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Re: Strange transformer problem

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"
>

Hi John,

	This is my guess, and it is just a "guess"...

Even though there is no capacitor connected to the output of the pig there
is still capacitance there.  Let's just guess that it is about 1nF.
Transformers not only transform voltage but they transform impedance.
Since the winding ratio of the pig is 14400/240 = 60.  The impedance is
transformed by N^2 or 3600 times.  So on the primary side of the
transformer, the 1nF looks like 3600nF.  Now one just has to find the
inductance that will resonant with it at 60hZ.  It is 1.95H.  Which is
probably just about what the inductance of your variac is!  You may have
made a very high power resonant circuit.  This would explain why you see
high voltage across the variac and pig.  Is is like a series LC connection.
 You see about the line voltage across each but they are out of phase.

Be VERY CAREFUL!!!!  God only knows what the output voltage of the pig is!!
 I think it will be about 360/240*14400 = 21600 volts.  You have a lot of
power there so having strange things going on can be very dangerous!!!

At least, that a guess from me sitting here with a calculator without
really having ever heard of this before....

Cheers,

	Terry