estimating voltage from HV transformer

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Tesla729-at-cs-dot-com 

Hi all,

I know this is pretty basic stuff but I was hoping some
of you could give me some insight into this situation. I
have a 140 kVP, 500 mA single phase Picker x-ray
transformer that I have removed the diode sticks from
and am running it AC to power a wicked Jacob' ladder.
Anywho, I was wanting to get a pretty good ball-park
estimate on the actual AC voltage that it puts out with
a given input voltage from the variac input. I know that
the basic rating of an x-ray trannie is for very short du-
ration and that their ratings have to be considerably
downgraded for cont. operation. The name plate on
mine says that it's only good for 3.6 kVA continuous
output but I'm sure that this means w/out the diodes
removed  and as a DC power supply. The transformer
itself is a monsterous 980 lb. 22" tall x 22" wide x 36"
long oil-filled steel tank. The tank holds 62 gallons of
transformer oil (est 400 lbs of oil). I'm sure that the
huge transformer core (guestimate 300 to 350 lbs dry
weight) could put out considerably more than 3.6 kVA
on a continuous basis. The tiny diode sticks have got to
really limits this behemoth's cont. output. The
diodes will burn out almost immediately when the HV
output is shorted w/ the diodes still installed. Running it AC
to drive a Jacob's ladder, it will jump a minimum 7.5" gap
to start the JL w/ about 120 to 130 volts input. I know
that the rectified, filtered DC voltage is going to be higher
by a factor of 1.41 x the AC rms voltage but I think this
must be higher than only ~ 100 kVAC w/ a full 240 volts
input when around 125 volts in will make it jump a 7.5"
gap between two 1"w x 1/8" thk aluminum rails.

I tried hooking up my 10 kVA, 14.4 kV distribution trans-
fromer to one HALF of the HV side of the HV secondary of
the x-ray tranny (had to since the HV output is mid-point
grounded). With 14,400 volts into half of the high voltage
side of the x-ray tranny, I measured 39 volts at the low
voltage input terms. of the x-ray tranny. Doing the math
this showed that the full high voltage side of the x-ray
tranny should have almost 90 kV open circuit voltage w/
only 120 volts input (14400/39 = 369.23 "HALF sec"/pri turns
ratio). 120volts input x 369.23 x 2 (to include the voltage of
the second half of the HV sec. coil) = 88,630 volts. I assume
that "open circuit" voltage are the key words here and that the
140 kVp DC rating takes into account the voltage drop that
would be associated w/ drawing .5 amps from the transfor-
mer's HV side, even w/ 240 volt input. I have been told to
NEVER apply the full rated primary voltage to an x-ray
tranny with the HV output open circuit as this will cause
a great possiblity of internal flashover from over voltage
and this would make sense. I have ruined a couple of
smaller x-ray trannies in the past by doing this ;^((  so
now I know from experience also to not do this!

Sorry for the long post that's getting borderline OT, but
I was hoping to get some input from some of the more
knowledgable list members to confirm (or correct) my
assumption regarding this matter.

Keep 'em sparkin'
David Rieben