Motor Torque Calculator Formula
Understand the math behind the motor torque calculator. Each variable explained with a worked example.
Formulas Used
Full-Load Torque
torque = power_kw * 1000 * 60 / (2 * pi * speed)Torque in lb-ft
torque_lbft = power_kw * 1000 * 60 / (2 * pi * speed) * 0.7376Variables
| Variable | Description | Default |
|---|---|---|
power_kw | Motor Rated Power(kW) | 15 |
speed | Motor Speed(RPM) | 1455 |
How It Works
Motor Full-Load Torque
Torque is calculated from the motor nameplate power and rated speed.
Formula
T = P x 60 / (2 pi N) = 9549 P / N
where P is power in watts, N is speed in RPM, and T is torque in N-m. The constant 9549 comes from 60/(2 pi) x 1000 for kW input.
Worked Example
A 15 kW motor running at 1455 RPM.
- 01T = 15,000 x 60 / (2 x pi x 1455)
- 02T = 900,000 / 9,142 = 98.44 N-m
- 03T = 98.44 x 0.7376 = 72.61 lb-ft
Frequently Asked Questions
What is the difference between full-load and starting torque?
Full-load torque is the continuous rated torque. Starting (locked rotor) torque is the torque at zero speed during startup, typically 150-300% of full-load torque for standard induction motors.
Does motor efficiency affect torque?
The nameplate kW is the mechanical output power. Electrical input power is higher (input = output / efficiency). The torque formula using output kW gives the shaft torque directly.
How do I select a motor for a given torque requirement?
Determine the required torque at operating speed, add a service factor (typically 1.15-1.25), then select a motor whose full-load torque exceeds this. Also check starting torque, breakdown torque, and duty cycle.
Learn More
Guide
How to Calculate Torque: Formulas, Units, and Real-World Applications
Master torque calculations for engineering applications. Covers torque formulas, moment arms, units conversion, motor torque, bolt torque, and the relationship between torque, speed, and power.
Ready to run the numbers?
Open Motor Torque Calculator