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Looking for mechanism by which motor run capacitors would be slowly destroyed
johansen:
Standard off the shelf cheap motor run caps, 40uf at 370 or 440volts.
They filter a saturatable inductor at perhaps 8khz pwm to produce a 240v sine wave.
Nominally the capacitor will have 4 amps at 60hz and perhaps 4 amps rms at say 8khz. But during 150 amp overload, the capacitor may have my guess is 20 or 40 amps rms at 8khz.
Failure is gradual, but once the capacitance drops too far the customer complains, for obvious reasons.
Only idea i can come up with is thermal expansion cyclical stress in the capacitor.
Any other ideas? Over voltage isnt possible.
Psi:
Maybe just spec a 650V cap.
How are you sure overvoltage is impossible?
Maybe the motor has a minor fault, or is just very inductive, and is producing high current pulses which are causing higher than normal voltage spikes over 375/440V. Or could be spikes coming in from external dirty power at the factory.
Or could just be cheap china motor run caps, have you tried a better brand?
Could also check what temp the cap is running at in their installation. Maybe it's just getting too hot and failing from accelerated old age
It's a bit messy, but you can take the faulty cap apart and unroll it to see if its full of areas that have been blown away from overvoltage or not.
NiHaoMike:
Try using a pair of 80uF in series?
woofy:
Maybe heat and/or poor quality capacitors. These are non-polarized aluminium electrolytic capacitors and if the electrolyte drys out they will degrade.
Gyro:
--- Quote from: johansen on November 01, 2023, 04:22:38 am ---Only idea i can come up with is thermal expansion cyclical stress in the capacitor.
Any other ideas? Over voltage isnt possible.
--- End quote ---
Motor run capacitors are metalized Polypropylene film. Under transients they self heal by evaporating the metalization around any film puncture. This leads to a loss of capacitance over time. It could be that if the capacitors experiences currents of 20A+ the current density in the metalization becomes high enough to vaporize or disconnect areas of it. This would account for the reduction in capacitance that you're seeing.
Several manufacturers, eg. Epcos, produce ranges of film capacitors that are specifically designed for high frequency, high current / high pulse applications. They will be nowhere as cheap as bog standard motor-run capacitors though.
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