The Motor Constant Km is the best metric to size a motor, as it defines the torque-power relationship. The units are Nm/sqrt(watt). The Torque Constant defines the torque-current relationship of a motor and is in Nm/amp. The motor and torque constants are related by the formula Km = Kt(trap)/sqrt(R), where R is the phase-to-phase resistance of the winding. Two motors with the same Km can have dramatically different torque constants and Torque-Speed curves due to the wire gauge used and the number of turns. Kt is useful when calculating the required current based on the required torque. Km is useful when comparing motors and in calculating temperature rise based on dissipated power.
