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Re: NST's shorting out.



Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Martin,

Tesla list wrote:

Original poster: MARTINFRYML@xxxxxxx
These are follow up questions are for Terry regarding your email:

Two follow up questions here. Is there a difference in the resonant cap size between running a "modified" single NST (removing some shunts and running only 15-20 sec to prevent overheating) at 120 ma versus running 2 NST's in parallel where you have two cores operating? Does the Java program take this into account?


No, it's the same. The program will compute the capacitance which is resonant with the transformer. So, for paralleled NST's, add up the currents and use that as your program input current. Now, there may be some slight differences between NST's and the actual value will be off a little high or low, but it should be pretty close.

Second, what if I run at 180 ma ?
The primary resonant capacitor size would now be 0.0398 and my smaller .03 cap would be considerably below that value. Would this be less likely to cause the resonance voltage spikes?

I think you'll still be inside a hefty resonant window. I think you would be better off in terms of keeping out of a resonant problem by going to .025uF. Think of resonance as a bell shaped curve. The center of that curve has the highest resonance and tapers off on each side from the center. The higher the resonance, the higher the voltage. Running on either side of the curve outside of resonance will keep away from resonance between NST and Cp. Even with say a static gap LTR Cp value (typically 1.6 x the resonant C value) there is still some resonance occurring and one would probably have to go beyond or below 2 x to escape it fully.

Remember, when running a small cap size, you will quickly charge the cap to the transformer output voltage. Because of this, it would be a good idea to set the presentation time somewhat near this since your using a timing system (RSG). A static gap is safer because the gap width sets the maximum voltage. In a timing system, the timing sets the maximum voltage. If for example you have a good deal of resonance factor involved, a slow timing system as compared to how quickly you charge the cap can cause the cap to charge up towards the resonant factor and overvolt the NST(s).

Besides the efficiency difference, is there any other reason to get rid of the async gap? Do you think it is contributing to the kickback?

I certainly believe so. The async is constantly rotating it's firing angle and there will be missed firings. These areas can cause the voltage to move up towards the resonance as mentioned above. NST's are sensitive to overvolting and is why async rotary's are not recommended. On beefier transformers it's usually not a problem, but again, it's all in the big picture. If you stay out of the resonance window, it may not be a big problem.

You also mentioned previously that the rotary disc was warped but that you keep it to a minimum (less than safety gap). Hopefully, a magnitude less! This actually may be a problem depending on the distance. My electrodes pass within 1/8" or less (for comparison).

It was a significant improvement to my static gap ( SG wasn't air quenched, though) so I really hate to get rid of it just yet. A synchronous gap is not on the horizon right now.

As far as another approach to eliminating the primary resonance, what if I add an extra turn to the primary coil? Then the system becomes detuned even further, with the reference cap size going down from .026 to .021. Would this make the system less prone to these resonant spikes? (Java run below):

To get the cap away from resonance, you will definitely need to change the cap size (nothing else will do it). But, of course, you will need to then consider how this affects your primary and secondary coils. You will need to make changes, but don't try to detune too high. Some coils are ok with 16%, but others don't like detuning beyond 5%. What would it take to tune nearer to the resonant frequency of the coil?

Or alternatively, if I go away from the (nice) Maxwell .03 mfd pulse capacitor I am using, I have three other caps, each 10 kv: 0.5, 0.3 and 0.3 mfd which I could run hook up in series giving me a total of .115 mfd (but only 30 kv total- I worry about that). Reducing the primary turns to 5 would make this system:

Well, I think I would stay with the Maxwell if I were you and run with a single NST. If your running two 12/60's for 12/120 operation, then your resonant at 0.028uF. As has already been pointed out, the 0.03uF is right smack in the middle of the worst place you could be (center of that bell shaped curve). My recommendation would be to use a static gap in this mode of operation until you can attain a second 0.03uF Maxwell which you can then parallel. I know that's not what you want to hear, but when it comes to killing NST's, overvolting is #1, and both async rotary's and running with a resonant sized cap will cause it. Your NST deaths are yet again proof of that.

Take care,
Bart