Motor Synchronous Speed and Slip Calculator
Work out the synchronous speed of an AC motor, its slip at the nameplate speed, and the drive frequency needed for a target speed.
Synchronous speed and slip
Drive frequency for a target speed
Uses the pole count above. Slip is ignored, so the shaft will run slightly slower than this under load.
How it works
An AC induction motor's rotating magnetic field turns at the synchronous speed, set by the supply frequency and the number of poles. The rotor always lags slightly behind, and that difference is called slip. The nameplate speed (for example 1750 RPM on a 60 Hz four-pole motor) already includes full-load slip.
slip % = (synchronous − actual) / synchronous × 100
frequency = RPM × poles / 120
Worked example
A four-pole motor on 60 Hz has a synchronous speed of 120 × 60 / 4 = 1800 RPM. If the nameplate says 1750 RPM, slip = (1800 − 1750) / 1800 = 2.78%.
To run a four-pole motor's field at 1200 RPM, the drive frequency is 1200 × 4 / 120 = 40 Hz.
Common questions
Typical synchronous speeds on 60 Hz
| Poles | Synchronous RPM (60 Hz) | Synchronous RPM (50 Hz) |
|---|---|---|
| 2 | 3600 | 3000 |
| 4 | 1800 | 1500 |
| 6 | 1200 | 1000 |
| 8 | 900 | 750 |
Can I run a motor above its base frequency?
Many drives allow it, but torque falls off above base speed because voltage stops rising. Check the motor's and drive's limits for mechanical speed and cooling at low speeds.
Calculations are provided for convenience and education. Verify results against manufacturer data before relying on them.