Motor RPM Calculator
Find an AC motor's speed from its number of poles and supply frequency, including slip.
Inputs
Formula
- Synchronous speed Ns = 120 × f ÷ poles
- Actual speed N = Ns × (1 − slip)
- Torque = 9550 × kW ÷ RPM (Nm)
An induction motor's magnetic field rotates at the synchronous speed set by frequency and pole count. The rotor always lags slightly behind — that lag is the slip, typically 3–5% at full load, which is why a 4-pole motor runs at 1,440 RPM rather than 1,500.
Step-by-step calculation
- 1
Synchronous speed
120 f ÷ poles
1,500 RPM
- 2
With slip
× (1 − 4%)
1,440 RPM
Worked example
- 14 poles on 50 Hz → Ns = 120 × 50 ÷ 4 = 1,500 RPM.
- 2With 4% slip the shaft runs at 1,440 RPM.
- 3A 1.5 kW motor at 1,440 RPM produces 9.95 Nm of torque.
Tips
- A VFD changes the frequency, so the speed changes in direct proportion.
- Slip rises with load — a motor running very slowly is usually overloaded.
Frequently asked questions
Why is my 1500 RPM motor showing 1440?
1,500 is the synchronous speed; 1,440 is the loaded speed after about 4% slip.
How do I find the poles?
Divide 120 × frequency by the nameplate RPM and round to the nearest even number.
Can I run a 50 Hz motor on 60 Hz?
It will run 20% faster with reduced torque, and the fan load rises sharply. Check with the manufacturer.
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