GB2046035A - Converter supplying arc welding current - Google Patents

Abstract

In an arrangement for direct- current and/or alternating-current 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 (22, 23) and at least one commutating capacitor (20, 21) and its output is connected via a transformer (13) to a workpiece (15) and a welding electrode (16). In order to prevent the switching elements (22, 23) from being destroyed at frequencies of more than 20 Kilohertz and during welding operations, the primary winding (13a) of the transformer (13) is divided in accordance with the number of switching elements (22, 23) and one primary winding section (31 and 32) each is connected to one switching element (22 and 23) and one commutating capacitor (20 and 21). Full bridge converter arrangements are described with reference to Figs. 2 and 3 (not shown). <IMAGE>

Description

SPECIFICATION Arrangement for direct-current and/or alternating-current arc 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 alternatingcurrent 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 at least one commutating capacitor and, furthermore, the converter output being connected, via a transformer and, if necessary, a rectifier or second converter, to a workpiece and to a welding electrode.

This pre-characterising clause refers to welding arrangements which have become known, for example, from German Offenlegungsschrift 17 65 775, German Offenlgegungsschrift 26 51 510, German Offenlegungsschrift 27 20 347, German Offenlegungsschrift 27 20 942 and RCA Application Note An 66 28, entitled "Design and Applications of High Power Ultrasonic Converters Using ASCR's".

The advantage of these arrangements for arc welding, which have become known from the literature, consists in that the converter makes it possible to raise the input frequency for 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 1 8 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.

In the above-mentioned literature converters are described which must generate an alternating output current having a frequency of up to 30 Kilohertz. In the Application Note, too, a frequency of more than 20 Kilohertz is specified. However, the arrangement of this type which has hitherto become known in practice only works with a frequency of 1000 Hertz. This frequency is within the audible range and an unpleasant noise is produced during welding. The above-mentioned literature can induce an expert to think that this noise can also be prevented by increasing the frequency. Trial constructions containing thyristor converters and having output frequencies of more than 20 Kilohertz have shown that particularly in those power ranges necessary for welding, the semiconductors (thyristors) used in the converter were destroyed.

Investigations have shown that this destruction is the result of excess currentsi excess voltages and the stress conditions produced by these in the semiconductor (de/dt and di/dt). The expert knows that, in order to limit the stress condition de/dt, resistors, capacitors and diodes can be connected to the semiconductors of the converter and that the only significant disadvantage of these components lies in the power losses incurred.

Care must also be taken that the de/di circuit is not over-sized since this, in turn, will again generate an excessive current in the thyristor when the thyristor fires, producing a stress condition dVdt. It is also known to provide saturable chokes in series with the thyristors in order to limit the stress condition di/dt. in order to limit the increase in current to values which are safe for the thyristor, the circuit elements, also called "cascade chokes", themselves again generate considerable excessive voltage (de/dt) which then leads to the destruction of the thyristor. These facts provide an opportunity to find a circuit compromise, by extensive investigations, to avoid the above-mentioned excess voltage, excess currents and stress conditions in dependence on the components of the overall circuit used in each case.Apart from the design expenditure, this solution has the particular disadvantage that another new circuit compromise must be developed when the component specifications are changed within the circuit.

It is the object of the present invention to construct an arrangement of the type mentioned initially, in such a manner that it can be used to obtain in the simplest manner with the known circuits reliable control of the above-mentioned stress conditions, excess voltages, excess current and the like which can lead to a destruction of the semiconductor.

According to the present invention, in order to achieve this object it is proposed that the primary winding of the transformer associated with the converter output is divided according to the number of switching elements (semiconductors) provided in the converter and that one primary winding section each is connected to one semiconductor and to the commutating capacitor of the converter. This circuit advantageously achieves that, on the one hand, the semiconductors are loaded with a lower de/dt and, on the other hand, the load choke can be designed with more generous dimensions for preventing a high di/dt at the thyristor since the excess voltages arising are now absorbed by the commutating capacitor. Seen overall, the semiconductors are subject to less loading, ensuring reliable converter operation.

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

The arrangement for arc welding shown in Figures 1,2 and 3 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 13a, 13b and a rectifier 14 to a workpiece 1 5 and a welding torch 1 6.

Essentially, the direct current section 10 consists of a rectifier 1 7 followed by the smoothing capacitor 1 8 and a smoothing choke 19.

According to Figure 1, the converter 11 is configured as a half bridge converter and is provided with 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 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 1 8 Kilohertz. The design of such control units is known and thus does not need to be explained in greater detail.

The output 30 of the converter 11 is connected to the primary winding 1 3a of the transformer 13.

According to the invention, the primary winding 1 3a is provided with a number of winding sections 31,32 which corresponds to the installed number of switching elements (thyristors 22,23). The winding 31 is connected, on the one hand, to the thyristor 22 via the load choke 28 and, on the other hand, to the commutating capacitor 20. The winding section 32 is electrically connected via the choke 29 to the thyristor 23 and also to the commutating capacitor 21.

By dividing the primary winding into two winding sections 31 and 32 which are mounted on a common ferrite iron core the thyristors 22, 23 are prevented, as already mentioned initially, from being overloaded when the converter is operated at high frequencies (higher than 18 Kilohertz) and at a power which is adequate for welding. This is due to the fact that now at the firing time of the thyristor 22 no longer the potential of point A is applied to the thyristor 23, as in the circuits which have hitherto become known, but that of point B. In this way, the thyristor 23 is loaded only with the de/dt at point C which acts at point B. In addition, this makes it possible to design the di/dt load choke 29 with very generous dimensions since the excess voltage generated by the choke 29 is absorbed by the capacitor 21.

In the illustrative embodiment of Figure 2, the converter is constructed as a thyristor converter in a full bridge circuit and, accordingly, is provided with fourthyristors 33,34,35 and 36 and their corresponding load chokes 37 to 40. Each of the thyristors 33-36 is associated with a de/dt circuit which is configured in accordance with the illustrative embodiment of Figure 1. The thyristors are driven by a control unit 12, not shown. In this illustrative embodiment also only one commutating capacitor 41 is provided which is connected to the four winding sections 42, 43, 44 and 45 of the primary side 46a of the transformer 46. The remaining configuration of the arrangement according to Figure 2 corresponds to the configuration of Figure 1 so that it will not be discussed in greater detail.

In the illustrative embodiment of Figure 3, the converter 11 is also constructed as a full bridge circuit containing thyristors. In contrast to the illustrative embodiment of Figure 2, two transformers 47, 48 are provided each of which has two primary windings 49, 50 and 51, 52, respectively. The secondary windings, 53, 54 of the transformers 47, 48 are connected via rectifiers 55 to a direct-current rail which, in turn, is electrically connected to a workpiece and the torch 15, 1 6. The remaining configuration corresponds to the circuits of Figure 1 and 2 so that it will not be discussed in greater detail.

In the above-mentioned illustrative embodiments the output transformers 13,46,47, 48 are connected via rectifiers 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 (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, converters which are arranged in different circuits. Converters using switching elements which are not thyristors, for example using transistors as switching elements, can be used advantageously.

The above-mentioned arrangements for arc welding are not restricted, as shown in Figure 1 and 3, to arc welding using a non-fusible electrode but can also be used to weld with fusible electrodes according to the manual electrode welding or the gas metal-arc welding method (as indicated in Figure 2). The arrangement according to the present invention can be employed advantageously also for submerged-arc welding, plasma welding and cutting.

Claims (6)

1. An arrangement for direct-current and/or aiternating-current arc welding, comprising a converter which can be connected to alternatingor 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 at least one commutating capacitor and the converter output being connected, via a transformer and, if necessary, a rectifier or second converter, to a work-piece and to a welding electrode, characterised in that the primary winding (1 3a, 46a, 49-52) of the transformer (13, 46---48) is divided in accordance with the number of switching elements (22,23, 33-48) and one primary winding section each (31, 32 or 42 45 or 50-52) is connected to one switching element (22, 23 or 33 to 36) and the commutating capacitor (20,21 or 41).

2. An arrangement according to Claim 1, characterised in that the alternating output current of the converter (11) has a frequency which is higher than 1 8 Kilohertz.

3. An arrangement according to Claim 1 or 2, characterised in that the converter (11) is constructed as a thyristor converter in a bridge circuit (Figure 1, 2, 3).

4. An arrangement according to Claim 3, characterised in that the thyristor converter (11) is constructed as a full bridge converter and that two transformers (47, 48) are provided each of which has two primary windings (49, 50 and 51,52) and one secondary winding (53 and 54) (Fig. 3).

5. An arrangement according to Claim 4, characterised in that the secondary windings (53, 54) of the transformers (47, 48) are connected via rectifiers (55) t,o a direct-current rail (56).

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