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Re: AC-rating for MMC caps // EMMC vs the GTL-WIMAs

Hi Stefan,

Comments and many snips..

At 03:02 AM 5/27/99 +0200, you wrote:
>Hi Terry, Coilers,
>
>
>
>I guess with 'V a. c.' they mean only sinusodial waveforms. So 
>I think it means that you should not exceed the rms value 
>regardless of the waveform. With other words, you could go beyond
>1.4 times the rated VAC with your peak voltage if your waveform 
>is not sinusodial. This applies for example for a 'bipolar 
>rectangular' signal (+U,0,-U,0,+U,0,-U...) with t_off=3*t_on.
>But their formula is not a very reliable thing because it would
>allow a far to high voltage for t_off=100*t_on :-(

I would never let the voltage go above the DC level in any important
application for even a microsecond.  However, in a Tesla coil where things
are a little less critical, I would push them to where I though they would
still work reliably without a great chance of failure.  About 1.5x... 


>
>Well, today I got a phone call from WIMA and we discussed the things.
>The guy said that there ARE partial discharges occuring above a 
>certain voltage level. This phenomenon takes place ONLY at AC.
>Above a certain VERY low frequency (about 1Hz, he guessed), it is
>INDEPENDENT of frequency. So we have the same problem at 50/60Hz 
>as well as in the 100kHz range.
>
>This Corona Inception Level depends on the internal geometry of 
>the cap. With one floating electrode (= series connection of two 
>plates like the FKP1-type from WIMA), it is 700VAC. With an internal 
>series connection of 4 plates, it only raises up to 1200VAC.
>
>I told him that we want the caps to last for 2000 hours and asked 
>him how hard we could drive them above the rated 700VAC. At first 
>he said factor '1.5 above', later he only suggested factor 1.25
>or with other word 900VAC. BUT: he couldn't tell me how much
>of the caps will die in this time (50% or 1% or whatever...). Of 
>course I nailed him down to tell me that important information in
>the next days. I think the number of dead caps will be quite low
>as they count failures per 10^9 cap hours. So I think we can go
>beyond these 125% AC stress.
>
>Due to the low duty cycle we typically have (<5%), we don't have to
>derate the caps HV wise. Only in high breakrate systems we have to 
>check the temperature (as Terry said).
>
>The max. amperage (pulse risetime) is not as critical as the 
>Corona Inception Level is, he told me. 

WOW!  great information!!  It is great that they called back.  I would love
to be there as people are trying to figure out what in the world we are
doing with their caps :-))

Of course, the guy would not want to be wrong and have you blame him.
Expect a very conservative answer.


>
>----------------------------
>
>
>How high is the factor Udc/Uac for the caps you mentioned? There
>are types where it is pretty low (1000Vdc/600Vac) and where this 
>should be no problem as the peak voltage (850Vpeak) is nearly the 
>rated dc voltage (1000Vdc). The caps the GSTCs ('German Speaking 
>Tesla Coilers) ordered are 6kVdc/700Vac and therefore there is a BIG
>difference!

We build hundreds of types of switching power supplies and use hundreds of
different WIMA types.  I will run a search through the failure analysis
files for the word "WIMA" and see what I come up with.  I remember one
problem two years ago but that was our fault where a part was overheated...
 WIMAs are regard as being very reliable and tough parts.

>
>
>-----------------------------

>
>What about using the WIMAs in an XEEMMC-arrangement with just
>two 6kVDC caps (33nF) in series on an 8kV-neon? This would meet 
>your 'peak voltage < DC rating'-requirement (peak 11.3kV for 
>2x 6kVDCr) and will result in an 16.5nF cap for only 1.70$US ;-)
>I think I have to perform this test just for fun. The 700VAC
>rating compared to their 6kVDC rating is very strange and if
>the caps withstand the test for any reasonable time, those
>caps seem to be the ultimate TC-caps. I hope they'll do....
>(we should get them in KW26)

I bet I can blow them up in the EEMMC configuration.  I just really like
the little cap and I hate to kill it.  Sooner or latter I'll do it.  I know
from DC destructive testing that they should start to arc internally and
the self healing may or may not protect them...

>
>Terry> And you now see why the manufacturers have so much darn 
>       trouble rating capacitors :-)))  It just isn't easy....  
>       Take your best guess as they do....
>Yes, that seems to be true. The guy had my data for three weeks 
>and still couldn't say how high the risk due to the partial 
>discharges will be. Hope he gives me some more concrete nuumbers
>in the next days when he looked over their statistics.

Or when they call us about our statistics...  I am very glad WIMA called
you back.  They are really nice people.  If I get a call from them asking
about this, I'll just tell them it only works with Tesla coils! :-)))  It
is interesting that Tesla coils are an excellent test bed for these
parts...  WIMA may be very interested in how they perform for us.  Perhaps
you could get them to send you some parts to test for them for free!!  If
they do fail unexpectedly, perhaps WIMA would be very interested in what
went wrong.  Tesla coils and our switching power applications are very
similar except we run at around the 300-600 volt level.  If Tesla
applications have a problem.  Only the voltage should be different.

Cheers,

	Terry