In-depth study of the torque constant for permanent magnet machines

The torque constant, together with the back-EMF constant, was originally used in permanent magnet DC commutator motors (PMDC motors) to couple the electric circuit equations with mechanical equations. But it is still an open question whether the concept of the torque constant and the back-EMF constant can be applied to brushless DC (BLDC) motors and permanent magnet (PM) AC machines. This paper presents an in-depth study of the two constants under various real conditions in PM machines. The torque constant at various load conditions is computed using transient 2D finite element analysis (FEA). It is shown that the torque constant is not a constant for BLDC motors and PM AC machines.