Capacitor exploded, any ideas why? / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

Capacitor exploded, any ideas why?

1 2 3 next

Move Thread

LAN_403

Neil

Tue Jun 24 2008, 08:52AM

Registered Member #1458 Joined: Wed Apr 30 2008, 09:47AM
Location:
Posts: 51

Hi All,

Recently finished my capacitor bank project and was testing it out, had done about 7 shots with it, slowly working my way up to full power. Working voltage is 5000, and these caps can go to 7500 apparently peak. At the time I had got to 4000 and was just about to fire it when one capacitor exploded and did some damage to my bank. Here is the Video of my bank, at the end shows when it exploded, though unforunately had the camera zoomed in on the can I was about to crush at the time.

I'm not so worried about fixing it, what worries me is if one blew up, will the others. If anyone can help me decide or help me work out exactly what caused it please do. I would hate to fix it and just have another one go on me. I was lucky this time it only broke the acrylic on the end, it would have been a shame to have wrecked the side plate or the floor that everything screws to.

Here are some pics of the capacitor that exploded

Some info from where I bought it

Model Number of the capacitors are K75-40a

POLARITY: Any DC or AC

Peak Working Voltage: 7500 V

Discharge current amplitude: up to 1400 A

Dissipation factor at f=50Hz: <0,008 !

Time constant: >3000 MOhm.uF

Insulation resistance: >15000 MOhm

Single shocks witch acceleration: up to 500 G

Temperature range: -60...+70Â°C (-76...158Â°F)

Atmospheric pressure of work: 0,00012 to 294 000,0 Pa

Working Position -Any

Material of electrodes:Composite of metals witch a special film coats

Electrode insulator: Polyethylene-tereftalate

Marko

Tue Jun 24 2008, 11:43AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Woah Neil, and I just wanted to post an admiration to your project...
There's nothing I hate more than a nicely and painfully constructed project which messes up in the end. If it was me now, I would have probably given a good amount of shots to those caps just sitting on the floor before building an enclosure like that.

And yes, if you lost a cap so fast, I would be really concerned about the rest.

Have you estimated how much peak current are you drawing per capacitor? 1400A doesn't look like much of a rating for pulse capacitors.

OZZY

Tue Jun 24 2008, 12:35PM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

Hi Neil

Are these foil capacitors or Metallized film capacitors?

Foil caps are suitable for pulse discharge (can crushing, coin shrinking etc.) but for caps that size the max peak current would be more than 1400A. Metallized film caps are not suitable for very high currents (can crushing, coin shrinking etc.) but can be used for slower discharges (coilgun, railgun etc.).

How much voltage reversal are they rated for? Most pulse caps are only rated for 20% and will need some protection, or be operated at lower voltage. Are they new or used? How old are they? You may want to be sure they are in good condition before charging to 5000V.

Possible failure modes:
If they are foil caps and all 9 are conected in parallel, then when the dielectric in one cap fails the energy of the whole bank will be dumped into the arc. Bang!
If they are metallized caps they will self heal by vaporizing some of the metallization, but if this happens a lot then pressure will build up inside the cap. Bang!

I hope this helps.

Ozzy

teslacoolguy

Tue Jun 24 2008, 01:36PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

It looks like he is using a bunch of these caps

...

Tue Jun 24 2008, 02:59PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Those look like oil caps, and aren't suited to high peak powers. They are designed for things like smooting your power supply, dc blocking, etc.

Neil

Wed Jun 25 2008, 10:08AM

Registered Member #1458 Joined: Wed Apr 30 2008, 09:47AM
Location:
Posts: 51

Thanks Marko for your comments. I didnt fire it enough times to get the chance to try and work out peak amps. I have an osciliscope that I had all ready to test it to work out discharge times and voltage reversal but it exploded before I had the chance to test.

Ozzy, as far as I know they are foil capacitors, I guess out of assumption at the lack of finding anything saying otherwise. I'm not sure what voltage reversal they are rated for, I assumed some would be ok, for a while at least given their apparent rating for AC and description of what they were designed for. They were all hooked up in parallel so yes unfortunately all of the power from the bank got dumped into that one poor capacitor which didnt fair too well. They are brand new, old stock caps.

Teslacoolguy, yes they are exacttly the capacitors I have been buying up. For me personally, I found it very hard to find capacitors that suit the application we put them into when building this sort of equipment. I often look through ebay both in Australia and overseas and so far these were all I could find that I could buy in a quantity that would fulful what I was after, They seemed ok from their descriptions though admitadly I was a bit hesitant when I seen their peak amps were only 1400, I thought they should probably be more for a capacitor of this application.

Im thinking perhaps I should be looking at fitting out new capacitors to this unit...

Thanks for your comments which have been very helpful

Neil.

Shaun

Wed Jun 25 2008, 02:27PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

That's really too bad about such a nicely constructed project. The first sign, for me, that something was wrong was when you only squeezed those cans inward a little with multiple kilojoules of energy. 2kJ of pulse capacitors would have ripped them in half and thrown the halves across the room.

If the rest of the capacitors are undamaged, they would make a nice coilgun. With all of them in parallel the 1400A peak would multiply by however many capacitors you have. Plus with reverse protection diodes you wouldn't need to worry about voltage reversal killing your capacitors.

Fraggle

Sat Jun 28 2008, 05:51PM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

Neil, dont overlook microwave oven caps - you could easily achieve that can-crushing result with as few as six and they`re easy to get hold of.

Myke

Sat Jun 28 2008, 05:54PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Six? That's 12J...
I don't think 12J is enough to crush a can. I could be wrong though.

Fraggle

Sat Jun 28 2008, 06:12PM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

Yeah but you can charge them repeatedly to at least 9kV, I charge mine with three MOTs in series and an HV diode (behind 2 building blocks just in case).

1 2 3 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob