GB1574190A

GB1574190A - Electric welding apparatus

Info

Publication number: GB1574190A
Application number: GB3937977A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-09-21
Filing date: 1977-09-21
Publication date: 1980-09-03
Also published as: FR2364733B1; FR2364733A1; ATA700776A; DE2740529B2; AT349577B; DE2740529C3; DE2740529A1

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRIC WELDING APPARATUS (71) I, HOLGER REEH, a citizen of the Federal Republic of Germany, of Adalbert Stifterstrasse 1, D-8034 München-Unter- pfaffenhofen, Federal Republic of Germany, do hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an electric welding apparatus particularly, but not exclusively, suitable for d.c. welding shielded by inert gas, for avoiding end craters at weld joints.

As a result of continually increasing demands on welded joints, of the use of materials which are difficult to control and which are prone to cracking, of increasingly more stringent test conditions and not least, of a decline in the average proficiency of welders, particular attention has to be paid to the avoidance of end craters, insofar as concerns the welding equipment. Rough weld seams are caused by the discharge arc, by the sudden cooling of the seam and by the material flow being maintained undiminished to the end of the welding operation. In particular, end craters are formed which are not only visually disturbing but also are often the starting point for cracking, if the weld point is stressed.

According to the present invention there is provided a d.c. welding apparatus comprising: a motor-driven d.c. generator which provides current for the welding circuit and has a field coil the current through which controls the welding current: a current decay circuit connected via switch means to the field coil and including a resistance-capacitance circuit; two welding current controllers which can be selectively connected to the field coil to provide a greater and a lesser current respectively for the field coil so as to produce in the welding circuit an operating and a reduced welding current respectively, the arrangement being such that the current decay circuit causes the field coil current to decay exponentially from the greater to the lesser value when" by operation of a control, the second current controller is connected to the field coil in place of the first, and when by further operation of the control, the current controllers are both de-energised the current in the field coil decays to zero.

The welding current is preferably decreased to zero upon reaching the end of a weld seam. By gradually decreasing the arc parameters, particularly the power consumption, it is possible to completely fill an end crater, and thus produce a uniform finish for the weld seam. The case where the welding current is decreased continuously, without an intermediate period at the reduced value of welding current, is referred to herein as "one-step fade-out" or "onestep decay"; and correspondingly "twostep fade-out, or decay" where there is such an intermediate period. The total duration of the fade-out can be altered by changing the duration of the intermediate period wherein the welding current is constant. Thus by "holding" the welding current at this constant reduced value different crater-filling conditions can be obtained without having to alter the settings of the welding apparatus i.e. the value of the reduced welding current, and the decay characteristics of the welding current, and thus an operator can compensate for a somewhat premature fade-out procedure by holding the current at the intermediate value for a correspondingly longer time.

In a preferred embodiment, the RC circuit of the decay circuit includes a variable resistor for adjusting the exponential decay function. The stored energy is fed from the capacitor via the variable resistor to the field coil. The control is preferably a pushbutton switch mounted on the torch of the apparatus, to facilitate manual control. The control preferably connects the field coil which is in parallel with the series-connected resistor and capacitor, to the tappings of a selected one of a plurality of separately adjustable potentiometers connected in parallel to a common power supply. When changing over from a higher voltage tapping to a lower voltage tapping, the voltage at the field coil slowly decreases to the lower voltage as a result of the discharge of the charged capacitor and the welding current correspondingly decreases. It remains at this value until the field coil together with the decay circuit is disconnected from the tapping whereupon the capacitor continues to discharge according to the time constant RC, and, accordingly, the welding current correspondingly decreases until the arc extinguishes. Thus, even with a deliberately incorrect current setting and torch guidance, it is no longer possible for an end crater to occur, except when there is a spurious interruption of the arc due for example to a failure of the supply mains. Even in electric welding (E-welding), particularly in the processing of bar electrodes of lime-based type with a distinct tendency toward the formation of end craters, the end location of the weld takes the form of a smooth Bat hump. In contrast to the usual troughs with a tendency toward shrinkage, the seam end remains free of any cavities.

In the case of WIG-welding (inert-gas welding using tungsten electrode), the end location has the shape of a tiny round projection. Even with oversize and almost continuous puddles on thin metal sheets, a shallow and very well rounded trough is produced. In both cases it is possible to detect concentric circular structures on the surface of the hardened puddle, which indicate voltage deficiencies. In contrast to this, radial structures are customary in the case of known end craters. The crater-filling time is correspondingly longer with higher operating currents and also with greater heatretention of the puddles and tungsten electrodes. The fade-out of the arc takes place between 10A and 2A as a function of the retained arc length and the condition of the electrode tips.

The amplitudes of a high frequency (HF) superimposed locating arc and a work arc are adjusted by means of respective current adiusters. For preheating it is possible for the initial current (as is advantageous with copper) to be selected higher than the operating current.

An advantage of a preferred welding apparatus is that the operator can select with a single pushbutton switch two different current values, with any desired amount of repetition, without it being necessary to re-ignite or to alter a setting. This is especially advantageous in the case of pipe welding and repair welding under conditions which cannot be accurately surveyed before -hand, or with a constantlv varving gan between the members to be welded. These advantages are apparent both in WIGwelding and with E-welding.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 shows a general wiring diagram of an apparatus according to the invention, Fig. 2 shows a diagram of the control circuit of the apparatus of Fig. 1, Fig. 3 is a graph of welding current versus time showing the decay procedure for preventing the formation of end craters, and Fig. 4 is a graph of welding current versus time showing a second welding operation closely following a first welding operation and starting before the completion of the decay procedure.

An asynchronous motor (ASM) 1, which drives an exciter 2 and a generator magnet wheel 4, is provided for producing the electrical energy required for the electric welding (WIG-welding, MIG-welding i.e. inertgas manganese welding, or welding without inert gas) at the terminals N and P, to which the welding electrodes are connected. The field coil 9 of the exciter 2 is coupled via a voltage controller 7 and a rectifier 8, to an auxiliary winding 6 of the ASM stator winding 5, and, is coupled to parallelconnected current controllers 10 and 18 which are coupled to a transformer 14 in the current path of the generator stator winding 16. This transformer supplies no voltage as long as there is no current in the welding circuit. A rectifier 3 is disposed in the supply lines between the exciter 2 and the magnet wheel 4. According to the setting of the controllers 7 and 10, there occurs either a non-load voltage at the terminals N-P or a welding current in the welding circuit. The settings are initially selected depending on the requirements of a workpiece. In order to avoid the formation of end craters there is provided a decay circuit comprising a capacitor 19 and a variable resistor 20. For electrical isolation there is provided a relay control arrangement which comprises relays Cl and C2 having respective sets of contacts cl and c2.

The controller 7 is coupled to the rectifier 8 by parallel-connected contacts lel and lc2 which are shown open. (The contacts in Figures 1 and 2 are shown in their positions just prior to start up of the apparatus.) The resistor 20 is connected to the field coil 9 via series-connected contacts 2cl and 2c2 which together are in parallel with a contact Id of a stepwise operable relay D also forming part of the control arrangement. Controllers 10 and 18 are coupled to the transformer 14 by change- over contacts 3cl and 4c,: while controller 10 is coupled to change-over contact 3cl by contact 3c2, and to change-over contact acl bv contact 4c,.

With complete isolation from the auxiliary winding 6 and transformed 14 by means of contacts lel, 3c2, 4c2, 3cl and 4cl, the discharge of capacitor 19 takes place continuously via resistor 20 and the field coil 9, so that as a result of the reducing excitation, the arc energy gradually decreases until the arc is extinguished. The decay circuit including the capacitor 19 is controlled by the relay control arrangement as described later.

The arc can be controlled by switching by means of contacts 3c2, 4c2, 3cl and 4cl, from controller 10 to controller 18 which is set to produce a reduced welding current value, preferably half the operating welding current.

The current decreases from the value I2 set by the controller 10 to the reduced value I1, under the control of the decay circuit 19, 20. A diode 21 ensures the capacitor 19 cannot discharge below the voltage set by the controller 7 until contact 1c1 is opened after which the welding current gradually reduces to zero. The relay control itself is actuated by a pushbutton switch having a contact T and mounted on the torch of the welding apparatus.

The operation of the welding apparatus will now be explained with reference to Fig. 3.

The actual welding operation at the oper ating current I2 is preceded by the locating of the work piece and the ignition of the arc by means of a HF superimposed locating arc (section A) at a reduced welding current value I1. When the weld seam is approaching its end (section B), crater filling is started (periods C + D) by continuous pressure on the torch switch, whereupon the welding current decreases from the operating cur rent "I2" to the preselected reduced current "it". This current is maintained as long as the switch is pressed. In this switch position it is still possible for welding wire to be added. After releasing the pushbutton switch, the current decreases until the gradual ex tinction of the arm (period D). This provides a two-step decay of the welding current.

If at the end of welding the torch switch is pressed for a short period only (i.e. the contact T is opened before the decaying current falls to Ii) the welding current decreases continuously from the operating current "12" to extinction of the arc (dashed line). This provides a one-step decay.

For inert-gas shielded arc welding a selector switch 22 (Fig. 2) is set to WIG.

When using an air-cooled torch the contact of a water-failllre switch Q is by-passed at terminals 1 and 2.

The operation of the relay control will now be described. When contact T of the push button switch is closed, the relay Cl is energised. Ifs contacts complete circuits for the controller 7 (via let), the controller 18 (via 3cl and 4cm), the stepwise operable relay D (via 5cm), a solenoid valve Si for the inert gas and an indicator light h (via 6cm), and the HF ignition (via 7cm), The contacts id and 2d of the energised relay D isolate the capacitor 19 and apply voltage to the relay C2, respectively. The capacitor 19 remains isolated, since the contact 2cl in series with contact 2c2 is open. The contacts 3c2 and 4c2 prepare the circuit through the controller 10 to be completed when relay C1 becomes de-energised. The field coil 9 has a voltage applied to it from the auxiliary winding 6, accordingly a voltage is applied between the electrode and work-piece. The welding operation can now commence. As soon as welding current flows, the voltage developed by the transformer 14 and set via the controller 10 acts on the field coil 9.

After releasing the pushbutton switch, thereby opening contact T, the relay Cl is de-energised. Accordingly, the HF is cut off and the controller 10 becomes operative in place of the controller 18. Adjustment of the setting of the controller 10 varies the excitation and therefore the welding current.

The capacitor 19 thus now charges to the exciting voltage set by controller 10, via the resistor 20.

Upon renewed closure of the contact T, the relay C1 is again energised, resulting in a further operation of the stepwise operable relay D. Relay C now de-energises, The HF is switched on by contact 7cl, S1 and h remain energised via contact 6c1 and the controller 18 becomes operative (via contact~ 3cl and 4cl) in place of the con- troller 10. Since the capacitor 19 connected via contact id had charged to the voltage supplied by the controller 10, the voltage across, and the current in the field coil 9 decays exponentially according to the time constant RC (ignoring the impedance of field coil 9) until the voltage set by the controller 18 is reached. The decay time thus depends on the setting of the variable resistor 20.

When the contact T is opened, the relay C1 is de-energised. Since relay C2 is also energised, the three controllers 7, 10, 18 are isolated from their voltage sources, the charged decay circuit 19, 20 supplies the field coil 9 with a decay voltage and diode 21 blocks discharge via the controller 7.

The welding current thus slowly decays to a value at which the arc can no longer be sustained. The decay time depends on the setting of the resistor 20. An additional supply container continues to supply inert gas for a predetermined period of time, after the main gas supply has been switched off by means of the solenoid valve Sl. A time switch could also be provided to delay the de-energising of solenoid valve Sl.

In the E-mode of operation, the relay C2 is permanently energised. The voltage controller 7 for the no-load voltage and the current controller 10 for operating current are operative, independently of the operation of the pushbutton switch. If the pushbutton switch is connected at terminals 1 and 2 (Fig. 2), the welding current can be switched between Il and 12 by contacts 3c, and 4cl as in WIG-welding.

Fig. 4 is a graph of welding current versus time for instances in which the welding current needs to be reduced only temporarily after the crater filling, since a fresh welding operation is to follow. As in Fig. 3, the welding period B is followed by the decay period C with subsequent hold period at intermediate current Il (corresponding to the 2-step decay). If during the hold period the switch T is opened, closed and then reopened in quick succession the periods D and A are passed through rapidly to arrive at the next operational welding period. If the switch is opened and closed during the decrease of welding current towards the intermediate value, the welding current will fall quickly to the intermediate value because contact 1c1 will be closed and contacts id and 2cl will be open.

WHAT I CLAIM IS: 1. A d.c. welding apparatus comprising: a motor-driven d.c. generator which provides current for the welding circuit and has a field coil the current through which controls the welding current; a current decay circuit connected via switch means to the field coil and including a resistance-capacitance circuit: two welding current controllers which can be selectively connected to the field coil to provide a greater and a lesser current respectively for the field coil so as to produce in the welding circuit an operating and a reduced welding current respectively, the arrangement being such that the current decay circuit causes the field coil current to decay exponentially from the greater to the lesser value when, by operation of a control, the second current controller is connected to the field coil in place of the first, and when by further operation of the control, the current controllers are both de-energised the current in the field coil decays to zero.

2. A welding apparatus as claimed in claim 1 in which the decay circuit is further connected to an adjustable voltage source comprising a voltage controller coupled in series with at least one controllable switch to a source of d.c. voltage, and the control is arranged to open said at least one controllable switch to isolate the adjustable voltage source from the decay circuit and permit the decrease of the current in the field coil to zero.

3. A welding apparatus as claimed in claim 2 wherein said source of d.c. voltage comprises an auxiliary winding of the motor for the motor-driven generator, and a rectifying circuit.

4. A welding apparatus as claimed in any previous claim in which the d.c.

generator excite an a.c. generator the rectified output of which provides the wefd- ing current and the current controllers are energised from the output of the a.c.

generator via a transformer.

5. A welding apparatus as claimed in claim 4 wherein there is provided at least one controllable switch for isolating the decay circuit from the transformer.

6. A welding apparatus as claimed in any preceding claim wherein the resistance of said RC circuit is adjustable.

7. A welding apparatus as claimed in any previous claim wherein the control is arranged to increase the welding current to its operating value in a first step to the intermediate value and a second step from the intermediate value to said operating value.

8. A welding apparatus as claimed in claim 7 wherein the control is arranged to effect operation of the apparatus in a sequence of steps under the manual control of an operator.

9. A welding apparatus as claimed in claim 9 wherein the control includes an onoff switch for actuation by the operator and is arranged to produce a step in said operation sequence at each make or break operation of the on-off switch.

10. A welding apparatus as claimed in claim 9 wherein the on-off switch is mounted in the handle of a torch of the apparatus.

11. A welding apparatus as claimed in either claim 9 tr claim 10 wherein the con trol is arranged such that the step of gradually decreasing the welding current from its operating value by exponentially decreasing the current in the field coil from the greater to the lesser current is initiated by a make operation of the on-off switch, 12. A welding apparatus as claimed in any of claims 9 to 11 and further including an adjustable voltage controller operable to set the voltage across supply terminals for the welding electrodes in a no-load condition at a presettable level, a solenoid valve operable to control the supDly of an inert gas to the welding region, a HF ignition device oDerable to supply HF current to the electrodes, and a pump operable to activate a water cooling circuit. said voltage con troller, solenoid valve, HF ignition device and pump being operable by said control.

13. A welding apparatus as claimed in claim 12 in which the control is arranged: upon a first operation of the on-off switch to - operate the HF ignition device, the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. In the E-mode of operation, the relay C2 is permanently energised. The voltage controller 7 for the no-load voltage and the current controller 10 for operating current are operative, independently of the operation of the pushbutton switch. If the pushbutton switch is connected at terminals 1 and 2 (Fig. 2), the welding current can be switched between Il and 12 by contacts 3c, and 4cl as in WIG-welding. Fig. 4 is a graph of welding current versus time for instances in which the welding current needs to be reduced only temporarily after the crater filling, since a fresh welding operation is to follow. As in Fig. 3, the welding period B is followed by the decay period C with subsequent hold period at intermediate current Il (corresponding to the 2-step decay). If during the hold period the switch T is opened, closed and then reopened in quick succession the periods D and A are passed through rapidly to arrive at the next operational welding period. If the switch is opened and closed during the decrease of welding current towards the intermediate value, the welding current will fall quickly to the intermediate value because contact 1c1 will be closed and contacts id and 2cl will be open. WHAT I CLAIM IS:

1. A d.c. welding apparatus comprising: a motor-driven d.c. generator which provides current for the welding circuit and has a field coil the current through which controls the welding current; a current decay circuit connected via switch means to the field coil and including a resistance-capacitance circuit: two welding current controllers which can be selectively connected to the field coil to provide a greater and a lesser current respectively for the field coil so as to produce in the welding circuit an operating and a reduced welding current respectively, the arrangement being such that the current decay circuit causes the field coil current to decay exponentially from the greater to the lesser value when, by operation of a control, the second current controller is connected to the field coil in place of the first, and when by further operation of the control, the current controllers are both de-energised the current in the field coil decays to zero.

4. A welding apparatus as claimed in any previous claim in which the d.c.

generator via a transformer.

11. A welding apparatus as claimed in either claim 9 tr claim 10 wherein the con trol is arranged such that the step of gradually decreasing the welding current from its operating value by exponentially decreasing the current in the field coil from the greater to the lesser current is initiated by a make operation of the on-off switch,

12. A welding apparatus as claimed in any of claims 9 to 11 and further including an adjustable voltage controller operable to set the voltage across supply terminals for the welding electrodes in a no-load condition at a presettable level, a solenoid valve operable to control the supDly of an inert gas to the welding region, a HF ignition device oDerable to supply HF current to the electrodes, and a pump operable to activate a water cooling circuit. said voltage con troller, solenoid valve, HF ignition device and pump being operable by said control.

solenoid valve and the pump and to increase the welding current to said reduced value; upon a second operation of the on-off switch to increase the welding current from the reduced value to the operating value end and remove the HF current; upon a third operation of the on-off switch to decrease the welding current to the reduced value, and to operate the HF igniting device again, and upon a fourth operation of the on-off switch to cause the welding current to decay to zero; and thereafter to switch off the HF ignition device, the solenoid valve, and the water pump.

14. A welding apparatus as claimed in claim 13 in which the control is responsive to operation of the on-off switch to effect a brief dwell in a particular switch state, to advance the operation of the apparatus to the next step in the operational sequence.

15. A welding apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.