ThinGap’s sister company, Sierramotion recently published an insightful article about motor constant, and has given permission to repost part of it. Many motor specifications define how a brushless DC motor will perform, but only one allows a true motor to motor comparison of torque, associated power dissipation and temperature rise. That specification is the motor constant (Km). The units of the motor constant are Nm per square root of watts dissipated in the form of heat (these being I2*R losses, known as copper losses).
This metric is a form of efficiency, but it only considers the copper losses. In a traditional, slot wound brushless DC (BLDC) motor, there are at least two other loses, those being core losses, which include eddy current and hysteresis, and mechanical frictional losses, those being viscous friction, coulomb friction, and sometimes windage. A third form of loss that is not addressed here, is created by the pulse width modulated method of driving BLDC motors.
This is an effect that is highly dependent on the inductance and frequency of the PWM rate and could also be considered under the right conditions. An Effective Motor Constant (Kme) is introduced to include core and copper motor losses (frictional and PWM losses ignored) and offers a more meaningful comparison of motor performance at speed.