6 foot spark (it was perating out of tune by this point). After implementing my tuning trick and lowering the secondary impedance to maintain better tuning, i make bigger sparks (almost 7 feet now) with 150A (same driver voltage).

Uspring

Sat Aug 10 2013, 04:37PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Steve Ward wrote:

I think your model might do better if you put branches in it to simulate "bushy" versus uhhh... "non bushy" sparks. BIG tesla coils will often make huge arcs with not a whole lot of branches.

I tried that once in order to find out, if the incorrectly predicted phase shifts between arc voltage and current would improve. It didn't help. The capacitance per unit arc length needed for a fit did increase.
I do have big difficulties with the Electrum measurement, mainly due to its much lower capacitance per unit length. Your remark about big TCs likely is part of an explanation.

Do you suppose any of the high-rate wiggling of power levels in the high-power measurement is due to the low coupling of tesla coils in general? Id expect there to be some back and forth exchange over 1/k cycles or so, which may modulate the streamers power.

That could well be so. Another idea is, that the voltage drop after the initial surge puts the secondary out of tune again, so that primary current will rise. Since there is some delay in the response of the arc to the then rising voltage, it will "overshoot", which then leads to another surge similar to the initial one. That produces oscillations of a frequency dependent on the rise speed of primary current and the delay between arc voltage and current.

As for tuning, I think it's a great idea to run at the upper pole while tuning the primary low. As Steve said, this keeps primary current peaks low.

Steve Conner

Sat Aug 10 2013, 07:40PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Steve, so you're saying you finally got round to trying a PLL driver with current control and you like it? Only took you like 7 years

Yes, I am looking forward to doing some serious damage with Odin.

Dr. Drone

Sun Aug 11 2013, 07:02PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Uspring

Mon Aug 12 2013, 09:57AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Eric wrote:

When I finally get to testing this 1/8th scale QCW in about a week or two I will forward on any data I collect. I plan to use the process I outlined above and we will see how that turns out.

Thank you, I'm looking forward to that.

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