Core relationships
- ω = 2πN / 60 where N is rpm and ω is rad/s.
- P = T · ω where P is watts and T is N·m.
- Rearranged: T = P / ω, N = (60P) / (2πT), P = (2πNT) / 60.
Gearbox estimate
- N_load = N_motor / ratio
- T_load ≈ T_motor · ratio · η (η is efficiency as a fraction)
- P_load ≈ P_motor · η (simple loss model)
- Select what you want to solve for.
- Enter the other two values with correct units.
- Optionally add gear ratio, efficiency, and service factor.
- Press Submit to see motor and load results.
- Use CSV or PDF buttons to export the report.
Sample calculations
| Scenario | Given | Computed (motor) | Gear (ratio, η) | Computed (load) |
|---|---|---|---|---|
| Pump drive | Power 2.2 kW, Speed 1450 rpm | Torque ≈ 14.50 N·m | 1:1, 100% | Speed 1450 rpm, Torque 14.50 N·m |
| Conveyor | Power 1.5 kW, Speed 900 rpm | Torque ≈ 15.92 N·m | 6:1, 92% | Speed 150 rpm, Torque ≈ 87.93 N·m |
| Mixer | Torque 120 N·m, Speed 60 rpm | Power ≈ 0.75 kW | 3:1, 90% | Speed 20 rpm, Torque ≈ 324.00 N·m |
Frequently asked questions
1) Why does torque change when speed changes?
For constant power, torque is inversely proportional to angular speed. If rpm drops, torque rises to keep power the same. Real machines may also have changing efficiency and load curves.
2) What units work best for motors?
Use kW and rpm for nameplates, then convert torque to N·m. If your data is in horsepower or lb·ft, the calculator converts accurately using standard mechanical definitions.
3) Is load power always motor power times efficiency?
It is a practical estimate, not a full loss model. Some losses vary with speed, temperature, and lubrication. Use measured efficiency curves when accuracy is critical.
4) How do I choose a service factor?
Service factor depends on shock loading, starts per hour, and duty cycle. Higher factors are common for conveyors, crushers, and mixers. Follow your gearbox or coupling vendor guidance.
5) Can I enter speed in rad/s?
Yes. Select rad/s and enter angular speed directly. The calculator converts to rpm internally and returns both rpm and rad/s in the result summary.
6) What happens with very low speed values?
Torque can become very large for a fixed power as speed approaches zero. Always confirm your machine can handle starting torque, thermal limits, and mechanical strength at low speeds.
7) Does this replace a full motor sizing study?
No. It helps with quick checks and unit conversions. For final sizing, consider load profiles, acceleration, inertia, thermal ratings, and safety margins.