Patent GB-711829-A (GB)
Title: Improvements in or relating to current convertors of the mechanical switching type
IPC: H02M1/26
Filed: 1952-03-28
Abstract: 711,829. Converting. SIEMENS SCHUC KERTWERKE AKT.-GES. March 28, 1952 [March 30, 1951; May 10, 1951; May 29, 1951; May 31, 1951; Sept. 14, 1951; Oct. 18,1951; Nov. 17, 1951; Dec. 11, 1951; Feb. 5, 1952], No. 8033/52. Class 38(2) In a contact converter arrangement having saturable chokes to give a current stage of negligible value for opening the contacts without sparking, this value is reduced to zero by premagnetizing the choke by a magnetizing force comprising a constant part and a part dependent on the voltage across the.choke. The constant and voltage dependent parts of the premagnetizing force are made proportional respectively to Ho and He, which are the values measured as shown in Fig. 1 on the curve of coercivity (Hc) of the choke core plotted against the rate of change of flux dB/dt which is proportional to the voltage across the choke (Cd). A constant closing current stage for preventing closure on current and voltage may be provided similarly. This magnetizing force is produced by currents in auxiliary circuits which replace the magnetizing current forming the normal current stage which would otherwise flow through the main choke to produce an e.m.f. opposing the commutating voltage. In Fig. 2 a D.C. load 5 is supplied from a three-phase source, each phase including a choke 3 with a saturable core 2 premagnetized by windings 6, 3a carrying respectively constant current from battery 7 and varying current produced by the voltage across choke 3. By tapping into choke 3 (at its centre point for greatest efficiency) part a of the choke replaces winding 3a, Fig. 3. Valve 12 prevents choke saturation before contact closing where a closing stage is provided. Valve 25a may control magnetization for a contact-closing stage or may be replaced by valve 15a (Fig. 2) to saturate choke 3 during closing where no closing stage is provided. Resistance-capacity circuits 17. 17a which may include inductance may be coupled to the choke so that the effective hysteresis loop of its core is made more rectangular. Fig. 4 shows, for a converter operated magnetically by a coil 44. 44a. the same arrangement for producing a current opening stage by constant and voltage-dependent magnetization parts VA-. VA~ respectively, together with a closing stage produced by constant and voltagedependent magnetization parts VE-, VE~ Fig. 5. The constant part VE-is produced by the discharge upon firing of thyratron 25 from a condenser 53 through auxiliary winding 33 of a current maintained constant by a premagnetized choke 55. Condenser 53 is charged through transformer 62. Valve 25 is fired through transformer 67 when voltage-dependent current VE~ starts to flow through winding 13, after the phase voltage of winding 1 has risen to a value at which reactor 10b is saturated, at an interval from the beginning of each cycle determined by the controlling premagnetization current from battery 65. Holding current i also flows through winding 44a and after delay angle d which is determined by the value of the magnetizing current in reactor 10b, contacts 4 are made, whereby main current I can flow. It can be seen from Fig. 5 how the contacts open at A when holding current i is sufficiently reduced by the effect of the voltagedependent premagnetization current VA~ opposing the voltage across reactor 3. In the absence of a valve 25 choke 55 may be connected in series with reactor 10b which then supplies both VE- and VE~, windings 13, 33 being combined, Fig. 6 (not shown). Alternatively VE~ may be increased and VE- suppressed. In an alternative arrangement VE- derived from the voltage across the supply transformer winding is given a phase advancement by a component derived from the following phase, and the circuit including valve 25 for supplying VE~, is connected in parallel with the circuit connection supplying VA~, the same premagnetization winding being fed, Fig. 7 (not shown). Instead of superimposing a greater and reversed magnetization VE- on VA-, as in the preceding arrangements, these two quantities may result from the equal or unequal flat parts of a trapezoidal wave, as shown in Fig. 17. This may be produced by applying the phase voltage from winding 1 to two saturable chokes 71, 72 in series, which are premagnetized in opposite directions beyond their saturation points, by D.C. through windings 71a, 72a which passes through winding 66 and thus provides the biasing premagnetization V- (Fig. 17) in choke 3, Fig. 9. Also the main contacts may be operated by a grid-controlled valve 51, or by valve 25, Fig. 11 (not shown). Battery 47, which supplies D.C. to choke 70, 71, may compensate the voltage drop in valve 25. Many modifications in the circuit arrangement are described, including choke 3 on the load side of the main contacts, with change of position of battery 47, using part of winding 3 instead of separate winding 13, and amplification when the contacts are motor driven. Fig. 23 'shows a threephase bridge connected converter circuit with three chokes only, in which the voltage-dependent circuit opening premagnetization VA~ is derived by circuit connections 32 to the transformer winding of the incoming phase, and not to the load terminal as in the preceding arrangements, which in this case would result in an ineffective voltage dividing effect, Fig. 22 (not shown). In such a bridge circuit separate magnetic circuits, either to the same or different chokes, must be provided in each phase. To prevent interference when one core is unsaturated to produce current stage, the other must be saturated, if necessary by further windings. Fig. 25 shows a three-phase converter circuit in which each main choke 3 has cores 2, 2a for circuit opening and closing respectively. Windings 6, 6a, which together supply VA- and an opposite impulse to saturate core 2 during closing, are fed with trapezoidal current waves having half-cycles of unequal length, from saturable reactors 20. Windings 33, 33a, energized from saturable reactors 40. provide VE-, and winding 23. 23a fed from saturable reactors 50 saturate core 2a during, opening. Voltage-dependent magnetization VE~, VA~ are provided by circuits connected to the centre points of the choke 3 and include respectively thyratron 25 fired by means of an auxiliary, contact 4a and a valve 12. They also include saturation windings 22a, 22. Various combinations and modifications of these various features, including arrangements for econoinizing in power in the control circuits, are described.