Patent ID: 7345388
Filing Date: 2008-03-18
Classification: G11B,H02K

Abstract:
1. A brushless motor comprising: a housing member made of ferromagnetic material; a stator fixed relative to the housing member; a rotor hub supported in the motor so as to be rotatable relative to the housing member around a central axis of rotation, the rotor hub including a cylindrical portion which extends longitudinally in the direction of the axis of rotation, and the cylindrical portion having a step in a distal end thereof, wherein the cylindrical portion has a first cylindrical circumferential surface, and a second circumferential surface disposed coaxially with the first cylindrical circumferential surface, the second circumferential surface constituting the distal end of the cylindrical portion, and the diameter of the rotor hub at the second circumferential surface being different than the diameter of the rotor hub at the first cylindrical circumferential surface such that the second circumferential surface defines the step; an annular rotor magnet fixed to the rotor hub so as to form a rotor therewith, the annular rotor magnet extending alongside the first cylindrical circumferential surface of the cylindrical portion of the rotor hub, the annular rotor magnet having a circumferential face opposing the stator as spaced radially therefrom, and one axial end face opposing the housing member as spaced axially therefrom, and the rotor magnet being radially polarized so as to have one polarity at an outer circumferential portion thereof, an opposite polarity at an inner circumferential portion thereof, and an inter-pole, magnetically neutral area located between the inner and outer circumferential portions of opposite polarities; and an annular shield plate fixed to the rotor hub, the annular shield plate extending alongside the second cylindrical circumferential surface of the cylindrical portion of the rotor hub so as to be located between the rotor magnet and the housing member and so as to have a circumferential face that opposes the stator, and the circumferential face of the annular shield plate that opposes the stator being juxtaposed in the axial direction with a region of the rotor magnet located between the magnetically neutral area and the circumferential face of the rotor magnet that opposes the stator, whereby the annular shield plate is disposed coaxially with the rotor magnet.