GB770614A - Improvements in and relating to circuit arrangements for supplying d.c. networks from an a.c. source - Google Patents

Abstract

770,614. Converting; transformers. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. June 28, 1955 [June 28, 1954], No. 18703/55. Class 38 (2). [Also in Group XXXVII] A three-wire D.C. system, to which may be connected two traction or rolling mill motors 11, 21, is energized from an A.C. source 1 through a supply transformer, the outer lines 16, 26 being connected to the anodes and cathodes respectively of a number of pairs of electron discharge devices 13, 23, the other electrodes of each pair being connected together respectively to one of the terminals 4, 5 of the secondary winding 3 of the supply transformer, which winding has a neutral point connected to the third D.C. line. The discharge devices may be grid controlled to alter the voltage output, and they may act as inverters, thus allowing regenerative braking. The neutral point may be the star point of a polyphase winding connected as shown, or the central tapping of an interphase transformer joining a number of star-connected groups of secondary windings, Fig. 3 (not shown). Alternatively, the anodes and cathodes connected respectively to the outer D.C. lines may be divided each into two groups, the lines then being connected to the centre tappings of interphase coils connected between the pairs of groups, Fig. 4 (not shown). Fig. 5 shows the connections for supplying parallel connected pairs of traction motors 311a, 311b; 321a, 321b from a single phase source 301. Voltage from a tapping in autotransformer 309 is supplied to the primary winding 302 of a transformer having a secondary winding 303, the two ends and centre tapping of which are connected respectively to the two outer and the inner D.C. lines. The connections to the outer lines are made through eight rectifiers connected in parallel pairs such as 312a, 312b, inductances 334a, 334b ensuring current division. Wheel slip may be indicated or controlled from a current sensitive member 360. The connection to autotransformer 308 may be made by way of on-load tap-changing equipment, but more efficiently the tap-changing is done on the secondary winding of the main transformer, Fig. 7 (not shown). In a further voltagecontrolling arrangement the secondary winding is divided into three sections 503, 513, 523. Windings 503, 513 connected by the centretapped inductance 508 have equal and opposite voltages induced in them. The voltage induced in winding 523, which may be varied by taps up to a maximum of twice the voltage induced in each of the windings 503, 513 is applied across the ends of inductance 508 in one or other direction determined by change-over switch 520, Fig. 8. In a further modification of the tap-changing arrangement, finer voltage steps may be obtained by adding or subtracting voltage developed in an auxiliary transformer to or from the voltage derived from a main winding tap, Fig. 9 (not shown).