Find the shaft speed a given power produces against a known torque load.
Sizing the electrical supply for the same machine? Use the kVA to kW calculator.
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Rearranged from the torque equation; you must know the load torque.
15 kW against a 50 N·m load settles at roughly 2,865 rpm. Raise the load torque and the speed falls; raise the power and it climbs.
This is the ideal constant-power answer. An induction motor on a fixed supply cannot exceed its synchronous speed regardless of how little torque the load demands.
A machine runs where the driver characteristic meets the load characteristic. Raise the available power and the speed rises; increase the load torque and it falls.
An induction motor does not choose its speed freely. Synchronous speed is fixed by the supply frequency and the number of poles; the rotor then runs slightly slower because slip is what induces rotor current in the first place.
| Poles | 50 Hz synchronous | 60 Hz synchronous | Typical full-load |
|---|---|---|---|
| 2 | 3,000 rpm | 3,600 rpm | 2,850 / 3,450 |
| 4 | 1,500 rpm | 1,800 rpm | 1,440 / 1,750 |
| 6 | 1,000 rpm | 1,200 rpm | 960 / 1,160 |
| 8 | 750 rpm | 900 rpm | 720 / 870 |
Full-load speed sits 1–5% below synchronous. That difference is the slip.
| kW | 25 N·m | 50 N·m | 100 N·m | 200 N·m | 400 N·m |
|---|---|---|---|---|---|
| 5 kW | 1,910 | 955 | 477 | 239 | 119 |
| 10 kW | 3,820 | 1,910 | 955 | 477 | 239 |
| 15 kW | 5,730 | 2,865 | 1,432 | 716 | 358 |
| 30 kW | 11,459 | 5,730 | 2,865 | 1,432 | 716 |
| 55 kW | 21,008 | 10,504 | 5,252 | 2,626 | 1,313 |
| 90 kW | 34,377 | 17,189 | 8,594 | 4,297 | 2,149 |
| 160 kW | 61,115 | 30,558 | 15,279 | 7,639 | 3,820 |
Your power figure against a range of load torques.
| N·m | Shaft speed (rpm) | Angular velocity (rad/s) |
|---|---|---|
| 5 N·m | 28,648 | 3,000 |
| 10 N·m | 14,324 | 1,500 |
| 20 N·m | 7,162 | 750 |
| 30 N·m | 4,775 | 500 |
| 50 N·m | 2,865 | 300 |
| 75 N·m | 1,910 | 200 |
| 100 N·m | 1,432 | 150 |
| 150 N·m | 955 | 100 |
| 200 N·m | 716 | 75 |
| 300 N·m | 477 | 50 |
| 400 N·m | 358 | 37.5 |
| 600 N·m | 239 | 25 |
| 800 N·m | 179 | 18.8 |
A variable frequency drive changes the supply frequency, and with it the synchronous speed. Below base speed it holds the voltage-to-frequency ratio constant so the motor keeps full torque; above base speed it runs out of voltage and enters field weakening.
Because power follows the cube of speed, running a fan at 80% speed uses roughly half the energy. That relationship is the entire business case for variable speed drives.
Where a drive is not an option, gears, belts and chains change speed and torque in inverse proportion — power in equals power out, minus the transmission efficiency.
Speed falls to 571 rpm and torque rises to about 251 N·m, ignoring belt losses.
29 calculators built on the same maths as the tool above. Each one has its own inputs, interactive diagrams and worked examples.
Common questions about converting kw to rpm.