Patent GB-590527-A (GB)
Title: Improvements in or relating to current indicating and control circuits
IPC: B23K11/25
Filed: 1945-03-12
Abstract: 590,527. 'Current measuring apparatus. BUDD MANUFACTURING CO., E. G. March 12, 1945, No. 6133. Convention date, Feb. 24, 1944. [Class 37] A current indicator system comprises a load circuit, a control circuit operable on energization to supply current to the load current in the load circuit for indicating a current limit of a predetermined load circuit range or for opening the load circuit at that limit. A seam welding circuit A, Fig. 1, is fed from A.C. mains 10, 11 through a transformer 13, the secondary winding 14 of which is-connected to roller electrodes 15, 16 engaging the workpiece 17. In series with the primary winding 12 of the transformer 13 are ignitrons 18, 19 connected in inverse parallel, and controlled from a D.C. source 23. A control circuit B includes the secondary winding 30 of a current transformer, the primary 31 of which is in the welding circuit, the ratio being variable by switches 34 connected to taps and through resistors 35, 36 to the earthed side 37 of the secondary winding. An additional variable shunt resistor 38 and a short-circuiting shunt 39 we also provided. Connected between the earthed side 37 of the current transformer and the movable arm 41 of the potential divider 36 is a rectifier 40, the output of which is connected in series with a variable resistor 42, a resistor 48 and a potential divider 49. Connected across this circuit are a voltmeter 47 and a bank of capacitors 44, 45, 46. A constant potential reference circuit C is supplied from A.C. mains 62 through a transformer 60 having a primary winding 61 and secondary windings 63, 64, 65, 66. The secondaries 64, 65 supply a rectifier 69, the output of which is smoothed by a filter comprising a choke 71, capacitor 72 and resistor 73, a glow tube 76 maintaining the potential constant. The load of this rectified constant potential circuit includes two potential dividers 77, 78 in parallel, a resistor 79 and a triode 80, the grid 86 of which is connected through a resistor 88 to the movable arm 51 of the resistor 48 in the control circuit so that the current through the valve 80 is proportional to the potential across the resistor 48. The variation in current in the load circuit is used' to indicate the limits of a predetermined current in the welding circuit by means of an operating branch circuit D which includes two relays 90, 91, the relay 90 being normally energized during the welding operation and the relay 91 being normally de-energized during the normal operating range. The relay 90 indicates by means of visual or audible signals the lower limit of current variation, the higher limit being indicated by the relay 91. The windings of the relays 90, 91 are connected in the anode circuits of gas-filled tetrodes 104, 105, the control grids of which are connected to the contact arms of the potential dividers 78, 77. The welding operation is started by closing a switch 96 which energizes a timer relay 97 and thereby closes the timing circuits of the ignitrons 18, 19. Current then flows in the welding transformer 13 and in the current transformer 30 causing a D.C. voltage across the resistor 48 proportional to the welding current. This voltage is imposed on the grid 86 of the valve 80 and causes a corresponding variation in the current through the resistors 77, 78. When the current exceeds its upper limit, the valve 104 is operated, while the valve 105 is operated to stop current flow through it if the welding current falls below the lower limit of its normal range. The corresponding relay 90 or 91 is thereby operated and a signal given by the lamp 98 and bell 99, or a recording instrument may be employed. The relays 90, 91 may cause lock-out of the welding circuit as shown in Fig. 2, in which a relay coil 130 is substituted for the relay 97, Fig. 1, and is connected in parallel with the lamp 98 and bell 99. The relay 130, when energized at the current limits, opens the weld roller motor circuit contacts 131, 132 and also the contacts 133, 134 in the circuit of the relay 135. The contacts 125, 126 of the timer circuit 127, 128 are thereby opened, causing the welding circuit to open.