An axial electric fan rotates an impeller inside a stationary frame. Air travels approximately along the motor shaft. This calculator compares operating points using the fan laws: airflow follows speed and required shaft power follows speed cubed under the stated similarity assumptions.
Electric Fan Interactive Calculator
Compare airflow and fan shaft power at another speed using supplied reference data. Motor and drive electrical losses are excluded.
Equation Used
- Same fan and diameter.
- Unchanged fluid density.
- Corresponding operating points and applicable fan similarity.
- Reference power is delivered fan shaft power.
- Motor and drive efficiency and changed-system operating points are not solved.
Power is fan shaft power, not wall-plug electrical input. Same fan, density and corresponding operating points are assumed.
Rotor, fixed supports and axial airflow
The upper drawing looks into the intake. Seven curved blades and their hub rotate together, while the square mounting frame, shroud and rear motor supports stay fixed. The lower section separates the rotor from the fixed motor support and shows the axial airflow direction.
The drawing is a generic fan, not a replica of a named product. Blade sweep, pitch, clearances and frame details are illustrative. They do not determine the calculated airflow. Moving air markers show direction rather than a computed velocity field; the visible rotation is slowed for inspection.
Speed changes the animation rate and the calculator results. Reference airflow and shaft power are supplied operating data, not values inferred from the illustrated blades.
Compare speeds for the same fan
Enter reference airflow, reference fan speed and reference power delivered to the fan shaft. Choose an operating speed as a percentage of that reference. Keep fan diameter, air density and the relevant similarity conditions unchanged.
The results are most useful for corresponding operating points, such as a fan operating against a comparable quadratic system resistance. A changed duct, filter, damper or static-pressure requirement can change the operating point and requires fan and system curves.
Motor electrical input and fan shaft power are different quantities. If only a wall-plug power reading is available, do not silently use it as shaft power. The motor and drive losses must be treated separately.
Speed, airflow and shaft-power ratios
Let s=N₂/N₁ be the operating-to-reference speed ratio. Then N₂=sN₁, Q₂=sQ₁ and P₂=s³P₁ for corresponding operating points of the same fan at unchanged diameter and density. Here P is fan shaft power, not wall-plug electrical input.
The reported shaft-power reduction is 100(1−s³) percent. It becomes negative above the reference speed because the estimate is then an increase. This percentage is not a claim about total electricity savings over a duty cycle.
If motor and drive efficiencies are known at the relevant load, electrical input is related to shaft power through those efficiencies. Their values and variation are outside this calculator. The zero-speed case reports zero ideal-law airflow and shaft power; standby electrical consumption is not predicted.
Default calculation
The illustrative reference values are 60 CFM, 1500 rpm and 2 W of shaft power. At 50% speed, the model gives 750 rpm, 30 CFM and 0.25 W of shaft power, an 87.5% shaft-power reduction.
At 120% speed, the estimates become 1800 rpm, 72 CFM and 3.456 W. The reduction reads −72.8%, meaning a 72.8% increase relative to the reference shaft power.
These values are example inputs, not specifications for a particular fan. The cubic power rule does not establish a safe overspeed, a motor rating or a guaranteed electrical energy saving.
Where the comparison stops
This model does not solve blade aerodynamics, pressure distribution, stall, sound level, temperature, motor efficiency or bearing life. It does not compute a new operating point after changing system resistance.
Efficiency, operating life and manufacturing tolerances depend on the actual fan design and service conditions. Use the selected manufacturer’s performance curves and speed limits for a real product.
The cross-section is explanatory: fixed blade-section silhouettes show axial arrangement while the upper intake view shows actual illustrated rotation. Air markers pause at zero speed.
Electric fan questions
Why does shaft power fall faster than airflow?
For the stated similarity conditions, airflow is proportional to speed and shaft power is proportional to speed cubed. Half speed gives half the airflow and one eighth of the reference shaft power.
Is this the electricity drawn from the socket?
No. The power result is delivered fan shaft power. Motor and drive losses are not included.
Why can the reduction be negative?
Above 100% reference speed, the estimated shaft power exceeds the reference value.
Does changing reference airflow change blade size?
No. It changes supplied operating data. The generic drawing is not a blade-design solver.
Does zero speed mean zero electrical consumption?
No. It means zero airflow and shaft power in this ideal comparison. Standby controls or other electrical loads are outside the model.
References
- Greenheck: Elements of motor sizing — fan-law speed and required horsepower relationships.
- Greenheck: Fan laws FAQ — flow, pressure and brake-horsepower ratios.
- Greenheck: Field testing fans — shaft power and electrical measurements.
- Noctua: Airflow direction — intake, support side and direction of rotation.
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