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The "Fat Coil" QCW Tesla Coil

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Steve Ward

Sat Nov 15 2014, 09:45PM

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

Id like to share my latest Tesla Coil, its a higher power QCW type. You can find some pictures of it on my flickr:

Like most of my projects, its in a prototype stage as I learn what works and what doesnt. Here are the specs:

Secondary: 12"x8" winding of 17awg on a fiberglass form. 165 turns.

Primary: 16" ID, 18" OD "Basket" primary, 6 turns, 24uH.

Topload: 4.5"x33" ugly drain tube and foil tape.

MMC: 11 strings of 20 series 27nF 1600VDC MMKP type, total 14.85nF at 13kVAC (RMS).

Impedance matching: 8 ferrite transformers. Primaries have 10 turns, there are 3 secondary turns (all in series) so total ratio is 10:24.

H-bridge: 8x H-bridge of FGH60N60SMDs, each drives its own transformer.

Buck: 30mF 700V input capacitance. 2x FZ600R12KE3 switches and 2 x BSM400GA120DN2 bricks (used for diodes only). 250uH output choke (big powdered iron core) and 55uF film capacitance on the h-bridges. Fswitch = 20khz.

Buck Snubber: During a ground strike, or fault, the tesla h-bridge can shut off instantly, the buck output voltage may overshoot to dangerous levels for 600V silicon if there is current in the buck inductor (and there likely is a lot). The snubber module (zip tied to the wooden frame) has an electrolytic cap bank (500V, 600uF) to absorb the stored energy in the buck inductor safely. The charged snubber cap is then boosted back to the main 700V bus with its own little boost converter circuit (another IGBT/diode and inductor).

Performance so far (edited 11/19): Peak power level from the buck has been 350V @ 460A which is ~160kW peak. The h-bridges are limited to 636A total (80A per device); with the impedance matching this gives about 265A on the primary. Sparks have reached 11 feet pretty easily, so i suspect it can reach further still. Operating the coil in bursts of 75% duty cycle shows that the arcs tend to form a hot trunk like typical tesla coil sparks, just much more plasma. This allows sparks to reach somewhat further than they could on a single shot-basis from what i've seen. So far, a 15kW power supply has been very insufficient for powering the coil, i think 30-40kW average is what it would take to sustain 10'+ sparks with 50% duty cycle (what the electronics are designed to withstand). A new power supply is in the works, 240V 3ph @ 100A is available

.

Next experiment is to change the impedance match so that i can drive the bridges to 450-475V. At 460A thats about 210kW peak power available for the streamers. I should be able to do a multi-kJ spark if i charge the input caps to 700V. For now im still operating in the ~1kJ/spark range.

zrg

Sat Nov 15 2014, 10:56PM

Registered Member #4762 Joined: Sun May 06 2012, 05:59PM
Location: Russia
Posts: 93

Awesome, as always. Hope we'd be able make something comparable to your QCWs one day

Sigurthr

Sun Nov 16 2014, 12:03AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Stunningly beautiful results as always. Perhaps I missed it, but I don't think I ever saw your buck converter schematic on your site. Would love to see it some day.

Steve Ward

Sun Nov 16 2014, 02:53AM

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

Also, a crappy video from my cell phone during a test awhile back:

I'll post more as it shows up.

The loud squealing is a side effect of the current limit protection circuit droping the buck frequency to 10khz. More software tweaks are in the works...

Sulaiman

Sun Nov 16 2014, 08:18AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Your TCs always amaze me ... great stuff.

From reading your post I see that you have a really good understanding of arc impedance and growth, have you documented your thoughts on these in a simple manner for us to learn.

How do you get such short secondaries to shoot out long arcs that do not arc around to the bottom end of the coil or primary and even ground ... awesome.

Steve Ward

Sun Nov 16 2014, 07:28PM

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

Your TCs always amaze me ... great stuff.

From reading your post I see that you have a really good understanding of arc impedance and growth, have you documented your thoughts on these in a simple manner for us to learn.

Thanks! I don't think my understanding is as good as some other forum members here, but I do enjoy building the stuff so I "make it happen". I think i have some practical knowledge on how to chose coil parameters now, mostly from all of the experiments. I do plan on writing something about it someday, when i have a better understanding of it all. For now i go off of experience and "instinct" rather than facts and analysis, so it isn't science yet.

How do you get such short secondaries to shoot out long arcs that do not arc around to the bottom end of the coil or primary and even ground ... awesome.

That's the fun part! High-frequency is key to developing big sparks from less voltage. The voltage is pretty low, otherwise it would arc over where we dont want it to (and it has happened when the electronics malfunctioned and allowed the voltage to go too high). I did a good bit of design work with FEMM to check the field stresses on the coils, which is how i arrived at my primary with a partial torus shape to it. The design should likely fail at 120kV of topload voltage. My estimate on spark growth under these conditions is that it takes 40kV to initiate, and only ~5kV/foot of spark length after that, so I estimate its operating in the 90-100kV range now.

Extreme Electronics

Sun Nov 16 2014, 10:33PM

Registered Member #74 Joined: Thu Feb 09 2006, 09:17AM
Location: Nottingham UK
Posts: 99

Steve, Stunning Stuff.
Do you have (even a block) schematic. I would love to know the details of your bridge. My attempts at paralleling IGBT's have all ended as bricks.

Uspring

Mon Nov 17 2014, 11:37AM

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

Very impressive work

I did some quick calculations from the specs you provided. It seems, like you didn't use a secondary MMC, which implies (by some more calcs), that arc loading would increase top capacitance by a factor of 2 or 3. How did you deal with that amount of detuning? My guess would be upper pole operation together with some heavy primary low tuning, e.g. 30 or 40%.

Edit: I think I lost a factor of 10 calculating the top load, so likely a secondary MMC was used, decreasing arc capacitance effects strongly.

Comparing arc length to peak power relationships with your previous coil (20kW, 5-6 footers), there seems to be a deviation from the square root law, i.e. arc length ~ sqrt(P). Any idea about this?

Steve Conner

Mon Nov 17 2014, 12:31PM

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

The sqrt(p) thing is only for sparkgap coils and DRSSTCs. The relationship between spark length and power also depends on frequency, break rate (in the limit, for single shot the average power is almost 0, yet the spark length is not 0), burst length, and last but not least the ratio of spark length to coil length. If you want a huge spark from a small coil, you have to make compromises that hurt the efficiency of streamer growth. This coil takes 1kJ to make a spark length that a larger resonator could do with 100J.

It is very cool though. :)

Uspring

Mon Nov 17 2014, 02:29PM

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

You're right about the fact, that Freaus equation was established with short bursting SGTCs and relates average power input to spark length. So why should it carry over for QCW coils and peak power?

If you think of an SGTC running at various input voltages, average and peak power would be proportional, so a sqrt peak power law would apply also. Varying burst rates will change the average to peak power ratio, which will cause Freaus law to fail drastically at e.g. very low rates, but a relationship between peak power and arc length will not fail to the same extent.

Steve W. QCWs are basically one shot devices (he does generate multiple shots, though, but I think, that the increase of arc length probably is small), so average power is not a very useful concept. Both of his coils run at about the same frequency and have similar burst length, the main difference being peak power.

Peak power for a given arc length depends much on the burst length. The slow arc growth in QCWs creates less branching arcs and thus less energy is wasted in feeding branches. Also longer burst times allow for more heat accumulation in the arc, which will make it more conductive and less power is necessary to carry the voltage to its tip. In terms of bang energy, slow growth is costly, though.

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