Re: The death of a classic

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Finn, all,
              It is also worth noting that the manufacturer of the 
caps I use (Arcotronics) derates the caps considerably as the 
operating frequency goes beyond 10kHz. As much as 50% by the time you 
get to 100kHz in fact. And that overlooks the fact that they are 
being hammered by step functions.

     I think this incident is a timely reminder that the only good 
design is a conservative one. I see nothing wrong with using MMCs in 
a pro design but it's as well to remember that pro caps don't cost 
the earth for nothing. Probably the rightful place for MMCs that push 
the limits performance and cost-wise is the lab.

Regards,
Malcolm
 
On 11 Aug 2002, at 15:22, Tesla list wrote:

> Original poster: "Finn Hammer by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<f-h-at-c.dk>
> 
> Perhaps this is bending the numbers to the advantage of the
> circomstances, but I did just check the data sheet for the capacitors,
> and the AC rating is not 600 volta as I recalled, but 500 volts.
> This puts the expected lifetime to 3 hours.
> 
> I am using a formula which Terry made sometime, and are axcessible
> somewhere.
> 
> It says "10^6((Cap AC rating * no. of caps in string)/transformer RMS
> voltage)^15"
> 
> There are 20 caps in a string, and the transformer puts out 20kV.
> 
> 
> Finn Hammer wrote:
> > 
> > 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://wimshurst-dot-com/mmc1.jpg
> > 
> > http://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
> 
> 
> 
>