Electronics > Repair

Motor start or motor run capacitor

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--- Quote from: SeanB on May 16, 2021, 09:46:17 am ---Even the run capacitors will fail, as they slowly self heal from the voltage spikes at turn on and turn off. The 330VAC capacitor will survive this better, so should last a lot longer in service as a result. If you can get a 440VAC rated one that fits there even better, but anything over the original 250VAC rated will be better.

--- End quote ---

Why would there be spikes across the capacitor in this situation and if there are, why aren't they a problem in every PSC motor?

Some 50 µF is a rather large capacitance and I would thus not expect very high spikes as the motor is likely not that large.
At least in 230 V land, 40-50 µF would be about suitable for a 3 kW motor. With 115 V it would be less (e.g. 900 W) but still quite large for a garage door opener.
The exact suitable capacitance can vary on how the motor is build.

A garage motor is likely not switched that often to make the cap age very fast. A little higher voltage can help, as the capacitor can start out with some voltage before an extra spike from switching.
The motor caps do age, at least with 230 V use. It may be better with 115 V, as they may have more reserve (PP caps are rarely available with low voltage ratings).

A properly rated capacitor should last many years. I know of multiple 35+ year old garage door openers that are still going strong on the original capacitor. Even with daily use they don't accumulate all that many hours.

Looks like they have specified 2 different caps based on 2 different motors. I assume you have checked the motor P/N according to the chart in that bottom left corner.

Unusual to see only 5 year lifetime, likely it's just a cheap part- but electrolytic start capacitors have a bakelite case, large rubber bung+phenolic lid and do dry out, no "self-healing", they're not the greatest.
The same motor run capacitor is commonly used for many other 1/3HP door/furnace fan motors. It's a standard EIA case size (not metric).
Wayne Dalton/Challenger/Allstar 260572, 43-52 MFD 220-250VAC, 3.375" x 1.4375", 86x37mm.  Mallory/Cornell Dubilier PSU4335B USD $17.75 Mouser

I would change over to a polypropylene film run cap, they don't dry out like electrolytics. But the physical size is bigger and many are 370-470VAC which is huge, i.e. Kemet C276.
It seems to be nominal 45uF +10/-5% to get the "43-52 MFD" range they give. But I saw a reference to same Westinghouse 1/3 HP motor 322P317 type 123C12 using 54uF 250VAC for the run cap. I wonder if 50uF would be better.

China’s national capacitor and resistor standard GB/T 2470-1995 (GB2470-81) is used for their parts. They have completely taken over the motor capacitor business.
You had a type CD60 and changing to a CBB60 is what I would do, if it will fit. Their parts are metric.
CD60  Motor Start Capacitor, electrolytic, bakelite case (same as old part)
CBB60 Motor Run Capacitor, polypropylene. Will withstand 2X voltage surges.
CBB65 Motor Run Capacitor, polypropylene, oil-filled aluminum can, pressure safety switch. (these are huge)

Original part is 37x86mm, I see CBB60 45uF 250VAC are 40x70mm, 50uF is 45x90mm. 45uF 400VAC 50x106mm.
CBB65 45uF 370VAC oil-filled can is 50x110mm, 13mm or 1/2" bigger dia.


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