Re: NST filter failure?

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

Thanks for the info, Terry!  I concatenated the two posts and responded
below.


>         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!!

27kV PER LEG?!?!  I hope you mean total!  I'm not completely sure, but
I'm pretty certain that all the NSTs are fine.


>         I wonder if the NSTs were drivin into that odd SLTR region where the
> current limiting on the NSTs goes away.  I would suggest adding fast blow
> fuses to the NST inputs (~7A for a 15/30).  The fuses will not blow
> normally but if the NST starts to draw like 4X the current, they will save
> save the NSTs.  Even though LTR coils have large caps sizes to prevent
> overvoltage, if the NST shunts saturate, they become little pole pigs and
> will charge anything till something blows.

I have a 20A fuse (fast blow, I think) in my variac, which runs the
NSTs.  I lost two while trying to get the 4 NST system tuned (which is
one reason why it was removed), but there was no severe "over-current"
problem with the 3 NST system.  What still puzzles me is that we tripped
the 15A circuit breaker while this "failure" was going on, yet it hadn't
tripped with all 4 NSTs hooked up (while blowing two 20A
fuses), and the motor going full bore (5+A).  I did have 300uF of PFC
caps hooked up (normally have 240uF for a 3NST system), which I forgot
to disconnect when I removed the 4th NST, :-o  but I can't imagine that
would do anything other than force me to tinker with the motor phasing,
which I had to do anyway.



>         I suspect you are right about a gap problem.  If the sync gap
when out of
> alighnment at full power it could explain this too.  In other words, if
> something came loose and the rotor's timing changed things could go very
> wrong and stuff would start blowing right and left (fuses hopefully).
> Check the gap timing carefully.  I think you will find it no longer fires
> where you originally had it set.

Like I said in my last post, this isn't the first motor failure.  It was
replaced in a friend's furnace, too.....  The rotor is anchored with a
1/4-20 set screw.  There is a flat on the motor's shaft that the set
screw
presses against.  I tightened the set screw about a week before the demo. 
The motor cannot turn more than about 180 degrees before the wires stop it
(and we would have seen the motor move, as we were staring at the rotor the
whole time).  The screws that mount the motor to its base were tight, and
had to be loosened to move turn the motor by hand.  In short, I *highly*
doubt that anything came loose, but I will definately doublecheck next
weekend.

I have a 10A fuse (slow blow?) in the motor variac, which is about twice
the operating current.  I use the variac to ramp up the motor, as I have
routinely blown 10A fuses in the past (draws ~22A startup current), and
also to try to keep inertial forces due to a hasty startup to a minimum.

Thankfully the street lights allowed us to visualize the motor phasing (the
Demo was done on a street corner in the middle of DT Kalamazoo!  People
honked while driving by, even! :-).  This is how I know it was hunting. 
After cutting NST power, the motor would re-sync.  It's a 1/3HP 1800RPM
motor, and it has a LOT of extra power for a 10" rotor.


>          So fix it all up, Add tranny fuses, and reset the gap timing
(fix what
> came loose too) and all should be well.

I hope so.  I may have to look into getting a new motor....



> The power resistors "normally" can run at 350 degrees C.  So unless they
> are obviously melted down or something, they are fine and don't need to be
> replaced.  You may want to at least check the resistance.

I didn't figure that these were a problem, since they were well under
350C, and still have their glossy epoxy coating.  But they seemed to be
running MUCH hotter than they had during previous runs of 2-3 minutes
STRAIGHT (in tune, etc) with 3 NSTs.  Total run time for this demo was
about 5-7 minutes over an hour, with about 2-3 minutes under the higher
power.  I need to add a muffin fan to cool the resistors off....


> The MOVs save the caps on the filter too but visually check the cases of
> the caps for obvious damage.  You can check the values too if you have a
> meter.  The resistors across the caps should not be blackened.

I'm pretty sure the caps and bleeders are all OK.  I haven't checked the
bottom of the board, since it's fastened down (loosely), but the caps on
top showed no problems, and ran pretty cool.


> However, the MOVs will "ALL" have to be replaced. When they get really
> hot, the internal solder connections melt and the solder flows across the
> bulk material reducing their turn on voltage but providing even greater
> protection when it is most needed (they don't give up easy ;-)).  You might
> as well order a new set of MOVs.


This is pretty much what I was affraid of.  Chris and I tried to set the
SRSG electrodes apart since the motor wasn't staying in sync, but
couldn't get the gap to fire unless the elctrodes were almost directly
opposite each other (roughly .055" total gap), and then performance
*SUCKED* big time (many 12" WEAK purple streamers).  This was about the
time I (and a couple observers) noticed a little bit of smoke coming
from the NST filter....  After thinking about it, I decided that the
MOVs had at least partially failed, causing a lower firing voltage, and
then sucking up a lot of the juice (which they aren't designed to do, of
course).


> Properly set the safety gaps and KEEP them set there ;-))  Normally the
> safety gaps would protect the MOVs.  It can be argued that the MOVs really
> are not needed, but things like this show they are! :o)

I think my gaps are set about .160" per leg to ground (and I'm roughly
sea level).  They don't fire too often when it's runing well and in tune
(and apparently when it isn't....), and usually let fly a BIG snap (like
a 9mm bbeing fired).  I *think* I recall hearing a weak snapping
"barrelish" sound (safety is closed on all but one side, which faces the
1/8" plexi of the MMC container)....  I've thought for quite a while
that they need to get closed a bit, and I guess I was right :-(


Mark Broker:  Master Procrastinator.



> >Well, I actually just reassembled the base, and placed it in an
out-of-the-way
> >location in the laundry room (the only first-floor location that's out
of the
> >way).  I doon't have access too the tools required until next week.
> >
> >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
> >
> >
> >