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Thoughts on the Aerovox RBPS snubbers for an MMC

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Feathers

Wed Feb 12 2014, 01:43AM

Registered Member #10052 Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78

Many of us use these capacitors

for, well, snubbing. And they seem to do alright.

I remember taking a look at one of ArcAttack's show coils at a Maker Faire, which used a few dozen tab-mount IGBT snubbers as an MMC.

Polypropylene dielectric
VDC: 1000V
C: 2uf
Ipk: 854A
Irms: 37A

At first these numbers look GREAT for the price (around $2 per cap!)

BUT I heard that TeslaOrchestra experienced a nasty failure running them in an MMC.

I was planning on using these in a design for a large 10-15KW DRSSTC:

Tank Capacitor Array by HighVoltageFeathers, on Flickr

Any thoughts or experience with these units?

HV Enthusiast

Thu Feb 13 2014, 07:07PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

They will certaintly blow up if you use them like this.

Mads Barnkob

Thu Feb 13 2014, 09:00PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Just from the numbers given in the data sheet I see no problem using these in a MMC, one thing have me worried though.

There is no X vs. frequency data to find on their homepage or in data sheets. The AC voltage rating is also given at 60Hz, that tells us the AC rating could be pretty low at 300 kHz.

Hydron

Thu Feb 13 2014, 11:47PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

I built a MMC out of them for my coil, 2 parallel strings of 12-24 in series (depending on coil setup), running up to about 4kW (tank current ~700A peak). Coil runs at ~65-80kHz depending on which pole I tune for, I'm assuming you'd be running lower frequencies with the higher capacitance setup you have rendered.

They seem to work just fine, no sign of heating with reasonable run lengths (up to maybe 10-15 min continuous at ~2kW). 1/4" primary coil was quite warm while the caps were cool to the touch.

I may be able to do some better characterization of one of the caps this weekend (impedance/capacitance vs frequency, not sure if I can do tan delta), but I need to fix some of my test gear first.

If you're getting them off CTR surplus (ebay) then I had a good experience dealing with them, I paid $99 for 50 used caps (all of which tested fine) and they threw in a few extra too. You may get a better deal if you make them an offer or something.

Edit: as for the voltage rating, I'm running much below 1000V/cap in my coil, so I didn't really worry about it. The number in series I am using is more to do with getting the capacitance low enough than keeping the voltage rating up.

Feathers

Fri Feb 14 2014, 02:15AM

Registered Member #10052 Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78

The coil in mind will run just above 30kHz, and they are polypropylene caps rated for handling high peak-currents.

They certainly aren't meant to dissipate heat very well, but they are so inexpensive that I should be able to build a BULLETPROOF bank.

Thanks for the word on experience Hydron! So far its one-for-one on these caps.

Hydron

Fri Feb 14 2014, 02:36AM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

I did have a bit of a look at the forums and the pupman list before I bought mine, there was some speculation about using them in a MMC but nothing about if they worked in practice, so I just took the gamble. I didn't know anything about the Tesla Orchestra experience, would be interested in any info about that failure, incase it's something relevant to me.

Anyway, they worked out well for me, hope it works for you!

HV Enthusiast

Sat Feb 15 2014, 09:29PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Interesting. I always thought that these capacitors, being snubbers, had a relatively large R in series with the C which is typical with snubbers.

Wolfram

Sat Mar 08 2014, 02:50AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Mads Barnkob wrote ...

Just from the numbers given in the data sheet I see no problem using these in a MMC, one thing have me worried though.

There is no X vs. frequency data to find on their homepage or in data sheets. The AC voltage rating is also given at 60Hz, that tells us the AC rating could be pretty low at 300 kHz.

The RMS voltage vs. Frequency graph is usually just a way to express the RMS current handling capability of the capacitor.

Steve Ward

Thu Mar 13 2014, 02:30AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

The RMS voltage vs. Frequency graph is usually just a way to express the RMS current handling capability of the capacitor.

I agree with this, however, stressing caps in both voltage and frequency could be a recipe for fast failure. The faster voltage reversal at high frequency would likely stress the dielectric worse when running "at the edge" due to space charge injection into the dielectric. From what i understand, when you charge up a cap, its likely that some small charge can leave the plate and get "stuck" in the dielectric. Then later, when the voltage reverses, this charge gets sucked back out, causing some small damage to the dielectric. Repeat fast enough and it could go pretty quick.

AFAIK the snubbers use pretty much the same self-healing film technology as any other MMC caps. The benefit of the snubbers is the heavy duty end connections, allowing for better current handling. I prefer them to the CDE 942 series, more effective KVA rating for your money (even when buying brand new snubbers).

1000VDC caps usually have like a 500VAC RMS rating, i would stay below that 500VAC in operation.

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