Can anyone explain.

[thedoc298]

 I forgot to add that these were not running resonant capacitor but almost double at 56 nf. As an example, I have one 15000/30 ma neon transformer. Manufacture says it draws something like 3.5 or so amps with the two ends shorted together. I short the ends together and it draws really close to what the manufacture says.

With a power factor cap of 80uf across the ac inputs and the high voltage shorted, the amp draw drops to below 1 amp, disconnect one leg of the cap, and amp draw goes back to 3.5 amp. This is normal.

With 4 of the above transformers all paralleled and each with 80uf cap across the inputs I get the same reduction of current, time 4


Hear is what I don't get, with the 4 paralleled transformers with pf correction in my working coil, the amp draw easily pulls 15 to 20 amps at 70 or 80 volts off the 30 amp variac and peaks over 30 amps if I slam the variac to higher voltage.

The question, with 4 transformers shorted how is the amp draw going above the dead short rating of the transformers. Like if you short a transformer, you can't get any more current than the dead short....?

The only thing I can come up with and have to test out, is removing the variac.     

[klugesmith]

You didn't identify your load that is not a "dead short".

What if you draw a NST equivalent circuit with the current-limiting effect modeled as a discrete L in series with secondary.
There are capacitive loads that draw much more current (primary and secondary) than a short circuit.

Here's a model I drew a few months ago.  No time now to explain details.  There are probably better ones somewhere in this forum or other tesla coil forums.

[thedoc298]

As an example, I have one 15000/30 ma neon transformer. Manufacture says it draws something like 3.5 or so amps with the two ends shorted together. I short the ends together and it draws really close to what the manufacture says.

With a power factor cap of 80uf across the ac inputs and the high voltage shorted, the amp draw drops to below 1 amp, disconnect one leg of the cap, and amp draw goes back to 3.5 amp. This is normal.

With 4 of the above transformers all paralleled and each with 80uf cap across the inputs I get the same reduction of current, time 4


Hear is what I don't get, with the 4 paralleled transformers with pf correction in my working coil, the amp draw easily pulls 15 to 20 amps at 70 or 80 volts off the 30 amp variac and peaks over 30 amps if I slam the variac to higher voltage.

The question, with 4 transformers shorted how is the amp draw going above the dead short rating of the transformers. Like if you short a transformer, you can't get any more current than the dead short....?

The only thing I can come up with and have to test out, is removing the variac.   

This is what I have, The rotary gap shorts the transformer 120 times a second and the rest of the time is spent charging the cap and waiting for the next discharge.
I don't understand what was posted above, but still looks impossible with this to draw more than the manufactures stated amp draw. Anyone with a explanation I can understand. Thanks for your answer, i just don't get it.

[acmq]

Power factor correction is to be done to compensate the no load reactance (magnetizing inductance) of a transformer. Operation at some particular power level can be considered too. If the compensation is done at the worst case of the output short-circuited, at any other condition the current through the capacitors dominates the input current.

[klugesmith]

Could you believe that the NST secondary current equals no-load secondary voltage divided by the sum of (load impedance + transformer output impedance)?  It's nominally about 30 mA when load impedance is zero.
You might think the current can't get any higher than that, because load impedance can't be less than zero. Wrong! 
Time to learn some 19th century electrical engineering concepts.
With alternating current, those impedances are vector quantities. 
If load is a capacitor, its impedance in series with the transformer impedance can have magnitude much smaller than either term by itself.

I just simulated my model after replacing the diodes (from somebody's cap charger application) with a capacitor directly between Vp and Vn.  As expected, the transformer primary current blows up, to many tens of amperes, as the load C value approaches resonance at 60 Hz.
At about 5.5 nF, with the parameters in my model.

Here is an experiment you can do with parts on hand, and no sparks or very high voltage.
Start with a single NST, output shorted, and turn your variac up enough to get 1 ampere of primary current.
What's the primary current when NST output is open (and same variac setting)?
How about with NST connected directly to your MMC (no spark gap, tesla coil primary, etc.)?
Repeat with different capacitive loads, like intermediate taps in your MMC (which typically give you C values higher than the whole),
or MMC with one or more parallel branches disconnected.

[thedoc298]

Power factor correction is to be done to compensate the no load reactance (magnetizing inductance) of a transformer. Operation at some particular power level can be considered too. If the compensation is done at the worst case of the output short-circuited, at any other condition the current through the capacitors dominates the input current.

I have 80uf on each transformer and that improved a lot but will still suck up lots of juice and don't know where the extra is going. I did testing with 80uf and with the output of the neon shorted was when the least current was used. The 4 transformers with the caps use the current expected, but in the working coil, the amp draw jumps to 30 to 40 amps. This makes for nice display.

On the floor the 4 transformers dead shorted with 80uf pf correction pull about 4 to 5 amp, in the coil  up to 40 amp. I just don't want each transformers primary being over drawn.

I will do that testing tonight, I know you are right, I just want to get my mind around it. Sound like some fun testing. Thanks, will report back.

[johnf]

If you are really interested then you could buy

https://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/eval-ade9153b.html#eb-overview

gives all the info you need when driving your NST

[thedoc298]

So after all said and done, is it possible to run the 4 transformers at full voltage. The only reason I brought this up, is because I have never heard anyone complaining that they could not run there transformers at full voltage. Most even talk about the little extra voltage they gain from there veriac. I have just been running the transformers at a amp drawl of 16 amps to be safe. Seems like im getting getting good performance at the lower voltage

Is it even possible to run them at rated 120 volts via more or less power factor, or changing the recommended primary cap.
Just seems strange that I can't find anyone who experience the same problem. Guess just need to find a small pig.
Thank everyone who took time to show me the light. This is my first coil.

[thedoc298]

I found something that makes sense with high mains current being used by the neon transformers.

This was found on Gary Lua's web page http://www.laushaus.com/tesla/index.htm

A late and important footnote -

Recent discussions on the Pupman Tesla List now suggest that neon sign transformers exhibit a non-linear behavior when primary voltages are used near maximum and are used to charge capacitors. The result is that far greater currents than normal are passed through the NST's secondary (and primary), resulting in far greater power being processed. This is believed to be due to the NST's current shunts saturating, and somewhat circumvents their current limiting ability. This effect may be more pronounced when using larger than mains-resonant sized capacitors. While some suggest that caution be exercised so as not to burn out the NST, I think this is a godsend, and I finally have an explanation for the much hotter than expected operation of my protection resistors.

The simulations above do not take this behavior into account, so the optimal cap values suggested by these simulations may or may not be valid. Sorry to have dragged you through all of this only to tell you this now, but isn't that what makes coiling so interesting?

 

Back to Gary Lau's main Tesla Coil Page

[thedoc298]

Up date: was only able to run the 4 parallel 15000/30 transformers at 70 to 80 volts, this was about 15 amps. If I ran higher than that voltage, the transformers pulled 40 + amps


 Built me a nice 30X7 toroid and the problem went away, now can crank the variac  up to 120 (thats where it is wired for now).  Amp draw is about 17 amps, but the voltage is kick ass now and im very happy with the proformance.

The bottom is the old toroid.

[davekni]

Great to hear that you have it working well now!

I'd guess that your earlier high-current events were due to resonant charging with sparks at some harmonic of line frequency.  Resonant frequency depends on current (input voltage) as the iron shunts start to saturate.  (My SRSGTC is designed for resonant charging at 120Hz.  But, if the rotary spark gap gets out of phase much, current goes way too high and transformer cores hard-saturate.  With a static spark gap, I'd expect that to occur more often.  Before breaker tripping, the high power does make higher voltage, so would trigger the spark gap to keep running in that high-power phasing.)

[MRMILSTAR]

Nice looking toroid. This looks like an outdoor-only operation coil.

[thedoc298]

Nice looking toroid. This looks like an outdoor-only operation coil.

LoL, yes on the outside. Started was supposed to be a desktop, then a garage model and now strictly outside. That toroid made a big difference. I will post some video clips before and after.

[Mads Barnkob]

Great to hear that you have it working well now!

I'd guess that your earlier high-current events were due to resonant charging with sparks at some harmonic of line frequency.  Resonant frequency depends on current (input voltage) as the iron shunts start to saturate.  (My SRSGTC is designed for resonant charging at 120Hz.  But, if the rotary spark gap gets out of phase much, current goes way too high and transformer cores hard-saturate.  With a static spark gap, I'd expect that to occur more often.  Before breaker tripping, the high power does make higher voltage, so would trigger the spark gap to keep running in that high-power phasing.)

Ready through the initial problems and the discussion, my first thought was also resonance with the NST secondary, this is why MMC design for SGTC/RSGTC has a term coined Larger-Than-Resonant or LTR for short.

Check out: http://classictesla.com/home/javammc/ and do some calculations on it, the last two results are for SGTC or RSGTC LTR MMC values.

Static Gap LTR Value (uF)

This output identifies the recommended LTR value for a Static Gap as based on the transformers input data. I thought it might be good to have this info handy when designing an MMC.



SRSG LTR Value (uF)

This output identifies the recommended LTR value for a SRSG (synchronous rotary gap) as based on the transformers input data. I thought it might be good to have this info handy when designing an MMC.

[thedoc298]

Great to hear that you have it working well now!

I'd guess that your earlier high-current events were due to resonant charging with sparks at some harmonic of line frequency.  Resonant frequency depends on current (input voltage) as the iron shunts start to saturate.  (My SRSGTC is designed for resonant charging at 120Hz.  But, if the rotary spark gap gets out of phase much, current goes way too high and transformer cores hard-saturate.  With a static spark gap, I'd expect that to occur more often.  Before breaker tripping, the high power does make higher voltage, so would trigger the spark gap to keep running in that high-power phasing.)

Ready through the initial problems and the discussion, my first thought was also resonance with the NST secondary, this is why MMC design for SGTC/RSGTC has a term coined Larger-Than-Resonant or LTR for short.

Check out: http://classictesla.com/home/javammc/ and do some calculations on it, the last two results are for SGTC or RSGTC LTR MMC values.

Static Gap LTR Value (uF)

This output identifies the recommended LTR value for a Static Gap as based on the transformers input data. I thought it might be good to have this info handy when designing an MMC.



SRSG LTR Value (uF)

This output identifies the recommended LTR value for a SRSG (synchronous rotary gap) as based on the transformers input data. I thought it might be good to have this info handy when designing an MMC.

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The 4 transformers had a 54 nf cap as recommended by javatc . The safty spark gaps never fired even at 30 to 40 amps (for just a few seconds) It just kept making larger and larger streamers. I new it would not run long at that rate. But certainly the effects of the shunt was going to zero. Now with the new top hat can turn the variac full and pulls 15 to 17 amps. I can live with that. This was my first coil and I really appreciate all the help you guys gave me. The old toroid was 7X26 and the new is 7.5 X 30. It has been real fun figuring out all the problems that pop up. During the first fire it was one hell of a rush and still is even more so.