General > General Technical Chat
The unfortunate shrinkage of capacitors
Ed.Kloonk:
--- Quote from: james_s on January 11, 2022, 04:47:16 am ---
--- Quote from: IanB on January 11, 2022, 04:11:17 am ---I suppose I should check the one in my A/C unit. However, it seems to work fine and has been in use for at least 20 years.
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Mine died when it was about 12 years old. A mouse had got in there and fried itself across the terminals and in the process it peed on top of the capacitor and that rusted the terminals off.
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Occasional visual inspection can save ya. One of my A/C's on the other side of the house has 10 more years seniority and needed it's TLC a number of years ago.
Poking around I found evidence of Jerry the mouse who had recently moved in nearby. There was no Tom the cat, but some strategically placed baits sure sorted him out and prolly saved me a mint cos we found a bit of power cable being gnawed on.
IanB:
--- Quote from: T3sl4co1l on January 11, 2022, 04:21:51 am ---If it's across the line, it's probably exposed to a lot of transients, maybe a MOV across will do? Then choose voltage rating to meet/exceed MOV peak (surge) voltage, and it should last "forever". Shouldn't be a problem where it's behind a winding (run cap?, ferroresonant, etc.), the inductance should do well as filtering?
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This was a run cap for the furnace blower motor. It had definitely failed, the motor was sluggish to start and would sometimes stall. The cap was rated at 7.5 µF but measured 2.2 µF. After the cap replacement the motor runs so vigorously it is like a hurricane.
--- Quote from: floobydust on January 11, 2022, 04:37:21 am ---Was it an electrolytic or polypropylene run cap?
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I'm not sure it is electrolytic. I think they are made from aluminized plastic film and filled with castor oil
I looked at a video where someone disassembled a failed capacitor, and it had visible bulging and distortion of the internal structure.
Kleinstein:
The run capacitors are usally non electrolytic, to day polypropylene. In the old time paper (with oil) capacitors were used and these were considerably larger.
It looks like they improved (more consistent) the production of thiner foils and this way allowed a smaller form factor. A foil that is 10% thinner would be a 20% reduction in volume as less area is needed.
The starter capacitors are sometimes non polarized electrolytic types as they are not used that long and often need higher capacitance. With reduced size for the PP types it can be possible to use a film type instead.
As far As I understand it the capacitors suffer from overvoltage spikes, e.g. when the motor is switched off.
Chances are the loss in capacitance in nonlinear in time - the smaller the capacitance the easier the capacitance can rise too high. So once too small the detoriation can speed up.
magic:
--- Quote from: IanB on January 11, 2022, 03:45:37 am ---I'd rather pay twice as much, and not have to worry about replacing it.
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There may be a way if you are willing to pay 4x more ;)
TERRA Operative:
You can always use a new capacitor with a higher voltage rating to get something with thicker plastic laminations inside that will last longer.
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