Single Phase AC MOTOR stuck

[Christopher]

I am trying to fix a single phase ac motor but need some help. When powered on and put off load, motor humms but doesnt rotate, its sort of magnetically locked because even trying to rotate by hand it very hard to turn (motor was removed from housing and therefore I have stator and rotor exposed)

When switched off, rotor turns freely and smoothly by hand, therefore bearings are not an issue.
Motor is a capacitor run. Checked capacitor and both windings, all looks fine (see image attached). Coils ohmage measured was 15 and 40 ohms and capacitor reads 10microF OK.

Can someone please help what could be the problem? Thanks!

 

[westward10]

That diagram shows a permanent split capacitor motor, the most common cause of what you are describing would be a failed capacitor.

 

[Christopher]

That diagram shows a permanent split capacitor motor, the most common cause of what you are describing would be a failed capacitor.

Hi westward10, capacitor checked with multimeter on Farads setting - reading = 10microFarads as shown on casing. Aslo charged it with 240V and shorted pinouts to check if it is holding charge, also OK as pinouts sparked. Any other ideas?

 

[pc1966]

Is there a start/run centrifugal switch?

 

[freddo]

If the bearings, windings and capacitor are ok that doesn't leave much else! If the rotor locks when power is supplied only one of the windings has power or they are both in phase due to a faulty capacitor. I would ignore the capacitor test results and try a substitute capacitor.

Is there a start/run centrifugal switch?

Any switch would appear to at least be closed by the winding resistance readings so something should happen.

 

[Strima]

What are the connections on the motor? T2-T3 & A1 & A2 or similar?

Was this motor working when you got it?

 

[Christopher]

Is there a start/run centrifugal switch?

No centrifugal switch
[automerge]1589401840[/automerge]

Is there a start/run centrifugal switch?

Hi, no there is no centrifugal switch, see new image attached.
[automerge]1589401905[/automerge]

What are the connections on the motor? T2-T3 & A1 & A2 or similar?

Was this motor working when you got it?

Hi, yes this was working fine. There are 4 wires coming out of the motor, 2 for capacitor and 2 for L+N, as shown in schematic. Thanks for your help. Any ideas?
[automerge]1589402026[/automerge]

If the bearings, windings and capacitor are ok that doesn't leave much else! If the rotor locks when power is supplied only one of the windings has power or they are both in phase due to a faulty capacitor. I would ignore the capacitor test results and try a substitute capacitor.

Any switch would appear to at least be closed by the winding resistance readings so something should happen.

Thanks for the replies. Actually i noticed that the rotor does turn very very slowly while humming. I did not notice this because I am switching it off immediately since its heating up a lot quite easily and i m trying to avoid the damage.

 

[Lucien Nunes]

I also suspect the capacitor is breaking down when subjected to full working voltage and would second Freddo's suggestion to test by substitution. If the motor is uncoupled from what it drives, and you can do so safely, power the main winding only (with the capacitor disconnected) and give the shaft a shove in the normal direction. With no mechanical load it should accelerate to normal running speed, although will lack torque. If it then runs quietly without excessive current or heat, the main winding at least is probably OK.

Has there been any sign of overheating or overcurrent that would point to shorted turns in the winding?
And a quick test to eliminate sillies: if you haven't done so, measure the resistance between the ends of the windings not connected together and confirm it's 55Ω.

 

[Christopher]

Image of disassembled motor
[automerge]1589402288[/automerge]

I also suspect the capacitor is leaky when subjected to full mains voltage. If the motor is uncoupled from what it drives, and you can do so safely, power the main winding only (with the capacitor disconnected) and give the shaft a shove in the normal direction. With no mechanical load it should accelerate to normal running speed, although will lack torque.

Has there been any sign of overheating or overcurrent that would point to shorted turns in the winding?

Hi, I did try to power it with capacitor removed. Same thing happens (hum and locked). Yes both stator windings and rotor heat up very quickly, after 10seconds ON its very hot

Yes I had already checked resistance of both windings, 55ohms OK

 

[Lucien Nunes]

Unfortunately then I suspect the 15Ω winding has shorted turns if it heats that quickly. The shorted section behaves like a shorted secondary of a transformer, causing heavy current to flow in the remaning intact turns. You might be able to see some sections of the winding discoloured. It probably isn't economically viable to repair.