Motor constants

The constants K_M (motor size constant) and K_v (motor velocity constant, or the back EMF constant) are values used to describe characteristics of electrical motors.

K_M is the motor constant (sometimes, motor size constant). In SI units, the motor constant is expressed in (N⋅m/sqrt(W)):

where

The motor constant is winding independent (as long as the same conductive material used for wires); e.g., winding a motor with 6 turns with 2 parallel wires instead of 12 turns single wire will double the velocity constant, K_v, but K_M remains unchanged. K_M can be used for selecting the size of a motor to use in an application. K_v can be used for selecting the winding to use in the motor.

Since the torque is current x Kt then Km becomes Kt / ( motor resistance)^0.5.

K_v is the motor velocity constant (not to be confused with kV, the abbreviation for kilovolt), measured in RPM per volt or radians per volt-second (rad/V-s): ${\displaystyle K_{\mathrm {v} }={\frac {\omega _{\mathrm {No-Load} }}{V_{\mathrm {Peak} }}}}$

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