115v ac induction motor binds under power 1

[Salthart]

I have a .75 hp electric motor that hums when power is applied. But unlike other motors like this one, If you apply power and try to spin it (by hand), you find it tight and slow to spin. It is free before power is applied. I did replace the capacitor with one from a .5 hp motor and it started once. Any idea why it binds under power ?

 

[jraef]

Bad centrifugal switch, it's stuck OPEN instead of being closed when at standstill. The capacitor and Start Winding are put into the circuit at standstill through the centrifugal switch contacts. The centrifugal switch opens the contacts at about 80% speed because once the motor has STARTED spinning, it stays spinning. The caps and start winding however are necessary to make it START spinning. So with the switch stuck open, it never starts. The force you feel when trying to spin it is the magnetic fields just changing back and forth, which from a standstill have equal opposing forces, hence feeling "stuck".

If you are mechanically inclined you can remove the non-drive end bell housing and look at the mechanism. Often times they just get gummed up with crud, so a little electrical spray cleaner will free up the mechanics and get you running. If not, you can often order a new centrifugal switch if the motor is not too old. Or just take it to a motor shop and have them do it.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Or; Is this motor on or near a grinder? I have seen motors do this when the saw-filer cleaned his grinding machine with an air chick. He inadvertently blew some metal dust into the motor. The motor spun free but when the power was applied, the metal dust magnetized and locked the rotor. We could spin the motor quite freely and with the motor spinning, when the power was applied the motor stopped and would not turn.
While some motors may lock up with a broken centrifugal switch, I have seen many motors with a broken switch started by spinning the motor. These motors work until someone makes a mistake and then the smoke comes out.

Bill
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"Why not the best?"
Jimmy Carter

 

[Salthart]

I purchased this motor, along with others, over the summer. All used. This one has had someone working on it. After reading both replies to my post, I went out to the shop and pulled the motor apart. The switch seems to be working freely. It is the type that has a "D" shaped hole and fits a very shallow "D" cut onto the end of the shaft. The contact point is on a spring steel disk with a bend in it.
After testing the ohm's with my meter, And if I understand correctly, the non switched full time hot wire was connected to the windings with the highest ohm's and the switch and cap were wired to the lower ohm's windings. Reversed ?

I swapped the wiring and then learned that the switch was in fact open. No idea as to how bending the steel contact disk will work out. (no other adjustment otherwise as far as I can tell) Working the switch with a screw driver, the motor seems to run well enough.

Thanks to you both for your replies.

 

[waross]

Yes reversing the start and run windings will give those symptoms. Forgive me for forgetting. The last time I saw that problem with a motor was 50+ years ago.

Bill
--------------------
"Why not the best?"
Jimmy Carter