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 a constant thickness and having two ends and a side wall; wherein the side wall surrounds a magnet and includes four straight portions and at least four connecting portions, the motor frame being a magnetic material and being a magnetic constituent portion of the motor;
wherein each of the connecting portions is located between adjacent two of the four straight portions;
wherein the magnet includes at least four poles and has an outer surface conforming to the motor frame and an inner surface forming a circular opening and wherein the width of the magnet between the inner and outer surface varies;
a rotor having a shaft and rotatably disposed in the circular opening,
wherein the side wall includes the same number of connecting portions as the number of poles contained in the magnet, wherein each of the connecting portions is a segment of a circle concentric with a center of the shaft;
wherein an air gap provided between the inner surface of the magnet and the rotor has a constant distance.
2. A small DC motor according to claim 1 , wherein the magnet has a maximum thickness portion at portions corresponding to the connecting portions of the motor frame and a minimum thickness portion gradually decreasing from the maximum thickness portion toward circumferentially opposite end, a ratio of a width of the minimum thickness portion to a width of the maximum thickness portion is between about 1:2 to about 1:4.
3. A small DC motor according to claim 2 , wherein the ratio of the width of the minimum thickness portion to the width of the maximum thickness portion is between about 1:2 to about 1:3.