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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Nice piece of theory Uspring.
I'd just add that we mainly care about performance with heavy streamer loads. The performance at light streamer loads only needs to be good enough to grow the streamer until it gets "heavy", without tripping the OCD.
I'd just add that we mainly care about performance with heavy streamer loads. The performance at light streamer loads only needs to be good enough to grow the streamer until it gets "heavy", without tripping the OCD.
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Uspring wrote ...
Possibly this is just what PJ is struggling with.
That is EXACTLY the case. Well, sort of.. From whats been said in this thread, it seems as if detuning ~20% allows for best arc load. At the moment, tuned to resonance with the secondary I get 41" arcs with no compensation for arc load. Tonight I will tune down a little bit, 5~10%, and see what happens. Mark my words.... I WILL get 60" outta this thing!!!
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
That is EXACTLY the case. Well, sort of.. From whats been said in this thread, it seems as if detuning ~20% allows for best arc load. At the moment, tuned to resonance with the secondary I get 41" arcs with no compensation for arc load. Tonight I will tune down a little bit, 5~10%, and see what happens. Mark my words.... I WILL get 60" outta this thing!!! [/quote1365124687]
Mark my words.... I WILL get 60" outta this thing!!!
Mads Barnkobs DRSSTC1 looks similar to your coil. He gets 60" while running with the OCD set at 500A. It still puzzles me, why there should be such a difference to yours. He uses a larger MMC than you, much as Wards DRSSTC1, which actually should increase his current requirements compared to yours.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Bennem wrote ...
Hi there, So you are using Dan McCauleys H-bridge circuit which according to his Schematic, he uses a 0.1uf cap on the output of his bridge? try removing that cap and test again to see if it makes a difference. leave the caps that bypass from either or both + and - to ground as they help bypass primary strikes.
Just my tuppence worth.
Mel.
Yes, for the half-bridge, i recommend using a 0.1uF, 2kV capacitor between the DC+ and DC- on the half-bridge and also a 0.1uF, the output of the half-bridge directly to chassis ground. This will act as a shunt and help protect the half-bridge especially from primary strikes.
Also, keep in mind, these DRSSTC 1 and DRSSTC 2 coils were designed back in the very beginning days of the DRSSTC probably around 2004??? Wow has it been that long? DRSSTC technology has evolved quite a bit since then, so it is important to recognize that and understand that even though circuits as is work very well (we used these for corporate and educational demonstration pieces for several years), there are always improvements that can be made.
One of the big differences in this coil is that the OCD circuit i designed uses time averaging for the current. Current is integrated over an entire pulse. However, the new OCD circuits I use (as well as others) monitor current on a pulse-by-pulse basis.
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
One thing I noticed when reading Mads drsstc1 page is that his primary is tuned to 64kHz which is about 8-14% detuned for his system (70-75kHz). And he also mentioned best tuning was found by tapping the primary in small increments of 5cm at a time. Those are two similarities I've noticed, since I've found best performance by tapping in increments of 1/8 turns. And things go wrong with my system when tunes below 15% or around there. As far as the current and MMC parameters go, I'm stumped too. Maybe the current will 'chill out' when I have mine more fine tuned? Probably not, who knows... I will find out later.
The puzzling aspects of my system are: the high currents, and low coupling.
EDIT: I tried detuning a bit tonight. I cant honestly say that performance necessarily decreases, but current just gets way too high for my system that I cannot find out the full potential without blowing stuff up. I will have to keep finding ways to lower current to check this out. I did get 55" tonight by increasing the OCD to about 850A and pushing the PW to 150uS. But this performance is with the primary still tuned to near exact resonance with the secondary(tuned to 76kHz), the OCD trips frequently too. The primary current waveform looks excellent though. when sparks are about 2-3' there is a slight notch and some ringing at the end of the pulse, but once I turn the variac to full power, spark length grows considerably, the notching disappears and theres no ringing, or just ringing for like one cycle. I can note that performance 'appears' to greatly decrease at 5-6% detuned (small sparks and huge currents) but like I said I cant really tell because im getting +850A with 2' or less streamers, if I could turn the power up without having to worry about exceeding 1000A and blowing another bridge, then I'm sure I could easily get over 60"... or so it seems
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Been pondering a few things said:
...The primary is tuned only a liitle bit below the secondary. This will ensure initially a good energy transfer to the secondary, but as the arc grows, the secondary frequency will shift below the primary freq und put the coil out of tune, limiting further spark growth...
...To recap, the effect of arc load on primary current is determined by a) The Lsec/Lpri ratio: A lower ratio leads to lower currents. b) The tuning: Getting the primary and secondary resonant frequencies closer together will lower the pri current. This is a moving target, since the growing arc changes the secondary resonant freq. Optimal tuning is a compromise between differing arc loads during ramp up. c) The coupling constant: The effect is somewhat involved. On the one hand increased coupling will make the arc load more "visible" to the primary, thus lowering primary current. On the other hand, it will have the effect of lowering the lower poles frequency, which is the frequency your coil operates at (if there is not much notching). This will have a detuning effect, which counteracts the intended decrease in pri current(see b). At larger arc loads this detuning effect will disappear again, but your IGBTs may have blown up before that....
See in this updated video the current waveform looks fine to me. But again, pri is tuned 1-2kHz lower then sec. So typically, this would be limiting my spark growth when in fact, it seems this is the edge of my performance. Tuning any lower and I see higher currents and smaller sparks. This is also contradictory to a lower Lsec/Lpri ratio leading to lower currents because that is the exact opposite of what I am seeing in the primary. But I'm not saying that the Lsec/Lpri ratio is the problem. Current increases drastically as I detune, which fits the case of lower currents when pri and sec are closer in tune, but higher Lsec/Lpri ratio. One would think that as spark growth occurs and secondary frequency drops into resonance with the primary, the current would, sort of, even itself out or be at a relatively 'reasonable' level. Regarding the coupling, it seems that any effect of Ipri gets cancelled out. For my system I must have a low K factor if I dont want racing sparks, at the sacrifice of higher Ipri.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Physics Junkie,
You say that when you tune lower, you get higher currents and shorter arcs. However, is that only at lower input voltages? Because that is exactly what you want to see initially.
If you are using a variac, if you are tuned considerably lower on the primary, you will initially see higher current and shorter current (or more current required to make the same length arcs as at other tuning points closer to secondary fres). But as the arc grows, it will pull the system more into tune, and the current won't change as much, but the streamer growth will really take off.
See in this updated video the current waveform looks fine to me. But again, pri is tuned 1-2kHz lower then sec. So typically, this would be limiting my spark growth when in fact, it seems this is the edge of my performance.
I would not be too concerned about the detuning of your secondary freq due to the arc. Once the arc has gotten big, it will load the secondary and lower its Q, so that the secondary resonance will become wide. That makes tuning at that point less critical.
Tuning any lower and I see higher currents and smaller sparks. This is also contradictory to a lower Lsec/Lpri ratio leading to lower currents because that is the exact opposite of what I am seeing in the primary.
During rampup, when the arc is still small, you have little arc loading so the secondary resonance peak is still quite pointed. Small changes in tuning can have a large effect. The dependency on the Lsec/Lpri ratio is much slower. You'll likely only see it, when you change your MMCs and correspondingly your Lpri to stay at the same tuning.
But I'm not saying that the Lsec/Lpri ratio is the problem. Current increases drastically as I detune, which fits the case of lower currents when pri and sec are closer in tune, but higher Lsec/Lpri ratio. One would think that as spark growth occurs and secondary frequency drops into resonance with the primary, the current would, sort of, even itself out or be at a relatively 'reasonable' level. Regarding the coupling, it seems that any effect of Ipri gets cancelled out. For my system I must have a low K factor if I dont want racing sparks, at the sacrifice of higher Ipri.
Still, low coupling could be a part of the problem. Ward quotes a value of 0.19 for his DRSSTC1 and probably yours is lower than that. It'd be interesting to know Mad Barnkobs DRSSTC1 coupling. Since you can't increase it, you could try reducing your MMC, preferably by lenghtening your strings instead of taking strings off, since that increases the current load on your caps.
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Strike rail arcs to secondary. -coupling & tuning-
