Re: DANGER! Strange transformer problem

Terry Fritz wrote:
> Hi again John,
>         I just realized that you measured the current!!  Give yourself an
A+ and a
> pat on the back for getting that vital bit of information!!

Thank you :-)

Bear with me.  I'm just yr hmbl nuke who picked up this power EE
stuff by osmosis from my colleagues in the power plants.

> This tells us the impedances:
> Variac inductance 0.5H
> Pig primary apparent capacitance 14uF
> Pig secondary capacitance 3.9nF

I sorta follow this.  The pig ratio is 60:1 (14.4kv/240).  Wouldn't
the apparent capacitance be 3.9nF * 60 = 234nF?  At least this
formula works when i'm making small filter capacitors look like big
filter caps on low voltage PX circuits.

> I guess that is reasonable for the secondary capacitance given the size of
> the pig, the winding capacitance of the secondary, and the capacitance from
> the secondary to the case.
> Your pig's open load current is 1/4 amp at 240 volts giving an impedance of
> 960 ohms or 2.55H for the primary inductance.  The pig's secondary
> inductance is N^2 times that or 9167H (that a lot of inductance :-))
> Now we can Spice model it...
> **YIIIIPS!!!!!!!  DANGER!!!   DANGER!!!**
> Output voltage 400kV!!!
> Primary voltage across transformer 6kV!!!!

Now this I don't understand.  I measured the voltage across the
primary at 275 volts with the variac growling.  Where does the 6kv
come from?  I do get a nice corona buzz off the bolt thread on the
pig horn but that would be expected with 275 volts on the input.

> Obviously, the model is ideal with rather guessed values and doesn't take
> into account core losses, saturation,... but...  There is definitely some
> BAD things that could potentially be going on there!!!!
> That "growling" noise may be a serious warning to duck!!!!
> Be SUPER careful!!!!!!!!!

Hey, I'm allergic to Ready Kilowatt!  Wanna see my lineman's gloves,
blankets, mats and remote controls?  Those photos on various web
sites of guys drawing arcs off of pig horns with wires held in the
bare hand make me shudder..

I know something strange is going on here but I can't figure out
what.  Saturation would be the first thought but the variac should
handle it since it's rated for 25 amps.  I thought the variac might
be breaking down but today I borrowed a 5KV hipot set and tested
it.  Nadda.  Some sort of resonance comes to mind but I can't
imagine where, given only stray capacitance in the circuit.  I've
thought of slotting the variac core but it's going to be a pain in
the a** to get to plus I hate to hack this nice variac until I know
what's going on.

Some more tidbits of information.  The pig nameplate impedance is
2.1%.  I removed the variac and choke from the circuit and hooked up
a welder transformer with the shorted secondary as the current
regulator.  Worked fine.  Smoothly controlled the shorted secondary
current from 200 ma to about 2 amps.  Open circuit performance is
smooth with no bumps across the range of the welder.  Putting the
variac back in line with the welder set on max and the pig sec open,
the growling and high voltage returns.  This is pointing back to the
variac, I think.  This variac is the type where  the whole outside
circumference is the slider contact circuit with a bank of brushes
spanning the height of the core.  Could the turn that the brushes
short possibly be doing this?


John De Armond
Neon John's Custom Neon
Cleveland, TN
"Bendin' Glass 'n Passin' Gas"