If the motor hums but won't turn when power is applied,
it indicates that one phase of a three-phase motor is open, or the start
winding of a single-phase motor is open. You can use a clamp-on ammeter
to locate the line that is not drawing current in the three-phase motor.
After you have determined which line is not drawing current, you can
look for a problem such as an open wire or open fuse. It's important
to remember that if the motor continues to try to start, additional fuses
may also blow and the motor will no longer hum.
If the motor is a single-phase motor and it hums but won't start, you
can remove the voltage source and test the start winding for resistance.
If the winding has high current or is open, you will need to change the
centrifugal switch, or change the motor. Be sure to test the capacitor
also if one is used to start the single phase motor. This problem may
be caused by having too large a load for the motor or a bearing that