Patent GB-1252594-A (GB)
IPC: H02M1/06
Filed: 1968-11-11
Abstract: 1,252,594. Invertor commutations. LA TELEMECANIQUE ELECTRIQUE. 11 Nov., 1968 [20 Nov., 1967; 22 Oct., 1968], No. 53244/68. Heading H2F. An invertor, for supplying an inductive load B (Fig. 1) (e.g. a motor windings); and comprising a thyristor bridge OD, is provided with an extinction circuit DE wherein L 1  forms an auto-transformer having a 1 : 1 ratio when centre tapped, a commutating capacitor C 1  and switching thyristors The and TH C1 . A diode D is connected across L 1  to return to the D.C. source U current circulating in the inductive load so that overvoltages are prevented. The extinction circuit has two functions during every commutation operation: (i) to isolate the load B from the D.C. input U and thus avoid a short circuit path via anti-parallel diodes D I , D II  &c. in the invertor. (ii) Rapid blocking of the conducting thyristors of the invertor by lowering the current therethrough below a threshold value. In operation, Th c  is fired to discharge C 1  and block the invertor thyristors. Th C1  is fired to charge C 1 . When for example Th 1  and Th 1 <SP>1</SP> are blocked, Th C1  is fired and C 1  is charged to the source voltage U whereupon Th C1  will turn off. When Th I  and Th I <SP>1</SP> are again fired, a current I o  will flow in B. To turn them off, The is fired and C 1  discharges into winding MO producing a counter e.m.f. across L 1  which isolates the load B from the source U. This counter e.m.f. thus produces a periodic current i 1  opposing I 0  in a path through the source U. In addition a magnetizing current I m  flows in L 1  in the same direction as I 0 . The net current in the invertor thyristors is therefore i = I 0  + I m  - i 1 ; (i 1  being a rising function of C 1  and I m  being a decreasing function of L 1 ). It is thus possible, by selection of C 1  and L 1 , to make i zero or negative whereby Th I  and Th II  will be blocked if i remains zero or negative during their extinction period, i.e. if the time constant L 1 C 1  is greater than the extinction period. As the current i goes negative the anti-parallel diodes D I , D I <SP>1</SP> conduct applying a reverse voltage across their associated thyristors. When Th I  and Th II  are fired again current I 0  flows in B. Th C <SP>1</SP> is fired and C 1  is charged via winding NM and a counter e.m.f. is produced across L 1 , again isolating the load B from source U and causing a current i<SP>1</SP> 1  in winding MO opposed to I 0 . This current i<SP>1</SP> 1  flows via diodes D I  and D II <SP>1</SP> and the turned on thyristors Th I , Th I <SP>1</SP> can be extinguished thereby, the net current through the invertor thyristors being reduced. The magnitude of the reverse currents i 1  and i<SP>1</SP> 1  may be increased if the transformer ratio is made greater than unity (Fig. 3, not shown) thereby aiding the extinction facility. The two functions of the extinction circuit may be separated, e.g. Fig. 4 embodiment, wherein L 1 C 1  function to isolate the load from the source and LC function to extinguish the invertor thyristors. To ensure adequate quenching current particularly when the invertor is supplying a synchronous or a synchronous motor load an arrangement as shown, Fig. 6 (and Figs. 7, 8, not shown) may be provided, wherein the quenching current circuit including LC is supplied from a separate DC source U<SP>1</SP>. Diodes D 3 , D 4  are provided to isolate the two sources from each other. The quenching current bridge may be fed via a thyristor.