GB309959A - Improvements in and relating to electrical frequency changers - Google Patents

Abstract

309,959. British Thomson-Houston Co., Ltd., (Assignees of Merrill, F. W.). April 18, 1928, [Convention date]. Rotary transformers.-The invention relates to frequency changers of the type described in Specification 302,904. According to the prior Specification, a frequency changer comprises a machine having single stator and rotor elements, and the primary element, usually the stator, is provided with independent motoring and generating windings while the rotor carries some form of starting winding and, in addition, is provided with polar salients which co-operate with the motoring winding to produce synchronous rotation and also set up a flux which generates currents of higher frequency in the generating winding. A rule is deduced that the number of generating polar salients on the rotor is equal to the frequency conversion factor minus one. Therefore, to change from CO to 120 cycles with a motoring winding wound for two poles, the rotor would have only one generating salient and this design would be impracticable on account of the absence of mechanical balance and insufficiency of synchronizing torque. In accordance with the present invention, the rotor is provided with the proper number of generating salients as determined by the rule given and in addition with auxiliary polar salients so distributed as to balance the rotor and to provide a return path for the generating salient flux, but arranged so that their generating influence in the generating winding cancels at every instant. As shown applied to a machine changing from 60 to 120 cycles, the stator 11 is provided with a two-pole motoring winding 10 connected to terminals 12 and a four-pole generating winding 13 connected to terminal 14. The rotor has a repulsion motor starting winding 15 which is completely short-circuited at synchronism and acts as a damping winding. The rotor has also one main generating salient 19 comprising eight complete teeth, and also two auxiliary salients 20, 21 comprising four complete teeth each, the intermediate teeth being cut off. Fig. 2 shows diagrammatically the spacing of the stator windings 10 and 13, and also shows in degrees the spacing of the rotor salients. With the arrangement shown, the machine will rotate synchronously at 3600 r.p.m., salient 19 constituting one pole and salients 20, 21 together the opposite pole, and the resultant effect of the salients 20, 21 on the generating winding 13 will be zero.