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Some really burned RIFA capacitors, but not X class

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cybermaus:
So my brother trades in old machinery, and he recently got two Bridgeport CNC routers from ' 89
Pretty early for CNC machines in fact. They had been in use until very recently at a university, where he picked them up himself.

Firing the first one up, after 20 minutes of fiddling with its mechanics, it released the magic smoke. And *lots* of it.
Being in an enclosure, it took some time to detect, and some more to respond and douse the fire, see for yourself.

Checking the 2nd machine, with a IR camera at the ready pointing at the same components, indeed, the capacitor themselves very quickly shot to 50C, and crawled to 80C in less then a minute. Normally not a really temp, but it was too quick, and nothing else was even getting warm yet.


Of course, it is always the question when finding a burned component: was this the cause, or a consequence.
Well, given this video and this thread, and the fact nothing else got hot, we think/hope this is the cause. They will be replaced for sure, all 12 of them.

Slightly worry however is that I measured the non-burned ones, and they measure a nice, exactly 0.1uF and 0mA leakage at 360V (that is as high as my leakage test setup goes)
So they measures very well. Except for the ESR, that seems a bit high to me, but I am unsure what is normal for a film capacitor like this.

So question: what would be a normal ESR value for a 80'ies RIFA 0.1uF non-X class film capacitor? Or any such film cap from that era?

I did check some equivalent modern ones, and they are lower, but I am not sure if I should compare 2010 devices to 1980 devices.

Thanks

Gyro:
The failure mode of Rifa MP (Metallised Paper) capacitors is moisture ingress due to the epoxy package cracking. As you would expect, this is most likely to occur after a period of storage. If there is visible cracking then you have no choice but to replace them as a matter of urgency.

With values as low as 100nF, it isn't really a matter of measuring ESR, but Q and dissipation factor [Edit: damp dielectric is likely to cause increased dissipation I suspect]. In this case, it looks as if the capacitors are being used in RC snubbers across the SCRs, where the resistor should swamp such factors. Metallised paper caps have a slightly higher dV/dt withstand capability than metallized film, but again it shouldn't matter in this case.

Either way, you're probably going to want to replace the caps with MKP Polypropylene film - although you do also have the option of using newly manufactured Rifa MPs which will probably be reliable for a few more decades of active use.

eti:
I see a smoking RIFA 😄 but as opposed to a reefer, one doesn’t get a choice, and it’s potentially far from worse for one’s health.

cybermaus:

--- Quote from: Gyro on October 07, 2021, 08:06:54 pm ---it looks as if the capacitors are being used in RC snubbers across the SCRs
--- End quote ---
correct.


--- Quote from: Gyro on October 07, 2021, 08:06:54 pm --- but Q and dissipation factor ... dV/dt withstand capability
--- End quote ---
Ok, got some studying to do, those terms are new to me (in the context of capacitors)

My reasoning of suspecting the ESR is/was (other then they measure a little high, but really not extremely so):
To get hot, there must be energy dissipated, and a high resistance in combination with current constantly flowing in- and out of the capacitor (either AC or spikes) gives a simple I2R

Anyway, obviously I already have polyfilm's on order (I only had 450V rated ones lying about)

Edit: relevant link

richard.cs:

--- Quote from: cybermaus on October 08, 2021, 06:29:01 am ---My reasoning of suspecting the ESR is/was (other then they measure a little high, but really not extremely so):
To get hot, there must be energy dissipated, and a high resistance in combination with current constantly flowing in- and out of the capacitor (either AC or spikes) gives a simple I2R

--- End quote ---
These normally fail the other way -  the leakage resistance in parallel with the capacitance causes enough current to flow for heating. The ones that measure no-leakage are probably OK for now, but will inevitably fail the same way at some point, this type are infamous for it.

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