Motor Torque

Formula

Description

The torque produced by a DC motor is directly proportional to the armature current, with the torque constant Kt as the proportionality factor. Kt is a fundamental motor parameter specified in the datasheet and depends on the motor's magnetic field strength, number of turns, and geometry. Interestingly, the torque constant Kt (in N·m/A) is numerically equal to the back-EMF constant Ke (in V/(rad/s)) when both are expressed in SI units. This duality arises from energy conservation in electromechanical conversion.

Variables

• τ — Motor torque (N·m)
• Kt — Torque constant (N·m/A)
• I — Armature current (A)

Practical Notes

For BLDC motors, Kt usually refers to the peak torque per phase current. Stepper motors specify holding torque (at rated current with rotor locked) and detent torque (with no current). Motor current increases under load, so ensure the driver and wiring can handle the maximum expected current. Continuous torque rating is limited by thermal dissipation in the windings.