Re: Maxwell AC vs. DC ratings

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Ed,

At 10:37 AM 4/6/2004, you wrote:
>Tesla list wrote:
>
>................
>To be fair, I was talking about MMC
>failures in general. In the recent past MMCs have been touted on the
>list as being the answer to every coilers prayers and folks were pretty
>much told not to bother with a commercial cap, MMCs were better. Now it
>seems like there have been about as many MMC failures as there have been
>in the over stressed commercial cap realm.

There are probably close to 20,000 MMC caps out there.  Unexplained 
failures number about 5 ;-))  with a little more time, we will figure out 
these interesting failures with John's coil too!  Last year we asked 
everyone if there were any problems they were having with MMC or the 
resistors and no new or serious problems were reported.

Of course, if one over-stresses MMCs or commercial caps, the chance of 
failure is 100% ;-))  We really do not over-stress MMC caps typically, we 
just use them within their specs very hard.  Not unlike the Maxwell 37667's 
that show up on the surplus market all the time.  They are just replaced 
when their hard use predicts they may begin to fail.  MMC will fail 
eventually too, but we try to put the time to failure for a normal coiler 
at decades at least.

>MMCs are not better, just
>different! I wish I'd had MMCs when I was getting started.

You are not a beginner now and you use the caps best for your advanced 
coils.  MMCs really wiped out homemade oil/poly caps practically 
overnight.  Commercial caps always had a following of their own for those 
that could afford them and use them right.  But those folks are only about 
5% of us.  MMCs really go well with NST LTR systems.  Big pole systems do 
get to the point where you would need so many MMC caps that it gets to be 
cheaper just to go commercial.  Neither is really "best", now we just have 
a nice choice depending on our needs ;-)


>In the past 10 years I've popped zero commercial caps, so it's been a
>long time since I had to have the "company" build me a new one. I
>haven't had to take any minutes to get up and running again from a
>failed cap. :^)

Knock wood and throw salt over your shoulder :o))


>The CP caps on my maggy have well over twenty hours of total run time
>(actual hour meter) and the break rate on the system is from 750 to 800
>BPS! The very same caps had seen extensive duty on several classic coils
>before that. During the Teslathon last year I put almost two hours of
>run time on the maggy in a single afternoon while people tried out the
>"Cage of Death" and the only part of the system that overheated and shut
>down was the series rotary gap cooling fan motor (which is necessary
>only because of the safety cover over the gap).
>
>All that said, I can't wait to fire up the maggy for the first time this
>year after I get the lab open and have one of the aforementioned
>commercial caps go up in a blaze of sparks and smoke. :^) That would be
>just my luck!

Chances are very good they will do fine.  If a cap is really going to blow 
up from stress, chances are it will be within minutes.  After all this 
time, I bet your coils and caps have proven to be able to live happily 
together.

I think the real key to all this is to understand the caps one is using and 
use them well.  In the past, so many people just somehow got their hands on 
a big cap and just wired it up with very little knowledge as to currents, 
voltage peaks, cap construction, etc.  Now that we know so much more about 
coils and the way they work, we can be far more intelligent about how to 
get the the right caps use them properly.

I do wish good solid specs were a bit easier to find on the commercial 
caps.  There is practically no detail of MMC caps that one cannot find out 
right down to the materials and OEM suppliers of the films.  Heavy 
competition has pretty much weeded out "bad" suppliers...  Commercial cap 
makers live in a bit more shielded world and don't have to be as forth 
coming in supplying really good solid information.  I note that some 
details of commercial caps are only found when someone calls a pal at the 
factory...  for MMC caps, a Google search on "film capacitors" pulls 
140,000 hits and just about every excruciating detail one would want to know.

BTW - The Maxwell 37667 only has an RMS current rating of 20 amps and a 
dissipation factor of 0.002...  That might be why they tend to fail in 
Tesla coil applications rather than any purely voltage related cause.  In 
that case, their wonderful 25,000 amp "peak" rating does not help us at all.

Cheers,

         Terry


>Ed Wingate RATCB