Patent GB-243800-A (GB)
Title: Improvements in thermionic rectifier systems
IPC: H02M7/06
Filed: 1924-09-04
Abstract: 243,800. Robinson, E. Y., Burch, C. R., and Metropolitan-Vickers Electrical Co., Ltd. Sept. 4, 1924. Vacuum or low-pressure apparatus, arrangements of; phase, changing; polyphase transformers.-Relates to thermionic valve svstems for rectifying polyphase current, and consists in arrangements whereby the production of lower harmonics of large amplitude is avoided. The Specification refers to a known arrangement for rectifying three-phase current in which the six valves of two three-phase half-wave rectifiers phase displaced with respect to each other by 180‹ are, supplied with current from a transformer having two sets of secondary windings interconnected through choke coils to ensure that each rectifier group operates as a separate unit, each valve passing current for one-third of a cycle. According to the present invention, more than two M-phase rectifiers are employed, M being a number greater than two and a submultiple of an even number equal to the number of phases of the current supplied to the rectifier. According to one arrangement, Fig. 7, 18-phase current is produced by an arrangement of transformers and supplied to six three-phase half wave rectifier groups phase displaced with respect to each other by 40‹. The secondary windings 16 - - 21 of the transformers are arranged in pairs, one pair feeding each group of rectifiers and the windings of each pair are phase displaced with respect to each other by 180‹. In order to ensure that each valve shall pass current for one-third of a cycle, the star points of each pair of windings are connected together by an interphase reactance 22, and the mid-points of the three interphase reactances are connected through choke coils 23 to the terminal 9 of the rectified current output circuit, whilst the other terminal 8 of the output circuit is connected to the cathodes 7 of the valves. The current in the line resulting from the three components due to the three dual rectifier units is shown in Fig. 11. A still closer approach to a sine curve with a corresponding further reduction of harmonics is obtained by a further arrangement described in which 30-phase current obtained from a three-phase supply by an arrangement of transformers is rectified by ten three-phase half wave rectifiers. The higher harmonics remaining may be eliminated by tuned acceptor circuit connected between the lines, and the remaining harmonics in the output circuit may be removed by choke coils and condensers. The desired phase relationship between the groups is obtained by displacing the secondary voltages of the transformers in relation to the primary voltage by a winding arrangement as shown in Fig. 13. The core carries two sets of secondary windings 38 - - 40 and 41 - - 43, both sets being starred whilst the other ends of the windings on the same limb are connected to different lines. By suitably proportioning the number of turns of one set with respect to that of the other set, the desired phase relationship between the primary voltage and that of the secondary windings 48 - - 50, which are connected to the anodes 52 - - 54 of the valves, may be obtained. In an alternative method, the parallel arrangement of primary windings is replaced by a series arrangement.