What would cause a power fan motor to burn out prematurely?

Generally, when power vent motors burn out early it is caused by a lack of intake ventilation.

Generally, when power vent motors burn out early it is caused by a lack of intake ventilation. If there is not enough intake for the CFM rating of the power vent (CFM = cubic feet of air moved per minute), the fan will be working harder than it should.  Another consequence of insufficient intake ventilation for a power fan is the power vent could pull air from the conditioned living space. Another cause of premature motor failure is bearings seizing up. If the power vent is not equipped with a humidistat it may sit idle for many months of the year without ever turning on because only the thermostat is turning it on. The bearings could stiffen up over that period. When it does come time for the power vent to come on it will try to turn but the bearings will not move. This presents a locked rotor condition at which time the motor windings start to heat up and eventually get to a point where the temperature is enough to trip the thermal cut-off. Once the thermal cut-off has been tripped the motor will no longer run (per Underwriters Laboratories Safety Standards) and the motor will need to be replaced.