To prevent heating from a core for miniaturization and higher output by generating a rotating filed in a space encircled with the tips of a plurality of teeth along the distribution of the teeth, and applying alternating-current voltage, containing phase difference, to each of electric coils.

A plurality of teeth ST1 are formed on the inside surface of a ring-shaped core 1 the teeth which are radially projected toward the center of the core 1. Tooth end slits TS1 are formed which extends from the top faces of the teeth ST1 in the direction of the radius of the ring. In this state a rotating field is generated in the space encircled with the tips of the teeth ST1 along the distribution of the teeth, and an alternating-current voltage having phase difference is applied to each of a plurality of electric coils C1 wound around the core 1. This enables the prevention of heating from the core 1, and achieves miniaturization and higher output.