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Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
I have had a failure in my MMC cap bank. I thought it was puzzling that the 33M x 3 one watt resistors had blown and had arced over because it implied that the cap was open circuit to AC.
I performed an autopsy and found no obvious blowouts or external damage. Strangely the capacitance was only 92pf rather than 330 nF (330,000pF). That 92 pf was at least partly the leads and my hands. What appears to have happened is that the film layer joining the two caps in series has completely eroded away, separating the halves by almost 1/4 inch.
Click on the enlarged pics to see the eroded film.
Is anyone aware of this failure mode? It doesn't bode well for the rest of my MMC which is now 4 or 5 years old running an 18kW TC.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Peter -
This could be the result of combined over-voltage and over-current, with the overvoltage being the triggering event. Overvoltage will result in punctures of the polypropylene dielectric film. A dielectric puncture will vaporize small circular areas of metallization on the "floating" plate surrounding the site of the puncture (C-D's "self-healing" feature). These microscopic punctures usually occur along the edges of the metal foil plates, where the field gradient is highest. As more and more punctures occur and more of the metallization is eroded from the floating plate, eventually a point is reached where the remaining metallization cannot carry the peak current, there is significant internal arcing across the metallized film (as opposed to arcing through the dielectric film), and the cap goes "open circuit".
Sometimes the last stages of this internal arcing can be seen as flashes of light from inside the capacitor, but this is usually not noticed given all the other excitement when a coil is running.
If you've got access to a quality stereomicroscope with a good illumination system, you may be able to see the punctures in the dielectric and the circles of vaporized metallization on the floating plate. This might be difficult on the sample shown in your photos, as there is almost no metallization left on the floating plate.
Somewhere I've got photomicrographs of C-D 942's that suffered this type of failure, but I can't find them right now.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Peter -
Mein Gott!
23.6KVAC into a cap bank rated for 24KVDC?? How many hours of operation at full throttle until the first signs of cap failure were seen?
It's not clear in your MMC photo if bleeder resistors are used across each cap. If not, where were the 33M Ohm resistors located, schematically and physically?
I think there's a good chance that if you pull a few more of the caps and measure their capacitance, you'll find them to be low. If so, it's likely that all the caps have been permanently damaged. Over time, have you had to continually re-tune the coil to compensate for lost capacitance?
(If you don't have access to a good microscope system, feel free to send me the remains of the cap by snail-mail, and I'll finish up the failure analysis and take some photomicrographs for you.)
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Herr Zapp wrote ...
... How many hours of operation at full throttle until the first signs of cap failure were seen? It's not clear in your MMC photo if bleeder resistors are used across each cap. If not, where were the 33M Ohm resistors located, schematically and physically? I think there's a good chance that if you pull a few more of the caps and measure their capacitance, you'll find them to be low. If so, it's likely that all the caps have been permanently damaged. Over time, have you had to continually re-tune the coil to compensate for lost capacitance? Herr Zapp
I originally built this one for a 4 MOT 6 inch coil runing 8 kv so it would have been in spec then. When I built my 18 inch coil running 16kV I used it knowing it was on the "bleeding edge". But it worked and that was about 4 or 5 years ago. All my big coil stuff uses it. No idea of total run time. It uses a bleeder resistor of 10 MOhm for each cap (3.3M x 3 1 watt). I don't see how the capacitance in the cap should vary until the film connection is finally broken, at which stage it will suddenly drop to near zero. They have a spiral construction.
More importantly it shows that the 0.33uF series are good in TC service. Although the rating is 1600V rather than the 2000V of the gold standard 0.15 series, they seem to be better value for money. Robin Copini in Australia also uses them.
Now the problem: I need a new tank cap fast - before a show on March 20. How can I get 120 of these CD 942C16P33K fast (needed in 4 weeks in Australia)? Digikey appears to be the only stockist and they are on the not available list.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
More importantly it shows that the 0.33uF series are good in TC service. Although the rating is 1600V rather than the 2000V of the gold standard 0.15 series, they seem to be better value for money. Robin Copini in Australia also uses them.
That is good news, that seems to mean my .68uf 1600v CDE caps are good. I hope so, i will be trying to get at least 6' arcs using 16 of these caps.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Peter wrote: "I don't see how the capacitance in the cap should vary until the film connection is finally broken, at which stage it will suddenly drop to near zero. They have a spiral construction."
The reason the 942 caps will slowly decrease in capacitance value is due to the unique internal construction that provides the "self-healing" or "clearing" capability when the dielectric is punctured by overvoltage. The self-healing function is provided by allowing a circular area of metallization on the floating plate surrounding a puncture site to be "vaporized". This loss of conductive material from the floating plate prevents arcing through the permanent new hole in the dielectric, but it also reduces the effective plate area by a tiny amount. Every time there is a dielectric puncture, there is a tiny loss of plate material, and a tiny loss of capacitance. Thousands or tens of thousands of punctures in the dielectric will eventually remove enough material from the floating plate to cause a measurable loss of capacitance.
Desolder one end of a few capacitors in your MMC and measure the capacitance. I think there's a good chance that you'll find them "low" due to loss of material from the floating plate.
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Strange MMC cap failure with zero capacitance
