Patent GB-632150-A (GB)
Title: Improvements relating to electric valve control circuits particularly suitable for the control of intermittently energised load circuits such as welding circuits
IPC: B23K11/24
Filed: 1945-09-11
Abstract: 632,150. Valve control circuits for intermittently energized loads. BRITISH THOMSONHOUSTON CO., Ltd. Sept. 11, 1945, No. 23395. Convention date, Sept. 12, 1944. [Class 38 (iv)] A timing and control circuit for intermittently energized loads includes a number of pairs of back-to-back valves for transmitting A.C. and so connected that conduction initiated in one valve progresses from one to another at the rate of one valve per half cycle of the supply ; certain preselected valves can all be made initially conducting so that one particular valve will be alternately conducting and non- conducting for certain numbers of cycles, to a desired pattern. The Figure shows the invention applied to the control of the supply of current impulses to the resistance welding transformer 1 from the A.C. supply 3 through the reversely connected mercury vapour valves 4 and 5. The control grid 8 of each of these valves is connected to the cathode 7 through a secondary winding of transformer 10, supplying a hold-off voltage 180 degrees out of phase with the anode voltage, the control transformer 12 (or 35), the peaky wave transformer 14 (supplied by transformer 10 through an adjustable phase-shifter, and the resistance-condenser combination 15-16. Each of the valves 4 and 5 is made conductive for a half-cycle by temporary energization of its control transformer 12 or 35 through control valve 48 or 49 of the timing and controlling circuit. There are eight control valves, in pairs 44-45, 46-47, 48-49 and 50-51. They are supplied from lines 40-41 connected to the outer secondary terminals of transformer 37 and lines 42-43 connected to transformer tappings. The valves of each pair are reversely connected, i.e. the anode of valve 44 is connected to line 40 through inductance 52 while the anode of valve 45 is connected to the opposite line 41 through an inductance 53. The control grid of valve 45 is connected to the anode of the preceding valve 44 through resistance 59 ; the control grid of valve 46 is similarly connected to valve 45 but through the selector switch 54, and so on, the connection of the grid of valve 50 being through switch 56 while the grid of valve 44 is connected through switch 55 to the anode of valve 51. The grid voltages normally keep the valves non-conductive. When valve 44 conducts, there is a phase displacement due to inductance 52 and at the beginning of the next half-cycle the grid makes valve 45 conductive, and so on. If the conduction progresses from valve to valve, one conducting at a time, continuously around a closed loop, the pair of valves 48-49, and the welding supply valves 4 and 5, will be "on" for one cycle of the supply voltage and " off " for three cycles repeatedly. Switches 54, 55, 57 and 58 are for varying this pattern of operation. To initiate conduction, the hand switch 86 is closed to energize relay 751; this energizes transformer 71 in series with condenser 77 and valve 73, and connects the secondary of the peaky wave transformer 78 across the resistance 83 between the cathode and the grid of valve 73. At an instant determined partly by the reactor 80 in the primary circuit of transformer 78, an impulse is applied to the grid of valve 73, which allows condenser 77 to charge and so a single impulse is applied, through transformer 71, across resistance 75 between lines 41 and 76. Thus valve 46 becomes conductive because its grid is connected to line 76 through switch 54. If switches 57 and 58 were in their upper positions the result would be the above-mentioned " one cycle on, three cycles off " welding pattern, repeated until switch 86 is released to de-energize relay 75<1> ; the latter disconnects the condenser 77 from valve 73 and connects it across discharge resistance 87 and also by contacts 89 connects a blocking potential to the grid of valve 48 to stop the sequence of operations. Other patterns of operation can be selected. For example, with the selector switches all in the positions shown, switch 86 will initiate conduction of the three valves 46, 48, 50 together ; the valves 47, 49, 51 will conduct on the next half-cycle, and so on, giving a welding pattern of " three cycles on, one cycle off ". Repeated operation, of the valves in a closed loop, for seam welding, is obtained while switch 56 is in the lower position ; for spot welding, the switch 56 is moved to the upper position to disconnect the grid of valve 50 from the anode of valve 49 so that operation ceases after valve 49 conducts. The Specification sets forth the different positions of the selector switches for obtaining different patterns in which the sum of the number of cycles " on " and the number "off" for each seam welding operation is equal to or a factor of the number of pairs of valves (i.e. is 2 or 4) and also for obtaining other patterns by putting valves 50 and 51 out of action. Valves 48 and 49 determine the particular half-cycles during which valves 4 and 5 are conducting but the instant during the half-cycle at which conduction begins is adjusted independently by the phase shifting circuit supplying transformer 14. Certain of the inductances 52 and 53 can be transformers serving, like transformers 12 and 35, to control further welding equipments.