[TCML] MOV destruction

[Lau, Gary]

Hi Dennis,

Even though you're using a significantly LTR cap, I still think its possible for a too-wide static gap to allow mains resonance ringup to create a voltage above what a 15kV NST (and 8 MOV's) should endure.  It definitely sounds like the MOV's threw themselves on the sword!  8 MOV's (per side) is the right stuff for a 15kV NST, even at 130V.  I would recommend recalibrating the main static gap width, and probably the safety gaps too.

Regards, Gary Lau
MA, USA



> -----Original Message-----
> From: tesla-bounces at pupman.com [mailto:tesla-bounces at pupman.com] On
> Behalf Of otmaskin5 at aol.com
> Sent: Sunday, April 06, 2008 5:51 PM
> To: tesla at pupman.com
> Subject: [TCML] MOV destruction
>
> Hi, I'm trying to figure out what happened in my most recent coil melt down and was
> hoping someone out there could help me out.? I had just replaced my old capacitor
> bank with a 0.02uF MMC on my 15/60 coil.? I thought I had the coil reasonably well
> tuned & was running it for about 3 or so minutes.? The coil & it seemed to be
> behaving fairly well, producing strikes that somethime reached over 50
> inches...when it just stopped running.? I detected an oil type odor & first thought I
> had blown a cap somewhere.? Eventually I found the MOVs on my Terry filter were
> destroyed.? There was remnants of molten metal on and around the MOVs, several
> of them had cracks or splits in them & an oily substance had been ejected from the
> MOVs.?
>
> I tested the NST & both legs seem to be healthy, so the MOVs apparently died in
> the line of duty protecting the NST (thankfully).
>
> I have not heard of MOVs coming apart in this manner.? From everything I've
> heard, they just quietly do their job when there is a voltage surge.? Before I replace
> the MOVs and start running the coil again, I'm trying to get an idea of what caused
> this to happen & what I need to correct in the system going forward.?
>
> I had 8 MOVs?(1800V ZNR) in series to ground on each leg of the transformer.?
> This essentially duplicated MOV arrays?that I have seen?others' using on their
> 15/60 coils.? My variac maxes out at 130v - also,?I didn't see any strikes to the
> primary or lower deck?at the time this occured.
>
> I am wondering whether I should have had more MOVs in series to handle greater
> voltage.? If so, what is the right number?
>
> Or were the MOVs adequate, but was there some other problem that caused this
> which I need to correct?
>
> I would appreciate any ideas.? Thanks, Dennis Hopkinton MA???
> _______________________________________________
> Tesla mailing list
> Tesla at www.pupman.com
> http://www.pupman.com/mailman/listinfo/tesla