PURPOSE: To lower vibrations by constituting an armature core by alternately laminating cores, in which the opening sections of grooves are formed obliquely to the left and the right at the same pitch at every determined length along the axial direction, and alternately reversing the phase of electromagnetic exciting force working between a magnetic pole and the armature core in the axial direction.

CONSTITUTION: The opening sections of the upper sections of the winding housing sections 7 of grooves 2, in which armature windings 2a mounted at regular intervals on the circumference of an armature 1 are housed, are shaped obliquely only by half groove pitch sections, and cores 11a with openings 8a formed obliquely to the right and cores 11b with openings 8b formed obliquely to the left are laminated alternately at every determined length in the axial direction. When a magnetic pole is brought near to the tooth 4 of one armature core 11, the other armature core 11 is brought near to the groove 2, the armature core 11 is shifted only by the same pitch, and the groove is moved to a position precedently used as the tooth and the tooth is moved to a position precedently employed as the groove, thus keeping the total amount of electromagnetic force working to the armature cores constant at all times. Electromagnetic exciting force is divided into two and added mutually in antiphase, and the total of electromagnetic exciting force working among the magnetic poles and the armature cores 11 is brought to zero, and dissipates.