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Registered Member #55
Joined: Thu Feb 09 2006, 04:56AM
Location:
Posts: 149
I think this would be a nice place for folks to share methods they use for tuning and calibrating their DRSSTCs using the instruments they have available. Do you use a Rogowski coil or CT for active measurements, if so how do you hook it up? How do you find the lower pole and upper pole of the coupled coils etc, what is the best method? Hopefully to be explained better by someone esle here is what I do: I usually just find the fres of the seconday with the topload, then find the Fres of the primary and mmc wired in parallel and match those two numbers as best I can by adjusting the tap. Then with both coils set up in their respective places I drive the primary with the function generator and look for the two points above and below the measured Fres indicating the upper and lower poles. Then I eyeball the tap about .25 a turn greater than the lower pole point and run the coil. Any Tesla Gurus want to give their opinions?
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
I dont bother measuring the frequency of anything when tuning my coils. I usually just tune experimentally, but there are a few things that i find produced more efficient results, particularly for larger coils. Usually i tune the primary just a little bit lower than the secondary. I watch the primary current for somewhat of a notch after about 7-10 cycles. This "notch" (really, more of a dip, *near* 0) is an indication of an almost complete energy transfer to the secondary coil, making it the best time to shut off the drive. I only see this notching effect when i have sparks shooting out of the coil.
Small coils cant seem to achieve really long sparks with only 7-10 cycles, so i detune them pretty severely. What i think happens is that you accumulate a large amount of energy in the primary (it runs for 10's of cycles, without the secondary pulling much energy out). Eventually you get the secondary voltage up high enough to start forming small streamers. The small streamers begin to lower the Q (and lower the Fr) of the coil, such that it will now "accept" the energy being stored in the primary at the time. The coupling usually has to be pretty high, maybe around K=.2-.25. This then causes all of the primary energy to transfer to the secondary in about 3-4 cycles, at which point you should see a sharp decrease in primary current. Also, this kind of tuning has the characteristic of "all or none". When i had my DRSSTC-3 maxed out, it would only make some little purple corona at about 85% Vin, but once you hit about 90-95%, the sparks jumped out to 3' or more (its a very sudden increase).
Registered Member #55
Joined: Thu Feb 09 2006, 04:56AM
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Posts: 149
Thanks Steve and EVR! Thats kind of the info I was hoping to concentrate in this thread. EVR I have read your paper a few times and its great. Some folks just dont have the know how or equipment to make that kind of analysis and I was hoping we could create a repository here for future reference. Anyone want to takle a method of observing a running DRSSTC with a scope?
From EVR via another thread:
SECONDARY RESONANT FREQUENCY
1. Attach 1k ohm resistor to base of secondary and feed into + input of signal generator. - input of signal generator doesn't have to attach to anything. Hook oscilloscope to secondary base end of 1k resistor. Oh yeah, put two opposing LEDs in series with this connection. Adjust signal generator frequency until LED is brightest. Remember series resonance is when circuit is lowest impedance (i.e. short circuit) so LED will be brightest at resonance. Use this to roughly determine your resonant frequency. Make sure your topload is on it as well and its in the spot where you would normally run the coil.
PRIMARY RESONANT FREQUENCY
1. Disconnect everything from the primary coil, primary capacitor. Connect the primary coil and primary capacitor in parallel. One end connects to GND of the signal generator. The other end through the LEDs and 1k ohm resistor. Parallel resonance is where impedance is at its highest, therefore, adjust signal generator frequency until LED is dimmest.
These will get you good ballpark numbers to compare resonant frequencies. Try this if your IGBTs are both okay.
If you have an oscilloscope, you can monitor voltage at the resistor as well to get a more accurate number, but shouldn't be necessary.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Anyone want to takle a method of observing a running DRSSTC with a scope?
I do it all the time using CTs. The CTs are fine even at full power, there is really no risk to the scope either. In fact just tonight i was retuning my big coil (after a series of mishaps, finally getting it back in shape for upcomming tesla thons) simply by viewing the primary current through a 1:1000 CT. It takes 1250A at 690VDC running 6 RF cycles at ~40khz to achieve 10' sparks(just thought id make note of that). The actual input power from the converter was 4.5kW. My efficiency could go up greatly if i reduced the resistance in my primary coil (it burns up about 1400W with no secondary installed!).
What i seemed to notice tonight was that if the primary was tuned too high, the current was lower, but i couldnt get the longest sparks. If i tuned a little lower, the primary current peaked too high too fast, causing my 1250A limit to limit the sparks too much. Tuning just a half turn lower still, changed the shape of the current envelope so that i could get solid 10 foot sparks, though still limiting constantly (hey, thats what its there for). I should mention all of my tuning points were within a 1 turn area, so it can be a little picky when you go for the best possible performance.
Registered Member #55
Joined: Thu Feb 09 2006, 04:56AM
Location:
Posts: 149
It takes 1250A at 690VDC running 6 RF cycles at ~40khz to achieve 10' sparks(just thought id make note of that).
sic
Do you load down the CT with a 10 Ohm resistor like you do for the feedback CT? Another thing I noticed and I will take a look at with a CT tomorrow is when how I tune my small DRSSTC with no breakout point. it will just sit there and buzz at what I percieve as the lower pole (apprx 215Khz for this coil), I have to bump it up to about the 225Khz point for it to actually breakout and start dancing, is this becasue with tuning to the lower pole I need a higher K, something I can not have without a breakout point?
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
If you have a simple current transformer, you will need to provide a burden resistor.
This can be chosen for the proper voltage vs. power dissipation as necessary.
Remember, if you have 100A in the primary with a 1:100 turn current transformer, you'll have 1A in the secondary winding. Put a 10 ohm resistor as a burden, and you get 10V (ohms law) Of course, you'll also dissipate 10W !!!
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Yeah, the 1:1000 CT is loaded with 10 ohms, my 1:100 CTs are usually 1 ohm. Dan is right, keep the dissipation in mind, but with DRSSTCs, the dissipation is usually very low because the duty cycle is very low. I just go with a standard ratio, (either 1:10, 100, or 1000) and look for something that will work well with my scope (which only goes to 5V/div).
I have to bump it up to about the 225Khz point for it to actually breakout and start dancing, is this becasue with tuning to the lower pole I need a higher K, something I can not have without a breakout point?
Sounds logical to me. Something to keep in mind is, without a breakout point, you will force it to a higher energy level before you get streamers anyway, so it might sort of work out the same. In any case, i think most people will have to experiment with the tuning and find what works for them. All of my coils seem to tune a little different.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
To hook a scope to a DRSSTC, I've used a current transformer measuring primary current (a 33:1 with a 0.33 ohm burden) and a 100:1 high voltage scope probe looking at the output from one leg of the H-bridge. I display the CT signal on one channel of a dual trace scope and the probe signal on the other. The 100:1 probe was about $75, made by MultiContact. I originally bought it for some work I was doing on tube audio amps.
I use an odd tuning method that works with the features on my PLL driver. First I eyeball things to get the primary tap roughly right. Then I fire the thing up at a low DC link voltage. I tune across both poles and see if one draws more primary current than the other (as shown by the meter, also the coil buzzes louder if it's drawing more.) If this is the case, I turn off and adjust the tap, and repeat until I get both poles about the same magnitude. The primary and secondary are now tuned to the same frequency.
Then I add another turn or so to the primary (which would make the lower pole bigger) and retune the driver so it locks to the higher of the two pole frequencies.
wrote ... I have to bump it up to about the 225Khz point for it to actually breakout and start dancing, is this becasue with tuning to the lower pole I need a higher K, something I can not have without a breakout point?
As you tune the primary lower below the secondary, it takes more primary current to achieve a given output voltage. That's why you need to tune the coils closer to each other to get breakout.
In other words, tuning the primary lower down gives a lower voltage, higher current output from the topload, which can sometimes feed a longer spark than a closer tuning would, provided that you use a breakout point to get it started.
The above explanation assumes your feedback is driving the system at the lower pole frequency: otherwise it's reversed so that lowering the primary tuning steps the output voltage up and the current down. My PLL driven coil behaves like this, although the coils that Finn built with my drivers use the more normal setup at the lower pole frequency.
A Steve Ward style feedback driver, sensing primary current, will drive the lower pole if the primary is tuned lower than the secondary, and the upper pole otherwise. The PLL driver always drives whichever pole you choose, which makes it fun with twin coils: you can have sparks coming from either coil or both.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
wrote ...
A Steve Ward style feedback driver, sensing primary current, will drive the lower pole if the primary is tuned lower than the secondary, and the upper pole otherwise.
Not entirely correct. In either case, both poles are still excited, and this can be clearly seen using either a FFT analysis (simulation), or using a spectrum analyzer when looking at the current waveforms in the actual DRSSTC. Depending on how far you drive the primary freq above or below the natural (uncoupled) frequency of the secondary determines how much of each pole is excited, but they are both excited. In my DRSSTC III system, even though my primary is tuned well below the uncoupled resonant frequency of the secondary, there are still quite a bit of harmonics in the primary current waveform at the upper pole. Just ballparking it, its probably at 75% / 25% split from eyeballing it (linear scale, not log)
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DRSSTC tuning FAQ
Some folks just dont have the know how or equipment to make that kind of analysis and I was hoping we could create a repository here for future reference. Anyone want to takle a method of observing a running DRSSTC with a scope?
