Brushless d.c. motor

Claim 1

A brushless d.c. motor comprising: (a) a flat annular rotor magnet secured on a rotary magnetic yoke, said rotor magnet being endless and having 2p contiguous poles of alternating N and S polarity, where p is an integer of at least 1, said poles defining equal areas, having a predetermined width, of non-skewed magnetization; (b) a stator yoke disposed in a face-to-face relation with said rotor magnet in an axial direction; (c) an air-gap and a printed circuit member positioned between said rotor magnet and said stator yoke; (d) at least one armature coil of the air-core type arranged on said printed circuit facing said rotor magnet, said armature coil having a sectorial shape and conductor portions contributing to generation of torque, said conductor portions defining an opening angle having a width substantially equal to the width of each of said poles of said rotor magnet; (e) means for detecting one of the N and S poles of the rotor magnet and causing electric current to flow in a predetermined direction in said armature coil, said detecting means comprising a sole position sensor arranged along one of a said conductor portion of said armature coil and an in-phase position relative to a said conductor portion; (f) a power control circuit having electrical elements arranged on said printed circuit member; (g) means for orienting said rotary magnetic yoke such that, when no electric current is caused to flow in said armature coil, the areas of the stator yoke opposite to the N magnetic poles of the rotor magnet are substantially equal to the areas of the stator yoke opposite to the S magnetic poles of the rotor magnet, and, when no power is fed to the motor, the central axis which divides the stator yoke in two is located at a point approximately one-fourth of the width of each magnetic pole of the rotor magnet behind, relative to the direction of rotation of said rotor magnet, a point opposite to a said conductor portion of the armature coil which contributes to the generation of torque, said orienting means comprising a cut-off portion of predetermined shape in said stator yoke.