GB1193163A - Improvements relating to Static Inverter Circuits - Google Patents

Abstract

1,193,163. D.C. to A.C. converters. INDUSTRIAL INSTRUMENTS Ltd. 9 April, 1968 [13 Feb., 1967], No. 6785/67. Heading H2F. In a 3 phase static invertor, as shown, Fig. 1, after an initial starting period the charging and discharging of commutation capacitors takes place through the load, to substantially reduce energy loss. In operation, initially, thyristors 1, 2, 5, 6, 11 and 12 are gated to conduct simultaneously and supply the load (the conduction periods being indicated in Fig. 2, relative to secondary firing pulses for the commutation of conducting thyristors and which are applied to turn ON thyristors 9 and 10 during this period). Thyristor 10 is fired at the instant indicated so that point R is connected to the negative supply line. Thus commutation capacitors C2, C3 are charged through 5, 10, 11, 12 and 1, 10, 11, 12 respectively, points P, Q becoming positive with respect to R, by a voltage equal to the supply voltage. Subsequently Th9 receives a drive pulse and connects C3, C2 to the positive supply line so that thyristors 1 and 5 are reverse biased and turn off. Load current now flows via 9, C3, 2, load, 11, 12 and via 9, C2, 6, load, 11, 12, in the interval between switching off 1 and 5 and the new conduction period. The current flow through C2, C3, during this commutation interval, falls as each capacitor is first discharged and recharged in opposite directions. Thyristors 2, 6, 11, 12 cease to conduct when the current falls below a holding value. At the start of the next conduction period after commutation is complete, thyristors 1, 2, 7, 8, 11, 12 are gated and turn on. Two sets of thyristors connected to the negative line are now conducting and the next commutation will involve the negative side thyristors, the procedure similar to the above being repeated.