A device capable of reducing cogging torque of brushless direct current motor is provided to improve gap structure by forming tilt surface in one side of stator pole piece. A stator(20") includes a coil(22). The coil is winded in a plurality of protrusions(21a). A plurality of protrusions is formed with a ring shape according to an edge of a central aperture of a core(21"). A stator pole piece(21b") is formed in a line end of each protrusion in order to prevent a secession of the coil, and is parallel formed with an inner surface of a ring shaped permanent magnet of a rotor in the line end of each protrusion. An air gap is formed between the permanent magnet of the rotor. A stair shaped jaw(23) is formed in a center of the stator pole piece. Difference as much as a differential air gap(D) corresponding to a height of the jaw is in a minimum air gap(d1) of one side and a maximum air gap(d2) of the other side. A tilt surface(24) is formed in a line end of one side of the stator pole piece in order to form the maximum air gap between the permanent magnet of the rotor.
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