Electronics > Repair

Motor start or motor run capacitor

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I have an ancient Sears garage door opener, which may outlive me if I treat it well.  It's built like a tank, but every few years I have to replace the motor capacitor.  I think I may have been buying the wrong replacement capacitor, and could use some help on getting the right type.

The parts list in the manual very clearly describes it as:

Capacitor, motor start
Rating 43-53 MFD, 250VAC

and I've been buying the Dayton 2MER3 from Grainger, which is exactly that.

However, the schematic (see below) shows it as a "motor run capacitor", and describes it as:

50 MFD, 250VAC

I've looked at the online manuals for a couple other Sears models, and they do the same thing.  The schematic labels them as run capacitors, but the parts list describes them as start capacitors.  I wonder if the explanation is that since the opener motor only runs for a few seconds at a time, a start capacitor is thought to be good enough - even though it's actually wired as a run capacitor.  I've confirmed that the capacitor is wired directly across the motor leads, with no centrifugal cutout switch.  Perhaps someone here will know for sure, but it seems to me that such wiring is for a run capacitor, as the schematic describes it.

So I think I should be using the 2MEE4, which is a 50 MFD, 330VAC run capacitor (there is room for it).  Could someone more expert on motors than I am weigh in on this issue?  I understand that it should be ok to use a run capacitor in place of a start capacitor (but not the other way around) even if it's really wired as a start capacitor.  So it may be that the run capacitor, which presumably would last longer, would be the way to go in any case.  But if it's really wired as a run capacitor, then that's what I should be using no matter what the parts list says.  The 2MER3 that failed (short) this morning has lasted five years.  That's not awful, but it seems it should have lived a bit longer than that.

Thanks for any help.

Yes, I think your reasoning is 100% sound and if the part you have selected is physically larger than the other, then I think it may well last a lot longer.

What is the construction of the original cap and how is it failing? They can be electrolytic or metallized PP film type, oil-filled, dry, aluminum can, plastic etc.
If they suddenly go open-circuit (yet read only the bleeder resistor ohms) there is a safety pressure disconnect switch inside canned types that may have tripped.

I think the main difference between start and run capacitors is power handling. With the high ripple current, the cap's ESR makes heat and the challenge is getting rid of the heat for a long lifetime. A start cap will overheat if used as a run cap. Run caps have better cooling, why they are larger oil-filled, bare cans.
For the 30 seconds the door opener might run, it's probably fine pushing a smaller start cap.

It's a run capacitor, garage door openers typically use a PSC motor and to reverse the rotation they swap which winding is fed through the capacitor. A capacitor start motor will have a centrifugal switch that will cut out the capacitor as soon as the motor comes up to speed. Start capacitors are designed for intermittent use and will not hold up to being used as a run cap.

Edit: I just looked at the schematic and it says PSC motor right on the schematic, so yes, that's definitely a run capacitor, no question about it. You can also see how it uses a relay to swap which winding is fed through the capacitor to reverse the rotation.

It's only a start cap if the motor has a centrifugal switched start winding that disconnects at speed. I guess you kinda got away using a start cap in a run (continuous duty) application as a door opener only runs like 20 seconds at a time, possible the designers did the same and that's where the naming confusion comes from.


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