where, vd and vq are the d- and q- axis voltages, id and iq
are the d- and q- axis stator currents, Ld and Lq are the d- and
q- axis inductances, λd and λq are the d- and q- axis stator flux
linkages respectively, Rs is the stator resistance per phase, ψf
is the permanent magnetic flux, and p is the derivative
operator