Author Topic: Can anyone explain.  (Read 540 times)

Offline thedoc298

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Can anyone explain.
« on: July 19, 2020, 11:04:14 PM »
 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.     

Offline klugesmith

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Re: Can anyone explain.
« Reply #1 on: July 19, 2020, 11:44:40 PM »
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.
« Last Edit: July 19, 2020, 11:50:40 PM by klugesmith »

Offline thedoc298

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Re: Can anyone explain.
« Reply #2 on: July 21, 2020, 07:22:19 PM »
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.





Offline acmq

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Re: Can anyone explain.
« Reply #3 on: July 21, 2020, 08:04:57 PM »
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.

Offline klugesmith

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Re: Can anyone explain.
« Reply #4 on: July 21, 2020, 08:54:50 PM »
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.
« Last Edit: July 21, 2020, 09:01:01 PM by klugesmith »

Offline thedoc298

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Re: Can anyone explain.
« Reply #5 on: July 21, 2020, 09:39:34 PM »
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.
« Last Edit: July 21, 2020, 11:03:12 PM by Mads Barnkob »

Offline johnf

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Re: Can anyone explain.
« Reply #6 on: July 22, 2020, 10:36:22 AM »
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

Offline thedoc298

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Re: Can anyone explain.
« Reply #7 on: July 23, 2020, 01:39:49 AM »
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.

Offline thedoc298

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Re: Can anyone explain.
« Reply #8 on: July 24, 2020, 05:40:52 AM »
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

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Re: Can anyone explain.
« Reply #8 on: July 24, 2020, 05:40:52 AM »

 


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