Re: The death of a classic

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Hi Finn,

Sorry to hear about the failure of your museum coil MMC - and it was
such a pretty MMC too! 

It's a bit difficult to tell just which failed first from the pictures,
but judging from the greater damage on the resistor side of the
capacitor matrix, it sure looks like one or more resistors failed
catastrophically, began arcing, and the flames/arcing spread to other
nearby areas.

It's possible that a single capacitor may have actually caused the
event. For example, suppose one cap suddenly shifted to a markedly lower
capacitance than its peers in the chain. By any chance are these metal
film caps or do they use the more robust film-foil construction? I can't
remember which type you were using. If they were metal film, their
desirable self healing behavior might eventually cause a failure since.
The capacitor with the lowest capacitance will see an abnormally higher
voltage across its terminals once the gap fires. The excess voltage may
have overvolted the resistor across this cap, causing arcing on the
associated resistor. If the coil continued to operate after the resistor
began arcing (these look like little blowtorches when they fail!) the
heat and flame likely caused the other failures and the rest of the
damage seen in the pictures. Terry's Failure Mode Analysis should
quickly pinpoint to the root cause. Any idea how many hours of use your
MMC might have had? 

Good luck and best regards,

-- Bert --    
-- 
Bert Hickman
Stoneridge Engineering
"Electromagically" Shrunken Coins!
http://www.teslamania-dot-com  

Tesla list wrote:
> 
> Original poster: "Finn Hammer by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<f-h-at-c.dk>
> 
> Perhaps it is not so good an idea to fit a museum coil with a MMC style
> capacitor.
> 
> The capacitor for The Museum Coil is a classic, in that it was one of
> the first really big MMC`s 3 years back. It has put up with all my
> initial mistakes, a lot of abuse, and an estimated 10 hours of total run
> time. I had thought it would last 470 hours, but alas, I must have been
> treating it worse than I thought I did.
> 
> Recovering from a Weekend bicycle race yesterday (160 miles -at-9h22min)the
> museum manager called me this morning to tell me, that the coil was
> smelling.
> "Red Alert"
> So I hurried down to the museum, carrying with me the cap from the
> Ambassador coil in order to have something to rig up, so the visitors
> could get what they came for.
> Now it is gone for good.
> 
> http://www.wimshurst-dot-com/mmc1.jpg
> 
> http://www.wimshurst-dot-com/mmc2.jpg
> 
> What went wrong?
> Perhaps I just pounded it too heavily, I didn`t always hook up the
> fiberprobes, so I really have no idea about how often they were
> overvolted.
> They never were hot, even one time when they ran for an hour, without
> stop, during EMC measurements.
> So probably, one cap gave up, and this lead to heat inside it, being
> transmitted to nearby caps, and the avalance started.
> 
> This is the reason that I will not use a MMC again on a professional
> coil: It is not reasonable to ask, that the operator should be able to
> spot that first bulging cap when it pops out amongst the others, A pro
> coil damands a pro cap.
> 
> With the caps from the Ambassador, rearranged for 28nF, and 5 strings
> from the original MMC, i managed to get the coil up in an hout or so,
> and am now anxiously awaiting the arrival of the backups, Maxwell 37667,
> that I bought from Tesla Systems Research.
> Got 6 of those, and am going to use 4 in a quad, and 2 as spare. Then it
> is only my hope that they will last untill we get the Maxwell 35309,
> which is a 40nF 100kV cap.
> 
> Don`t ever deal with a professional setup without a backup!
> 
> Cheers, Finn Hammer