Motor run capacitors - why do they fail ?

Thread Starter


Joined Nov 29, 2005
In a couple of months, I have replaced capacitors in a blower motor, compressor, 2 ceiling fans. All OK after.

What makes them fail ? Are they designed with not enough overhead on voltage, temperature rating, other ?
Replacing them with what extra rating will ensure longer life ?

Am sure maaany appliances are discarded or replaced with new just because the owners believe they are beyond repair, or the repairman person prefers to $ell new equipment.
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Joined Jul 18, 2013
In a couple of months, I have replaced capacitors in a blower motor, compressor, 2 ceiling fans. All OK.

What makes them fail ?
There was a time when it was extremely rare to replace start/run capacitors, as opposed to the bi-polar start only caps, which only Occasionally failed.
But with the increase of offshore imported equipment it is getting to be a very regular occurrence for both types.
For a Motor I own, I would ensure that the replacement capacitor was from a manuf. with a good reputation and of N.A. origin.
I recall some years ago an importer of some Chinese machinery, the first thing he did was to replace the motors.


Joined Jan 15, 2015
Why do they fail? I am with Max on this one. My home air conditioner is about 10 years old. This summer I replaced a dual start capacitor for the Fan & Compressor. Not really bad sine the unit is outside in the elements 24 / 7. Winters bring sub zero F and summers 90 F. I really should place a small roof over the AC unit and the emergency generator. Anyway I went with a capacitor branded Cornell Dubilier Electronics a name I have trusted for decades. As to why they fail? Likely poor quality at time of manufacture. In my case possibly the constant expansion and contraction of the can season to season over a decade.



Joined Sep 22, 2013
If I had several start caps blow around the house, within a short time....I would suspect a line high voltage spike. You might want to inspect your power panel terminals for tightness and arcing....and any other sensitive electronics. Check out input sine to panel with scope. Might consider line conditioning.

Our electric service much less reliable than it used to be. The sine is not symmetrical.....and very noisy. It's like third world.


Joined Nov 30, 2010
A/C was my day job for decades and I can say a bad capacitor was in the 1% area for failures. Still, I carried them in stock at all times because 1% for me resembled one every month. When you lose a batch of them, look for an outside influence like lightening surges.

ps, The last time I looked, over-rating the voltage on a capacitor for increased lifetime is valid up to 200% of the applied voltage. You use 240 VAC? Anything up to a 480 V label rating is OK, but 370 volts is what you will be more likely to find.
In a couple of months, I have replaced capacitors in a blower motor, compressor, 2 ceiling fans. All OK after.
With that many failures all close to each other, I think you got a big spike.

Is a whole house surge suppressor with a connected equipment warranty worth it?

All I know is, I put an bidirectional Transorb on the R & C terminals at the furnace and an RFI filter. Both missing on a Carrier Infinity furnace. No wonder why they fail.

I've replaced ceiling fan switches, capacitors and fan direction switches. The direction switches fail because they aren't rated to switch a running fan. The age of the caps easily exceed 10 years.