Patent GB-851413-A (GB)
Title: Improvements in or relating to alternating current control systems
IPC: H05B7/11
Filed: 1957-02-05
Abstract: 851,413. Control circuits for resistance welding; automatic regulation. IGRANIC ELECTRIC CO. Ltd. Feb. 5, 1957 [Feb. 6, 1956], No. 3914/57. Class 38 (4). In an electric seam welder, the welding current is automatically maintained at a value proportional to a pre-selected supply voltage by a saturable reactor in series with the supply, the reactor being controlled by a self-saturating transductor in dependence on the difference between a signal proportional to the supply voltage and a signal proportional to welding current; and before welding commences, the control current in the saturable reactor is made to correspond approximately to its working value so as to reduce starting transients. The two modes of operation are switched in sequence by a time-delay relay. The welding transformer 9 is connected (whenever the main supply contacts W2 are closed) in series with the A.C windings of a saturable reactor 8 between one supply line L1 and a pre-selected tapping on a supply auto-transformer 5. Each tapping is arranged to supply a particular range of welding currents with a further control 31 (see below) to select the particular desired value within the selected range. The D.C. control winding of the saturable reactor 8 is supplied from an autoself-excited bridge rectifying transductor 15 ... 21. In addition to a fixed bias winding 15c this transductor has a control winding 15d energized by the preselected reference voltage and either a welding current signal (during welding) or a feedback signal (prior to welding). The changeover is effected by relay contacts ITR2 ... ITR6 (see below). The welding current signal is tapped off a potentiometer 29 and is obtained by rectifying at 30 the output of a transformer 51 measuring the voltage drop across the welding transformer 9. This signal aids the fixed bias and opposes the reference voltage which is obtained by rectifying at 26 the output of a potential transformer 48 connected between line L1 and the selected tapping on transformer 5 so as to measure the selected welding supply voltage, and so change the controlled range of values of welding current when the tapping is changed. Particular values of welding current are selected by adjusting the slider of a potentiometer 31, which, together with a bank of variable resistors 34 ... 39 form a potential divider across the rectifier 26. With the slider of the potentiometer 31 at the " rectifier " end of its track, the maximum reference voltage is applied to the transductor 15 and the maximum welding current for that tapping is supplied. The minimum welding current for each tapping is determined by the setting of the appropriate one of the bank of variable resistors 34 ... 39, these resistors being selected by a switch 7 ganged with the supply transformer tap changer. Prior to the energization of the welding transformer, relay ITR is not energized and the contacts ITR2 ... ITR6 are in the positions shown so that the welding current signal is disconnected at contacts ITR5 and ITR6 and it is replaced via contacts ITR3 and ITR4 by a feedback signal tapped off a potential divider 28 which is connected across the output of the transductor 15. The magnitude of the feedback signal is such as to reduce the output of the transductor 15 and so reduce the starting current transient when welding commences. Additionally a control ITR2 places a variable resistor 23 in series with the control winding of the reactor 8 to further reduce the control current. Resistor 23 is ganged with the currentselecting potentiometer 31. The sequence of control operations is as follows: closing the " start " switch 55 energizes relay DT which closes contacts DT3 immediately, closes contacts DT4 after a time delay and opens contacts DT2 after a further time delay. Contacts DT3 energize relay D which connects the supply transformer 5 and reactor 8 to a dummy load 14 over contacts D2; provides a temporary self-hold circuit for switch 55 over contacts D4; and prevents operation of relay W at this stage by opening contacts D3. After the first time delay, contacts DT4 close to energize relay WCR which provides a long-term self-hold circuit for switch 55 over contacts WCR4; and prepares an energizing circuit for relay W over contacts WCR2. After the second time delay, contacts DT2 open to de-energize relay D and so open contacts D2 to remove the dummy load 14 from the supply, and close contacts D3 to energize relay W via already closed contacts WCR2. Relay W closes contacts W2 to supply current to the welding transformer and energizes relay ITR to effect the control system changeover described above.