Wechselstromschalteinrichtung

730,612. Rectifying. FKG FRITZ KESSELRING GERATEBAU AKT.-GES. Feb. 2, 1953 [Feb. 2, 1952; March 22, 1952 ; March 22, 1952; Aug. 19, 1952], No.2936/53. Class 38 (2). [Also in Group XXXVII] In an A.C. switching arrangement, e.g. a rectifying system, the main contacts 3 in series with a unidirectionally conducting device 7, e.g. a dry metal rectifier, are bridged by a circuit including unidirectional conducting devices 8 and an inductor 9 or other means for developing an auxiliary E.M.F. having such characteristics that when the load current (I) has reached a low value and is to be interrupted, it is passing through the bridging circuit (12) and the current through the main contacts 3 (I1) is zero. Fig. 2 shows voltage U supplied from transformer winding 1, and a leading voltage U0 derived from an auxiliary winding 14 which, through rectifier 12 and resistance 13 energizes coil 6 to close the contacts 3 at time t1, and current I which passes through load 10 and divides into components I1, I2 which respectively traverse main contacts 3 with its holding coil 5, and the bridge circuit. Current 12 begins to increase at time t11 and rises until it equals I at time t2 when I1 has fallen to zero, then falling to zero dt time t3. Thus between times t2 and t3 there is a zero step in I1 when contacts 3 can be opened without sparking. The current I2 carried by rectifiers 8 is always less than the maximum load current I and the reverse voltage on rectifier 7, which reaches a maximum at that instant between t2 and t3 when the contacts 3 open, is always, less than the maximum valu of U. The inductor 9 may have a secondar winding connected across the contacts, in which while I2 is decreasing, a voltage is induced whic produces a current equal in value to the reverse current through rectifier 7 which is thereby diverted from the contacts, Fig. 6 (not shown). To reduce losses in the bridging circuit, it may include a valve 27 (e.g. a caesium vapour valve), which is ignited shortly before t2 by a voltage derived from winding 24 coupled to the choke 23 in which the magnetization reverses as the main current falls, Fig. 3. The effect is assisted by transformer 35. The valve may be a triode whose grid is energized, 143, Fig. 12. The current through rectifier 31 is reduced to a small value by connecting a saturable choke 32 in parallel, another saturable choke 29 being provided in series with the contacts, to give a Step of small value to the reverse current. The E.M.F. in the bri