A control system for a synchronous motor comprising: a synchronous motor; a power converter coupled to the synchronous motor and configured with a plurality of switching elements; and a controller operable to output a voltage command to the power converter to control the synchronous motor, wherein the control system further comprises a first phase estimation means operable to select two energized phases among three phase windings of the synchronous motor, operable to apply a pulse voltage, and operable to estimate phase and velocity on the basis of a non-energized phase electromotive force at the time of application of the pulse voltage; a second phase estimation means operable to estimate phase and velocity on the basis of a velocity-induced electromotive force of the synchronous motor; a first driving means operable to drive the synchronous motor by 120-degree energization; a second driving means operable to drive the synchronous motor by 180-degree energization; a first selecting means operable to select one of the first phase estimation means and the second phase estimation means; and a second selecting means operable to select one of the first driving means and the second driving means, characterised by wherein, when the rotational velocity of the synchronous motor is not less than a prescribed value V1 set up in advance, the first selecting means selects the second phase estimation means and the second selecting means selects the second driving means, wherein, when the rotational velocity of the synchronous motor is not greater than the prescribed value V1 set up in advance and not less than a prescribed value V2 lower than the prescribed value V1, the first selecting means selects the second phase estimation means and the second selecting means selects the first driving means, and wherein, when the rotational velocity of the synchronous motor is not greater than the prescribed value V2, the first selecting means selects the first phase estimation means and the second selecting means selects the first driving means. a synchronous motor; a power converter coupled to the synchronous motor and configured with a plurality of switching elements; and a controller operable to output a voltage command to the power converter to control the synchronous motor, wherein the control system further comprises a first phase estimation means operable to select two energized phases among three phase windings of the synchronous motor, operable to apply a pulse voltage, and operable to estimate phase and velocity on the basis of a non-energized phase electromotive force at the time of application of the pulse voltage; a second phase estimation means operable to estimate phase and velocity on the basis of a velocity-induced electromotive force of the synchronous motor; a first driving means operable to drive the synchronous motor by 120-degree energization; a second driving means operable to drive the synchronous motor by 180-degree energization; a first selecting means operable to select one of the first phase estimation means and the second phase estimation means; and a second selecting means operable to select one of the first driving means and the second driving means, characterised by wherein, when the rotational velocity of the synchronous motor is not less than a prescribed value V1 set up in advance, the first selecting means selects the second phase estimation means and the second selecting means selects the second driving means, wherein, when the rotational velocity of the synchronous motor is not greater than the prescribed value V1 set up in advance and not less than a prescribed value V2 lower than the prescribed value V1, the first selecting means selects the second phase estimation means and the second selecting means selects the first driving means, and wherein, when the rotational velocity of the synchronous motor is not greater than the prescribed value V2, the first selecting means selects the first phase estimation means and the second selecting means selects the first driving means.