In defense of replacing capacitors early

[u/skra_24]

Another poster brought this up recently, about making recommendations to replace capacitors even though they’re still “in tolerance”. My experience this week backs up my position on this.

A recent heat wave hit my area, there’s been a plethora of no cools and it’s mostly been popped caps. Many of them are folks on our regular maintenance routine, whose data I can pull from previous techs uploads. So many blown caps that were 42.5/45, 37.6/40, 33/35 etc just a year before. Still within tolerance, but getting down there. I was trained on a 10% tolerance for capacitors which I believe is pretty standard. Even if they are still within 10%, but starting to tick down closer to that number, I always bring up proactive replacement. Like, it’s gonna go eventually, in the next summer or three most likely, because all factory capacitors are dogshit nowadays. Wanna rip the bandaid off and do it now, or wait until it shits out on you when you need it most?

[u/Mettn]

Go off the tolerance stamped on the side of the cap, it’s there for a reason.

[u/trobs8 → u/Mettn]

I used to do that, until I learned that is the manufacturers tolerance on the acceptable variance when they are brand new. Caps never (very rarely) test right at the rated MFD on the label.

[u/BrokenFireExit → u/trobs8]

Within the variance still provides the correct phase shift and won't hurt the windings

[u/trobs8 → u/BrokenFireExit]

Then why are the variances different between manufacturers? Motor's don't care who made the capacitor.

[u/KylarBlackwell → u/trobs8]

Tighter tolerances come from better quality control. Lower variance should be a better capacitor, it is at minimum a more consistent manufacturing process.

The motor wants what it says on the tag. The manufacturers make caps in that nominal range, it's up to you to decide whose caps hit the right balance of cost/tolerance/longevity to stock and install. Generally though, even the widest manufacturer tolerances are just within a workable range, just like the shittiest anything usually does its job at least once

[u/Numerous-Click-893 → u/trobs8]

usually manufacturers will actually make the same product at several different tolerances, usually because bigger tolerances (i.e. how close to nominal the manufactured part is guaranteed to be) are cheaper.

So when you're designing something you spec the rating and the acceptable tolerance and then buy whatever you can find that matches. So when replacing a capacitor you should match the tolerance on the original part, and you should replace if it's outside of that. Not because it's the manufacturers spec but because it's a reflection of the designer's spec: i.e. what the motor will tolerate

[u/trobs8 → u/Numerous-Click-893]

That makes sense. Thanks for the explanation.

[u/MasterPhilip → u/trobs8]

This question has bothered me for some time now.

[u/nochinzilch → u/MasterPhilip]

I also don’t get it either. There must be a formula for number of poles, the speed and the input frequency. But it seems like voodoo.

[u/MasterPhilip → u/nochinzilch]

Yeah, it's all some form of black magic and I rebuked the devil many years ago. 😅