As Ray mentioned...Sail Switch
It is a small switch with a 'paddle' that sticks out into the room air flow
When motor comes up to 75% speed there should be enough airflow to move the paddle and close the sail switch
Small white box accessible with exterior panel removed then remove cover over motor (4 screws)
Sail Switch will have a Blue wire (DC+ to Switch) and a White wire (DC+ FROM switch to connector on circuit board)
Many times the switch is just dirty ....stuff builds up under paddle where it makes contact with switch