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Capacitor safety vent audible sound
morwind:
Hi I am a degree student in the UK, I am looking at designing an early detection sensor for the detection of failing capacitors in variable speed drives in lift shafts, during operation. Albeit the sensor could be used in a wide range of remote unmanned applications (substations etc).
So the entire basis of my sensor is on the theory that when capacitors vent due to electrolyte breakdown they start to produce hydrogen gases inside the can which is then vented gradually through the safety vent over a period of time. Note: not instantaneously like all the "blowing up capacitors!!!" videos around the internet.
I am talking about during routine operation, not reverse biasing, over voltage etc....
My sensor would detect the audible noise emitted during the venting process and when detected an event would occur such as: disconnect vsd supply voltage. Preventing further damage to the circuitry.
TLDR: Am I right in thinking that there is a detectable/measurable audible noise when the aluminium electrolytics vent? Which increases gradually as the capacitor vents.
Ian.M:
Your assumptions are incorrect. There may be an initial 'pop' of some sort - either the bung to can seal failing or the can splitting at the stamped vent lines, but once that's happened there is unlikely to be sustained noise from the failing capacitor detectable over the usual background noise level in a plant room, let alone a lift shaft, and the pop is going to be near impossible to distinguish from random background noises. Increased ripple on the DC bus may result in increased noise from other components but that's going to be extremely model specific.
You might get somewhere with a chemical sensor to detect vapors of typical capacitor electrolytes ...
It may be better to tackle this at the VFD design level. It would be reasonably simple to detect excessive DC bus ripple and depending on its severity, either indicate a fault and enter a degraded performance 'limp home' mode or trip out.
wraper:
If capacitor fails gradually, there will be barely any noise when vent ruptures, if it ever ruptures at all (in most cases it does not). Noticeable sound will happen only if capacitor is put under extreme stress which will create rapid pressure increase within the capacitor.
--- Quote ---My sensor would detect the audible noise emitted during the venting process and when detected an event would occur such as: disconnect vsd supply voltage. Preventing further damage to the circuitry.
--- End quote ---
If your sensor will be so sensitive to have ability to detect anything at all, there will be orders of magnitude more faults created by false positives, than actual failures detected. It's a tech which has no use in the real world.
ogden:
Many most electrolytic capacitors don't die with bang. They gradually "dry out", lose capacity until circuit can't tolerate ripple anymore.
Benta:
I'd think that a more viable way of keeping an eye on electrolytic capacitor health would be monitoring ESR.
Just an idea.
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