While the permanent magnet synchronous motor (PMSM) operation in the synchronous mode rotor speed average value coincides with current frequency in the windings by definition. However instantaneous value of rotation speed can differ from the average one significantly in the process of oscillations occurring in the PMSM rotor and stator electromechanical system. This article describes a refined mathematical model of permanent magnet synchronous motor (PMSM) operating in the synchronous rotation mode allowing to explore rotor speed oscillations arising in the synchronous mode. On the basis of the mathematical model the article provides a theoretical foundation and the example of specific implementation of one of the oscillation damping ways of the PMSM rotor with permanent-magnet excitation in the synchronous mode by introducing a current feedback to the impact on the stator phase voltage. The transfer function of the compensating device, affecting the voltage phase on the motor windings and providing a significant reduction of PMSM rotor rotation irregularity is synthesized on the basis of matrix transfer function of interrelationship between output variables and input signals.