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POWER TRANSFORMERS...

Quoting Dan Kline:

 DK> I got a pole-pig on loan today. 

Congratulations!

 DK> It's rated 15 kVA with 8300 V output. I will be putting 240 V 
 DK> into it. How does this sound? Is the output voltage high enough? 

While the output voltage is a little low, and the kVA rating is a 
little high, it still beats the tar (pun intended) out of a neon.
With a little fiddling around in the supply circuits, you can make
this quite serviceable.

First you can try wiring your variac so that it delivers higher 
voltage out than the line voltage. Usually the variac can take 
you from 10 - 20% over the line voltage if you wire them right.
Then think about ballasting this puppy down, and at the same time 
bumping the voltage up some more. 

The problem with the existing voltage of 8300 volts is that by the time 
you ballast down, the output voltage has dropped to the point that the 
main system spark gap adjustments are going to be very very tricky. The 
gaps are going to be devilish to quench because the current will be 
extremely high compared to neons. Pigs pump out some current, even 
after heavy ballasting.

Fortunately these pole pigs are so conservatively rated, that the 
plate specs are almost a joke. You could practically double the output 
voltage AND current and still not strain these transformer in Tesla 
coil applications. What you need to do here is to limit the current 
while bumping up the voltage. One way to do this is to shop around for 
a step up/down transformer that has a core rating around 2kVA, and 
something like a .75 to 1  turns ratio. The low voltage step-up xfmr 
can then be put on the 240 volt winding of a pole pig to raise your 
output voltage up to the neighborhood of 12kv after resistive and/or 
inductive ballasting. Low voltage step up/down transformers like this 
frequently show up as surplus "boat anchors" from old tube sets in 
our local electronics surplus places, and can be had for pocket 
change if you are inclined to carry them out. They can also be used
to reduce the input voltage of a pole pig that is too high. 

Another alternative would be to wind some step up coils out of some 
#10 magnet wire to slip onto an old neon core. The last (and perhaps 
the easiest) method to bump the voltage up is to run the 240 volts 
supply straight into a 120 volt winding. You can get away with this 
if you use lots of resistive ballast on the primary supply circuit, 
and you wire your variac so that it does not put out voltages higher 
than the line. The concern here is that too much output voltage may 
damage the tank circuit capacitor.

After you get the step/up down part solved, then you can start to 
ballast the primary supply circuit down. I prefer to ballast on the
line side of the variacs to keep the current load on the supply 
circuit components (line filters, power relays, variacs, cabinet
fuses/breakers etc.) to a minimum. For starters you can wire a 
up a couple of electric heaters in series with the supply circuit.
I like salvaged oven elements too. As the weather warms up you will
look hard at inductive ballasting. Henry L. Transtrom's book:
ELECTRICITY AT HIGH PRESSURES AND FREQUENCIES is a virtual gold mine
of variable inductive ballasting techniques, and Chapter 10 is really 
a must read for anyone considering inductive ballast on a shell wound
step up transformer (like a pole pig).

Good Luck!

Richard Quick


... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12