A small DC motor includes a motor frame having a cylindrical portion, the cylindrical portion having a constant thickness and having a cross section in a shape that comprises four sides and connecting portions, each of the connecting portions connecting adjacent two of the four sides and being located inward from a corresponding corner in a quadrangle comprising the four sides; and a magnet having a circumferential surface on an inside thereof and having conformable contact with the motor frame on an outside thereof.
1. A small DC motor, comprising:
a motor frame having two ends and a side wall; wherein the side wall surrounds a magnet and a rotor and is formed by four straight side portions and four connecting portions; wherein each of the connecting portions is located between a pair of the side portions;
wherein the magnet includes four separate adjacent magnet sections, each magnet section having a different polarity from an adjacent magnet section, wherein each magnet section has an outer surface conforming to the motor frame and an arc shaped inner surface forming a portion of a border for a circular opening containing the rotor so that the cross section of each magnet section through a plane perpendicular to the axis of the rotor varies in width according to the shape of the side wall of the motor frame;
wherein each of the side portions is in contact with at least two magnet sections and each of the connecting portions is in contact with only a single adjacent magnet section; and
wherein, in a plane perpendicular to the rotational axis of the rotor, each of the connecting portions extends in a direction substantially perpendicular to a line extending from the center of the circular opening through a midpoint of the arc shaped inner surface of the adjacent magnet section so that, for each magnet section, the ratio of a radial minimum thickness of the magnet section to the radial maximum thickness of the magnet section is between about 1:2 to about 1:3.
2. The small DC motor of claim 1 , wherein the cross sectional shape of the motor frame is octagonal.
3. The small DC motor of claim 1 , wherein each of the magnet sections is spaced apart from the adjacent magnet sections.
4. The small DC motor of claim 1 , wherein the rotor includes an armature assembly rotatably disposed within the motor frame, wherein the armature assembly includes an armature windings-molded element of a cylindrical coil wounded by a magnet wire; and an inner yoke comprising a cylindrical portion facing the armature windings-molded element.
5. The small DC motor of claim 1 , wherein the rotor includes an armature assembly rotatably disposed within the motor frame, wherein the armature assembly includes an armature windings-molded element of a cylindrical coil wounded by a magnet wire; and a cylindrical movable back yoke having contact with an inner side of the armature windings-molded element.
6. The small DC motor of claim 1 , wherein an air gap provided between the magnet sections and the rotor has a constant distance.