GB418297A - Improvements in and relating to electric converting systems - Google Patents

Abstract

418,297. Converting; motors. BRITISH THOMSON - HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Alexanderson, E. F. W. ; 8, Adams Road, Schenectady, New York, U.S.A.) March 13, 1933, No. 7606. Convention date, March 12, 1932. [Classes 35 and 38 (ii).] Machines with vapours or like rectifiers in place of commutators.-Relates to a frequency changer in which means are provided for varying the output frequency and has particular application to the speed control of an A.C. motor. The phase winding 12, Fig. 1, of a synchronous motor 11 is connected to an A.C. supply 10 through two full-wave rectifiers comprising valves 15, 16 and 17, 18, and the phase winding 13 is connected through two full-wave rectifiers 19, 20 and 21, 22 connected in the reverse direction to the first two. The centre points of the windings 12, 13 are interconnected through the armature field winding 14. The rectifiers are controlled so that in each phase winding, current flows alternately through the two parts, thus producing an alternating flux in the core. These fluxes are out-of-phase and produce a rotating field. The current flow is from the line through one of the rectifiers associated with the winding 12 and back to the line through one of the rectifiers associated with the winding 13 so that a pulsating direct current flows through the winding 14. The machine will therefore function as a synchronous motor. Two methods of grid control are described. In one method transformers 23-26 in the grid circuits are periodically connected to a bridge 30, energized from the supply 10, by means of a commutator 42 driven by the motor 11. By varying the resistance 33 of the bridge 30 the phase of the grid potentials with respect to the anode potentials can be varied to regulate the current flowing and thereby the speed of the motor. An additional adjustment of speed can be obtained by means of a variable resistance 55 shunting the field winding 14. This type of grid control is most satisfactory at frequencies less than half the supply frequency. For higher frequencies, the grids are controlled by means of transformers 44-51 supplied from a phase shifting transformer 53, 54 the primary of which is connected across the phase windings 12, 13. Short circuiting switches 52, 43 are provided so that the grid control not in use can be disconnected and the grid transformers shortcircuited. In a modification a 3-phase supply is used and in a further modification the motor has a 6-phase winding. In a further modification, Fig. 2, an induction motor of the squirrel cage type is supplied. The phase windings 63, 64 are interconnected through a reactance 65. To ensure the commutator 42 running at synchronous speed, it is driven from the motor 61 through a differential gear 66 the movable member of which is driven by an auxiliary motor 66a to compensate for slip. Reference has been directed by the Comptroller to Specification 395,917.