Motor Torque Calculator
Calculate electric motor torque, power, speed, and efficiency.
Covers DC motors, AC motors, and stepper motors.
Torque is the rotational force produced by an electric motor. It is one of the most important specifications for motor selection in robotics, automation, electric vehicles, and industrial machinery.
The Power-Torque-Speed Relationship
The fundamental relationship between mechanical power, torque, and rotational speed is:
P = τ × ω
Where:
- P = mechanical power (Watts)
- τ = torque (Newton-meters, N·m)
- ω = angular velocity (radians per second)
Since ω = 2π × n / 60 (where n = RPM):
P = τ × 2π × n / 60
Rearranging for torque:
τ = P × 60 / (2π × n) = 9.5493 × P / n
Torque Units
| Unit | Conversion |
|---|---|
| N·m (Newton-meter) | Standard SI unit |
| oz·in (ounce-inch) | Common in small motors: 1 N·m = 141.6 oz·in |
| lb·ft (pound-foot) | Common in large motors: 1 N·m = 0.7376 lb·ft |
| kgf·cm | 1 N·m = 10.197 kgf·cm |
DC Motor Torque
For DC motors, torque is proportional to armature current: τ = K_t × I_armature
Where K_t is the motor torque constant (available on the motor datasheet, often in N·m/A).
Stepper Motor Torque
Stepper motors produce different torques at different speeds. At low speeds, they produce rated (holding) torque. At higher speeds, torque falls off due to back-EMF. Always check the motor’s torque-speed curve when selecting a stepper for a specific application.
Efficiency
Real motors are not 100% efficient. Electrical input power is always greater than mechanical output power due to copper losses (I²R heating), iron losses (eddy currents, hysteresis), and friction:
η = P_mechanical / P_electrical = (τ × ω) / (V × I)
Typical motor efficiency ranges from 75% (small motors) to 97% (large industrial motors).