Re: NST filter failure?

[Terry Fritz <twftesla-at-uswest-dot-net>]

Hi Mark,

	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!!

	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 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.

	In all the fault studys I have done, I never did loosing sync at full
power or a sustained miss-timed gap...  Always something...

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

Cheers,

	Terry

At 10:13 PM 8/20/00 -0500, you wrote:
>> Original poster: "Christopher Boden" <chrisboden-at-hotmail-dot-com>
>>
>> >Original poster: Terry Fritz <twftesla-at-uswest-dot-net>
>> >I would be interested in hearing what with wrong with the NST filter?
>> >
>> >snip...
>> >
>>
>> Mark?
>
>Yes?  ;-)
>
>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
>
>
>