Brushless direct current motor

A brushless direct current motor, comprising:

• a stator which is provided with a plurality of teeth extending toward an inner side of a core and pole shoes each extending to be formed at radial inner end parts of the teeth;
• and
• rotors which are disposed at an inner side of the stator surrounded by the pole shoes while being spaced apart from the stator and have a plurality of permanent magnets coupled to cores thereof;
• wherein an outer circumferential surface of the rotor has a circular shape, an outer circumferential surface of a part of an area corresponding to a specific angle range with respect to a rotation center C of the rotor is formed as a rectilinear straight portion, predetermined areas of central parts of radial inner circumferential surfaces of the pole shoe have an arch shape;
• and
• the inner circumferential surfaces of the circumferential both end parts of the pole shoe are formed so that a distance of the inner circumferential surfaces of the circumferential both end parts of the pole shoe from the outer circumferential surface of the rotor increases as the inner circumferential surfaces of the circumferential both end parts of the pole shoe is far away circumferentially from a central line CL of the pole shoe.