Correct motor sizing prevents overheating and premature failure; it starts from the fan shaft power and applies defined safety margins.
Calculating Shaft Power
Shaft power is the mechanical power the fan needs at the driven speed, equal to the product of airflow and pressure divided by efficiency. In SI units, P = Q × ΔP / (η × 1000) in kilowatts, with Q in m³/s and ΔP in pascals, a formula worth memorising for quick checks.
Motor sizing starts from the fan shaft power, the mechanical power the wheel needs at the duty point, divided by the transmission efficiency to get the power the motor must supply. Belt, coupling, or direct drive each lose a few percent, and that loss must be in the calculation, not optimism, or the motor will run hot and short-lived.
A safety factor covers off-design conditions, belt slip, and dirt on the wheel that raises the load over time. A common practice adds 10-15% to the calculated demand, but over-sizing wastes capital and can land the motor on a flat part of its efficiency curve where it draws more current than necessary, so the margin is chosen with the duty variation in mind.
The service factor printed on the nameplate is not an invitation to run continuously at that load; it is a short-term overload allowance. For continuous duty the motor is selected so its rated power comfortably exceeds the expected demand, leaving the service factor in reserve for start-up and transient peaks rather than as the everyday operating point.
Efficiency class is now a specification item in its own right. IE3 or IE4 motors cost more up front but recover the premium over thousands of running hours, and on a fan that operates most of the day the energy saving is real money; the premium is weighed against the operating hours and the local electricity cost during selection.
The company's 80,000 m² production site builds fan sets under ISO 9001:2015 with CCC certification and supplies units to more than 50 countries, so motor frame, efficiency class, and starting method are documented per order and its engineers respond to selection enquiries within 24 hours with a sized drive recommendation.
Efficiency here is the fan's total efficiency at the operating point, not a nameplate best value. A fan forced to run well off its peak can have efficiency 20-30 points lower, which quietly enlarges the motor requirement and the energy bill over the equipment's life.
The motor must also cover drive losses. A belt drive loses a few percent to friction, while a direct-coupled arrangement avoids that loss but demands precise alignment to protect bearings and to keep the coupling from overheating during continuous operation.
Duty variability matters. A system whose resistance rises as filters load needs a motor sized to the worst-case pressure, not the clean starting point, or the motor will overheat late in the service interval when the building needs cooling or extraction the most.
Applying Margins
A service factor is the motor's allowed overload capacity, commonly 1.15 for general-purpose units, meaning it can carry 15% above rated power for defined periods without damage, a buffer for starting and brief peaks rather than for continuous overload.
Rather than rely on the service factor for normal duty, many specs add a 10-20% power margin so the motor runs below its rating, keeping windings cooler and extending insulation life, which is cheaper than an early motor change in an inaccessible plant room.
Altitude and ambient temperature reduce motor cooling. Above 1,000 m or in hot plant rooms, the usable output falls, so the selected rating should be de-rated or the margin increased to keep the windings within their temperature class during the summer peak.
Starting torque is another check: a direct-on-line start must accelerate the fan inertia within the motor's pull-up region, or the rotor overheats during each start, and a high-inertia impeller may need a soft starter or a variable-frequency drive to ramp safely.
| Item | Specification | Note |
|---|---|---|
| Formula | P=Q·ΔP/(η·1000) | kW, Q in m³/s |
| Fan efficiency | 40-75% | At operating point |
| Service factor | 1.15 typical | Short overload only |
| Power margin | 10-20% | Below rating extends life |
| Altitude | >1,000 m | De-rate motor |
| Drive loss | 2-5% | Belt friction |
Selection Checklist
- Use the fan efficiency at your operating point, not the catalogue peak.
- Size to the worst-case pressure, including filter and coil loading.
- Add a 10-20% margin so the motor runs below its nameplate rating.
- De-rate for altitude above 1,000 m or high ambient temperature.
- Confirm starting torque suits the fan inertia and starter type.
- Prefer direct coupling where alignment can be maintained.
About the Manufacturer
Keruite Air Conditioning Group, headquartered in Dezhou, Shandong, has more than 30 years of HVAC manufacturing experience and operates an 80,000 m² production site with its own performance testing laboratory. Products are built under ISO 9001:2015 quality management and carry CCC certification, and are supplied to customers in over 50 countries.
Our engineers respond to selection enquiries within 24 hours and can provide project-specific sizing, performance curves, and documentation to support specification and procurement.