Stepping motor

Claim 1

In a stepping motor of the homopolar design and having a housing, a shaft journaled in the housing, a rotor secured to the shaft, and a stator connected with the housing and concentrically surrounding the rotor with a working air gap therebetween, wherein the stator is a layered steel structure with a plurality of radially inwardly extending poles for the respective field windings and each of the stator poles is provided on the end face thereof at the working air gap with a plurality of stator pole teeth at a uniform pitch, insulation means is provided for covering the stator poles so as to isolate the field windings electrically therefrom, the rotor includes an axially magnetized permanent magnet and two magnetically conductive annular steel members sandwiching the magnet therebetween and provided on their circumferential faces at the working air gap with respective identical sets of rotor teeth at a uniform pitch but with the rotor teeth on each annular member offset by a half pitch with respect to the rotor teeth on the other annular member, the shaft is magnetically non-conductive, and the housing includes a pair of magnetically non-conductive bearing bells; the improvement comprising that: (a) the stator is a single-section hollow structure devoid of an external yoke for the magnetic return in the axial direction and has a generally square transaxial shape with straight sides and slightly rounded or chamfered corners; (b) each of the two bearing bells has a transaxial shape the same as that of the stator, and means are provided for securing the bearing bells to the stator at the respective opposite faces of the latter and in alignment with the stator and each other to provide the motor with a generally cubic form; (c) the stator has only four stator poles and four field windings are wound about said stator poles, respectively, and each of the stator poles extends from the middle region of a respective one of the four straight sides of the stator and has four or five stator pole teeth; (d) the pitch of the rotor teeth is expressed by the relation τ r =360°/Z r and the pitch of the stator pole teeth by the relation τ s =360°/Z s =360°/(Z r -1±k), where Z r is the number of rotor teeth, Z s is the theoretical number of stator pole teeth consisting of the sum of the actual number of stator pole teeth plus the number of such teeth omitted between adjacent stator poles and must be divisible by 4, k is an even integer with 0≦k≦6, and for a given value of either Z r or Z s , k is so chosen that the step angle of the motor is about 3.6°; (e) the permanent magnet of the rotor is a thin ring disc structure of a highly coercive magnetic material with an energy density (B×H) max ≧10 MGOe and a ratio of disc diameter to disc thickness ranging from about 4 to about 20; (f) the two annular members on which the rotor teeth are provided are ring disc structures which are of somewhat greater diameter than the permanent magnet and on their sides facing each other and the magnet are provided with respective axial projections beveled inwardly at an angle of about 30°-60° down to the outer circumference of the magnet; and (g) the insulation means is a composite annular structure consisting of two unitary annular sections fitted into the stator from opposite sides thereof and covering all parts of the stator poles except the toothed end faces thereof, the insulation structure being of such configuration as to leave between adjacent stator poles respective wire winding gaps each not less than 2 mm wide.