A small DC motor includes: a motor frame including a cylindrical portion, the cylindrical portion having a constant thickness and having a cross section in a shape that includes 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 including the four sides; field magnets; and an armature assembly, wherein the field magnets are provided so as to be spaced apart from each other, and the small DC motor includes an air gap between each of the four sides and a radially outermost surface of the armature assembly, the air gap being a minimum size needed to rotate the armature assembly.
1. A small DC motor, comprising:
a motor frame comprising 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;
field magnets, each being positioned adjacent to each of the connecting portions, having an arc-shaped surface on an inside thereof and having a cross section in an inside portion that has an arc-shape and in an outside portion that has a same curvature as an inside surface of the connecting portions, and
an armature assembly comprising an armature winding and a shaft and being disposed rotatably within the motor frame,
wherein the field magnets are provided so as to be spaced apart from each other,
wherein the small DC motor comprises an air gap between each of the four sides and a radially outermost surface of the armature assembly, the air gap being a minimum size needed to rotate the armature assembly, and
wherein a radius from the center of the shaft to an inside surface of the motor frame is the same as a radius from the center of the shaft to the arc-shape surface of the inside portion of the field magnets.
2. A small DC motor as set forth in claim 1 , wherein the armature assembly comprises an armature core, the armature core constituting the radially outermost surface of the armature assembly.
3. A small DC motor as set forth in claim 1 , wherein the armature assembly comprises an armature windings-molded element, the armature windings-molded element constituting the radially outermost surface of the armature assembly.
4. A small DC motor as set forth in claim 1 , wherein the field magnets comprise joining portions, each of the joining portions joining the inside portion of the field magnets to the outside portion of the field magnets and being disposed at the air gap.
5. A small DC motor as set forth in claim 1 , wherein a radius from a center of the shaft to the inside surface of the motor frame is the same as a radius from the center of the shaft to the arc-shaped surface of the field magnets.
6. A small DC motor as set forth in claim 1 , wherein a radius from a center of the shaft to the inside surface of the motor frame is the same as a shortest radius from the center of the shaft to the arc-shaped surface of the field magnets, the arc-shaped surface having an arbitrary curvature.
7. A small DC motor as set forth in claim 1 , wherein each of the connecting portions has an arc-shape.
8. A small DC motor as set forth in claim 1 , wherein each of the connecting portions has an arc-shape, the arc-shape having an arbitrary ratio to a radius of the arc-shaped surface of the field magnets.
9. A small DC motor as set forth in claim 1 , wherein each of the connecting portions has a straight line-shape.
10. A small DC motor as set forth in claim 1 , wherein each of the connecting portions has an arc-shape, the arc-shape constituting a portion of a circle, and a center of the circle coincides with a center of the shaft.
11. A small DC motor as set forth in claim 3 , wherein the armature assembly comprises:
an armature windings-molded element of a cylindrical coil wound by a magnet wire; and
an inner yoke comprising a cylindrical portion, the inner yoke being fixed in such a manner to face the armature windings-molded element.
12. A small DC motor as set forth in claim 3 , wherein the armature assembly comprises:
an armature windings-molded element of a cylindrical coil wound by a magnet wire; and
a cylindrical movable back yoke having contact with an inner side of the armature windings-molded element.
13. A small DC motor as set forth in claim 1 , wherein the air gap is substantially equal to a distance between the inside portion of the field magnet and the radially outermost surface of the armature assembly.
14. A small DC motor as set forth in claim 1 , wherein the air gap is substantially equal to a distance calculated by subtracting the radially outermost surface of the armature assembly from a shortest radius, which is a distance from the center of the shaft to an arc-shaped surface of the inside portion of the field magnets having an arbitrary curvature.