Patent GB-716905-A (GB)
Title: Improvements in or relating to mechanical contact converter
IPC: H02M7/36
Filed: 1952-04-07
Abstract: 716,905. Converting. SIEMENS-SCHUCKERTWERKE AKT.-GES. April 7, 1952: [April 9, 1951; April 28, 1951; May 2, 1951; Oct. 30, 1951; Feb. 11, 1952], No. 8884/52. Class 38(2) In a contact converter of the kind operated electromagnetically and having saturable chokes in series with each contact for producing circuit closing stages of negligible current, an auxiliary circuit is provided for closing the contacts before the contact current can rise, and when an opening stage is also produced, for holding the contacts closed until the contact current has fallen to its negligible opening stage value. A load 5 is energized from transformer winding 1 through saturable choke 2 and main and auxiliary contacts 3, 3a operated respectively by winding 4 which carries the load current and by winding 8 in an auxiliary circuit connected between points 1a, 1b. Energization of winding 8 is controlled by a grid-controlled caesium valve 6 so that as shown in Fig. 2 events fall into sequence relative to the supply voltage e of winding 1 which passes through zero at time t. Valve 6 is fired after interval a at a time t, after voltage e has exceeded voltage ez of battery 9 by a value equal to the valve ignition voltage. Control current i passing through winding 8 increases rapidly until te when contacts 3a close and load current I begins to flow. At time t2, which determines the control angle a1, choke 2 saturates, and current I which passes through winding 4 increases rapidly until main contacts 3 close at t3. Saturation of choke 2 and closure of contacts 3 effectively short circuits the auxiliary circuit and i drops to a value maintained by battery 9 at a constant level sufficient to hold contacts 3a closed when, towards the end of the half-cycle, I declines to value I11 at t4 when contacts 3 open, and then to a value at b5 at which choke 2 unsaturates to form the opening current stage. The reversed voltage now acting across the auxiliary circuit from winding 1 now reduces i to zero at t6, contacts 3a opening at.t4 when it reaches i11 within the current stage. Battery 9 may be charged during operation of the converter by connecting its negative terminal to a tapping or winding 1, or by an arrangement of rectifiers. Winding 9<SP>1</SP> may provide extra alternating voltage e1z for igniting valve 6, and capacitor 10 increases the rate of rise and fall of i. The auxiliary winding may be energized from a winding 11 on choke 2 or connected to point 1c so that i flows through winding 4. Contact 3a may be omitted, windings 4, 8 both operating contacts 3, Fig. 5. Control valve 6 may be replaced by a saturable choke having controllable D.C. magnetism, in series with a unidirectionally conducting device. Shaded areas F1, F2, Fig. 2, show the time voltage integrals for reversal of magnetization of the choke core. Contacts 3 may be bridged at break by a caesuim valve with a grid controlled by a winding on choke 2. So that control current i does not influence the phase position of the circuit-breaking current stage, a capacitor 16 and series resistance rectifier combination 20, 15 may be connected respectively in series and parallel with the auxiliary circuit, Fig. 5. Battery 9 circulates the constant holding current i through valves 15, 6, the current variation for contact closing and opening resulting from the flow of the charging current for capacitor 16 through valve 6 and rectifier 15 respectively. The charging current can be utilized so that the main contacts close at to, the coverter being controllable over the full half-cycle. This advantage may also be obtained by drawing a leading anode voltage for valve 6 from a capacitor resistor connection across winding 1, Fig. 6 (not shown), or in a polyphase circuit from the third phase during commutation between the first two phases, Fig. 7 (not shown), or from the winding 1, Fig. 9 (not shown). Choke 2 may carry various control windings energized in such ways as to control the stage currents.