Electronics > Repair
For those with experience in AC motors, what is the reason of this noise
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highcap:
Bought this old bench grinder and after connecting it to power (single phase), this is what happens when connected (video attached).

The motor is lacking a start capacitor so it needs a push. It makes that noise when connected, which otherwise disappears at power disconnection.

Other than that, it has a few issues like slightly worn bearings (which I cleaned of old grease and re-greased) and a slightly bent shaft in the grinding stone area (probably from a drop on the stone from an angle).

Electrically, there seems to be two identical windings with 3 wires each, coming from the left and right side of the winding, as seen from the top of the power switch box.

One winding was not connected. The one which is connected to the power switch, measures like this:
- Brown to Black 1nd = 28 Ohms
- Brown to Black 2nd = 28 Ohms
- Black 1nd to Black 2nd = open

I have only connected Brown with either one of the Black wires.

Video where the noise can be heard:

https://youtube.com/shorts/7nsSNtZ2pHw

And pictures attached.

Thank you for any inputs!

CaptDon:
Was that thing dropped on a hard surface? It looks like the armature has a big scuff mark from rubbing on the stator. That would sure make some noise!! I have had some damaged motors and some with badly worn bearings/bushings that allowed the stator to scuff due to magnetic pull, but when power was removed the motor quietly coasted down.
highcap:
So this suggests the bearings are off the charts by enough to make the rotor being pullet onto the stator when plugged to power?
The noise in the video clearly suggests some mechanical friction during powering up.

I don't know it's history, but most than likely it was dropped, due to the (slightly) eccentric movement of the grinding stone.
BrokenYugo:
I'd guess between the 220Δ mark and three contact switch this is a three phase motor.
factory:

--- Quote from: BrokenYugo on September 26, 2024, 01:39:44 pm ---I'd guess between the 220Δ mark and three contact switch this is a three phase motor.

--- End quote ---

I think your right, also there are three sine waves shown on the spec. plate, I wonder where the cloth yellow wire goes after the switch? The other ends of the windings (cloth wires) appear connected together.

Not sure I would trust a grinding wheel that might have been dropped either.

David
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