Re: NST filter failure?

["Mark Broker" <broker-at-uwplatt.edu> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Well, I've FINALLY sat down to analyze the problem a bit.  I had to get my new
apartment organized, and my housemate's girlfriend's belongings out of "the
shop"
first!  :-)  The story, for those who can't remember more than yesterday (like
myself, these days), is at te bottom.


>         If the MOVs burned up (they short thank goodness!) they were clipping
> about 27kV spikes.  For them to get that hot, something must have been
> seriously going overvoltage.  They did their job in saving the NST (the
> hard way).  YEAH!!

The MOVs are blown.  I hooked a 12kV NST up to each string ov MOVs, and set the
variac at about 80V (should produce ~11kV max).  One string got slightly
warm after a
few seconds, and the other string got even warmer.  The input current on
the NST was
high for the second string, indecating a short.  The current was pretty low
for the
first string.  The second string was also the one that appeared to have
gotten the
hottest during "The Demo From Hell," as the glossy coating is now matte.  I
will be
replacing both strings of MOVs soon.  At least it's $20 for new MOVs, and
not $20
each for four new NSTs (so there is a good side of the story).

I haven't tested either the safety or the SRSG yet.  I'll hopefully to that
tonight
after my classes and meetings and such.  I suspect that the safety is set
closer to
about 16kV per leg, although I haven't intentionally adjusted it since I
first set
the gap (to about .170 inches per leg, if I remember correctly).  I may have
inadvertantly increased the gap when I reattached teh wires to it....


More to follow later.

Mark

> >My filter is the exact one posted on Terry's page.  I'm running 15/90
> >currently,
> >although I did have the 4th NST hooked up at teh beginning of the demo
(Mark's
> >law of life 157:  if it a lready works well, then do *NOT* make it better an
> >hour before you need to use it).  This was not, IMO, a cause of the
> failure, as
> >the symptoms below occurred at least 20 minutes after I removed the 4th
> NST and
> >MMC bank, and it was running "normal" just after the removal.
> >
> >I surmise that I had a motor problem (lost sync when NSTs were running),
> >combined with a too-wide safety gap, and a poor power supply (tried to run
> 20+A
> >off a 15A (probably 14AWG romex) fuse).  The motor, FRT, I've had
trouble with
> >before (thermometer failed during first test run).  The resitors got MUCH
> >hotter
> >than they should have (for ~5 minutes run time over 45 minutes).  The MOVs,
> >which normally run cold to "bearly warm," were SEARING hot - hot enough to
> turn
> >the glossy epoxy a nice matte.  I fear that these are toast, and I will
> need to
> >replace all 14.  MOVs fail open, correct?
> >
> >It'll probably be about 10 days before I'll know anything more.  I'll post
> what
> >I find out.
> >
> >Mark
> >
> >
> >