Patent GB-645950-A (GB)
Title: Improvements in or relating to frequency conversion systems
IPC: H05B6/04
Filed: 1947-12-11
Abstract: 645,950. Frequency changing. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. Dec. 11, 1947, No. 32665. Convention date, Dec. 14, 1946. [Class 38 (ii)] - A frequency conversion system comprises a plurality of electronic valves for interchanging power between two alternating current circuits of different frequencies wherein each valve has an excitation electrode, an auxiliary anode, and at least one grid means to which is applied a cyclically varying voltage having a frequency the same as that of one of the circuits while the excitation electrodes and auxiliary anodes are energized at the frequency of the other circuit Six ignition type valves are arranged in two groups 1, 2, 3 and 4, 5, 6, the anodes being supplied in pairs, one in each group, from three phase A.C. bus-bars 7 fed from a normal A.C. supply 13 (e.g. 60 c.p.s.) through a transformer 8. The three cathodes in each group are connected together and across the two terminals so obtained is connected a load circuit 26 (e.g. an induction furnace). The excitation electrodes, or strikers, of the ignitrons are energized from the same supply as the anodes through three single-phase transformers 40 having their primaries connected in delta. The firing points of the ignitrons are advanced or retarded by the variable D.C. saturated reactors 45, the firing being initiated by a voltage building up on a capacitor 49 such that at a given instant the reactor 50 saturates, its impedance thereby being reduced, and the capacitor then delivers a heavy current impulse to the corresponding pair of coupling transformers 41, the peak value of which pulse is rectified at a rectifier 51 to energize the striker 35. An auxiliary arc is instantly picked up by the auxiliary anode 36 which serves to hold the arc after the pulse to the striker 35 has disappeared. The auxiliary anode is energized by means of the transformer 60 and rectifiers 70 and is so arranged that each auxiliary anode maintains the holding arc for 120 input frequency degrees plus a period to allow the commutation of the output current. The tapped reactor 42 is provided to ensure that the two corresponding ignitrons (one in each group), fire simultaneously. To effect frequency conversion each low-frequency conducting period is divided into a number of conducting and non-conducting periods by means of a commutating condenser 30 and grids 33, 34 so that the firing of each ignitron is alternately prevented and permitted, the grids being supplied with a voltage which has an A.C. component, of the frequency which is desired in the output circuit 26 and which is obtained by any suitable means (e.g. an oscillation generator 55) superimposed upon a constant negative D.C. bias. The grids of one group are in antiphase with the grids of the other group so that firing of the valves in the groups is prevented and permitted alternately, thus producing in the output circuit an A.C. of higher frequency than that in the input circuit and which is identical with the frequency of the grid voltage. A further embodiment is described in which the ignitrons are connected back-to-back, the anodes of three of the valves being connected together to form one bus-bar, the other three valves having their cathodes connected to form the other bus-bar. The two bus-bars are connected together through a centre-tapped reactor, the tap forming one terminal of the output circuit which comprises a commutating condenser serially connected with the primary winding of the output transformer to form a resonant circuit. The control circuits are similar to those of Fig. 1 except that where the valves do not have a common cathode bus-bar the grid voltages must be electrically separate from one another. Also the auxiliary anodes are energized without the parallel phase energization, thus reducing the range of adjustment of the valve firing instants. Specification 645,887 is referred to.