These two fan failures were caused by human error. One was rotating backward after tripping out on a motor overload issue, not thoroughly investigated as to the cause and reset remotely at the motor control center. The rotor eventually came apart due to an internal component failure. The other fan failed due to lack of maintenance. Something internal may have come loose triggering the failure.
Resetting any electric equipment without first investigating is dangerous! Investigate and use a second mechanic to monitor any tripped restart. These fans are great performers and failures are rare. They do however require yearly maintenance and rebuilding when necessary.