Synchronous micromotor

In a synchronous micromotor, outer coil means circumferentially surrounding a predetermined axis, a rotary rotor shaft extending along said axis and surrounded by said outer coil means, annular permanent magnet means surrounding said rotor shaft and situated coaxially between the latter and said coil means, said annular permanent magnet means having a series of north and south poles circumferentially distributed therealong with said north poles alternating with said south poles and all of said poles being directed outwardly toward said coil means, cylindrical wall means made of a soft magnetic material and circumferentially surrounding said annular permanent magnet means while being situated between the latter and said coil means, said cylindrical wall means having a pair of annular yoke portions spaced from each other along said axis and surrounding said axis, and said cylindrical wall means being formed between said yoke portions with a series of elongated separate apertures circumferentially spaced from each other and each extending from one to the other of said yoke portions up to but not beyond said yoke portions and said series of apertures being made up of two groups of apertures one of which is inclined helically in one direction with respect to said axis and the other of which is inclined helically in an opposite direction with respect to said axis and the apertures of said one group alternating with the apertures of said other group so that said annular yoke portions are bridged by and integral with a series of elongated tapered wall portions of said cylindrical wall means with said tapered wall portions alternately tapering in opposed axial directions, said cylindrical wall means and said annular permanent magnet means forming a pair of means one of which is connected with said rotor shaft for rotary movement therewith, to form a rotor assembly with said shaft, while the other of said pair of means and said coil means form part of a stator assembly.