Anyone tried resonant NST?

[AstRii]

Hey!
I was wondering... You can see so many people making arcs with resonant MOTs, has anyone tried to do the same with NSTs?
Unlike MOTs they have very low output currents, but maybe with resonant capacitors, the arcs could be monstrous (maybe even try to parallel a few of those for higher currents? )

I have only seen very few attempts of trying resonant NST arcs, and with almost no details or information about it.
Is there any reason to why not try resonance with NSTs?

[Mads Barnkob]

You will see from SGTC builders that sizing of the MMC for such a Tesla Coil is aimed at a term called Larger-Than-Resonant and is a term to design the MMC to NOT resonate with the NST secondary inductance, as it will destroy the transformer from over-voltage.

The MOT only has about 2 kVAC output whereas the MST has 9-15 kVAC, but with wire with about the same voltage rating, the MOT is much more robust against over-voltage than the NST is.

[AstRii]

Oh, is the voltage really rising so high?
Because MOT capacitors are rated ~2.1kVAC, yet people very often use only 1 series capacitor (and 2-3 in parallel) for MOT resonance.
Why is it that the capacitors survive?

[davekni]

There are smaller NSTs for small signs with lower voltage, down to 3kV.  Those are usually lower current too (for smaller neon tube diameter), so not of much interest for SGTC or arcs.

Especially with AC use, capacitor failure isn't typically instantaneous,.  Failure occurs with localized corona or similar dielectric breakdown at field concentration points within the capacitor.  This breakdown doesn't initially extend all the way to the opposite plate, just very locally at the sharp edge or corner.  Over use time, the insulation degrades and spreads until failure.  So, capacitor lifetime is more of an exponential function of voltage than an abrupt function.

Most MOT arc experiments last no more than a few seconds or minutes.  The caps would not survive long-term use in such over-voltage conditions.

[AstRii]

Interesting..
When i find some spare time, i will measure the voltage of resonant MOT.
I'm curious

[DashApple]

I have tried it , Three 6.3Kv NST's with a 200nF resonant capacitor ( 5 MOT caps in series ) All three get warm rather quickly   

[AstRii]

That's so sick!
Thank you for sharing.

[klugesmith]

Thank you.  Reminds me how making arcs with high voltage transformers could be fun. But take it easy with the arcs if you intend to use the transformer for something more productive. :-)

How did you choose the capacitance value?

Did you happen to measure the primary current while making those arcs?   
That might benefit someone here, who recently wondered how a NST could ever draw more current than it would with short circuit on output.

[DashApple]

Thanks

I never measured the primary current when i tried this as it was just a quick setup and for sizing the capacitor it was just trial and error for the most part : )

[johnf]

Series resonant circuits can be scary
I did a job for a startup that was making a superconducting magnet for a beam scanning system.
upshot was that an audio amp of 1kW was used to sinewave drive the magnet at approx 100Hz.
the amp was connected by a 25:1 turns ratio transformer to the series tuned circuit. The capacitors required were 20kV rated Plastic capacitors inc units. yep you guessed it we did puncture a few expensive caps due to voltage over excursion.
This is the extreme case with very high Q but similar can be expected with trying to resonate NSTs albeit with much lower Q and higher drive voltage.

[Mads Barnkob]

Oh, is the voltage really rising so high?
Because MOT capacitors are rated ~2.1kVAC, yet people very often use only 1 series capacitor (and 2-3 in parallel) for MOT resonance.
Why is it that the capacitors survive?

MOT capacitors are designed for abuse. I do not have a datasheet at hand, but got a copied table of specifications on my article on a MOT capacitor bank here: http://kaizerpowerelectronics.dk/pulse-power/333-joule-moc-capacitor-bank/

They are tested with 9030 VDC for 60 seconds between terminals.

I have tried it , Three 6.3Kv NST's with a 200nF resonant capacitor ( 5 MOT caps in series ) All three get warm rather quickly   

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I would argue that these do NOT count as regular NSTs Very nice arcs, but those huge iron cores with separated windings is a whole other level from the small casted resin units seen outside on buildings.

[johnnyzoo]

 

Series resonant circuits can be scary
I did a job for a startup that was making a superconducting magnet for a beam scanning system.
upshot was that an audio amp of 1kW was used to sinewave drive the magnet at approx 100Hz.
the amp was connected by a 25:1 turns ratio transformer to the series tuned circuit. The capacitors required were 20kV rated Plastic capacitors inc units. yep you guessed it we did puncture a few expensive caps due to voltage over excursion.
This is the extreme case with very high Q but similar can be expected with trying to resonate NSTs albeit with much lower Q and higher drive voltage.

Hmmm... superconducting tesla coil, anyone?

[abstruse1]

I hope this question is not a hijacking of this thread. It seems to me that these truly spectacular flame arcs are not the result of especially high voltage. You’ll notice that the spoon had to almost touch the other terminal to get the arc started. One interpretation of this is that the voltage is pretty low but the power is very high, easily ionizing a lot of air which gets the arc growing. Or maybe the resonance phenomenon (high voltage) doesn’t begin until the current starts flowing, after which it takes over, big-time.

 What am I missing?

[AstRii]

You are right, the voltage doesn't have to be high to create large arcs.
This is my setup with multiple MOTs and resonant capacitors:

This setup produces ~4kV. The electrodes need to be millimeters apart to start the arc.
But the higher current creates much more conductive plasma channel which is easier to sustain over big distances.

With this setup I was able to stretch over a meter long arc with only a few kV (but more than 16kVA of power).

Even though most NSTs generate larges voltages (up to 12kV) their power is much lower (~400VA) and therefore create only "small" arcs.

In general, if you want bigger output, you need bigger power. You cannot magically create large hot arc with a few Watts of power. 
Theoretically you don't even need high voltage to make huge arc.