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#7408 02/04/02 02:46 PM
Joined: Dec 2001
Posts: 50
My co-worker has a small 115 volt single phase motor on a small air compressor that would not start. It would just hum. We put another start capacitor on it. now it starts and runs just fine till you put any kind of a load on it. even a very slight load stalls it and flips the breaker. What could the problem be?

#7409 02/04/02 03:05 PM
Joined: May 2001
Posts: 717
Lots of things come to mind. is the motor hot during its quick run period?
Most important, I think, compressors require oil, sure its not been run low and locked up/about to lock up?

#7410 02/04/02 03:38 PM
Joined: Dec 2001
Posts: 50
No, the motor is not hot. Also, it is not connected to the compressor at this time. It stalls just by holding your foot against the pully while it is running free. It won't even get completely started when it is hooked up to the compressor

#7411 02/04/02 05:27 PM
Joined: May 2001
Posts: 717
Bad news probably, start winding switch may not be cutting out of circuit. By the time you buy one, ya might wanna go to Graingers or maybe Northern Handyman for a compressor motor. If it is a dayton, you might call graingers for a replacement part, they carry several other lines as well. Check the switch first of course, but if it is starting (without a manual 'shove') it is likely the start circuit not cutting out. If you have to shove it, the start winding is gone. Either way with that motor, it's likely cheaper to replace than repair. [Linked Image]

#7412 02/04/02 06:14 PM
Joined: Jan 2001
Posts: 1,044
Tom Offline
I'll ditto George's reply. I've found, generally, if the capacitors been replaced the next most likely problem is that pesky centifugal switch. The part doesn't cost that much, until you buy it at the 2000% inflated replacement price, & it has ruined many a single phase motor.


Few things are harder to put up with than the annoyance of a good example.
#7413 02/05/02 12:33 PM
Joined: May 2001
Posts: 176
Ditto to George and Tom.
My compressor's centrifugal switch went bad. Would begin to start, but jusut couldn't get there. I replaced the capacitor, start and run, then sent it to a shop. Didn't think about the switch. Cost me $40.

#7414 02/06/02 11:52 PM
Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and
A little more info here would be great!

First off, before the start cap was replaced and the motor's rotor would remain in stall [not turning], was the hum very light, or was it a growling loud and scarry hum?
  • Light hum = no start winding connection, either capacitor fried open, or start switch not closing... motor can remain energized for eons without problems;
  • Bad hum = start / aux circuit in tact with semi or fully shorted capacitor... motor heats up and either trips breaker or thermal O/L on frame.

Secondly, what HP is this motor? Is it large enough to run the pump?

Third, is the rotor [shaft] able to spin freely? Can you hold it at start? If so, try spinning it in the opposite direction.

Fourth, how fast is the motor spinning when you are able to bring it back to locked rotor [stall] by placing your foot on the pulley?
Looking at the start switch not opening problem as mentioned by others.
This would also cause the motor to make the "Starting" sound constantly, plus become excessively hot and trip OCPD / O/L.

Lastly, is this motor multi pole - meaning less than 3450 RPMs - more like 1725 RPMs? If so, make sure all stator coils are connected as they should be.

Good luck. Let us know of the outcome.

Scott S.E.T.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#7415 02/07/02 07:11 AM
Joined: Dec 2001
Posts: 50
The motor made more of a softer hum. After the start capacitor was replaced, the motor starts fine and you can hear the switch clicking when it comes up to speed. You can stall the motor at full speed and spin it and get it to rotate the opposite way. however the motor makes more noise that way. As far as what size the motor is and is the motor large enough for the compressor, I can't remember what size it was since it's been a while since he had it here. Possibly 2hp. I don't know if it is sized right for his compressor or not but would think it is since it is a factory setup and was run for years this way. He said he checked out the compressor pump and it turns freely as it should. It is a low rpm motor ( around 1725 rpm).
I beleive the information I provided is all correct but it has been a few month since I saw it. He is bringing it in sometime again so I can take a fresh look at it.

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