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Re: Strange Current Flow



Tesla List wrote:
> 
> Original Poster: "ross andrews" <sflourine-at-home-dot-com>
> 
>         Hi all,
> 
>         I wired an ammeter up to my coil today, and something strange
happens -
>         The tranny (a 15 kV, 25 mA NST), draws about 3A on the primary when
> shorted, but when used in my coil, the meter jumps from 3 to 5A,
> sometimes even going off the scale of the thing.  Since the NST is
> current limiting by itself, the only thing I can think of is that
> perhaps only actual power is limited by the NST, and at times a large
> reactive component is induced somewhere, allowing the NST to spit out
> more current as purely reactive power.
> 
>         Any comments apprecitated,
> 
>         Ross

Hi Ross!

You've sort of figured it out...  :^)

This behavior is normal. The tank capacitance of your Tesla Coil is
partially neutralizing the current-limiting leakage inductance within
the NST. This allows the transformer to deliver more current than it
would drive into a simple short circuit, and this effect also makes it
pull more current from the mains. The current increase is most
pronounced if you've "sized" your tank capacitor to the current/voltage
rating of your NST (which most design-aid programs will do) - sometimes
called resonant charging. Under this situation, the input current easily
exceeds 2X the normal short circuit. 

However, you also need to make sure that your safety gaps are properly
adjusted, since this condition also means that the transformer output
voltage may get to over 3X it's faceplate value if your main gap fails
to properly fire or is set too widely. This will almost surely let the
smoke out of your NST... 

Safe coilin' to you, Ross!

-- Bert --