Motor humms but wont engage - Pool & Spa

The start capacitor or start winding has gone bad. Or bearings are dry.
Don't run motor if it does not rotate full speed or motor will overheat and go bad.
Replace or repair motor.

Each motor has 2 winding circuits.
Start winding and run winding.
Both windings receive power same time.
Start winding has capacitor that releases sudden burst of electricity that starts motor rotation. Kind of like low speed on transmission.
Run winding then brings motor up to speed.
http://waterheatertimer.org/images/Motor-wiring-500.jpg
http://waterheatertimer.org/pdf/AO-Smith-motor-wiring.pdf

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7