Motor, fan and manufacturing method of the same

A motor comprising:

• a shaft supported to rotate and centered about a rotary axis, the radial direction of the motor being perpendicular to the rotary axis;
• a cup portion having a side wall;
• a boss integral with the cup portion, and disposed at a radially central part of the cup portion, the shaft being affixed to the boss such that boss transmits a force of rotation from the shaft to the cup portion whereby the cup portion rotates along with the shaft about the rotary axis;
• a rotor holder disposed within the cup portion, and having a substantially cylindrical portion;
• a rotor magnet, having a substantially annular shape, integral with the rotor holder and extending along an inner circumferential surface of the cylindrical portion of the rotor holder;
• and
• an armature disposed opposite to the rotor magnet in the radial direction of the motor to generate with the rotor magnet a torque on the rotor holder;
• and
• wherein the cup portion has a plurality of convexities spaced from each other in the circumferential direction of the cup portion along an inner circumferential surface of the side wall of the cup portion, and each protruding inwardly in the radial direction from the inner circumferential surface, and
• an outer circumferential surface of the cylindrical portion of the rotor holder is engaged by the convexities, and said outer circumferential surface and the convexities are engaged with each other over only a length in the axial direction which is less than half the axial length of the rotor holder.