The motor must cover the fan shaft power at the operating point plus a margin for the cold-start condition, when dense air raises the power draw. Sizing too tightly risks overload on start-up; over-sizing wastes capital and runs the motor inefficiently. The fan power curve, read at your duty point, is the basis for the calculation.
Direct drive couples the motor straight to the impeller — compact, efficient and low-maintenance, but fixed at motor speed. Belt drive lets you set fan speed independently of the motor, which is valuable when the exact duty point is uncertain or may change. We supply both arrangements and will recommend the one that fits your duty, layout and maintenance preference.