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single phase motor, rotor locked on startup

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  • single phase motor, rotor locked on startup

    Got a 3/4 bench grinder here, that had a damaged capacitor.... it is a capacitor start then has a centrifigal switch that throws it out after start up...

    I have replaced the capacitor but now when I turn the power on the motor buzzes and the rotor is locked solid..... Turn the power off and the rotor spins freely...

    Any ideas?
    Precision takes time.

  • #2
    Take the motor apart and clean the contacts with some very fine 400 grit sand paper and contact cleaner. The contacts are probably a little bit dirty or slightly burned a light sanding will usually fix it. My motor has no capacitor its a start winding problem. When you replace a capacitor it needs to be the same rating or larger.

    I have a 1/2 hp single phase motor that keeps doing that. I learned to turn it on/off about once every 2 weeks even if I don't need to use it. Turning it on/off once in a while keeps it working.
    Last edited by gary350; 10-23-2011, 09:00 PM.

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    • #3
      Did you use the same mfd value capacitor?
      I just need one more tool,just one!

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      • #4
        Switch

        Check the starting switch and the continuity of the starting winding.
        JRW

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        • #5
          Could be a problem between the capacitor, wiring, start switch, and start winding. Could also be worn bearings- energizing the run winding shouldn't make the motor lock up, but if the rotor is too close to the field on one side or another, it may be getting sucked right to it. Then it would lock.

          You should be able to turn the shaft by hand, even with the run winding energized. That's all the start circuit does- gets the rotor turning so the run winding can get a phase shift going on with the rotor and thus keep it turning.

          It's also possible the run winding is open- if it didn't hum before you changed the cap, then this is a possibility. I'm not sure, but I don't thing the rotor will turn even if the start winding does get energized, if the run winding is open.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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          • #6
            continuity between the start and the run winding and the windings and the case is usually a good idea too, continuity between either is bad. Also visual inspection of the winding is a good idea (black is bad)

            Roy

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            • #7
              If you have had the motor apart there is a small possibility that the rotor is not centralized in the stator and is dragging. Check the end bell seating carefully and if they have been rotated from original position try to change the positions. You might be able to see rubbed spots on the rotor and stator.
              Don Young

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              • #8
                Locking of the rotor when power is on and start winding is not energized is pretty normal.....

                It should be holding the rotor locked by something similar to the torque it has when running..... It's not really "locked", just hard to turn.

                If it IS "locked, rigidly held, check for end-wise shifting when the power is on, and for rubbing.

                But I think you just are not getting a start winding.

                Wrap a string around the shaft, get it spinning by pulling, and (making dad-burned sure the string is clear) while it is still spinning, turn on power.

                if the motor is basically OK, it will then run. That pretty much proves the problem is with the starting system...... winding, connections, capacitor and switch.
                1601

                Keep eye on ball.
                Hashim Khan

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                • #9
                  Originally posted by J Tiers
                  Wrap a string around the shaft, get it spinning by pulling, and (making dad-burned sure the string is clear) while it is still spinning, turn on power.

                  if the motor is basically OK, it will then run. That pretty much proves the problem is with the starting system...... winding, connections, capacitor and switch.
                  Yes it starts up with some rotational help... I will check out the starting system... I replaced the capacitor with one of the same type.. I will check the connections to make sure they are right...
                  Precision takes time.

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