This small electric motor is a puzzle 3

[DeeJais]

It's a 1/2 hp motor made by Sears. Probably 30+ years old. It runs a lathe that I got for free. Guy says it sat for many years and he was convinced the motor was shot. Motor model is 113.12790.

When I got it, the motor would hum/buzz, but not spin. I could start it turning with my hand and then it would spin. Aha! Start capacitor, right?

Put in a new start capacitor, same problem. So I opened the motor and fiddled with the centrifugal switch, put some oil on the shaft & springs and cleaned the little black contacts that move apart when the motor starts. Put everything together and it worked as it should!

Started using the lathe and after about 10 minutes, the start capacitor blew up with a POOF and lots of smoke and smelly fluid. Thinking maybe that new start capacitor was bad, I replaced it with the original capacitor that I kept. OK, everything worked fine again. 2 minutes later the capacitor started smoking and bubbling. Shut everything down. For some reason the capacitor was staying in the circuit and overheating.

Opened the motor again and saw the lever that moves the contacts was in a weird position, the contacts couldn't move apart. I must have reassembled it wrong, so I put everything back, installed a THIRD (brand new) start capacitor, and everything worked just fine again!

Starting using the lathe again with no issues, occasionally checking the capacitor temperature & it stayed cool. I thought it was solved.

After about an hour of running with brief stops, it went back to the hum/buzz but no spin. Frustrated, I took a break and went to lunch. Came back when everything had cooled off and the lathe started and ran perfectly again.

So, what's going on here? Will a start capacitor stop working if it gets hot, then work again when it cools off? And why did those other caps overheat to failure instead of just stop working?

All input appreciated. Thank you.
DJ

 

[itsmoked]

The centrifugal switch is bad, bad, BAD. It's flaky. Sometimes it sticks closed sometimes it sticks open. It's a motor killer and a start cap killer.

1) Replace it.
2) Tear it completely apart and understand it so you can fix it correctly.
3) Toss the motor and get another. They're everywhere if you look.
4) Stop using a 60Hz motor in 50Hz land?? (A possibility that would cause flaky operation)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dArsonval]

A couple/three theories:

In certain instances, the centrifugal mechanism (sometimes referred to as a spool) will not track or
rotate "squarely" with the internal switch contacts. For lack of a better description, as the spool
rotates and depresses the switch pads creating the start circuit connection, the spool occasionally
comes to a resting place or position that does not provide enough pressure to make the start circuit.
Powering the motor, it will buzz but fail to rotate.
Turning the shaft slightly causes the switch contact pads to fully depress the start switch, and the motor takes off.

Let's say there is nothing wrong with the start contacts after getting them reassembled correctly, and there is no issue with the spool.

The next theory is (and I kind of doubt it because of the motor size), the rotor may have an open rotor bar.

An open rotor bar on a single phase motor can be a frustrating element of troubleshooting for the layman.
The motor has to come to a stop at a particular position each time for the motor to not start again.
One may start the motor a "zillion" times, and upon de-energizing the device, it fails to roll to the dead rotor bar position.
And then, [that one] time... it's in that position and the motor will not start-n-rotate.

Another possible issue is "bearing". Many of the small machine tool motors used on Craftsman products
were sleeve bearing type motors. A worn out bearing at one end of the motor will allow the rotor to be
"pulled" to the side during energizing the device and the motor will just sit and buzz until something is
jostled.

There's a lot going on with a single phase motor. They've been known to stifle the most competent of repair people.

Unfortunately... as Keith suggests, it's likely the end of the line for Craftsman motor... No. 113.12790.

John

 

[waross]

I agree with Kieth.
Flaky start switch.
FYI the normal operation is for the contacts to be held closed by the centrifugal mechanism when the motor is at rest.
When the motor comes up to speed the centrifugal mechanism moves away from the contacts and allows the contacts to open.
The centrifugal mechanism may have a problem.
The contacts may have a problem.
Both of the above.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zlatkodo]

You can replace the centrifugal switch with some kind of external manual switch with three positions: 0 - 1 <-- start.
Also you need to check the auxiliary winding, in case if there is no run capacitor.
It is designed to be energized during the start only, as well as starting capacitor.
Winding Design and Motor Repair

 

[DeeJais]

John -- super helpful reply and a good primer on this motor. I usually stick with projects like this and try to the bitter end to fix them, but in this case, I found a guy with (allegedly) a good-quality identical specs motor for just a few bucks, so I'm gonna get it. I've paid more for start caps than he wants for this motor.
But I was also thinking about what someone else in this thread suggested: bypassing the centrifugal and wiring a switch that will trigger the start, and stay on for the run. Like a OFF/MOMENTARY-ON/ON switch? Don't even know if such a thing exists. Or what about a separate ON/OFF push button just for the start capacitor circuit? Push button and hit the power switch at the same time, release button when it starts spinning...just spitballing here.

 

[zlatkodo]

DeeJais
You need only one motor switch.
Winding Diagrams and Data Calculation

 

[waross]

Or what about a separate ON/OFF push button just for the start capacitor circuit? Push button and hit the power switch at the same time, release button when it starts spinning...just spitballing here.

That will work until the first time that someone not familiar with the start sequence burns the motor up.

You need one of these: Electronic Start Switch
or
You may be able to find a replacement here:Centrifugal Switches


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DeeJais]

Many thanks for all responses. This is an education and you guys know what you're talking about.
But as to one of my original questions: do start capacitors stop working when they get very hot? Then work again when they cool off? Or do they work until they fail, and then they're just dead?
The reason I ask: when the motor would only buzz and not spin the other day after a long time running, it did start fine later after everything cooled off. I noticed that the body of the motor was very hot, almost too hot to touch. The capacitor rests on the motor body beneath a metal cover. I wonder if I put some Thermo-Tec or other thin heat insulation between the cap and the motor body, that would prevent it from overheating.

Apologies if these are dopey questions. My lack of knowledge about start capacitors is showing.

 

[itsmoked]

Like a OFF/MOMENTARY-ON/ON switch? Don't even know if such a thing exists.

Of course they exist. Just about every dryer on earth has one. It's the knob you have to twist and hold until the motor has fully started.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

But as to one of my original questions: do start capacitors stop working when they get very hot?

Not as standard capacitors, it would need to have a thermal cutout built into the can. I've seen that on large caps, not small motor starting caps though. It would be unusual. What would be more likely along those lines is that one of your wire connectors to the cap is loose and when it vibrates with the motor, it makes the connection intermittent.

Your motor probably has it's own thermal cutout inside. Look on the label for the words "thermally protected" or a TP somewhere. If that was wired to the cap circuit accidentally instead of the main power line, that might do it too.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

The reason I ask: when the motor would only buzz and not spin the other day after a long time running, it did start fine later after everything cooled off.

Yes, a failing centrifugal switch may present that symptom. It could be either an electrical failure of a mechanical failure.
It has nothing to do with the temperature of the capacitor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DeeJais]

I know everyone has been tossing & turning wondering how this problem was resolved, so this is the complicated solution:
I bought a Century 1/2 hp motor from a Craigslist guy for $20. Bolted it to my lathe and switched it on. Eureka!
Now I'll gut the old one and the Carolina Wrens will probably make a nest in it.
Appreciate (mostly) all of your replies.