On the left is a belt-drive fan. To balance the airflow, you need to remove the belt, loosen the set screws on the adjustable sheave, spin half the sheave to change its diameter, tighten the set screws, re-install the belt, and turn the fan back on. Then you have to take an airflow reading, while crossing your fingers that it’s somewhat close to your target. If not, you do the entire process over again …and again …and again.
On the right is a direct-drive fan with EC Motor. Use a screwdriver to turn the blue speed control dial. When you hit the target airflow, you’re DONE!
The moral of the story? Whenever possible, specify direct-drive motors!!