How it works
Torque is the turning effort at the shaft. For a given power it is inversely proportional to speed: a slow motor needs more torque than a fast one to deliver the same power.
The page uses the exact relation T = P × 60 ÷ (2π N), which gives the familiar 9550 × kW ÷ rpm when P is in kW. HP is converted at 745.7 W per mechanical horsepower. Torque is also shown in kgf·m and lbf·ft.
An induction motor always runs a little below synchronous speed. The difference as a percentage of synchronous speed is the slip, typically 2 % to 6 % at full load for small and medium motors.
Formulas used
- Torque T (N·m) = P (W) × 60 ÷ (2π × N)
- T (N·m) ≈ 9550 × P (kW) ÷ N (rpm)
- Synchronous speed Ns = 120 × f ÷ poles
- Slip % = (Ns − N) ÷ Ns × 100
- Current I = P ÷ (√3 × V × efficiency × pf)
Worked example
A 7.5 kW four pole motor running at 1440 rpm on 50 Hz: torque = 7500 × 60 ÷ (2π × 1440) = 49.7 N·m. Synchronous speed is 1500 rpm, so the slip is 4 %. At 415 V, 90 % efficiency and pf 0.85 the full-load current is about 13.6 A.
The calculator opens with these example values, so you can compare its answer with the text above.
Frequently asked questions
How do I calculate motor torque from kW and rpm?
Torque in N·m is about 9550 times the power in kW divided by the speed in rpm.
What is slip in an induction motor?
Slip is how far the rotor speed lags the synchronous speed, shown as a percentage of the synchronous speed.
How many HP is a kW?
One mechanical horsepower is 0.7457 kW, so 1 kW is about 1.341 HP.
Is the current result exact?
No. It is an estimate from the efficiency and power factor you enter. The nameplate of the motor gives the true rated current.