Rotary Spark Gap Arcing Not Sustained (Solved)

[tbbrune]

Hello all, I'm building a Tesla coil with a 12kV, 30mA NST and a asynchronous rotary spark gap. I'm also using an MMC with some 0.1uF metallized polypropylene caps from KEMET. I'm currently working on testing the spark gap and trying to get sustained arcing (i.e it doesn't just arc a couple times and stop) but to no avail. Any time I start the coil the spark gap will go, sometimes for up to 3 seconds, before inevitably stopping. Stopping and starting it again gets it to begin arcing again, but obviously i shouldn't be needing to constantly toggle a switch to get it to fire. I'm currently using an angle grinder and a variac for the rotary spark gap, but previously tried using a static spark gap as well and encountered the same issue. I have the spark gap wired in parallel with a Terry filter and the MMC in series with the primary. I'm using galvanized steel hex bolts mounted on a bakelite wheel for the spark gap conductors and insulated copper wire for all the wiring. I can adjust the gap by turning the stationary bolts on either side of the wheel so the gap between them is quite small, so I don't think the distance between them is a problem. The power supply I'm using is an older NST from the late 80s and early 90s and it works perfectly fine. Please note this is my first Tesla coil and I am a novice at coiling so keep that in mind. I can provide additional information and pictures if needed.

[Hysteresis]

Just a guess. Maybe the NST has a ground fault circuit in it. And cycling power resets the fault. You need to safely test for high voltage when the gap stops firing.

[tbbrune]

Just tested it this morning using a divider and there is still voltage coming from the power supply during these 'stops'. Any other ideas of what it could be? At present I only have the primary coil hooked up in series with the cap bank and in series with the power supply and in parallel with the spark gap for testing purposes. Interestingly, it arcs perfectly fine when the cap bank isn't hooked up (just at a much lower volume and intensity lol)

[davekni]

If I understand correctly, spark gap is in parallel with NST output and you are measuring 12kV across NST output while gap is failing to fire.  Is that accurate?  Or is the voltage lower due to load of MMC+primary?  That would be one possibility, that MMC+primary lowers NST output voltage enough that it doesn't arc across spark gap distance.
If voltage is still close to 12kV, only two other possibilities come to mind:
1) Gap distance is too wide to reliably fire at 12kV.  A bit of oxide buildup or whatever crosses the threshold from working to not.
2) Asynchronous rotation is somehow locking to line frequency, making fire times close to 0V from NST.

[tbbrune]

If I understand correctly, spark gap is in parallel with NST output and you are measuring 12kV across NST output while gap is failing to fire.  Is that accurate?  Or is the voltage lower due to load of MMC+primary?  That would be one possibility, that MMC+primary lowers NST output voltage enough that it doesn't arc across spark gap distance.
If voltage is still close to 12kV, only two other possibilities come to mind:

Thanks for the response davekni, your understanding of the layout is correct. Actually now that you mention it I did notice the voltage was substantially lower coming out of the NST when the spark gap was failing to fire than under no load, so that's an interesting point. It's odd to me though that it would work briefly and then stop as if the voltage drop gets larger over time. The gap always works for a short time and then stops after half a second or so.

[Hysteresis]

I think davekni is right, the MMC might be loading down the NST. What is your total cap value? For that transformer it should be about 10nf I think. NSTs are soft transformers, the more you load them the lower the voltage drops. A regular transformer is not like that. The more you load it the higher the current gets and the voltage output stays about the same until it burns up.   

[davekni]

As Hysteresis points out, I think your primary (pun intended) issue is low primary impedance.  Lower capacitance and higher inductance will correct that.

It's odd to me though that it would work briefly and then stop as if the voltage drop gets larger over time. The gap always works for a short time and then stops after half a second or so.

I have no personal experience with this, but can make a guess.  Depending on line voltage at the instant of transformer connection to line power (switch turned on), transformers pull a large initial transient current.  That may lead to enough voltage to start spark gap.  Then resonant charging works for a while until timing of spark gap hits a low spot in line voltage.  Resonant charging is when the flyback behavior of transformer secondary inductance and MMC capacitance resonate for a half cycle between spark gap closures.  If no losses, could hit 2x normal transformer output voltage with resonant charging.  (I used resonant charging on my SRSGTC to boost voltage.)

[tbbrune]

As Hysteresis points out, I think your primary (pun intended) issue is low primary impedance.  Lower capacitance and higher inductance will correct that.

On the flip side would it be feasible to design a power supply to work with the existing system using MOTs or something similar? The NST I managed to get for free and MMCs are quite expensive to build (I dropped a decent amount on the one I've already got and it's resonant tuned with the secondary LC circuit based on my testing). NSTs also just seem to be more trouble than they're worth with how sensitive they are.

[Hysteresis]

NSTs are OK to use. What is your mmc cap value?

[tbbrune]

NSTs are OK to use. What is your mmc cap value?

It's 27uF but it's 3 strings of 9uF in parallel so I guess bringing it down wouldn't be an issue.

[Hysteresis]

OK, that's to high. Should .01uF or lower for that transformer.

[tbbrune]

OK, that's to high. Should .01uF or lower for that transformer.

Whoops that was a typo, it's 27nF total

[Hysteresis]

Try something 3 times lower. Everything will have to be tuned again. Fun!

[davekni]

On the flip side would it be feasible to design a power supply to work with the existing system using MOTs or something similar?

Find two more similar NSTs and parallel them.  Would likely be about right for 27nF.
Yes, series-connected MOTs can work.  But it is difficult to insulate them for series connection.  One lead of secondary is normally connected to core.  Can be disconnected, but insulation between inside layer of secondary and core often can't handle the voltage needed for series connection.  May be possible with carefully placed epoxy or other insulation.  I made an SRSGTC using two MOTs.  If ground is center-tapped, then two each direction might work.  Other issue with MOTs is that leakage inductance is lower than for NSTs.  (Mine was low voltage high current, 0.7uF MMC into 3-turn primary.  Also, I added external inductors in series with MOT primaries to increase inductance.)  May be OK presuming your rotary spark gap opens fast enough (high velocity at arc points).
MOTs are much more dangerous, though parallel NSTs can be too if current capability gets towards often-lethal values (200mA or so).

[thedoc298]

Been following this thread, could you post a picture of your setup...? Thanks

[tbbrune]

Try something 3 times lower. Everything will have to be tuned again. Fun!

Thanks for all the help, I really appreciate everyone's contributions!! I managed to get the spark gap working today after I dropped the capacitance to 9nF and it runs like a charm. Now I need to get around to retuning the dang thing haha

[Hysteresis]

Don't run your system without the spark gap motor running. Because you could be near 60/50Hz resonance with your new capacitor value and the inductance of the transformer. This will a cause rise in voltage across the NST and kill it! This happened to me once and the NST died. I was measuring 17kv across my 12kv NST. So start the motor first.