Sigma delta modulated QCW- bang without the buck? / Projects / Forums

Forums

4hv.org :: Forums :: Projects

« Previous topic | Next topic »

Sigma delta modulated QCW- bang without the buck?

previous 1 2 3 4 next

Move Thread

LAN_403

Steve Ward

Sun Dec 04 2011, 10:25AM

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

Hey guys,

I was thinking that i mislead you into believing that the Q of my QCW tesla coil was really low, this was because of my invalid math and not taking the time to figure it out! I finally did the numbers based on some figures i wrote down about its operation at the peak spark length:

Buck output voltage = 300V, output current = 100A, power = 30kW

Tesla primary current = 150Apk

Tank impedance (20nF at 340khz) = 23.4 ohms, peak voltage = 150*23.4 = 3510v.

Peak stored energy in primary is .5*20n*3510^2 = .1232J

**The mistake i made was thinking Q = 340khz*.1232J / 30kW = 1.4 **

I forgot the factor of 2pi, so its really Q = 2*pi*f*Estore/Pdiss = 2*3.14*340k*.1232/30k = 8.8.

In fact, this is only the energy stored in the primary during operation, i think the secondary energy must also be included (as its energy is stored in phase with the primary), this should raise the Q even higher i suppose. Lets consider the energy stored in the secondary and streamer capacitance of 23pF at a voltage of 66kV, the stored energy is another .05J. Is it coincidental that .05/.1232 = .4, which is the coupling coefficient of the system? I suppose then the total Q of the system is really:

Q = 2*pi*340k*(.1232+.0500)/30k = 12.3.

Gosh was i way off in my interpretation of this before! I was thinking my Q was like 6.28X less :P. Anyway, these are pretty rough numbers, but they are at least in the ballpark.

I need to set up the machine again with a few different secondaries and see what the benefit of the lower secondary impedance was, i unfortunately cant seem to find any good notes on that test, though i recall the primary current was much lower for the same spark length.

Anyway, i think this also means a base-fed resonator (as Marko was thinking of doing with some vacuum tubes) might not be a bad way to go. You probably still end up with a Q of 10 or so for the system (means you need a higher Q secondary, by design), but you no longer need a tank cap, you are offloading all that energy storage into a bigger toroid. I think the double resonant thing still works good for low voltage drives since the overall system Q is pretty much the same, its essentially just using resonance as a tool for impedance matching, the question is, how you gonna store the energy?? will it be in expensive, elusive mica capacitors on the primary, or in a huge field around the toroid? Might even go as far as considering an oil-based capacitor built into the secondary coil (this was Jimmy's idea anyway), or maybe a stack of doorknob caps to achieve something like 50pF. But its starting to seem like no matter what the Q of any tesla coil in this frequency regime is gonna have a total Q around 10.

Steve

Steve Conner

Sun Dec 04 2011, 11:45AM

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

Thanks for the info Steve!

it's interesting that the loaded Q ended up somewhere about 10 again.

Progress has been slow, but I'll hopefully get some busbars to make my DC bus cap bank today.

Base driving a resonator with tubes seems like a good idea in theory, but it could be a big engineering challenge in practice. Marko recommended a halfbridge with floating drive for the high side tube, but a single tube with a loading capacitor across it would probably work too. I built a tube plasma globe driver like this, and it's also how the tetrode "plasma tweeter" works, except the frequency of that is so high that the tube's own plate capacitance makes an acceptable loading capacitor.

Marko

Sun Dec 04 2011, 09:03PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys

Well, I still can't wrap my brain around the nature of spark loading yet, and for base drive secondary Q will be the thing that matters!

Projections of Q in between 1-2 just seemed bizarre to me though. It would almost turn out that the ultimate tesla coil is no tesla coil, and that you could attain the same result by using a big linear tube amplifier operating at 50kV or so and driving a breakout point directly!

Q of 10 should allow some pretty epic sparkage with base feed voltages of 10-20kV.

I would venture into something like that if had some good pulse tubes around, and, more importantly, some good big energy storage... I was thinking that a several kilojoule, 20-30kV can crushing cap would be in order here.

Once I finish all my major projects (which are kind of losing point more and more over time) I might try looking into this dreamware. Unless someone does it before me ofcourse :)

A "plasma tweeter" circuit on steroids could indeed be an easy thing to try for someone who has a GMI 7 and some HV energy storage caps lying around... ramp could be applied to grid. Not saying that a normal VTTC wouldn't work, but this circuit would also be much simpler and save hassle with primary coil and tank caps!

Marko

Dr. Dark Current

Sun Dec 04 2011, 09:09PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Marko,
I was planning a "standard" class-C oscillator VTTC with the GMI-7 tube, screen QCW modulation and a defibrillator cap for energy storage, but someone outbid me on the tube

at least I can finish all the other projects...

Marko

Sun Dec 04 2011, 10:04PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi Jan,

Dr. H. on here seems to own a batch of GMI-7's, I thought to ask him whether he would sell some. A base fed coil in a tweeter-like configuration would be interesting and very easy to attempt if anything.

I do have a big microwave tube at home, external air cooled anode... also it's the type with oxide filament and sensitive grids, which might blow off if pulsed operation was attempted due to their low thermal capacity.

Marko

Steve Conner

Sat Dec 10 2011, 03:31PM

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

Update... I fired the coil again with 13,200uF DC bus capacitance instead of 3300. (Operating voltage about 320V as before.) The sparks got thicker and brighter, bright enough to light up the lab, but they didn't get any longer. The spark seems to have reached its final length and structure by about 5ms, and any extra energy just makes it hotter and more meandery.

I didn't blow anything, except the 32A breaker I was running off, when I tried to charge my new monster capbank too quickly.

I discussed the low Q thing with Steve Ward. He says he got his math wrong and the Q was actually about 6!

Next experiment: tighten the coupling and see if the ramp helps to prevent flashovers.

Marko

Sat Dec 10 2011, 08:08PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi Steve

Any pics n vids of those big, bangy sparks?

Many ideas fall into water with magic frequency required being so high - we can pretty much forget about using bricks, and even the fastest igbt's suffer great losses in this application which makes scaling up the design to much more than what Ward did difficult.

The options we have left are paralleling igbt's or igbt inverters, using tubes, or cryogenic devices (rather experimental for now). Guess what's my favourite!

I think your burst length is getting so great that the coil is running pretty much in the CW mode at the end of the burst.

You said that peak power of your coil as DRSSTC was low, on order of 10-20kW... and since we know how CW sparks tend to bunch up I wouldn't be surprised that your coil won't be able to make greater arc length... I actually believe they might get shorter if you add even more energy and prolong the burst! (If that would make any sense as the igbt's are already pushed pretty hard).

Note that my Big Bad the SSTC only produces about 50cm CW spark with 10kW input.

Time for a 350kHz secondary I guess!

Marko

Steve Ward

Sun Jan 01 2012, 02:59AM

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

Watt the Fork, did you measure the secondary base current before and after the bus cap modification? Perhaps because of your modulation scheme, the sagging bus cap didnt matter a whole lot and you just stopped skipping as many cycles in order to maintain the target primary current?

Or, maybe the impedance of the system is limiting the peak power delivered to the sparks. What happens if you try to ring it up even higher? Will it do it? or will it just saturate the control and not ring up more primary amps in the long term?

Also, have you checked what the power factor from your H-bridge is when you are at the peak power level? Hopefully you are pretty much driving the whole time, and not recycling, as that would effectively lower the power factor of the inverter (those recycle cycles average in as negative 1 PF), which is lame. That would mean either 1) turn it up higher, or 2) increase the primary impedance, or maybe lower coupling (need to think about that again to be sure).

Steve Conner

Sun Jan 01 2012, 10:23AM

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

Happy new year all! Good questions, no idea of the answers

will check it out when we go back after the Christmas break. I'll get out the Picoscope and the 13 year old sacrificial laptop.

I think the control scheme worked to keep the ramp the same until the DC bus cap "ran out" at which point the RF current suddenly collapsed. With the extra capacitance this wouldn't happen, but I think the streamer had reached its final length before even the small capacitor ran out of charge.

Gregory

Fri Jan 06 2012, 11:20PM

Registered Member #2922 Joined: Sun Jun 13 2010, 12:08AM
Location:
Posts: 226

Projections of Q in between 1-2 just seemed bizarre to me though. It would almost turn out that the ultimate tesla coil is no tesla coil, and that you could attain the same result by using a big linear tube amplifier operating at 50kV or so and driving a breakout point

directly!

And, what about use a linear tube amplifier driven the base of a QCW ressonator?

previous 1 2 3 4 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob