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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!
 

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

Was this motor working when you got it?
 
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.
 
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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.
 
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Image of disassembled motor
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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
 

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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.
 
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.
If that is the case, does it make sense that rotor also heats up a lot? I can t see why rotor will also heat if windings in stator are shorted. What do you think?
 
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Heavy currents may be circulating through the rotor cage winding due to the distorted pole pattern around the stator resulting from the short. A substantial power current (in-phase) is flowing instead of just the magnetising current (in quadrature) which can also be the cause of the braking effect.
 
Heavy currents may be circulating through the rotor cage winding due to the distorted pole pattern around the stator resulting from the short. A substantial power current (in-phase) is flowing instead of just the magnetising current (in quadrature) which can also be the cause of the braking effect.
Hi Lucien, thanks a lot, much appreciated
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Obvious thing to do, reassemble and replace the capacitor.
Thanks for your reply.
 
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!
Have you checked that the start cap is OK and the centrifugal switch is operating correctly?
No centrifugal switch
[automerge]1589401840[/automerge]

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

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]

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.
So how or what disconnects the start winding during the initial start up procedure?
 
Have you checked that the start cap is OK and the centrifugal switch is operating correctly?

So how or what disconnects the start winding during the initial start up procedure?
It is a permanent split capacitor or single value capacitor motor there is no centrifugal switch the aux winding is permanently connected.
 
Clearly the start up is not working,
If their is no cetrofugal switch ? then it could be capacitor fault
Or even faulty start windings.
I would replace the capacitor first.
Hi dmxtothemax,
Thanks for your reply. Thank you I will replace capacitor just in case. Will it be expected for rotor to lock (and unable to turn by hand) if its a start up issue?
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Have you checked that the start cap is OK and the centrifugal switch is operating correctly?

So how or what disconnects the start winding during the initial start up procedure?
Hi Pete999,

Capacitor tested ok and not faulty, but will replace capacitor just in case.
 
Hi dmxtothemax,
Thanks for your reply. Thank you I will replace capacitor just in case. Will it be expected for rotor to lock (and unable to turn by hand) if its a start up issue?
[automerge]1589606421[/automerge]

Hi Pete999,

Capacitor tested ok and not faulty, but will replace capacitor just in case.
Are you trying to run this motor up with only 1 end plate in place? and the rotor only fixed at 1 end of the housing? an odd question I know, but sometimes people do do some strange things.
 
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