GB438632A - Improvements in and relating to electro-magnetic speed responsive devices - Google Patents

Abstract

438,632. Speedometers. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London. Dec. 5, 1934, No. 34958. Convention date, Dec. 6, 1933. [Class 106 (iii)] A speed-responsive device or tachometer of the electromagnetic drag type comprises means involving a number of differently graduated scales for measuring the speed of a rotating body with a high degree of accuracy over a plurality of speed ranges, the magnitude of the induced current in the drag element being selectively changed in a plurality of definite steps. The rotating body 10 drives a two-pole disc magnet 16 within a casing 12 carrying a laminated ring 21. In the annular air gap between the members 16, 21 is a rotatable electrically-conducting structure comprising four concentric cylinders 22, 23, 24, 25 of aluminium or copper, the cylinders having different lengths as shown and being provided with longitudinal slots 27. The cylinders are mounted so as to be rotated against the influence of a spring 36 by the rotating magnet 16, and by moving a rod 31 longitudinally until a ball 32 engages a selected notch therein, the four cylinders may be selectively moved into one of four definite working positions and the number of cylinders threaded by the rotating flux is varied accordingly. At the same time one or other of the scales 37, 38, 39, 40 on cylinder 25 is moved into view under a pointer 41 and a window 42. In a modification, Fig. 2, a stepped cylinder 44 is used in the air gap. In another modification, Fig. 4, the rotatable cylinder 49 in the air gap is made of insulating material and wound with a polyphase winding 50, means being provided for connecting an adjustable amount of resistance in series with each phase of the winding for the purpose of varying in steps the drag of the rotating magnetic flux, a different scale being viewed for each of the predetermined steps.