Capacitor Type Motor not Starting

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I have an old Dayton 1/3 HP 1725 RPM capacitor start 120V motor that I am trying to put into use as a grinder. When I plug it in it hums until I give the shaft a good twist, and then it runs great. I'm wondering if all I need to do is replace the capacitor to make it run right. Any help from you motor experts out there would be appreciated

Ed MacLaren

Senior Member
Re: Capacitor Type Motor not Starting

Yes, it could be the starting capacitor, but the first thing I would check is the start winding switch.

It is a centrifugal switch that is supposed to disconnect the start winding when the motor comes up to 75 or 80% of it's rated speed.

If the contacts get dirty, burned, or for some other reason they fail to close, the motor will act as you described.


scott thompson

Senior Member
Re: Capacitor Type Motor not Starting

Sounds like one of two things - possibly even both of them:

  • <font size="2" face="Verdana, Helvetica, sans-serif">Open Capacitor, or Capacitor's leads not connected to start switch / Start winding;</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Start switch is stuck open, or not connected to Capacitor / Run Winding.</font>
<font size="2" face="Verdana, Helvetica, sans-serif">
Something is open in the Auxiliary (Start) Circuit. Could be one of the items listed above, or worst case - Aux. winding is cut somewhere / somehow.

Also verify the leads are all snug and solid connections. Verify continuity through the Aux. circuit for open circuit checks.
Test the existing Capacitor with an Ohm Meter, set on R?1K or R?10K setting. Be sure to discharge the Capacitor first!!! Open Capacitor will show infinite R always. Shorted Capacitor will show very low R to zero R always. Good Capacitor will show low or zero R, then "Build Up" to infinite R.

Have fun!

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