DRSSTC Tuning Study - Initial Results
HV Enthusiast, Tue Jun 13 2006, 06:04PMI’m presently performing a detailed analysis of the tuning characteristics of a DRSSTC system. I’m still in the early phases of this particular task, but have come up
with some initial data of Primary Current and DC Bus Voltage vs. Arc Length for various tuning points in a DRSSTC system.
This data was collected at 9 separate tuning points (1/4 turn increments) on the primary coil of a small DRSSTC system. A strike target was placed beginning at 0.25†and increased
in two inch increments up to the maximum arc the system was capable of. For this particular system, 32†was the maximum arc, and was limited by either the active current limiting
circuit which was set to about 550A and/or secondary-to-primary arcing at which time the experiment was ceased.
Natural Frequency of Secondary with 13†x 4†toroid = 198kHz (Measured)
Coupling: Approx. 0.18 (estimated)
Primary Tuning Range (9.2uH to 5.3uH)
Natural Frequency of Primary Circuit (w/ 0.108uF MMC) (Measured)
9.2uH = 160kHz
7.9uH = 173kHz
7.7uH = 175kHz
7.2uH (I) = 180kHz
7.2uH (II) = 180kHz
6.2uH = 196kHz
5.8uH = 200kHz
5.6uH = 205kHz
5.3uH = 210kHz
Please note, there are two 7.2uH, but they are actually ¼ turn from one another. There was some fluctuation do to wire length / positioning of the connection wires which attributed to this.
These inductance value were measured exactly how the wires would connect to the full-bridge.
Initial Data:
Haven't had time to analyze the data yet, but in the first graph, you can clearly see the exact point where the arc length drastically affects the performance of the system due to tuning / detuning. For the higher number of turns (larger inductance), you can see the peak primary current begin to level off. It is at this point where the added C of the arc brings the system "into better tune" and allows arc length to grow considerably. And likelise, at the lower number of turns, you can see the points where arc length capacitance throws the system out of tune and these are the points on the curves where additional arc length requires increasingly more current.
Peak Primary Current vs. Arc Length
http://www.easternvoltageresearch.com/tuningstudy01.jpg
DC Bus Voltage vs. Arc Length
http://www.easternvoltageresearch.com/tuningstudy02.jpg
Re: DRSSTC Tuning Study - Initial Results
ragnar, Tue Jun 13 2006, 07:47PM
very nice, EVR =)
are there any pleasant surprises (curves) to be had by setting the current limiting at e.g. 650A and doing further tests?
ragnar, Tue Jun 13 2006, 07:47PM
very nice, EVR =)
are there any pleasant surprises (curves) to be had by setting the current limiting at e.g. 650A and doing further tests?
Re: DRSSTC Tuning Study - Initial Results
Steve Ward, Tue Jun 13 2006, 08:07PM
Given the trend at say 9.2uH, you only needed 300VDC to get the "max" spark length for this test. If you opened up the current limit to 600A maybe, then you would likely see the longest possible streamers at the maximum 400VDC input. This is sort of obvious, but it shows the benefits of detuning the primary lower on smaller coils when you are limited by input voltage or tank surge impedance.
What were the burst lengths during these tests? roughly the same? Or was it all over the place? How about PRF?
Anyway, nice tests. I did similar ones awhile back, but not in as much detail. I focused more on burst length, input power (real watts), primary current, buss voltage, coupling, and achieving 2 or 3 benchmark spark lengths to compare the data (24", 30" and 36" for my smaller coil). I found that detuning was slightly less efficient (lower "Freau" number) but gave the longest possible sparks from the system. I also found that lowering the coupling had a similar effect to detuning the system somewhat, though not quite as efficient. I did not set my current limiter to inhibit performance, so the lower tunings ran maybe 20% higher primary currents, but gave probably 20% longer sparks. When performing similar tests on my larger DRSSTC systems, i found that the detuning was not required, and less RF cycles per burst could achieve very long sparks, and also maintain a high Freau number (usually around 1.8-2.2). It would be nice to have a place where i could test my coils for hours on end and do more of these tests.
Steve Ward, Tue Jun 13 2006, 08:07PM
Given the trend at say 9.2uH, you only needed 300VDC to get the "max" spark length for this test. If you opened up the current limit to 600A maybe, then you would likely see the longest possible streamers at the maximum 400VDC input. This is sort of obvious, but it shows the benefits of detuning the primary lower on smaller coils when you are limited by input voltage or tank surge impedance.
What were the burst lengths during these tests? roughly the same? Or was it all over the place? How about PRF?
Anyway, nice tests. I did similar ones awhile back, but not in as much detail. I focused more on burst length, input power (real watts), primary current, buss voltage, coupling, and achieving 2 or 3 benchmark spark lengths to compare the data (24", 30" and 36" for my smaller coil). I found that detuning was slightly less efficient (lower "Freau" number) but gave the longest possible sparks from the system. I also found that lowering the coupling had a similar effect to detuning the system somewhat, though not quite as efficient. I did not set my current limiter to inhibit performance, so the lower tunings ran maybe 20% higher primary currents, but gave probably 20% longer sparks. When performing similar tests on my larger DRSSTC systems, i found that the detuning was not required, and less RF cycles per burst could achieve very long sparks, and also maintain a high Freau number (usually around 1.8-2.2). It would be nice to have a place where i could test my coils for hours on end and do more of these tests.
Re: DRSSTC Tuning Study - Initial Results
HV Enthusiast, Tue Jun 13 2006, 09:20PM
Thanks. These tests were all done at constant PRF (100Hz) and 300us pulsewidth. I will be doing additional studies later with varying PRF and pulsewidth, although i really don't see any arc length improvement above around 100us.
As far as current limit goes, i am probably going to stick around 500A or so for limit as thats about the reliabilty limit i have found in my experience for these devices (above that, and reliability decreases considerably, again in my experience with these devices)
My original goal for this system was 30", and it can easily do that, so i'm pretty satisfied. I will now probably choose a tuning point somewhere midway between the curves to keep current down to manageable levels etc... All a trade-off of course.
On a separate note, best spark length so far for this coil was about 4 feet so far, but with no current limiting. Of course, probability for strikes on base or primary are quite high in this mode and without current limiting, you are asking for trouble.
If i have time, i may run some additional iterations tonite with even more primary inductance and also probably increase the current limit a bit and see what goes . . .
UPDATE: Decided to relax the current limit a bit (up to 700A), and maximum arc length now is easily hitting 48", although i do get nervous a bit running this thing at 600-700A peak current. . .
HV Enthusiast, Tue Jun 13 2006, 09:20PM
Thanks. These tests were all done at constant PRF (100Hz) and 300us pulsewidth. I will be doing additional studies later with varying PRF and pulsewidth, although i really don't see any arc length improvement above around 100us.
As far as current limit goes, i am probably going to stick around 500A or so for limit as thats about the reliabilty limit i have found in my experience for these devices (above that, and reliability decreases considerably, again in my experience with these devices)
My original goal for this system was 30", and it can easily do that, so i'm pretty satisfied. I will now probably choose a tuning point somewhere midway between the curves to keep current down to manageable levels etc... All a trade-off of course.
On a separate note, best spark length so far for this coil was about 4 feet so far, but with no current limiting. Of course, probability for strikes on base or primary are quite high in this mode and without current limiting, you are asking for trouble.
If i have time, i may run some additional iterations tonite with even more primary inductance and also probably increase the current limit a bit and see what goes . . .
UPDATE: Decided to relax the current limit a bit (up to 700A), and maximum arc length now is easily hitting 48", although i do get nervous a bit running this thing at 600-700A peak current. . .
Re: DRSSTC Tuning Study - Initial Results
Part Scavenger, Wed Jun 14 2006, 02:44AM
Uh... MY HOUSE!!! Come right over anytime.
Part Scavenger, Wed Jun 14 2006, 02:44AM
It would be nice to have a place where i could test my coils for hours on end and do more of these tests.
Uh... MY HOUSE!!! Come right over anytime.
Re: DRSSTC Tuning Study - Initial Results
Desmogod, Wed Jun 14 2006, 04:26AM
How do you measure these and come to these figures?
Desmogod, Wed Jun 14 2006, 04:26AM
EastVoltResearch wrote ...
Primary Tuning Range (9.2uH to 5.3uH)
Natural Frequency of Primary Circuit (w/ 0.108uF MMC) (Measured)
Primary Tuning Range (9.2uH to 5.3uH)
Natural Frequency of Primary Circuit (w/ 0.108uF MMC) (Measured)
How do you measure these and come to these figures?
Re: DRSSTC Tuning Study - Initial Results
HV Enthusiast, Wed Jun 14 2006, 12:44PM
These were measured with network / impedance analyzer.
However, you can measure this with an oscilloscope and signal generator if you have them.
HV Enthusiast, Wed Jun 14 2006, 12:44PM
wrote ...
How do you measure these and come to these figures?
How do you measure these and come to these figures?
These were measured with network / impedance analyzer.
However, you can measure this with an oscilloscope and signal generator if you have them.
Re: DRSSTC Tuning Study - Initial Results
JimmyH, Thu Jul 06 2006, 07:40AM
Hey,
I finally got around to checking out your results, and had a few comments.
1. Why stop at 9.2uH? It seemed to be the best of all the values, and also the lowest value tried. I would have tested values on both the low side and the high side to ensure the optimum one is in the data set. The amp/inch graph appears to steepen at the end a bit, implying that a lower value might work even better.
2. The Amp/inch curve was done with a fixed pulse width, and fixed tank Z (duh), so that means the DC rail voltage must have been changing. Same idea goes for the volt/inch curve. Since the secondary cant differentiate between your tank, and a tank with 4x the Z and half the current, it would be nice to have a "peak volt*amps/inch" chart. That would be helpful if you know your max DC voltage, and the max current you're willing to push.
3. Going along with the second point, a Z/(V*Apk) curve would help in choosing your primary LC impedance knowing your max VDC and Apk
If the information was rearranged in this fashion, it'd be easy to pick out the % detuning that gives the lowest VA/inch, and then find the capacitor value that corresponds with that % detuning at your chosen peak VA.
There are too many dimensions to cover everything (havent touched ON time, and the various resonator characteristics like fres, height width, etc), but this should be a very good start for coils around this size.
If you don't want to add this to your paper, I'd like to play around with the data myself. Could you send me your excel file?
Thanks
JimmyH, Thu Jul 06 2006, 07:40AM
Hey,
I finally got around to checking out your results, and had a few comments.
1. Why stop at 9.2uH? It seemed to be the best of all the values, and also the lowest value tried. I would have tested values on both the low side and the high side to ensure the optimum one is in the data set. The amp/inch graph appears to steepen at the end a bit, implying that a lower value might work even better.
2. The Amp/inch curve was done with a fixed pulse width, and fixed tank Z (duh), so that means the DC rail voltage must have been changing. Same idea goes for the volt/inch curve. Since the secondary cant differentiate between your tank, and a tank with 4x the Z and half the current, it would be nice to have a "peak volt*amps/inch" chart. That would be helpful if you know your max DC voltage, and the max current you're willing to push.
3. Going along with the second point, a Z/(V*Apk) curve would help in choosing your primary LC impedance knowing your max VDC and Apk
If the information was rearranged in this fashion, it'd be easy to pick out the % detuning that gives the lowest VA/inch, and then find the capacitor value that corresponds with that % detuning at your chosen peak VA.
There are too many dimensions to cover everything (havent touched ON time, and the various resonator characteristics like fres, height width, etc), but this should be a very good start for coils around this size.
If you don't want to add this to your paper, I'd like to play around with the data myself. Could you send me your excel file?
Thanks
Re: DRSSTC Tuning Study - Initial Results
Sulaiman, Thu Jul 06 2006, 09:16AM
Dan,
Nice work.
Thanks for sharing the data,
Aren't TCs amazing?
Such a simple concept - so many practical considerations.
I'm finaly starting a drsstc
(based on my stock items such as 30N60 as I have no spare cash)
(The secondary is 6.2" dia x 28" high, 1800 turns, 0.1H, Fres 82 kHz with topload)
I'm not asking you to design my TC for me, but I do have a question-
How do you choose the primary impedance?
e.g. 55.5uH//68nF vs 5.5uH//680nF
Lower impedance for rapid ringup and high volts/turn but low "Q"
Higher impedance for more kVAR in the tank for a given maximum current.
There must be an 'optimum' primary impedance
but where do I start?
Sulaiman, Thu Jul 06 2006, 09:16AM
Dan,
Nice work.
Thanks for sharing the data,
Aren't TCs amazing?
Such a simple concept - so many practical considerations.
I'm finaly starting a drsstc
(based on my stock items such as 30N60 as I have no spare cash)
(The secondary is 6.2" dia x 28" high, 1800 turns, 0.1H, Fres 82 kHz with topload)
I'm not asking you to design my TC for me, but I do have a question-
How do you choose the primary impedance?
e.g. 55.5uH//68nF vs 5.5uH//680nF
Lower impedance for rapid ringup and high volts/turn but low "Q"
Higher impedance for more kVAR in the tank for a given maximum current.
There must be an 'optimum' primary impedance
but where do I start?
Re: DRSSTC Tuning Study - Initial Results
HV Enthusiast, Thu Jul 06 2006, 12:47PM
I usually start by selecting primary inductance to make primary and secondary tuned circuits equal. Then i add a few additional turns for good measure and then just play around with tuning until i get a good point (as in this study)
For tuning, i would use cardboard cut-outs and a temporary primary.
I don't worry about such things as Q, ring-up, etc...
HV Enthusiast, Thu Jul 06 2006, 12:47PM
I usually start by selecting primary inductance to make primary and secondary tuned circuits equal. Then i add a few additional turns for good measure and then just play around with tuning until i get a good point (as in this study)
For tuning, i would use cardboard cut-outs and a temporary primary.
I don't worry about such things as Q, ring-up, etc...
Re: DRSSTC Tuning Study - Initial Results
Steve Conner, Thu Jul 06 2006, 02:55PM
I just design the primary for a Q of about 10. So I choose the tank capacitor to have a reactance such that when the IGBTs are delivering the maximum current that I want them to, the voltage across the cap is about 10 times (4/pi) times the DC bus voltage, ie about 4-5kV if you're running off 300V. For the calculation, I assume the frequency to be the resonant frequency of the secondary, which isn't quite exact, but good enough.
I then choose the primary inductance so it resonates at the right frequency, which seems to be about 10% lower than whatever the secondary is tuned to.
I then set my current limiter to the maximum current I chose above, and fiddle with the primary tapping till I get the biggest sparks without tripping the limiter. Adjusting the primary tapping can actually change the ratio of output voltage to primary current (the "transimpedance") over quite a wide range, so it leaves a lot of leeway as regards matching the streamer load. I've proven this mathematically, but by accident when I was trying to work out something else :^)
Steve Conner, Thu Jul 06 2006, 02:55PM
I just design the primary for a Q of about 10. So I choose the tank capacitor to have a reactance such that when the IGBTs are delivering the maximum current that I want them to, the voltage across the cap is about 10 times (4/pi) times the DC bus voltage, ie about 4-5kV if you're running off 300V. For the calculation, I assume the frequency to be the resonant frequency of the secondary, which isn't quite exact, but good enough.
I then choose the primary inductance so it resonates at the right frequency, which seems to be about 10% lower than whatever the secondary is tuned to.
I then set my current limiter to the maximum current I chose above, and fiddle with the primary tapping till I get the biggest sparks without tripping the limiter. Adjusting the primary tapping can actually change the ratio of output voltage to primary current (the "transimpedance") over quite a wide range, so it leaves a lot of leeway as regards matching the streamer load. I've proven this mathematically, but by accident when I was trying to work out something else :^)
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