Motor Smoking

I spent the last few months rebuilding a 10k and this morning I made my first chips with it. I didnt get very far when i smelled that tell tale scent of an electrical problem and noticed smoke coming from the motor. I ran the motor with the drive belt off it, no load, and it got uncomfortably hot to the touch after 5 minutes. Doesnt appear to be the bearings as the shaft rotates freely. I guess the motor is shot? Anything i can do to troubleshoot it? I dont know much about ac motors and it seems like it would be a pain to find a replacement fo a 60 year old motor that would fit the existing mount. Its 1/2 hp, 115 v.

Adam Selene said:

I spent the last few months rebuilding a 10k and this morning I made my first chips with it. I didnt get very far when i smelled that tell tale scent of an electrical problem and noticed smoke coming from the motor. I ran the motor with the drive belt off it, no load, and it got uncomfortably hot to the touch after 5 minutes. Doesnt appear to be the bearings as the shaft rotates freely. I guess the motor is shot? Anything i can do to troubleshoot it? I dont know much about ac motors and it seems like it would be a pain to find a replacement fo a 60 year old motor that would fit the existing mount. Its 1/2 hp, 115 v.

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Sounds like the dirt-common single-phase motor issue of a failed start-switch system. Not hard to repair, but motors that size are so cheap and common they aren't really worth the time, given that all-else (bearings, perhaps "plain", end-thrust shims...) are worn too, insulation is old, etc.

Finding a new one that is a drop-in fit should be dead-easy. Look up the frame size and such off the nameplate on Grainger/Zoro or the like to confirm, then run a broader search.

Take the end bell off the motor opposite the shaft. There is a switch attached to the end bell that connects the start winding during starting and then opens the switch as the motor comes up to speed. Attached to the rotor, the part that turns, is the other half of the switch. There is a insulated disk that toggles back and forth with a couple of springs attached to fly weights. The switch works by the the springs holding the disk outward against the start switch that connects the start winding to the main circuit. As the motor approaches full speed, the weights overcome the springs and the insulated disk snaps back, allowing the the start contacts to open, disconnecting the start windings. Either the mechanical mechanism is not snapping back and forth or the start contacts are welded. Clean and lightlylubricate the mechanism and the contacts. Reassemble and test. There should be a snapping sound at near full speed when the start mechanism operates.

Tom

Edit:- follow this link , there is a sketch of what you are likely to see when you open the motor

Split-phase motor | HVAC Troubleshooting

If the motor runs with the belts off, even though it gets hot, there is probably not a problem with the starting switch. If the start switch does not work, the start winding will not be powered and the motor will just sit there and hum but will not rotate very much if at all. The fact that it gets hot running "free" and has magic smoke coming out suggests a shorted run winding coil someplace. Fixing that is not economically feasible.

The motor does not look that old although it can be hard to tell from a picture. Look up the frame size on the data plate and then you have enough information to start looking for a replacement.

My 9A has the same motor, and every once in awhile the start switch fails to snap over to the run condition, but when that happens, it makes a LOUD humming-buzzing noise, and I immediately turn it off. So far everytime I turn it back on, the switch operates normally and I go on with what I was doing. Since it is no real bother for hobby use, I haven't got around to taking it apart for repair. If you didn't notice a noise, and it took 5 minutes to get hot, I would agree this sounds more like a shorted turn. If you just like the look of the original motor as I do, you could look into having it rebuilt, if there is a motor shop in your area.

Thanks for everyone's input. I was not able to tell what the the issue was after investigating. Didn't see any burned windings and couldn't tell if the starter switch was bad or not. Like I said at the beginning though, I am no expert. The thought of trying to find a replacement part even if I discovered the issue turned me off to any more troubleshooting and I decided to go for professional help.
Fortunately, I discovered I have a electric motor service shop near me. Its a family owned business and they had no problem looking at my motor. Within a few hours they called me back and said that the problem were burned "instant reverse contacts" but that they can repair it. Not sure I have that correct but will verify when I pick it up.

Like everyone, their initial comment was just buy a new motor. When they found it was an instant reverse motor, they seemed more willing to work it. He even stated it was not a standard motor.

Adam Selene said:

... problem were burned "instant reverse contacts" but that they can repair it. Not sure I have that correct but will verify when I pick it up.

Like everyone, their initial comment was just buy a new motor. When they found it was an instant reverse motor, they seemed more willing to work it. He even stated it was not a standard motor.

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They have always been a "distinct minority" in the single-phase world, yes.

Good move!

The term plugging service was once used to designate an instant reversing motor. The plate on an older motor may use that nomenclature.