Permanent-magnet stepping motor

A permanent-magnet stepping motor comprising:

• a rotor having a columnar shape and including plural magnetic poles arranged in a circumferential direction of an outer circumferential surface;
• a first stator yoke having a cylindrical outer circumferential portion and an inner circumferential portion which surrounds the rotor and which has plural first pole teeth and plural second pole teeth, the first pole teeth extending in an axial direction, and the second pole teeth alternately engaging with the first pole teeth with a gap therebetween, the first stator yoke including an outside yoke having a U-shaped cross section composed of the outer circumferential portion, the inner circumferential portion, and a ring-shaped portion provided between front edges of the outer circumferential portion and the inner circumferential portion, and an inside yoke provided between rear edges of the outer circumferential portion and the inner circumferential portion;
• a second stator yoke having substantially the same structure as the first stator yoke and arranged coaxially and oppositely with the first stator yoke;
• a first bearing and a second bearing rotatably holding the rotor with respect to the stator yoke;
• a front plate for supporting the first bearing and fixed to the outside yoke of the first stator yoke;
• and
• an end plate for supporting the second bearing and fixed to the outside yoke of the second stator yoke;
• wherein
• the rotor has an outer diameter “d” and the stator yoke has an outer diameter “D”, and the ratio of “d/D” is greater than 0.6,
• the magnetic poles of the rotor are made of ferrite magnets, and
• the stator yoke has an outer diameter of not more than 55 mm and a thickness of not more than 0.8 mm.