motor theory

[russrmartin]

I have a little theory question for you. I have a single phase, motor, assuming that it is a cap run motor. The cap is supposed to be 7.5 uF, and checks out tentatively at 9uF. When nothing is energized, the motor turns free. When the cap is wired up, and the circuit is energized, the motor hums, and also locks where it is. When the cap is unwired AT THE CAPACITOR, the motor sits idle. Even when spinning the motor and energizing it simply coasts to a stop. When the cap is wired, spinning and energizing brings it to an abrupt halt. This is a tiny blower motor, and because of what a new one costs, we're not gonna mess with it. We do know that the cap is good, and we are ordering a new motor. However, just for knowledge's sake, I am looking for a logical explanation of what could make it act like this. Any ideas are welcome. Thanks in advance.

Russ

 

[jrwb4gbm]

I'm just guessing but it sounds like an open "Run" winding,(the one not connected to the capacitor).

 

[mwatkins]

In short
A single phase motor uses the Cap to establish a second phase 90 degrees out of phase of the main windings. This "appears" to the rotor as a rotating magnetic field, which is needed for a motor to run. The rotor has its own magnetic field and it is trying to lock on the stator filed. Being AC the stator field is rotating thus causing the motor to turn.
If any windings are open then the rotor will "lock" on to the only phase left and not turn. So it does sound like the main wingdings are the problem.

Hope this helps.