On the higher order transfer function of two-phase servomotors

By employing two reference frames for the motor equations, a sixth-order transfer function is developed for a two-phase servomotor, taking fully into account the electromechanical transients, rotor current and speed feedback, and unbalanced excitation with the reference winding excited continuously. In applications, the model can readily be approximated by a fourth-order transfer function. A numerical example illustrates the differences with several transfer functions available in the literature.