Calculation sequence
Step-by-step method
- Divide new frequency by reference frequency.
- Multiply loaded reference speed by the frequency ratio.
- Review motor and driven-equipment limitations.
ElectricalIntermediateMotors and Drives
VFD Frequency and Estimated Motor Speed Calculator
Estimate loaded motor speed at a new VFD output frequency from a documented reference point.
Why it matters: Frequency and speed are approximately proportional, but actual safe operation remains equipment-specific.
Calculated result
Engineering evaluation
Estimated loaded speed (N₂)1,312.5 rpm
- Frequency and speed ratio (r)
- 75 %
Engineering interpretation
The proportional screen estimates 1312.5 rpm at 45 Hz, or 75% of the reference point.
Important limitations
- This calculation does not establish safe frequency limits. Motor cooling, torque, volts-per-hertz, resonance, overspeed, and driven-equipment limits require manufacturer and professional review.
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
r = f₂/f₁
N₂ = N₁·r
Current substitution
r = 45/60 = 0.75; N₂ = 1750×0.75 = 1312.5 rpm.
Equation legend
Inputs and calculated quantities
- f₁
- Reference frequency — Hz
- N₁
- Loaded reference speed — rpm
- f₂
- New output frequency — Hz
- N₂
- Estimated loaded speed — rpm
- r
- Frequency and speed ratio — %
Applications
How this calculation is used
Building design
Screen a proposed operating-frequency range.
Construction
Check VFD output frequency and shaft-speed feedback.
Commissioning
Compare measured speed response with a transparent frequency ratio.
Quality controls
Assumptions and limitations
Assumptions
- Frequency-to-speed behavior is approximately proportional over the evaluated range.
- Slip fraction and driven-equipment condition are similar to the reference point.
Limitations
- Does not establish a safe minimum or maximum frequency.
- Motor cooling, torque, volts-per-hertz, resonance, overspeed, and driven-equipment limits require manufacturer and professional review.
Field use
Verification procedure
- Follow the approved electrical-safety program and use correctly rated instruments.
- Confirm wiring, ratios, units, operating mode, and measurement boundaries.
- Document simultaneous readings and compare with approved project and manufacturer data.
Common engineering mistakes
- Combining values from different time intervals, phases, or measurement boundaries.
- Treating a screening result as equipment selection, protective-device coordination, or code compliance.
Technical basis
References and source standards
- NEMA Motors and Generators StandardsMotor rating, speed, efficiency, and operating context; verify the current applicable edition.
- IEEE Standards AssociationPower quantity, harmonic, transformer, motor, battery, and metering standards; verify the current applicable edition during professional review.
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