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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
By now every lover of Tesla coils probably knows about Steve Ward's QCW. If not you ought to check this out, it is awesome.
The QCW is a dual resonant solid-state design. The long, elegant sword sparks are made using a long burst of RF, some 10-15ms. A highish frequency (over 300kHz) is used, and a ramp-shaped modulation envelope is applied.
The modulation is currently done by ramping the DC bus voltage using a synchronous buck converter, similar to a Class-D audio amplifier but much more powerful. (Steve's QCW used a bigass IGBT brick for the modulator.)
Anyway, enough background. My idea is to combine the Class-D modulator into the existing SSTC bridge. Instead of pulsing an IGBT in a modulator on and off, I intend to just pulse the RF on and off, and let the Q of the Tesla resonator smooth it all out, at least enough for government work. I call it sigma-delta modulation because as far as I can see, that's basically what it is.
My Mk2 PLL driver already has a current limiter that works like this, so it is basically a case of replacing the current limit trimpot with a ramp generator. I've decided to use my Mjollnir DRSSTC as a guinea pig for this.
^Mjollnir's driver board removed and hacked to add ramp generator.
^A "Ramp Time" dial added to the remote control.
^ Ramp voltages from the generator. (I use a window comparator instead of rectifying the primary current, so I need two voltages.)
^Low power test run in the lab, with DC bus reduced to 160V and secondary short-circuited, as I don't want to make any sparks right now. if one hit the LeCroy I would be in big trouble- it's not mine
^At the start of the ramp, the primary current is discontinuous- a series of "banglets" of 1 and 2 cycles. Hopefully when the secondary is resonating, it will blend them together into one bang.
^By the end it has sorted itself out and you can see a nice ramp up to approximately 250A peak here. The beginning is actually better than it looks. Most of the amplitude is just "hair" from switching spikes picked up by the CT. And, you can see the DC bus capacitor running out of energy at the end, collapsing the voltage waveform.
On Monday we will take it to the uni HV lab, cameras will be rolling and a fire truck on standby!
(Monday...) Well here are the HV lab test results as promised...
The weird thing is that the sparks fundamentally don't look that different to how they did without the ramp! Either the ramp generator is really bad (the scope shows the primary current, and as you can see it is quite bad ) or the 220kHz resonant frequency affects the spark appearance so strongly that no amount of envelope shaping can undo it.
They look pretty big and gnarly though, and no components blew up, so I think it's at least somewhat of a result. I intend to go on and improve the modulation somewhat, then tighten the coupling and add more DC bus caps and see if the sparks can be got any bigger that way.
Then I'll build a new resonator with a frequency above 320kHz, which according to Steve Ward is the magic frequency where the sparks suddenly go swordlike. If I don't get sword sparks, this will prove that the modulator isn't good enough.
The tank cap used in this coil is 2 strings of 10 470nF, 1kV PP film/foil caps, for a total of 0.1uF. I've managed to get hold of 10 conduction cooled mica caps rated at 22nF, 1kV RMS, 75A RMS, to make a new "MMC From Hell".
Stay tuned for more updates, if you pardon the pun!
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Great looking setup! I think that has answered all the questions I had floating round in my head, I was getting confused by different QCW topologies on the other thread.
So if you're applying delta-sigma modulation to the gate drive instead of modulating the bus voltage during the burst, what effect does primary/secondary coupling have - especially when the modulation is low? I would have thought that the H dridge, in it's off state would try to damp the output from the resonator, making the output spark size very much non-linear with regards to modulation (except at 100% when you're basically at CW)
Probably, maybe, I dunno
Edit: do you have by any chance a scope shot like the last one, but with B & A zoomed on the beginning of the pulse? ...or where abouts is the second to last scope shot in relation to the 10mS pulse?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hey Avalanche
There are so many unknowns! I don't really know the answers to these questions but I'm just going to fire it up and see what happens!
The way I see it, when the H bridge is on it functions as an inverter. When it's off, it functions as a rectifier. The power flow is the same magnitude, because the primary current can't change quickly because of the resonator Q. it just reverses direction. So you have +1, -1 lumps of RF going to the resonator, exactly like a "1-bit" sigma delta DAC in a CD player or whatever.
This is untrue for levels so low that the primary current dies out altogether. In that case the bridge doesn't rectify anything. But I think the error caused by this is in a direction that would improve the results.
Here's a shot from the very beginning of the burst. I should have done all of them like the last one, but it took me until the last one to remember how the LeCroy's zoom worked. You're seeing 1 cycle of "motor" followed by three-quarters of a cycle of "regen".
Dude500, that resistor got crisped years ago! I had trouble with the PLL chip latching up and sending DC to the gate drive transformer. That's the damping resistor across the DC block cap, so the DC cooked it. I got the latchup fixed, but when I measured the resistor it still seemed OK, so I didn't replace it.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
AWESOME. I'm glad some one finally tried it rather than our techno theoretical babble hehe
Steve this looks good so far, the ripple at the lowish frequency is still present as predicted by simulation. I'm curious how this will turn out with the secondary thrown into the mix.
The tricky part is going to be how this reacts with a higher coupling. I have a feeling that coupling is going to play a big role in how the final system operates.
What is your current resonator configuration? It seems like it's rather low coupling low Z and low freq like a regular DR?
Great work, keep us updated I'm itching to see how it turns out! Eric
Registered Member #358
Joined: Sat Apr 01 2006, 06:13AM
Location: UCSB
Posts: 28
Do you have a guess at what the loaded Q is going to be?
I think Ward's loaded Q is pretty low, and the simulations we did showed a lot of ripple once you have some big sparks. I'm not sure at all how much ripple is acceptable, but if the goal is long straight sparks, I wouldn't be surprised if it had to be pretty smooth.
It seemed possible to do a multi-layer secondary (parallel) with an oil cap inside (only 60kv or so!) to really get the energy storage up there, but even that seemed borderline.
Doing some rough numbers, I think Wards coil was about 2amps at 60kv or 120kVA and 60kw (300V 200A in). at 400khz, thats ~65mj per half cycle, which is the energy stored in a 35pF cap at 60kv.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
JimmyH wrote ...
Do you have a guess at what the loaded Q is going to be?
I think Ward's loaded Q is pretty low, and the simulations we did showed a lot of ripple once you have some big sparks. I'm not sure at all how much ripple is acceptable, but if the goal is long straight sparks, I wouldn't be surprised if it had to be pretty smooth.
It seemed possible to do a multi-layer secondary (parallel) with an oil cap inside (only 60kv or so!) to really get the energy storage up there, but even that seemed borderline.
Doing some rough numbers, I think Wards coil was about 2amps at 60kv or 120kVA and 60kw (300V 200A in). at 400khz, thats ~65mj per half cycle, which is the energy stored in a 35pF cap at 60kv.
These numbers don't suggest a high loaded Q =\
I'm excited to see how it works!
WOAH Jimmy is posting to 4hv that hasn't happened in 10,000 years :p
I agree that the Q of the system is going to suck rather hard when making sparks, I would figure this with most DRSSTC as well though.
As you mentioned my only concern with this drive is going to be that ripple! At <10KHz that seems nasty for sparks growth. Even with my system small glitches in class D ramp output (maybe 10 or 20V at the most) could really mess up your sparks! Branching is usually the thing that results.
I'm hopefully though because there is nothing I would like more than to get rid of all that extra hardware!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
WB, Jimmy!
To answer some of the questions: The coil previously was set up as a DRSSTC. It operates on the upper pole at 220kHz, it has a 0.1uF tank cap, about 6 primary turns, and the tightest coupling possible without flashovers.
This is quite a high frequency and impedance as DRSSTCs go, and I found that it already seemed to like long bursts. I ran them up to 2ms with the current controller limiting to 400A, and got some really thick, gnarly sparks.
I take on board the comments about ripple, Q and so on, and I guess perfect sword sparks are too much to hope for. But I think it's reasonable to expect that the long burst sparks will straighten out and lengthen at least a bit, or branch less than they did before, or maybe make some cool new spark effects that nobody was expecting.
Another advantage, as Steve Ward pointed out to me, is that the controlled ramp-up should reduce the voltage stress on everything, allowing the coupling to be tightened. But maybe that's not a good thing!
The limiting factor in the long bursts was always the single 3300uF DC bus cap. Even charged to the full 320V, it was mostly empty by the end. I'm getting another 4!
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
Do you plan to build another tesla with a higher f res if you get brancing sparks? The two things that have been mensioned is F res 350-500khz and a ramping power envilope to make sword sparks. what type of caps does ^Mjollnir's use? good work Steve
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