Calculation sequence
Step-by-step method
- Calculate synchronous speed from frequency and pole count.
- Subtract measured rotor speed for slip speed.
- Divide slip speed by synchronous speed.
ElectricalIntermediateMotors and Drives
Motor Synchronous Speed and Slip Calculator
Calculate induction-motor synchronous speed, slip speed, and percent slip.
Why it matters: Slip connects supply frequency and pole count with actual loaded shaft speed.
Calculated result
Engineering evaluation
Percent slip (s)2.778 %
- Synchronous speed (Nˢ)
- 1,800 rpm
- Slip speed (ΔN)
- 50 rpm
Engineering interpretation
At 60 Hz with 4 poles, synchronous speed is 1800 rpm and the entered rotor speed gives 2.777778% slip.
Important limitations
- Expected slip depends on motor design, loading, temperature, VFD mode, and manufacturer data.
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
Nˢ = 120f/p
ΔN = Nˢ−Nʳ
s = 100(Nˢ−Nʳ)/Nˢ
Current substitution
Nˢ = 120×60/4 = 1800 rpm; ΔN = 1800−1750 = 50 rpm; s = 2.777778%.
Equation legend
Inputs and calculated quantities
- f
- Electrical frequency — Hz
- p
- Motor pole count — count
- Nʳ
- Measured rotor speed — rpm
- s
- Percent slip — %
- Nˢ
- Synchronous speed — rpm
- ΔN
- Slip speed — rpm
Applications
How this calculation is used
Building design
Screen motor speed relationships.
Construction
Check nameplate frequency, pole count, and tachometer readings.
Commissioning
Calculate field slip at a documented load and frequency.
Quality controls
Assumptions and limitations
Assumptions
- Entered values describe one stable operating condition and use a consistent electrical or metering boundary.
- Measured quantities are simultaneous, representative, and obtained with appropriately rated instruments.
Limitations
- Normal interpretation assumes induction-motor motoring operation.
- Expected slip depends on motor design, load, VFD mode, and manufacturer data.
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