GB2046536A - Converter for supplying arc welding current - Google Patents

Abstract

In an arrangement for direct-current and/or alternating-current arc welding, comprising a converter (11) for generating an alternating converter output current with a frequency which is higher than the mains frequency, the converter (11) is provided with controllable switching elements e.g. thyristors (22, 23) or transistors which can be controlled via a control unit (12) by means of zero crossing signals supplied. In order to prevent the controllable switching elements (22, 23) from being driven simultaneously the zero crossing signals are tapped off at free-running circuits (24', 25') of the switching elements (22, 23). <IMAGE>

Description

SPECIFICATION Arrangement for direct-current and/or alternatingcurrent are welding, comprising a converter which can be connected to alternating- or direct-current mains The present invention relates to an arrangement for direct-current and/or alternating-current arc welding, comprising a converter which can be connected to alternating- or direct-current mains, for generating an alternating converter output current the frequency of which is higher than the mains frequency, the converter being provided with controllable switching elements and, furthermore, the converter output being connected via a transformer and, if necessary, a rectifier or a second converter to a workpiece or a welding electrode and to a control unit which is associated with the switching elements and which can be supplied with zero crossing signals.

This pre-characterising clause refers to a welding arrangement such as has become known, for example, from RCA Application Note AN 6628 entitled "Design and Applications of High Power Ultrasonic Converters Using ASCR's".

The advantage of this arrangement for arc welding known from the literature is said to consist in that the converter makes it possible to raise the input frequency to the transformer following it considerably with respect to the power frequency used, for example to such an extent that the frequency is outside the human threshold of hearing, that is to say above 18 Kilohertz. This increase in frequency makes it possible to reduce the dimensions of the subsequent transformer, advantageously resulting in a reduction in the weight of the whole welding arrangement so that the welding arrangement is now lighter by a factor of about 10, in contrast to a welding unit which does not work with a converter.

For providing the correct drive to the switching elements (for example thyristors) it is necessary to detect the zero crossing of the converter output current and in the proposed welding arrangement this is done by feeding the converter output signal to a primary winding of a saturating pulse transformer so that a pulse is generated with each zero crossing.

This pulse triggers a timing stage which triggers a flip flop after an adjustable period has elapsed, but at least after the circuit commutated turn-off time of the first switching element (thyristor) of the converter has elapsed. The flip flop then causes the second switching element (thyristor) of the converter to be fired. Laboratory investigations of this circuit have shown that during welding and the associated possible extreme jumps in loading on the transformer connected to the converter output magnetic collapses have occurred in the pulse transformer. These collapses lead to the spurious initiation of a zero crossing pulse and thus to the firing of the second thyristor at a time when the first thyristor is still operationally conducting. Thus the second thyristor is already fired before the first thyristor is cut off, leading to a short circuit in the converter.

It is the object of the invention to avoid the above-mentioned disadvantages and to produce a zero crossing detection system which prevents the respective disabled switching element from being fired too early.

In order to achieve this object, it is proposed that, according to the invention, in an arrangement of the type mentioned initially the zero crossing signals are tapped off at the free-running circuits of the switching elements. A current flows in the free-running circuits only during the free-running phase, that is to say only when the thyristor connected in parallel with the free-running circuit is in its cut-off phase.

This advantageously ensures that the firing pulse for the one thyristor of the converter is initiated only when the other thyristor is cut off. Collapsing fields and other instabilities of the transformer connected to the converter output have advantageously no longer any effect on zero detection and thus on the firing of the thyristors.

The invention is explained in greater detail in the description which follows, indicating further advantageous characteristics and referring to the drawings, in which: Figure 1 and 2 show an arrangement for arc welding comprising a converter in a half bridge circuit, and Figure 3 and 4 show an arc welding arrangement comprising a full bridge converter.

Each of the arrangements for arc welding shown in Figures 1 to 4 is provided with a direct-current section 10 which is followed by a converter 11 with its associated control unit 12. The output of the converter 11 is electrically connected via a transformer 13 and a rectifier 14 to a workpiece 15 and a welding torch 16.

The direct-current section 10 essentially consists of a rectifier 17 which is followed by the smoothing capacitors 18 and a smoothing choke 19.

According to Figure 1 and 2, the converter 11 is constructed as a half-bridge converter and contains two commutating capacitors 20,21 which are connected in parallel to the controllable semiconductors (thyristors) 22, 23. The thyristors 22 and 23 are associated, in a manner known in itself, with a de/dt load comprising the free-running diodes 24, 25 and a capacitor 26 and a resistor 27 of the appropriate sizes. In addition, the thyristors 22, 23 are associated with a load choke 28, 29 for limiting di/dt. The thyristors 22, 23 are alternately driven by the control unit 12, in such a manner that the output frequency is preferably greater than 18 Kilohertz. The design of such control units is known and thus does not need to be explained in greater detail.

The control unit 12 is here supplied with an input signal which indicates the zero crossing of the current at the converter output 30. According to the invention, for this the primary windings 31,32 of a current transformer 33 are connected in series with the free-running diodes 24 and 25 into the freerunning circuits 24' and 25'. The current transformer 33 is advantageously constructed as a saturating pulse transformer. The secondary side/secondary winding 34 of the pulse transformer 33 is electrically connected via a diode 35 to the input 36 of the control unit. In contrast to a linear current transformer, power losses are prevented by using the saturable pulse transformer.In addition, the pulse transformer generates at each beginning of current flow a positive pulse in the free-running circuit and this pulse can directly trigger the timing stage in the control unit 12. The negative pulse arising at the end of current flow is blocked by the diode 35.

In the illustrative embodiment of Figure 1 a single pulse transformer 33 is provided for both freerunning circuits 24', 25'.

Naturally, and advantageously, it is possible to allocate one saturating pulse transformer 33a and 33b to each of the free-running circuits 24', 25', as shown in the illustrative embodiment of Figure 2.

In the illustrative embodiment of Figure 3 and 4 the converter is constructed as a thyristor converter in a full bridge circuit and, accordingly, contains 4 thyristors 37, 38, 39 and 40 and their corresponding load chokes 41 to 44. Each of the thyristors 37-40 is associated with a de/dt circuit which is configured in accordance with the illustrative embodiment of Figure 1. The thyristors are driven by the control unit 12.

In this illustrative embodiment also only one commutating capacitor45 is provided. The remaining configuration of the arrangement according to Figures 3 and 4 corresponds to the configuration of Figure 1 and 2 so that it will not be discussed in greater detail.

In the illustrative embodiment of Figure 3, in accordance with the example of Figure 1 one pulse transformer 33 is provided for both free-running circuits 24' and 25' and in Figure 4, according to the example of Figure 2, two pulse transformers 33a and 33b.

In all examples, detection of the free-running currents causes the control unit 12 to be supplied with a signal which indicates accurate detection of the zero crossing of the alternating current at the converter output since a current can flow in the free-running circuits only when the respective thyristor (pair of thyristors) is cut off.

In the above-mentioned illustrative embodiments the output transformer 13 is connected via a rectifier to the welding torch and the workpiece. It is, of course, also possible and lies within the scope of the invention to provide, instead of the rectifiers, controlled semiconductors (for example thyristors) which are connected together to form a second converter.

It is also possible and lies within the scope of the invention to provide, instead of the proposed bridgeconnected converters having an output frequency of between 20 and 30, and preferably 25 Kilohertz, converters having these output frequencies and arranged in different circuits. Converters using switching elements which are not thyristors, for example using transistors as switching elements, can also be used advantageously.

The above-mentioned arrangements for arc welding are not restricted to arc welding using a nonfusible electrode but can also be used to weld with fusible electrodes according to the manual electrode welding or the gas metal-arc welding method. The arrangement according to the present invention can be employed advantageously also for submergedarc welding, plasma welding and cutting.

Claims (4)

1. An arrangementfordirect-currentand/or alternating-current arc welding, comprising a converter which can be connected to alternating- or direct-current mains, for generating an alternating converter output current the frequency of which is higher than the mains frequency, the converter being provided with controllable switching elements and the converter output being connected, via a transformer and, if necessary, a rectifier or second converter, to a workpiece and to a welding electrode and comprising a control unit which is associated with the switching elements and which can be supplied with zero crossing signals, characterized in that the zero crossing signals are tapped off at freerunning circuits (24', 25') of the switching elements (22,23,37-40).

2. An arrangement according to Claim 1, characterised in that the primary windings (31,32) of at least one current transformer, and preferably a saturating pulse transformer (33, 33a, 33b) the sec ondarywinding (34) of which is electrically connected to the control unit (12), are connected into the free-running circuits (24', 25').

3. An arrangement according to Claim 1 or 2, characterised in that the converter output current has a frequency of more than 18 Kilohertz.

4. # 4. An arrangement for direct-current and/or alternating-current arc welding substantially as hereinbefore described and as illustrated in the accompanying drawings.