Author Topic: Motor start or motor run capacitor  (Read 698 times)

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Online Peabody

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Motor start or motor run capacitor
« on: May 15, 2021, 10:54:18 pm »
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.
 

Online bdunham7

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Re: Motor start or motor run capacitor
« Reply #1 on: May 15, 2021, 11:12:40 pm »
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.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline floobydust

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Re: Motor start or motor run capacitor
« Reply #2 on: May 15, 2021, 11:50:41 pm »
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.
 

Offline james_s

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Re: Motor start or motor run capacitor
« Reply #3 on: May 15, 2021, 11:54:49 pm »
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.

« Last Edit: May 16, 2021, 12:42:25 am by james_s »
 

Online BrokenYugo

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Re: Motor start or motor run capacitor
« Reply #4 on: May 15, 2021, 11:57:39 pm »
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.
 

Online NiHaoMike

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Re: Motor start or motor run capacitor
« Reply #5 on: May 16, 2021, 12:09:49 am »
If you haven't already, you'll probably want to replace the receiver with one that supports encryption. The old ones send a fixed code every time and are trivial to hack.
Cryptocurrency has taught me to love math and at the same time be baffled by it.

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Online Peabody

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Re: Motor start or motor run capacitor
« Reply #6 on: May 16, 2021, 05:16:53 am »
Thanks for the responses.

The original capacitor is long gone, and all I remember is it was round and it was black plastic.  But I suspect it was a start capacitor because I remember the spec of the replacement matching it exactly.  At Grainger and elsewhere the "43-53" MFD is a typical start capacitor rating.  Run caps seem to be a specific number, not a range.

Reading up on PSC motors, it appears they only use a run capacitor because it's connected to the mains all the time.  So even though Sears apparently thought a start capacitor was good enough for this very light use, I think I will replace the dead one with the run capacitor I found so I don't have to go through this as often.  But if there's a reason not to do that, please let me know.

And yes, the remote stuff died long ago, and was replaced with whatever the latest thrid party remote was at the time.
« Last Edit: May 16, 2021, 05:19:11 am by Peabody »
 

Offline amyk

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Re: Motor start or motor run capacitor
« Reply #7 on: May 16, 2021, 05:37:38 am »
You can replace a start-rated capacitor with a run-rated one of the same other ratings and it will be fine. The opposite is not true (but in this application they may have cheated a bit)
 

Offline SeanB

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Re: Motor start or motor run capacitor
« Reply #8 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.
 

Online NiHaoMike

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Re: Motor start or motor run capacitor
« Reply #9 on: May 16, 2021, 04:03:45 pm »
And yes, the remote stuff died long ago, and was replaced with whatever the latest thrid party remote was at the time.
The receiver, not the remote. If you replace just the remote, it will still be using the old insecure protocol.
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.
A MOV across the cap should help, assuming it's even a problem to begin with since I would expect the capacitance to suppress the spikes.
Cryptocurrency has taught me to love math and at the same time be baffled by it.

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Online bdunham7

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Re: Motor start or motor run capacitor
« Reply #10 on: May 16, 2021, 04:19:08 pm »
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.

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?
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Online Kleinstein

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Re: Motor start or motor run capacitor
« Reply #11 on: May 16, 2021, 04:23:58 pm »
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).
 

Offline james_s

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Re: Motor start or motor run capacitor
« Reply #12 on: May 16, 2021, 04:31:05 pm »
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.
 

Offline andy3055

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Re: Motor start or motor run capacitor
« Reply #13 on: May 16, 2021, 06:23:18 pm »
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.
 

Offline floobydust

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Re: Motor start or motor run capacitor
« Reply #14 on: May 16, 2021, 09:26:15 pm »
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.
 

Online Peabody

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Re: Motor start or motor run capacitor
« Reply #15 on: May 17, 2021, 12:45:28 am »
I've ordered the 2MEE4 run capacitor, and should be able to pick it up in a day or two.  I think it should work just fine.  Thanks again for the comments and suggestions.
 

Online Peabody

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Re: Motor start or motor run capacitor
« Reply #16 on: May 17, 2021, 05:42:40 pm »
I installed the new 2MEE4 run capacitor, and of course it works fine for now.  I know I'm years away from knowing if it will make any difference, but at least I feel better knowing I put in the right part.

 

Offline floobydust

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Re: Motor start or motor run capacitor
« Reply #17 on: May 17, 2021, 08:29:18 pm »
That 50uF 370VAC oil-filled run cap looks much better than the original.
It's worthwhile actually keeping these older garage door openers fixed, assuming the remote control has good security.
Modern Chamberlain/Liftmaster are a tiny 12VDC brushed motor and gears, slow as molasses. The steel is so thin, it flexes and bounces even with the cushy belt.
All the money went from the mechanical to the WiFi App.
I did not like the need for a Chamberlain centralized webserver to authenticate security, to open/close the door. Looking forward to the day when hackers get all garage doors in the country to open lol.
 

Online Peabody

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Re: Motor start or motor run capacitor
« Reply #18 on: May 18, 2021, 02:39:49 am »
This old Sears opener is almost entirely mechanical.  It dates back to 1972.  Replacing the radio stuff  at both ends turned out to be pretty easy.  The big shortcoming is that it doesn't have the safety IR beam across the bottom of the door opening, depending instead on some pushback against the closing door to cause it to reverse.  I think code would require the IR beam, so when it comes time to sell the house, I'll have to replace the opener even if it's still working.  I had thought about building an Arduino project to add the IR beam, but in the end concluded that's not a good idea for such a safety feature.  Well, nothing lasts forever I guess.  It's just too bad nobody makes an official IR-beam addon.  But the 1.3HP motor certainly moves the door with authority, and there's nothing to go bad except the drive belt and this capacitor, both of which Grainger carries.

 

Offline james_s

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Re: Motor start or motor run capacitor
« Reply #19 on: May 18, 2021, 02:55:18 am »
This old Sears opener is almost entirely mechanical.  It dates back to 1972.  Replacing the radio stuff  at both ends turned out to be pretty easy.  The big shortcoming is that it doesn't have the safety IR beam across the bottom of the door opening, depending instead on some pushback against the closing door to cause it to reverse.  I think code would require the IR beam, so when it comes time to sell the house, I'll have to replace the opener even if it's still working.  I had thought about building an Arduino project to add the IR beam, but in the end concluded that's not a good idea for such a safety feature.  Well, nothing lasts forever I guess.  It's just too bad nobody makes an official IR-beam addon.  But the 1.3HP motor certainly moves the door with authority, and there's nothing to go bad except the drive belt and this capacitor, both of which Grainger carries.

That's not a shortcoming, that's an advantage. Those stupid mandated IR beams were railroaded through by some nanny state busybody after one single incident where somebody was crushed by an improperly adjusted door. As long as you bother to set up the opener and adjust the mechanical obstruction sensor it's perfectly fine. I had so many problems with those damn photo beam sensors getting misaligned, covered in cobwebs or other problems I reverse engineered mine and built a little circuit to bypass it. You don't have to replace the opener to sell the house, there is no requirement to bring old systems up to current code. If I ever sell my house I'm taking my modified openers with me so I don't have to go through that exercise again.
 


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