BLDC Motor Kv to Kt Conversion

Formula

Description

The torque constant Kt (N·m/A) and velocity constant Kv (RPM/V) of a brushless DC motor are reciprocally related through the factor 30/π ≈ 9.549. This relationship comes from the fundamental motor equation: back-EMF = Kv × speed, and torque = Kt × current, with power conservation requiring V×I = τ×ω. Kv is commonly used in the RC/drone community (higher Kv = faster, less torque), while Kt is used in industrial servo applications (higher Kt = more torque per amp). They are two ways of expressing the same motor parameter.

Variables

• Kt — Torque constant in N·m per ampere
• Kv — Velocity (back-EMF) constant in RPM per volt

Practical Notes

The 9.549 factor comes from converting between RPM and rad/s: 30/π. In consistent SI units (rad/s per volt for Kv and N·m/A for Kt), Kt = 1/Kv exactly. Example: a 1000 Kv motor has Kt = 9.549/1000 = 0.00955 N·m/A = 9.55 mN·m/A. At 10 A, it produces 95.5 mN·m of torque. Motor efficiency determines how much of the electrical power is converted to mechanical power.