Patent GB-1047923-A (GB)
Title: Inverter circuit
IPC: H02M7/523
Filed: 1963-11-11
Abstract: 1,047,923. Inverting. GENERAL ELECTRIC CO. Nov. 11, 1963 [Nov. 13, 1962], No. 44483/63. Heading H2F. An inverter utilizing silicon controlled rectifiers 12, 13, includes an inductance 11 (a " commutating interval current limiting reactor ") in series with the rectifiers and a commutating capacitor 21 (or 19) in series with an inductor 22-this LC circuit being tuned to a series resonance frequency greater than the operating frequency-which discharges through the inductance 11 and a conducting rectifier to shut this off. The operation of the circuitry will be explained with reference to Fig. 1 which shows a series inverter. In Fig. 1 a D.C. supply is applied to series capacitors 16, 17, centre-tapped at X, and to SCR's 12, 13, in series with inductor 11 centretapped at Z. The direction of current through a load 18 reverses as SCR's 12 and 13 alternately conduct. A pair of capacitors connected at Y shunt the supply and an inductor 22 connects Y to Z. With SCR 12 conducting capacitor 21 is charged to the full value E DC . If SCR 13 is now fired this will conduct and point 2 will attain a potential E DC / 2  so that load current ceases. Capacitor 21 now discharges through 11a and 11b, the current through 11a shutting off SCR 12 and excess current flowing through diode 23 which thereby maintains a back bias across SCR 12. The inductor 22 ensures that the commutating current is sufficient to shut off rectifier 12 over the shut-off time. The commutating frequency may be from 2 to 100,000 times the operating frequency. During the period when both SCR's conduct (the " commutation interval "#T in Fig. 9b) an additional current #i builds up through the rectifiers. The reactor 11 serves to limit this current hence term " commutating interval current limiting reactor." As the commutating current i c  builds up it starts to shut off rectifier 12. The current i c  reaches a maximum and decreases until t 1  when the current in 11a reaches zero and the diode 23 begins to block, this finally occurring at time t 2 . At this time t 2  the potential at Z falls from E DC / 2  to a value of zero volts or lower to a negative voltage depending on the value of #i. If #i is large the under-voltage may be excessive and to meet this a resistor 25 may shunt the winding 11. A further protection for the rectifiers 12, 13 may be provided by RC networks 26, 27, which provide current paths across the rectifiers when diodes 23, 24 block. In Fig. 2 (not shown) a single commutating capacitor 31 is used, this-in series with its inductor 22-being across the load. In addition a winding 32 is inductively coupled with the winding 11 and is connected in series with a diode 33 across the supply. This allows excess energy drawn during commutation to be fed back to the supply and is an alternative to the resistor 25 of Fig. 1. In a further embodiment (Fig. 3, not shown) a parallel inverter is disclosed, the winding 11 being in the positive D.C. line and its associated winding 32 and diode 33 shunting the supply. Fig. 4 (not shown) discloses a circuit similar to Fig. 1 except that the winding 22 is replaced by two windings in series with capacitors 19, 21. A three-phase circuit is provided (Fig. 5, not shown) and comprises three circuits similar to Fig. 1 without the capacitors 16, 17, all connected between the same D.C. supply. A threephase load is connected between the points Z. Fig. 6 (not shown) discloses a bridge circuit of 4 SCR's, the arrangement being similar to Fig. 2 except that the capacitors 16, 17 are replaced by circuitry similar to that on the right-hand side of the load. In further bridge circuits two (Fig. 7, not shown) and four (Fig. 8, not shown) separate LC commutating circuits are disclosed.