How to Size the Motor for a Fan? Power Formula & Selection Guide

Sizing the motor for a fan is not something you should guess: oversize it and you pay for wasted electricity every hour; undersize it and the motor burns out. Fan power is directly tied to airflow and pressure. This guide gives the fan motor power formula, the service-factor values and the matching checks so you can size it correctly first time.

1. Fan Motor Power: Start with This Formula

Fan shaft power (kW) ≈ Flow Q (m³/s) × Total pressure P (Pa) ÷ (1000 × fan efficiency η)

Motor power = Fan shaft power ÷ drive efficiency (1.0 direct-coupled, 0.95-0.98 belt) × service factor.

• Flow Q: the actual process air volume, in cubic metres per second, not per hour;

• Total pressure P: the fan total pressure difference across inlet and outlet, in Pa;

• Efficiency η: typically 0.6-0.85 for fans, available on the selection sheet;

• Service factor: normally 1.1-1.2; take 1.2-1.3 for frequent starting, dusty or strongly fluctuating duty.

2. Coefficient Differences by Fan Type

3. After the Math, Check These Three Matches

• Speed match: motor speed must suit the fan design speed, or use belt / VFD control. For adjustable speed, a VFD-duty motor has better low-speed cooling and bearing design;

• Starting method: large fan rotors have high inertia and direct-on-line starting can be harsh - see our motor starting methods guide;

• Efficiency class: fans run continuously, so efficiency directly shows up on the electricity bill. Prefer IE4/IE5 motors; compare them in our IE3 vs IE4 vs IE5 guide.

4. Common Sizing Mistakes

• Sizing from the nameplate flow: if actual duty flow differs from nameplate, the motor runs overloaded for years;

• Ignoring altitude and ambient temperature: above 1,000 m or above 40 C the motor must be derated, so the power needs a larger margin;

• Missing stall / jam protection: induced-draught and exhaust fans can jam on debris - set the overload relay to the actual running current.

5. How to Make the Selection Stick

Send the fan data sheet (flow, pressure, speed, efficiency) to the manufacturer and let them calculate shaft power and motor power against your duty - far more reliable than estimating yourself. At SHANGDIAN (Shanghai Electric Motor Factory) we supply low-voltage three-phase induction motors and energy-saving retrofit motors for direct-coupled or belt-driven fan packages. If you already have a fan and suspect the motor is undersized, share the current and duty data and we will re-check it for you.

For fan motor sizing support, contact the SHANGDIAN team with your fan parameters and we will provide a matched selection.