[TCML] NST Measurements
bartb at classictesla.com
Thu Jan 22 21:48:44 MST 2009
As others also stated, it's not possible to pull 15kV at 30mA. Those
faceplate ratings are Voc and Isc respectively, but because regulation
(by design) is poor, the 15kV will not maintain while increasing load
current. It doesn't take much of a load to start dropping the voltage.
The term "proper" load would normally indicate the correct tube length
for the gas used in the tube (load changes with gas type). You might
want to look at the Franceformer website. In their Technical section is
some good information regarding how NST's are loaded and why the voltage
is designed to drop after the gas conducts and thus presents the load to
the NST. Also in there are hookup tests for measuring current and how to
attain the proper load and what a proper load is for an NST (regarding
tubes of course).
With VA, keep in mind how low the power factor is with NST's (< 50). A
couple hundred watts maybe if a small load that would not drop the NST
voltage much. But TC loading is different from the resistive type of
load a tube provides. The faceplate VA rating (for example the 15/30 at
450VA) is simply the open circuit voltage x the short circuit current.
It doesn't reflect the power of the transformer because of the shunts in
"normal" tube operation.
Because of the TC circuit, we do manage to pull near the 450VA as
apparent by spark lengths we attain, bps values we measure, and the
comparison to other high regulation transformers like PT's and Pigs.
This probably "appears" contradictory. The load is simply very small for
the TC circuit and thus we attain near Voc up until the gap conducts (at
which point we know during gap shorting, we are drawing the full 30mA
with near zero volts from the NST). Then when the gap quenches, were
back to a small charging load. If it were not small, we would not attain
the spark lengths that we do (and that is a fact).
Your absolutely correct, our understanding of NST's is not as good as we
assumed. The NST is rather complicated thanks to those darn shunts.
MOT's follow similar complications for the same reason, but to a lesser
degree. It's not so much I2R losses in the core (their there), but how
the shunts affect the core that really complicates the transformation.
Shunted transformers are very interesting!
Lau, Gary wrote:
> Maybe the question I really want to ask is - under what conditions will a 15/30 NST deliver 15kV AND 30mA? Is that even possible? I would have assumed that with a "proper" load, Vload would be 15kV and would draw 30mA. Could it be that the open circuit voltage really is supposed to be something much higher but settles down to 15kV under load?
> You mention that "30mA would drop the voltage very low ". If that's true, then we should never be able to draw anything close to the faceplate-rated power from an NST. Have you measured this? Or does the faceplate VA rating reflect just what is being pulled from the wall, with much or most of that being dissipated as I2R losses in the core?
> Sorry for all the question marks, but it appears that our understanding of NST's is not as good as we had assumed.
> Regards, Gary Lau
> MA, USA
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