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DANGER! Strange transformer problem

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!!

This tells us the impedances:

Variac inductance 0.5H
Pig primary apparent capacitance 14uF
Pig secondary capacitance 3.9nF

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!!!!

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!!!!!!!!!

	Terry



At 04:07 AM 6/22/99 -0400, you wrote:
>
>I've run into a situation I've never seen before and hope some of
>you magnetic gurus can help me figure this one out.
>
>I'm testing a new setup involving using a medium sized variac as a
>variable choke to control the current to a 25kva pig.
>
>the following components are in series across a 240 volt line:
>
>* 25 amp, 240 volt variac connected between one winding end and the
>wiper (John Freau- the one I bought from you)
>* Fixed choke consisting of 175 turns of 0.2" dia magnet wire around
>a bundle of 7 lbs of 18" long mild steel gas welding rods (about an
>inch in diameter.)
>* 25KVA, 14.4kv/240 volt pole pig.
>
>In testing this setup with the pig secondary OPEN, things progress
>smoothly until the variac reaches about mid-range.  Then it starts
>stuttering.  That is, an irregular growling sound.  An ammeter in
>the primary shows spiked current draw with each growl.  sounds like
>a gap when it is just breaking down and the spark is still blue and
>irregular.  Advancing the variac a bit more makes the growl become
>steady and the primary amp draw is about 1.5.  Advancing toward the
>stop gradually reduces the growl AND the primary current so that
>when the variac is at the stop and therefore out of the circuit, the
>magnetizing current is less than a quarter of an amp.  
>
>Here's the really weird thing.  As I turn the variac back down, when
>it reaches approximately mid-scale, the growling returns, the
>magnetizing current goes up AND the voltage applied to the pig rises
>from below 240 volts to about 275 volts.  If the variac is gradually
>turned all the way down, this behavior continues almost without
>change so that when the variac is all the way against the lower
>stop, the growling is again steady and the voltages are as follows:
>
>* line - 240 v
>* across the pig - 275 volts
>* across the fixed inductor - 9.5 volts
>* across the variac - 360 volts!
>
>Measured with a Fluke 88 and checked with a Beckman DVM, both in
>current calibration.
>
>The primary current is again about 1 amp.  Once this condition
>establishes itself, jumpering out the fixed inductor makes no
>difference.  remember the pig secondary is OPEN and there is no
>capacitor anywhere in the circuit other than distributed
>capacitance.
>
>So what's going on here?  The way the variac starts to growl sounds
>like a breakdown but it meggers good.  The pig's been in service on
>my neon bench for a couple of years so I know it's good.  when the
>variac is taken out of the circuit and connected conventionally, the
>voltage rises smoothly as the rotor is turned.
>
>This 'un has me baffaloed! 
>
>John
>
>
>-- 
>John De Armond
>johngdSPAMNOT-at-bellsouth-dot-net
>Neon John's Custom Neon
>Cleveland, TN
>"Bendin' Glass 'n Passin' Gas"
>