Calculation sequence
Step-by-step method
- Divide motor sheave pitch diameter by fan sheave pitch diameter.
- Multiply motor RPM by the sheave ratio for ideal fan speed.
- Reduce ideal fan speed by the entered slip fraction.
MechanicalBeginnerFans
Belt-Drive Fan RPM Calculator
Estimate driven fan speed from motor speed, sheave pitch diameters, and entered belt slip.
Why it matters: Sheave selection directly affects fan speed, airflow, pressure, and power.
Calculated result
Engineering evaluation
Estimated fan speed (Nf)1,072 rpm
- Ideal no-slip fan speed (Nideal)
- 1,094 rpm
- Sheave speed ratio (rD)
- 0.625 ratio
- Entered belt slip (s)
- 2 %
- Maximum acceptable belt slip (smax)
- 5 %
Engineering interpretation
The entered drive geometry predicts 1072 rpm after 2% slip, compared with 1094 rpm at ideal no-slip conditions.
Motor-driven equipment
Estimate electrical demand and energy use
Use nameplate information and an operating schedule to screen motor input power, current, annual energy, and operating cost. Results update automatically.
Choose the motor nameplate output-power convention.
Rated shaft output, not electrical input.
Used to estimate current; it does not change the hp-based energy result.
Use efficiency at the evaluated load point when available.
Use measured or defensible operating load rather than motor nameplate size alone.
Used only for the estimated current calculation.
Assumes all selected motors operate concurrently at the entered load.
Replace the profile with the actual occupied, process, or control schedule.
$/kWh
Use the applicable marginal utility energy charge; demand charges are not included.Estimated result
Total motor input power (Pin,total)6.21 kW
- Current per motor (I)
- 9.18 A
- Total coincident current (Itotal)
- 9.18 A
- Daily energy (Ed)
- 50 kWh/day
- Annual operating hours (Hy)
- 2,080 h/year
- Annual energy (Ey)
- 12,925 kWh/year
- Annual energy cost (Cy)
- $1,551/year
Equations and assumptions
Nameplate-based screening method
Horsepower is converted using 1 hp = 0.7457 kW. This estimate assumes steady average load and the entered operating-point efficiency. Voltage and power factor affect estimated current, but hp, load, efficiency, quantity, and schedule establish the energy estimate. VFD speed profiles, cycling, standby power, demand charges, and process variation require separate evaluation.
Technical basis: U.S. Department of Energy — Determining Electric Motor Load and EfficiencyEngineering knowledge center
Understand the calculation—not just the answer
Governing relationships
rD = Dm/Df
Nideal = Nm rD
Nf = Nideal(1−s/100)
criterion s≤smax
Current substitution
rD = 5/8 = 0.625; Nf = 1750×0.625×(1−2/100) = 1072 rpm; 2% ≤ 5%.
Equation legend
Inputs and calculated quantities
- Nm
- Motor speed — rpm
- Dm
- Motor sheave pitch diameter — in. or mm
- Df
- Fan sheave pitch diameter — in. or mm
- s
- Estimated belt slip — %
- smax
- Maximum acceptable belt slip — %
- Nf
- Estimated fan speed — rpm
- Nideal
- Ideal no-slip fan speed — rpm
- rD
- Sheave speed ratio — ratio
- s
- Entered belt slip — %
- smax
- Maximum acceptable belt slip — %
Applications
How this calculation is used
Building design
Screen a belt-drive speed ratio before final drive selection.
Construction
Check installed sheave diameters and expected fan RPM.
Commissioning
Compare calculated and tachometer-measured fan speed to identify slip or incorrect drive components.
Quality controls
Assumptions and limitations
Assumptions
- Entered values describe the same stable operating condition.
- Units, pressure definitions, fluid properties, and equipment boundaries are applied consistently.
Limitations
- This tool supports preliminary evaluation and field verification; it does not replace manufacturer curves or project-specific engineering analysis.
- Project criteria, applicable standards, measurement uncertainty, and authorized professional judgment remain controlling.
Field use
Verification procedure
- Confirm the approved reference condition, instrument range, and calculation boundary.
- Stabilize the system and record calibrated field measurements.
- Compare the result with current manufacturer data and project requirements before disposition.
Common engineering mistakes
- Mixing measurements from different operating conditions or equipment boundaries.
- Treating a calculated screening status as automatic acceptance.
Technical basis
References and source standards
- U.S. Department of Energy—Improving Fan System PerformanceFederal sourcebook covering fan-system relationships, fan laws, power, efficiency, system curves, and field performance considerations.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement