[TCML] NST Measurements

[bartb]

Hi Gary,

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!

Take care,
Bart



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
>