[Prev][Next][Index][Thread]

Re: More Tuning/Debugging

In a message dated 96-09-12 00:48:21 EDT, you write:

Snip

<< Having had some experience in pushing inductors into saturation
 at 60 cycles I can list a practical guide. Assuming you are
 simply over-voltage static testing the equipment (meaning there 
 is no load) things will begin to hum rather noticeably as you
 push the inductor into saturation. As you continue to add voltage
 the hum gets louder and things begin to actually buzz. At this
 point if your inductor is small, things will begin to heat as the
 unloaded inductor begins to pull real current during the static
 test, a pretty clear indication of saturation.
 
 At this point small inductors in air (read 10 amp 120 volt
 variacs) begin to smoke some. Heavier inductors in oil (pigs)
 don't get hot very fast, but they begin to growl rather badly.
 
 Now I may be spinning pure BS (to use the technical term), but I
 get the impression that the field flux in an inductor that is 
 throughly saturated during over-voltage operation is not stable.
 You mention above that the possiblity of fairly high voltage
 spikes could be evident as a pig goes in and out of total
 saturation while placed in series with ballast. I would guess
 this would be emphasized to an even greater degree if the ballast
 was purely inductive. 
 
 This is the root of the problem I have been dealing with.
 Resistance added to the ballast soothes things down, but the
 results are still not satisfactory. To bring the "bumping" and
 "spiking" under control with resistance means giving up the
 14,400 volts output, the whole purpose of running with the
 overvoltage in the first place.
 
 This is perhaps compounded by the huge amount of inductance in
 the system. Locally, if you start with the 50KVA pad mount
 transformer that supplies my house (and a few neighbors), then
 add in the arc welder, 60 amps worth of 240 volt Powerstats, and
 25KVA worth of pig, we are talking about some serious inductance.
 The actual delay as measured on a stop watch exceeds 1.5 seconds
 from the time the welder is opened up to the time spark leaves
 the discharger on a coil. We are talking real horsepower tied up
 (hopefully tied up) in magnetic field flux throughout this power
 supply. 
 
 Just the right amount of resistance can get things working pretty
 well, but the balance is precarious at best. Any substanial
 altering of the system inductance, either in the current limter
 or voltage setting on the variac, throws everything out of wack.
 This is rather frustrating because not only do I love smooth
 systems, but I have too much invested in my power control/supply
 to want things welded up into a useless mass. This would also
 appear to be a handy way to lose a few line filters.
 
 Sooo.... And I tuck this down here where few will probably really
 read it... I admit defeat. The 25KVA 7200-120/240 pig is not
 satisfactory when run saturated to supply 14,400 volts. A major
 change is in the making and I have a question or two to pop out
 for those whom I have not yet put to sleep.
 
 1) Pros and cons on: a) plunking down $335 for a 14,400 10KVA pig
 and using that for the standard lab supply; b) plunking down a
 similar amount for another 7200 volt pig and running two pigs
 with the HV windings in series to get the 14,400 volts needed.
 
 2) Since I have already have an investment in the 25KVA pig which
 is rewired on the low voltage side so that both 120 volt windings
 are in parallel ("Star")... If I plunk for another 7200-120/240
 pig, would it be better to rewire the second pig to match the
 first, and run the two "Star" wired LV windings in series to get
 to 240 volts input... Or, should I rewire my existing pig so that
 the LV windings are back to the Delta configuration (both LV
 windings in series for 240 operation) and place the LV windings
 on the two pigs in parallel. Would it make any difference?
 
 Richard Quick
  >>

Richard,

Good discussion.  I am convinced I am experiencing somewhat the same problem.
 I don't know if I am saturating the pig or not, but my system is certainly
shaky right now and I don't like it.  I like things to be well ordered and
predictable.  Right now, sometimes my coil will run and sometimes it won't.  

I have noticed the welder really growls now that I am operating with no
resistance in the system.  The pig is a few feet farther away but I think it
is also making noise.

Ed Sonderman