Dents in Capacitor Cans

[Drivesrock]

Hi all.

I'm on a job replacing PWM filter capacitors - 20uF, 690V - for active front end drives. Recently, one of the filters was not working and the cause were the capacitors that although measured out within tolerance, were making the filter draw too low current and the protection detected this and stopped the VFD. New caps fixed the issue.

Now back to replace others as preventative maintenance but some of the spare caps have dents in their cans so I'm reluctant to use and have not - but now we're short of a few! One can't dent the can without a drop or a hit. None have holes. In my mind, they were roughly handled somewhere - maybe when transferring from original packaging to bubble plastic wrap.

I know that large dents close to seals are not good and large dents perhaps could give a hot spot when in operation? They are not bulged.

Anyone have any comments?

Thanks

 

[Skogsgurra]

Just checking an ASEA MF induction furnace H bridge inverter. There were capacitors that looked like this:

They all have dents in them. Obviously from the use of a polygrip instead of a prober (band) tool. They are quite old but checked out good. Meggered with 1 kV, measured capacitance, ESR, insulation to can etcetera. They all checked out good.
But it is not good, of course, to dent them. You never know what happens after a while.

They have been running for around forty years. No maintenance. Ever. It is about time, I think.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

read PROPER

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

Dents are going to put the plates and interposing dielectric into compression. Subsequent thermal cycling is going to work the dielectric out of the compression zone. This will reduce the capacitance some. The operating voltage is a function plate spacing, with them mashed closer it's likely the voltage rating has been reduced some finite amount.

I know that some caps swiftly explode when dented others live on with long lives. Outcomes are probably dramatically different based on the service a dented cap finds itself in. If the caps are run near their design limits they will likely fail considerably earlier than specified. If they live in a cool environment and don't have thermal cycles they could last a long time. If 690V caps are run by a 480V system you're running them only 11V off their ratings. Bad juju. If they get put into 400V service then they have 124 volts of head room. They might live full lives. Drives have a hard life with typically lots of thermal cycling. I'd find some undented caps unless the drives are old and will have shorter lives anyway.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

Dents are better than bulges.