If the Motor, motor wiring has been tested and no faults were discovered, it could well be down to the thermister installed during the motor's construction sensing unit gone ----- up. Al the large AHUs I worked on had this design, as do lots of medium to large, squirrel cage motors, dirty filters both primary and secondary filters in the AHU could be the cause, are there any records of filter changes? damaged belts could also be the isssue, before you go removing expensive motors etc I would certainly investigate these points of failure. Loose or damaged belts can be easily be heard by a squealing sound during start up, a poor airflow could be the indication as well, along with blocked / damaged filters. Is there a general maintenance regime in place, or is it a case of it works leave it until it breaks down.
