Patent GB-386309-A (GB)
Title: Improvements in electric induction motors
IPC: H02K17/10
Filed: 1931-05-01
Abstract: 386,309. Alternating-current induction motors. TRIGGS, W. W., 57, Lincoln's Inn Fields, London.-(Barber- Colman Co. ; River Street, Rockford, Illinois, U.S.A.) May 1, 1931, No. 12861. [Class 35.] Pole pieces; field magnets, dampers or poles of.-The invention relates to small squirrel cage induction motors having a two-pole coretype stator with split poles carrying shading rings, the pole portions not enclosed within shading rings having magnetic extensions so that the rotor is substantially enclosed within the stator pole faces and the extensions thereof ; the core type stator is defined as one in which the entire section of the magnetic circuit through the poles is enclosed by the energizing winding. According to the invention, the pole faces and the magnetic extensions are maintained substantially saturated, the shading coils have a resistance within the range 1 to 2 x 10<-4> ohms, the air gap is of minimum width, the ratio of iron to copper in the rotor is relatively large and the rotor can be worked at high current density around its entire periphery, and the magnetic extensions of the poles have, between the centre line between the poles and the pole-face portions not enclosed by shading rings, no restriction therein greater in magnetic effect than a radial air gap <1>/32 of an inch wide located at the centre line. The stator is made up of laminations 10 having overall dimensions of 2 inches wide and 2# inches long in a motor of preferred size ; each limb 11, 12 is ¥ inch wide and the laminations are built up to form a core of square section or having a thickness of <9>/16 inch. To facilitate assembly of the core 11 within the spool of the energizing coil 14, each lamination is cut along a single line 15. Adjacent laminµ are cut at opposite ends so that lapped joints are formed. In the form shown, the magnetic extension 20 of the unshaded pole sections are integral with the opposite shaded pole portions. The rotor has a diameter of about ¥ inch and comprises discs slotted to receive conductorbars 24 which are soldered at their ends to copper discs which may be of the same shape as the magnetic discs. The bars 24 are narrow and deep, and in the present example the depth is # inch and the thickness 0À04 inch. The total cross sectional area of the conductor bars should be approximately <1>/16 square inch in the motor of the size described so that approximately one-sixth of the cross sectional area of the rotor is composed of copper. The rotor shaft 22 is supported in bearings formed on two non-magnetic bracket plates 26 bolted to the stator. The air gap between stator and rotor is 0À011 inch. In the example shown, each stator pole has two shading rings 27, 28. Each shading ring comprises a length of copper wire bent to V-shape, the ends being soldered to a flat bar 31. Slots 32 are cut in the stator poles between the shaded and unshaded sections and between the single and double shaded sections. Where two shading rings are provided on a pole, the resistance of the smaller is within the range specified above, but a wider variation is allowable in the case of the larger ring. The extensions 20 have a radial width of about <3>/32 inch and in order to limit flux leakage, a gap of 0À013-0À030 inch width or a hole may be located therein. Such a gap orhole may be located between the centre line between the poles and the shaded pole tip. Reference has been directed by the Comptroller to Specifications 212,263 and 313,595, [both in Class 35, Dynamo-electric generators &c.].