Help diagnose split phase motor start problem

[gol4]

I have seen a lot of threads over the years on motor rebuilds so I know there are a lot of knowledgeable people on this forum.

I have a 110v 3/4 HP split phase motor on my lathe. Earlier right as I was about to make a final spring cut on a threading job I flipped the switch and the motor just hummed.
I yanked the motor off and took off the back plate. I could not see anything burnt but is was dirty. I blew it out and cleaned it all up. Then bench tested it. It started right up several times.
I was able to finish off the threading job but a while later she just hums again.
Since I was not in back gear anymore I hand spun the chuck and it took off. Once I shut it off it just hums again.
If this motor had a capacitor I would replace that. I have not idea on how to trouble shoot this issue. If you can advise what test I need to run or even a good you tube video I would appreciate it.

 

[jallyn]

I'm 90% sure there is a capacitor hidden inside somewhere.

Single phase motors can be shaded pole, capacitor (start or run capacitor), synchronous, or "universal motors". Shaded pole is only for very small, low-inertial loaded motors like bathroom fans. Universal motors have commutators and brushes and are great for things like grinders that need lots of power and high speed. Synchronous motors have no starting torque and were used for clock motors because they are perfectly in sync with our 60hz power system and can keep fairly accurate time. That was like 50 years ago, though and required you to spin them the right direction when you started them up.

Your 3/4 hp motor is most probably a capacitor motor.

 

[lilredex]

If it runs after a spin, look at the centrifugal switch that connects the start winding. Probably all dirty or the contacts are burned.

 

[Davefr]

The place to start is by posting images of the motor.

 

[gol4]

Here are some pictures. If I hand spin it it will run.

Motor windings

Motor

Back cover

Is this a capacitor?

 

[Techie1961]

I've seen those same little units on some pumps that we use in our industry. I'm curious to know what it is as well. I asked a motor guy once and he said it was some sort of switch that worked along with the staring switch and centrifugal actuator. They're used on low starting power requirement applications where it's easy to spin up the motor but the HP increase drastically once it reaches nearly full RPM. A turbine water pump is the application I have seen.

 

[lilredex]

Picture #3 looks like the centrifugal switch.....look closely at those contacts...follow the circuit.

Picture #4 is a "KLIXON" type of switch......if motor winding over heats, it shuts off the motor until it cools off.

 

[Techie1961]

I can't read it in the photo as it's blurry. Do you have the number on it? The part that you ask if it's a capacitor?

 

[Techie1961]

Picture #3 looks like the centrifugal switch.....look closely at those contacts...follow the circuit.

Picture #4 is a "KLIXON" type of switch......if motor winding over heats, it shuts off the motor until it cools off.

So this is a split phase motor without a capacitor than. Easy starting compared to needing a capacitor to help startup.

 

[gol4]

Thank you lilredex I think you hit it. The contacts that looked kind of like a set of points under that copper strip in image 3 were blacked and not pressing tightly together.
I cleaned them and bent the little copper piece so it makes firmer contact.

It now fires right up and after several tests keeps restarting.

Hopefully that was it.

 

[jallyn]

Glad you got it running. You guys are right. It is a split-phase motor with no capacitor, just a centrifugal switch connected to the start winding.

 

[lilredex]

Good to see you found the problem, should be good to go again now, for another 100,000 cycles.

Below is a typical drawing of that motor circuit, add that Klixon safety switch to a power leg and you have it.

Take the start winding out of that circuit (defective centrifugal switch) and the motor just sits there and hums. Manually twist the shaft CW and the motor runs CW, twist the shaft CCW and the motor now runs CCW. If you gave the motor a bump, the switch may close and it would likely start on its own.

You reverse the start winding leads when you need to change the motor's direction. That is what you are doing when the instructions say "move the brown wire from terminal A to B" etc.

 

[gol4]

I hope your prediction of 100,000 cycles hold true.

Funny how some stuff always breaks at the worst possible time.

Thanks again