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Re: Strange transformer problem


    I noticed while working with small audio transformers, and other styles
of transformers, that have audio frequency ranges that usually there is a
frequency where the core will oscillate piezo magnetically.  There does seem
to be a resonant state but, I have never heard of any literal study of the
effect for power conversion, only for sonar, tweeters, and ultra sonics.  I
do know that peizo electric microphones, utilize roshel salts, and I don't
know if I spelled that right but, when they are compressed electrons exit
the material.  If you just happen to be at some peizo resonant state that is
knocking loose the electrons from the core, it might heat up faster but, if
the electrons are free they can then contribute to the magnetic field as a
magnetizing current.  I wonder about what kind of core materials you are
using, and if it's laminated on your choke's core.  In most transformers
there is a lamination that prevents current flow through the core.  I have
never seen a conductive core used.  If you hit a piezo resonant frequency,
and were using an unlaminated core somewhere, then maybe.  An audio
transfomer typically works best driven in that range where it does growl
but, at a higher frequency.  Those are usually so small that the speed of
sound through the core is a higher frequency.  I wonder if any wants to do
an experiment that encompasses conductive cores, and the note it would hit
if it were a wind chime, or the mechanically resonant frequency of the core
based on sound resonance.  Other than that, I figure you are looking at a
phase relationship.

    If anyone has Electronics workbench software.  I know to set up a
chaotic resonant circuit that really proves itself on the computer..  Tune
the primary of a 1 to 10 step up transformer to the same frequency of the
parallel reactance of two secondarys and step it down and equal amount.  I
watched the signal on workbench for hours, and that thing would seldom drop
from over a kilo volt using a 10 amp resonant primary where inductive
reactance was the current limiter at like 24v AC driving the primary tank.
Tuning the Primary, first secondary, and the second secondary all to the
same frequency resulted in an output that was unbelievable.  It's like a
high q, low ring value, low q, high ring value, and back.  Using a single
pulse, you ring a tank circuit, and can find it's resonant frequency.  A
high ring value will return a few thousand cycles, and a low ring value only
returns 50 to 400.  I used an in phase ground, and it would wind up out of
phase, because after a few thousand cycles they would wind up 180 out of
phase, and start acting like doublers, and it would just chaotically go
along, and at times the voltage jumped to over 6kv on the final secondary.
All I can say is try it if you have the software.  It has to do with how
many cycles go by, and their phase relationship, and the current that
actually gets hung in the 10s place up, or down.  It consisantly seems to
pull power out of nowhere according to the software.  It may be a bug in the
software.  I haven't been able to test that physically yet, because the
values of capacitor is so odd for the first secondary, and second primary in
parallel, and volt watt ratings  from the first secondary to the second
secondary are so high to run the circuit I couldn't.


James.

Subject: Strange transformer problem


> Original Poster: Neon John <johngd-at-bellsouth-dot-net>
>
>
> 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"
>
>