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Looking for mechanism by which motor run capacitors would be slowly destroyed
floobydust:
Film capacitors that fizzle and fade, going low value- are common and due to the metallization failing from corona or corrosion. You get discharges that cut and slice up the metalization and reduce the plate area. Hurrah "self-healing" lol.
Metallized Film Capacitor Lifetime Evaluation and Failure Mode Analysis
SEM Lab Failure Analysis of Film Capacitors
It would be worthwhile to do an autopsy of your caps. If they're oil-filled I think that is a different bird, as run caps they can take much higher ripple current.
bdunham7:
--- Quote from: floobydust on November 03, 2023, 04:24:17 am ---It would be worthwhile to do an autopsy of your caps. If they're oil-filled I think that is a different bird, as run caps they can take much higher ripple current.
--- End quote ---
I think they're typically resin-filled and an autopsy would be a challenge. Edit: There are oil-filled as well, but I found none with detailed specs. I don't know about them taking much ripple current, they are made to be physically rugged more than anything else. Most of these don't have very detailed specifications, but let's look at a KEMET C870CF35400AA0J.
https://www.mouser.com/datasheet/2/447/KEM_F3063_C87-3316450.pdf
* 420/470VAC, depending on the life you would like.
* Specifically rated for 50/60Hz, implying a current rating (not explicitly stated) of about 7 amps. Maximum permissible current is listed as 1.3X. That says to me that the cycle current should never go over about 9A.
* dV/dt is 15 or 20V/µs, it appears to me that in the OPs circuit 240VRMS @ 8 kHz he'll have a dV/dt of about 17V/µs.
So we'd need actual measurements of the ripple current in his application. This capacitor might be marginally OK or it might be not acceptable under normal conditions. It would seem to not be adequate for the overload conditions if his description is accurate. The original capacitors they are using may be of much lesser quality since he describes them as 'cheap'. He doesn't say what sort of life they are getting out of the current models, I'd be curious if they are getting 1000 hours or 10,000 hours or what.
chinoy:
I had this problem. In the end it boiled down to the wave form that was being feed to the motor.
The generator / UPS that would power it was not putting out a clean sine wave. So I was changing caps every 3 months till we cleaned up the input power.
Other tricks I use is buy more UF and voltage than you need. That way as the caps ages it lasts longer.
Next mount the cap in such a way that the heat of the motor does not impact it. I had a compressor that kept burning thru caps. As soon as I installed the cap off the compressors heat and vibrations cap life was never an issue again.
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