Chris said:
The grinder uses a shaded pole induction motor. This kind of motor has not
brushes or capacitor. The poles of the rotor are fabricated in such a manner
as to mechanically alter the magnetic field for startup. The grinder will
spin freely as long as no current is applied. When current is applied, the
motor buzzes loudly and movement by hand is opposed with great resistance.
When you rotate it by hand against that "great resistance", does it
"snap" from one pole to the next pole on the rotor, or does it feel like
it's bound up mechanically?
If the later, then *something* is worn or shifted enough mechanically to
let the rotor get pulled against the stator under the attraction of the
magnetic fields. I'd open it up again and see if you can determine by
eye or with feeler gages whether the alignment of the rotor to the
stator has gone out of concentricity, especially when you push or pull
the shaft(s) "sideways".
Last year my own little Craftsman bench grinder,which I've owned for
maybe 45 years started "losing power". It uses a universal motor.
(Brushes and a commutator, with a small timing belt drive to reduce the
motor speed.)
I opened it up and saw signs of scraping on the armature. Turned out the
pot metal housing onto which the stator and motor bearings were mounted
had warped over the years and moved things out of alignment. A couple of
shims placed in the right spots put the rotor and stator concentric
again and it's good as new again.
The coil's DC impeadence is approximately 50 Ohms. I cannot detect a short
to frame so I do not suspect one.
Any ideas to resolve the problem are greatly appreciated.
Chris
Tampa, FL USA
Good Luck, let us know what it turns out to be.
Jeff
--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)
"If you can smile when things are going wrong, you've thought of someone
to blame it on."