A direct-current power supply (40) is connected between the neutral points of two three-phase coils (24, 26) of a 2Y motor (22) constituted of the windings of the two three-phase coils (24, 26), which are connected in Y-connection and wound on a same
stator, and to which three-phase
alternating current power is severally supplied with a
phase difference of a shifted angle between the windings from two
inverter circuits (30, 32) having a positive pole
bus (34) and a negative pole
bus (36) for common use. A
capacitor (38) is connected between the positive pole
bus (34) and the negative pole bus (36). The
electric potential difference between the neutral points of the three-phase coils (24, 26) is made larger or smaller than the
voltage of the direct-current power supply (40) through the switching control of the
inverter circuits (30, 32). Thereby, the
capacitor (38) can be charged or discharged. Consequently, an
inverter input
voltage can be adjusted within a wide range.