I am wondering at what fan size/speed a direct drive motor is no longer sufficient to drive a fan? Phrased alternatively, what size/speed of fan requires motors with a transmission? For an axial fan.
Often because the “standard” off the shelf motor has a running speed different to the required fan speed or most efficient fan speed.
Then it comes down to a choice of transmission or motor speed control which is influenced by many other factors.
Fixed speed or variable speed
Can all be relevant.