Patent ID: 7250701

Claim:
An apparatus for detecting a rotor angle of a reluctance motor with an encoder, said reluctance motor comprising: a rotor shaft being rotatably supported; a rotor core fixed around said rotor shaft, with a plurality of poles having lower magnetic reluctances and provided at a radial outer side of said rotor core; a stator core disposed at a radial outer side of said rotor core with a plurality of inward salient-poles facing toward said rotor core; and a plurality of coils causing said inward salient-poles to generate magnetomotive forces, so that said rotor shaft rotates integrally with said rotor core based on magnetic attraction between said poles of the rotor core and said inward salient-poles and the stator core, said encoder comprising: a magnet provided at an axial end surface of said rotor core and integrally rotating with said rotor core, with magnetic poles varying in a rotational direction; and a magnetism detecting element supported in a confronting relationship with said magnet in an axial direction for detecting magnetism generating from said magnet, so that said encoder detects a rotational angle of said rotor core based on the magnetism generating from said magnet detected by said magnetism detecting element, wherein an axial center of said rotor core is offset from an axial center of said stator core by a predetermined amount in an axial direction going away from said magnetism detecting element so that the magnetomotive forces of said stator core can induce a thrust force acting on said rotor core in a direction toward said magnetism detecting element and so that a gap between the magnet and the magnetism detecting element is maintained at a predetermined amount.