GB690195A - Improvements in and relating to control circuits for electric converting systems - Google Patents

Abstract

690,195. Converting. BRITISH THOMSON-HOUSTON CO., Ltd. Nov. 28, 1950 [Dec. 7, 1949], No. 29087/50. Class 38 (ii). An electric converting system interposed between an A.C. and a D.C. circuit or vice versa, includes means. such as saturable reactors arranged to control the rate of change of current through the circuit interrupting devices at the end of a conducting period and during the commutating period and includes further means. viz. resistors having non-linear volt-ampere characteristics, for controlling the inverse voltages appearing across the circuit interrupting devices following commutation, thereby to reduce the possibilities of " arc-back." Electron discharge tubes 5-10 are disposed between an A.C. circuit 1 and a D.C. circuit 2, each tube being supplied from a transformer 3 over a main winding of a respective saturable reactor 16, 17 or 18. The discharge vessels 5-10 are caused to operate in the normal fashion by grid control via a phase-shift device 11, transformer 13, and a battery 15 giving the requisite negative bias. The reactors 16, 17, 18 are pre-saturated by means of windings 28, 29, 30 so that the reactors operate to cause the current flowing in an electron discharge device to decay at a relatively slow rate as described in Specification 501,735. Each main winding of the reactors 16, 17, 18 has connected in parallel therewith a resistor 34, having a non-linear volt-ampere characteristic, of the type disclosed in Specification 292,110, [Claas 38 (ii)]. Therefore when the reversal of flux occurs in the core of a saturable reactor the voltage induced in the main winding thereof causes a current to be circulated in the winding and its associated resistor thus delaying the reversal of the flux and allowing such change to continue after current has ceased to flow in the discharge device. This induced voltage is of opposite sense to the inverse voltage appearing across the discharge device following a conductive period thereof, thus causing a reduction of this inverse voltage so preventing " arcback ". In another embodiment there is described a system having mechanical circuit interruption and having resistors 34 similarly associated therewith.