Hi Dan,If the safety gaps are set properly, they should seldom fire in a stable LTR system. If your safety gaps are firing frequently, partial cancellation of the NST current limiting function and main resonance are likely occurring.
There are two types of LTR approaches: standard LTR and inductive kick LTR. A good discussion of standard versus inductive kick LTR systems (versus classical mains resonant systems) can be found in the archives:
https://www.pupman.com/listarchives/2000/August/msg01085.htmlA regular LTR system just barely charges the tank cap to the maximum rated output voltage of the NST twice on each mains cycle. Either a static or rotary gap works well with this setup. The tank cap is ideally sized to 1.57 times the mains resonant value, so for your 15/120 system, this would be about 33.3 nF.
Because of the heavier capacitive load on the NST, the size of the PFC capacitor bank can be significantly reduced or PFC can be eliminated entirely. A standard LTR setup with a much smaller PFC bank is the configuration I would recommend. The PFC bank may, in fact, be causing part of the problems you're seeing.
If you want to use an inductive kick LTR setup, the tank cap size can be estimated using the following formula:
C = 0.83*I/(BPS x V) Where: C = LTR Tank cap (Farads) I = NST bank short-circuit output current (Irms, in amperes) V = NST output voltage (Vrms) BPS = Break rate (2X mains Hz or 120 BPS for your system)For your 15/120 NST bank the ideal LTR size works out to be about 55 nF or about 2.6 times the resonant cap size.
However, there are a couple of significant disadvantages using an inductive kick LTR system. It requires a properly phased SRSG where the gap fires 3-4 milliseconds after zero crossings. More importantly, the SRSG can be difficult to "start". The tank cap will only charge up to about 30% of the NST open-circuit peak voltage if the gap doesn't initially fire. However, if the gap does start firing, then the tank cap voltage will significantly increase to near the level of a standard LTR setup. This occurs as energy (stored in the magnetic field in the NST's secondary inductance) provides an inductive "kick" that boosts tank capacitor voltage. But this can only occur IF the gap starts firing at the right time.
If your rotary gap is configured to have a total of four gaps in series (two sets of gaps located 180 degrees apart on the disk), the total SRSG breakdown voltage may be too great to "start" your LTR system. This sounds like the problem you encountered when trying to drive your Maxwell 60 nF cap. To alleviate this, you may want to use only ONE set of stationary electrodes (i.e., only 2 total gaps). This will allow you to maintain adequate mechanical clearance while also allowing the gaps to initially fire at the reduced starting voltage in your LTR system.
I don't recommend an inductive kick LTR approach since its benefits may not justify the added setup complexity and gap firing difficulties.
Hope this helps and good luck! Bert Daniel Kunkel wrote:
Thank you all for the input so far! 1) Bert: In your opinion, with a SRSG/NST setup, how often should a safety gap pop? 10% of the time, or less? Based on what you are saying, I'd say the resonant rise symptoms match my situation. I can see the safety gap preventing things from 'rise' escalating, but that doesn't seem like a permanent fix? I like your previous suggestion of removing the PFC caps and starting over...but I am guessing the BPS and capacitance are not happy...so ultimately do I need to re-size the MMC? What do you suggest on sizing? 2) Gary: I am glad you brought this up. I had initially removed the MMC and swapped in my Maxwell .06 uF cap, but no amount of fiddling could get the gap to reliably fire, let alone produce streamers. I have seen situations like this before, and it was always due to the tank cap being too large. I have no idea how your 15/60 is charging and firing a .04 uF cap! For my NST setup with 120 BPS, JavaTC suggests a .0553 uF cap. For you, it suggests a mere .0277 uF. To answer your question, 'I suspect that you were unable to stop the safety gap from firing so you opened it up to where it stopped?', the answer is YES, you are correct. My cap options are MMC (100 caps of .068 @ 2,000 VDC) or Maxwell pulse cap, .06 uF @ 50,000 volts)...what do you suggest I try? 3) Phil: Thanks for digging that out of the archives! I always love reading material from Richie. I can't wait get a real transformer one day (it WILL happen) and then I can leave the NST's behind. I think this all confirms what everyone is saying...my tank cap needs to be altered. 4) To anyone else still reading... Here is a pic of coil running...I did manage to take a quick video and here is a single frame. By scaling on the photo (secondary is 26.5" inches of winding), the lower streamer alone is 62+" from the breakout point to the end of the frame. It is 72" from the break out straight down to the ground, and this streamer ALMOST hit the ground at an angle, so I'd say it was right around the the 6 foot mark. The amp meter was showing 40 amps...4.8 kVA YIKES! https://imgur.com/a/xQKOpbx ~Dan Kansas City area
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