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AC fan not starting unless given a little 'help' - Start capacit lickely fault?

Robert Hill

Mar 5, 2015
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Mar 5, 2015
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112
Hi all,

I've got an AC fan which is part of an extraction system. Most of the time it starts up fine. However recently (and seemingly in combination with cold misty weather (fan is outside)) The fan won't start when power is supplied. When I give the motor shaft a little spin and then supply power the fan then runs fine.

This lead me to think it is the start capacitor that is at fault. This is because once running the fan continues to run suggesting the run capacitor is fine as is the motor itself. Also because from a stationary position the motor cannot start turning but from a gentle spin the motor then can start turning. This would suggest that the start capacitor has grown weak meaning it can't get things going from a stand still but can get them going with a bit of help.

What do you guys think?

Want to make sure I'm dealing with the right issue or starting in the right place as it is quite an effort to access the fan and I'd rather not keep going up and down to it.

Thanks in advance.
Bob
 

dorke

Jun 20, 2015
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Jun 20, 2015
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First thing is to check that the fan is spinning freely without power.
Most times this is the cause of the problem and once started it gets worse and worse(dirt and gunk) .
If this is the case the shaft should be cleaned and lubricated with the correct lub material.

In small fans there is usually a single capacitor for both start and run.
Some photos may help.
 

Minder

Apr 24, 2015
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Apr 24, 2015
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As per Dorke, a small fan is usually a PSC motor which has a single cap in circuit all the time, have you actually see it to confirm it HAS a cap, as some are shaded pole motors.
The most common reason is the sintered bronze (porous) bearings run dry.
M.
 

Robert Hill

Mar 5, 2015
112
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Mar 5, 2015
Messages
112
Thanks for your responses.

Yes, it definitely has both a start and a run cap. I've replaced both but that didn't solve the issue.

The bearings/shaft etc have now been greased and this seems to have made things better. The shaft moves freely when there is no power.

I've had someone else look at it and they reckon the motor is pretty old and that the windings may be breaking down due to age.
 

Minder

Apr 24, 2015
3,478
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Apr 24, 2015
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This must be a larger type ventilation/radial fan, maybe.
If it does have a start cap and the cap is OK then most likely the centrifugal start switch has failed.
Especially if a spin by hand starts it.
M.
 
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