Patent GB-742389-A (GB)
Title: Arrangements for the control of current converters
IPC: H02M7/15
Filed: 1952-09-01
Abstract: 742,389. Converting. LICENTIA PATENTVERWALTUNGS-GES. Sept. 1, 1952 [Aug. 31, 1951; Nov. 6, 1951], No. 21946/52. Class 38(2). In a converting system with improved power factor at low loads, grid-controlled converters are fired by energization of the grids by impulses developed at the frequency of the main A.C. but released and blocked at a subharmonic frequency. Output is varied by two groups of controls, a precontrol group which first reduces current to zero in the anodes it controls, and a post-control group which exercises control over the anodes when the precontrol has reached its limit. So that the load may be distributed equally between anodes and phases, the anodes are arranged in two parallel. groups in converters Ga, Gb fed by winding groups A, B, anodes in the same phase being energized simultaneously by the pre- and post-control groups respectively, and the parallel anodes of the same phase, and the consecutively firing anodes of each group are controlled alternately by the pre- and post-control groups. Thus in Fig. 2 the black and outline rectangles show the times of pre- and post-control of anodes of phases I, III, V arranged in the two parallel groups A, B. Each grid must be energized once per cycle of the main frequency, say 50#, by pulses alternately from the pre- and postcontrol groups which must provide pulses at 25#. Fig. 1 shows control arrangements in which the 25# supply is developed from the 50# supply through rotating machinery. To avoid the difficulty of maintaining complete synchronism, according to the invention the control pulses are generated at 50#, alternate pulses only being utilized in a single phase. By the arrangement shown in Fig. 4, currents of 25# are used, as shown in Fig. 3 to block (shaded) or pass pulses developed at 50# to the grids of 180 degrees displaced phases I and IV, the upper and lower curves showing pre- and post-control respectively. Pulses are developed at 50# across resistances R, R for phases AI, AIV respectively from supply transformers 1, 2 for pre- and post-control respectively. The 25# supplies shown (phased 180 degrees apart in controls 1, 2) act to open once each cycle (50#) a shunt circuit across each resistance R comprising resistance R 1 and rectifier VI, the pulse being diverted from R to R11. To halve the number of control sets needed, a circuit is described, Fig. 5 (not shown), in which each unwanted pulse is deflected to the grid of the parallel anode. The pulse switching units may be dry rectifiers or saturable chokes. It is shown that the maximum range over which the post-control impulses are variable is 220 degrees during inversion, Fig. 8 (not shown). Fig. 9 shows at (a) that a 25# voltage of maximum amplitude three times that of a pulse can open the pulse circuit with an allowable timing error of Œ30 degrees (50#) but that with a pronounced third harmonic (b) the allowable error becomes Œ45 degeees when the maximum amplitude is twice that of the pulse. If the phase of the 25# supply is varied with that of the 50# pulses, its magnitude may be reduced. Fig. 10 shows a motor L fed through two three-phase converters A, B in parallel, the 50# impulses for pre- and post control being derived from networks of elements marked with symbols bearing suffixes v, n respectively, and switched between the two converters by a synchronous slip-ring device 9, 9, 10. Each network comprises two unequal secondary windings 1, 2 of transformers energized from separate phases of the three-phase supply, and supplying in series a resistance 3 and a phase-shift reactor 4 with a saturating winding and tuned by a capacitor 8, the pulses being derived from a resistor 7 connected in series with a capacitor 6 and saturable reactor 5 between a tapping on transformer winding 1 and the junction of resistance 3 and reactor 4. Both pre- and post-control is effected by varying grid voltage U g  of valve 12 to control current i g . When i g  is a maximum, currents i v , i n  through the D.C. windings of saturable reactors 4v, 4n are at a maximum giving fully advanced firing with maximum rectifier output. As ig is reduced, as shown in Fig. 11, i n  drawn from battery 16 remains constant at a value determined by the resistance of D.C. winding of reactor 4 n , and i v  drawn from battery 15 falls to zero when i g  is equal to the current passed through valve 13 from supply 17. Further reduction of i g  reduces i n  continuously to zero. Resistors 20, 19 determine the value of i g  max and prevent its being exceeded with increase of Ug. A reciprocating switch may replace the slip-rings and may alternate resistances 7 between pre- and post-control circuits. It is shown that the post-control must fire each phase at two points 60 degrees (50#) apart, for a precontrolled phase may fire intermediate these points, Fig. 6 (not shown). These may be derived in a polyphase system from suitably displaced phases, Fig. 13 (not shown). When the pre-control has reached the value where there is no intermediate firing, the 60 degrees delayed pulse supply may be disconnected, Fig. 16 (not shown), or the pulse may be retarded by more than 150 degrees, Fig. 14 (not shown). The 60 degrees spaced pulses may be developed across a resistance 7 connected to two postcontrol networks with a common saturable reactor 4, Fig. 16 (not shown), or common saturating current through two reactors, Fig. 15 (not shown), for varying phase simultaneously.