Blower motor conundrum

Air handler squirrel cage 240V blower stopped working. Started when I spun fan manually (free wheels with no grinding or wobble), so replaced 7.5 mf plastic case capacitor with 8mf metal case. Ran, but not for long. Replaced that capacitor with 7.5 mf metal case. Hums but won't run. Removed blower from air handler to bench and ran fine on 240V. Replaced in air handler and hums but won't run. Bypassed low voltage relay, bypassed exterior mounted shutoff switch, tested voltage at 240. Still hums, won't run. Removed to bench again and ran fine. Reinstalled in air handler and hums, won't run. WHAT am I missing please??

 

Either the starting torque of the motor has decreased or the resistance of the rotating portion of the air handler has increased or both . I need the motor nameplate info before I can try to help.

 

Have you checked voltage to the motor while in the air handler?

 

Can we assume a four wire motor.
Two wires to the 240vAC supply and two wires connecting directly to the capacitor ?
If yes.... you need to check the 240v lines with the motor connected and trying to start.

 

You could check the motor bearings by trying to move the motor shaft from side to side.
Any moment would make it possible that this is the source of the problem.