Synchronous speed
Ns = 60 * f / p
Rotor speed
Nr = (60 * f * (1 − s)) / p
Where：
Ns = Synchronous speed (RPM)
Nr = Rotor speed (RPM)
f = Frequency (Hz)
p = Pairs of poles
s = Slip factor
Calculate synchronous and actual speed of asynchronous motors based on poles, frequency, and slip.
Supports:
2, 4, 6, 8 pole motors
50Hz and 60Hz power supply
Custom slip (3%–6%)
Synchronous Speed = (120 × Frequency) / Poles
Actual Speed = Synchronous Speed × (1 – Slip)
Example:
4-pole motor, 50Hz, 5% slip →
ns = (120 × 50) / 4 = 1500 RPM
nr = 1500 × (1 – 0.05) = 1425 RPM
| Poles | 50Hz Sync | 60Hz Sync | Actual Speed |
|---|---|---|---|
| 2 | 3000 | 3600 | ~2850 / ~3420 |
| 4 | 1500 | 1800 | ~1425 / ~1710 |
| 6 | 1000 | 1200 | ~950 / ~1140 |
| 8 | 750 | 900 | ~712.5 / ~855 |
Accurate RPM determinations translate directly into mechanical and electrical design steps:
Mechanical Gearbox & Pulley Ratio Selection: Use the actual full-load rotor speed (nr) rather than synchronous speed (ns) to calculate accurate speed reduction ratios for belts, pulleys, and gearboxes.
VFD Programming & Speed Tuning: Validate motor feedback speed readings from encoders or tachometers against theoretical output during VFD commissioning and open-loop vector setup.
Pump and Fan Law Calculations: Flow rate in centrifugal pumps and fans varies linearly with RPM (Q1/Q2 = n1/n2). Precise speed calculations prevent under- or over-sizing of mechanical systems.
This calculator determines the rotational speed of an electric motor in revolutions per minute (RPM) based on the power supply frequency (f) in Hertz (Hz) and the number of magnetic poles (p) or pole pairs. It calculates both theoretical synchronous speed (ns) and actual rotor operating speed (nr) by accounting for mechanical slip (s) in AC induction (asynchronous) motors as well as synchronous motors.
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