Patent GB-742081-A (GB)
Title: Improvements in or relating to electric switching apparatus having switch chokes
IPC: H02M1/26
Filed: 1953-05-14
Abstract: 742,081. Converting. SIEMENS-SCHUCKERTWERKE AKT.-GES. May 14, 1953 [May 14,1952; July 7, 1952], No. 13556/53. Class 38 (2). A mechanical contact converter has in series with the main contacts, a switching choke for producing current steps during contact operation, the switching choke being premagnetized by current proportional to the voltage across the choke, which is reduced or suppressed at least part of the time outside the steps. In a rectifying circuit in which D.C. conductors P, N are supplied from a three-phase source 12 through switches 4, 4<SP>1</SP> in each phase operated at 180 degrees apart in effective bridge connection, each phase includes a switching choke 3 with a saturable core 2, and an associated promagnetization circuit including a resistor 7 and a choke 14 energized from a transformer 13 connected between the phase being switched out and the incoming phase. During the current step, the voltage in this circuit is mainly across the switching choke 3, but at other times mainly across choke 14. By designing the choke 14 so that it can absorb the time integral of a half-wave of its energizing voltage, and that the value of current needed to saturate it lies just above that needed to saturate the switching choke, it remains unsaturated and effectively maintains the premagnetization current at a low level. It may be made small if it is promagnetized, as at 28, with alternating current preferably of trapezoidal shape from a series transductor 30. The core of choke 14 is advantageously of material having a rectangular hysteresis loop enclosing a large area. Where the three supply phases are connected to a star point forming one D.C. terminal, the premagnetization circuit may be connected between the incoming phase and a tapping on the switch choke 3, and may include a rectifier in series with the choke 14 which is premagnetized by D.C., Fig. 2 (not shown). In such a circuit the choke 14 and rectifier may be replaced by a valve 31 or a transistor blocked by the voltage applied to its control electrode through a transformer 34 from the voltage between the phases, except during the current step when the blocking voltage is cancelled by the voltage across the switching choke 3, Fig. 6. The premagnetizing circuit may include a separate winding 3<SP>1</SP> on the switch choke core 2 and a rectifier 10 in parallel with choke 5 to stabilize the current, as shown in the single-phase circuit of Fig. 3. The premagnetization circuit is there connected across the switch contacts 4 and switching choke 3. Where alternating current passes through the switching choke, the premagnetization circuit may include two chokes 5, 5<SP>1</SP> associated respec. tively with rectifiers 10, 8 and with a common resistor 22, Fig. 3a. A biasing source of E.M.F. may be inserted in series with the chokes, Figs. 4, 4a (not shown). Alternatively a single choke in serieswith the D.C. source and resistor are connected in the premagnetizing circuit through an A.C. bridge rectifying circuit, Fig. 5 (not shown). The switching choke may be also premagnetized by trapezoidal A.C. The premagnetizing current may be controlled by A.C. periodically opposing a unidirectional voltage source in series with a rectifier, Fig. 7 (not shown).