Abstract: A phase- locked servo system , which is applicable to 3 - phase ,6 - state brushless DC motors ( without position transducers ), is introduced . The paper emphasizes on the operational principle of a frequency and phase detector consisting of D triggers and ANDORN gates , the Boolean algebra model and the reduced model for the power control network . The simulation study for the servo system is carried out by means of a computer system , and the effect of some parameters on the dynamics of the servo system is given . Finally , results of simulation are compared with test - results on the prototype .