I have an old furnace. Sometimes the blower motor makes a scary buzzing sound and does not start running. It eventually gets very hot and shuts off -- probably because the thermal protect engaged.

When the motor does start, it runs fine. Turning the motor shaft to a new position seems to let it start the next time it is needed. The motor and blower both rotate freely without any obvious friction.

I' baffled what might be happening. The only thing I can think of is that there is a bad section in the slip rings.

My question is: What can cause this motor to be intermittent and how do I fix it?


For reference, the motor is marked:
Emerson -- Motor Division: Model S60CXSGA-3171, Cat. 287???

 

If you are sure it runs free, it could be the start capacitor on the weak side.
What slip rings are you referring to?
It appears to be a common induction motor.
Have you oiled it regularly as the plate mentions?
Max.

 

If you are sure it runs free, it could be the start capacitor on the weak side.
What slip rings are you referring to?
It appears to be a common induction motor.
Have you oiled it regularly as the plate mentions?
Max.

OK -- no slip rings. I am showing my (complete) ignorance of motors. I had no idea what kind it was.

It may have gone a period without oiling but it seems fine. Can you give any hints on how to be sure the bearings are still good?

Similarly, any hints on testing the start cap? Any ideas on where the cap is installed in the motor?

I think I can hear the centrifugal start switch actuate when the motor slows down after turning it off. Is it common for this switch to fail intermittently?

 

I think it could be dirty or worn contacts on the centrifugal switch. It is also possible that there is oil or grease mixed with dirt making the contacts not close properly so the sort winding would not be energised to get the motor started. This would fit with the fact that moving the motor clears the fault. I suggest dismantling the motor to inspect the centrafugal switch.

Les.

 

I wonder if it even has a centrifugal switch for just a 1/4 HP, these are often PSC motors.
IF it does you can usually hear it click closed when power is removed and it slows to a few 100 rpm.
Max.

 

OK -- no slip rings. I am showing my (complete) ignorance of motors. I had no idea what kind it was.

It may have gone a period without oiling but it seems fine. Can you give any hints on how to be sure the bearings are still good?

Similarly, any hints on testing the start cap? Any ideas on where the cap is installed in the motor?

I think I can hear the centrifugal start switch actuate when the motor slows down after turning it off. Is it common for this switch to fail intermittently?

I missed the details in this post somehow, if it does have a Cent. Sw then Les may be right, I have never seen one on a motor this small?
The start cap is usually just attached to the motor, and if it has a Cent.Sw then I would expect it to be visible.
If it has oil ports then I would make sure it it well lubricated, if you dismantle it for the SW then you could clean and lubricate the bearings and check for wear, on that motor I would expect them to be sintered bronze.
Incidentally if it is a start cap with switch, and the sw is open, it should technically start to rotate either way if you give it a spin just as you apply power.
Max.

 

I think it could be dirty or worn contacts on the centrifugal switch. It is also possible that there is oil or grease mixed with dirt making the contacts not close properly so the sort winding would not be energised to get the motor started. This would fit with the fact that moving the motor clears the fault. I suggest dismantling the motor to inspect the centrafugal switch.

Les.

Thanks. It did not occur to me that moving the shaft might make the start switch close.

I tend to over-lubricate so excess oil might have gotten into the switch.

 

I missed the details in this post somehow, if it does have a Cent. Sw then Les may be right, I have never seen one on a motor this small?
The start cap is usually just attached to the motor, and if it has a Cent.Sw then I would expect it to be visible.
If it has oil ports then I would make sure it it well lubricated, if you dismantle it for the SW then you could clean and lubricate the bearings and check for wear, on that motor I would expect them to be sintered bronze.
Incidentally if it is a start cap with switch, and the sw is open, it should technically start to rotate either way if you give it a spin just as you apply power.
Max.

Thanks for your help.

Some more background... The motor could be as much as 95 years old -- that is when the house was built. I have no idea when the furnace was installed or if the motor is original.

I don't see a start cap. I suppose that it could be hidden behind or below the motor but I have not noticed it while cleaning and adjusting the motor position. I will look more carefully.

I need to see if I can feel any roughness as I rotate the motor shaft. If there is that would be a clue that is is binding sometimes.

I can try your spin-start test. By the way why would it start either way if I start it but not normally?

 

My impression is that capacitor start was more common in years gone by for motors of a size that would now be permanent capacitor split phase. I suspect is has to do with capacitor reliability. A start cap has to work moderately hard but just for a few seconds at a time, whereas a run cap is always in-circuit.

 

Hi Richard,
It will run in either direction if you start in manually. You will probably need to spin it at a reasonable speed to start it.

Les.

 

As long as the switch does not close as you are spinning it, in the wrong Dir!
Typically now the start switch is used in motors 3/4 - 1hp and up.
Max.