Academic Level: 3. YearIntermediate 8m Read
Motor Power Calculation (Torque and Speed Relationship)
Sizing an electric motor requires understanding the dynamic relationship between rotational speed and the torque required to overcome load resistance and friction.
Governing Formula
P = (T × n) / 95509550Conversion constant
PPower (kW)
TTorque (Nm)
nRotational speed (RPM)
Step-by-Step Calculation
- 1Determine the required output torque (T).
- 2Determine the operational speed (n) in RPM.
- 3Apply the formula to find the power in kW.
- 4Account for transmission efficiency if necessary.
- 5Select a motor with a power rating higher than the calculated requirement.
Worked Example
Input Parameters
- T = 50 Nm
- n = 1450 RPM
Calculation
P = (50 × 1450) / 9550 ≈ 7.59 kW
Why This Matters
- Prevents motor burnout due to undersizing.
- Avoids excessive energy costs and initial investment due to oversizing.
Common Mistakes
- Forgetting to include the efficiency of the gearbox or transmission.
- Using HP units directly in a kW formula without conversion.
- Failing to account for peak starting torque.
Reference Material & Handbooks
Mechanical Engineering Handbook - Drives Section
Technical Q&A
How do I convert HP to kW?
1 HP is approximately 0.746 kW.
Live Simulation Engine
Open the Motor Sizing Tool to find the perfect drive for your system