Re: MMC's vs CW and Duty cycle

["David Trimmell" <davidt-at-pond-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Thanks for the info Terry. I think I will finish my (2) string MMC, but
meanwhile the single string one keeps chugging away. I suspect that when
running the tube coil in Audio mode (the caps do get warm on extended
runs), the "duty cycle" is relatively low. These MMC's are quite robust, I
had my doubts in the beginning, but real life experience has sold me over
(hey, I am a sceptic!), not to mention all the great work of people here on
the list.

Regards,

David Trimmell
www.ChaoticUniverse-dot-com

At 03:45 PM 8/25/00 , you wrote:
>Original poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
>Hi David,
>
>	You hit on a good point here.  The ratings given in the charts are for 24
>hour operation or until the temperature reaches a steady state value.  In
>my cap testing, that was about 30 minutes.  I ran the numbers for your caps
>under the conditions you mention and 5.6 amps should drive them up 10C.
>With double that current, they should go to 40C.  However, you have plenty
>of time until they get that hot.  They even may last hours at 40C but that
>would be pushing it...
>
>Current destroys the caps by overheating and melting the polypropylene
>dielectric inside the caps.  But if you start with cold caps, you have a
>considerable time that they can be run over current before the centers
>start to meltdown.  Designing for such things is really hard because the
>numbers are all fairly "fuzzy".  One would really have to actually try and
>test the caps to see if they can run so close to destruction in a
>particular application.  It sounds like you system is running hard but
happy...
>
>When MMCs die from too much current it is usually not a 2X or even 3X
>overload.  It's those 5X to 100X+ overloads that kill them...  Unlike
>voltage, they can take a very high sort term current load as long as they
>are cool to begin with.  The fact that these caps are designed to last
>millions of hours and we use them for only a few hours really works to our
>advantage...
>
>Cheers,
>
>	Terry
>
>
>At 08:45 AM 8/25/00 -0700, you wrote:
>>Hey all, Bert, Terry,
>>
>>The recent posts by Bert on the VTTC Tank thread has got me wondering. Now
>>as Bert has pointed out, the fact that I am running a single string of
>>MMC's in my VTTC, I should have had failiar due to over current. I am
>>stumped. The theory and reality do not mesh...
>>
>>Last night I did some tests:
>>
>>First I hooked up the Vac coil to run in filtered DC mode, two 833C's
>>powered by a 125# plate transformer. Measured Tank capacitance (7 series
>>0.015 uF Panasonics): 2.12 pF. Measured Primary Inductance: 81 uH. 9K in
>>grid leak. 16 uF filter cap.
>>
>>
>>1: Brought up plate voltage to 2300V, 6-7 in flame discharge, but would
>>trip 20 amp breaker after 10-15 seconds of run time, barely perceptable
>>color on plates. No detectable heating of Caps. That should be 11.8 amps
>>on the caps.
>>
>>2: Rewired Plate Transformer for lower output. Brought Plate voltage up to
>>1825V, 4+ inch flame discharge. Ran (5) 40 second tests, with 15 second
>>breaks to test temp of caps. After all runs there were some notcable
>>heating of the leads, but no real heating of the caps them selves. This
>>should have put 9.34 amps on the caps.
>>
>>Do these results make sense? Do I need to go for longer run times?
>>
>>Sugestions, anyone?
>>
>>Regards,
>>
>>David Trimmell
>>www.ChaoticUniverse-dot-com 
>>
>
>