WELDING TRANSFORMER, WELDING DEVICE AND WELDING PROCESS FOR WELDING AT LEAST ONE COMPONENT

Abstract

Welding transformer (30; 30A; 30B) for welding at least one component (5, 6), with a primary winding (31) for connection to a power supply (27), a first secondary winding (32) which is inductively coupled to the primary winding (31) for connection to a first welding electrode (11) of a welding tool (10), a second secondary winding (33) and a third secondary winding (34), each inductively coupled to the primary winding (31) and connected to the first secondary winding (32) and provided for connection to a second welding electrode (12) of the welding tool (10), in which at least one component (5, 6) is to be contacted with the welding electrodes (11, 12) for welding, a first rectifying branch (41) for rectifying an electric current (I2) flowing between the first secondary winding (32) and the first welding electrode (11), a second rectifying branch (42) for rectifying an electric current (I2) flowing between the second secondary winding (33) and the second welding electrode (12), and a current-limiting element (36, 37) which is connected to the third secondary winding (34) to limit an electric current through the third secondary winding (34) to a predetermined maximum value.

Description

[0001] The present invention relates to a welding transformer, a welding device and a welding method for welding, in particular resistance welding, at least one component.

[0002] Welding, particularly resistance welding, of various sheet metal combinations is used, for example, in automated manufacturing. In production lines for furniture, radiators, etc., metallic parts are joined by welding using a welding tool or welding fixture. In automated vehicle manufacturing, welding fixtures are used to create at least one welded joint on, for example, the body of a vehicle such as a car, truck, aircraft, etc.

[0003] Alternatively or additionally, a resistance welding device can be used in single-item production.

[0004] Such a welding device has a rectifier circuit to supply a direct current to the welding tool. The rectifier circuit can be designed as a diode rectifier. To increase the efficiency of the rectifier circuit, four thyristors can alternatively be used. In this case, two antiparallel thyristors are used instead of each diode of a diode rectifier. However, the rectifier circuit with four thyristors requires twice the installation space compared to a single diode rectifier.

[0005] The patent application DE 102 19 444 A1 discloses a method for operating a welding power source which enables rapid load changes even in the lower load range without the risk of an asymmetrically magnetized output transformer and does not cause excessive strain on the supply network.

[0006] The patent application JP H03-189 079 A discloses a welding device in which, in order to provide a high welding current, inverters are connected in parallel, synchronized by PWM control and fed to a welding transformer with multiple windings.

[0007] The patent application US 2014 / 0328088A1 discloses a power supply circuit with a plurality of output voltage terminals, comprising a transformer which has a plurality of output windings, wherein the first and second output windings have the same number of windings and are coupled together.

[0008] The applicant's patent application DE 10 2018 209 983 A1 discloses a resistance welding device, wherein a series circuit of two transistors is connected between a welding device and an output of the welding transformer, and a control device for switching said series circuit of two transistors.

[0009] Direct current used in welding can cause magnetization effects in the welded parts or components. This can complicate further processing of the welded metallic components. The potential magnetization of system components can also lead to contamination and malfunctions in the welding system.

[0010] Welding aluminum requires breaking through the aluminum oxide layers that form on the surface of an aluminum component. This necessitates a sufficiently high open-circuit voltage at the output of the welding transformer, ensuring adequate current flow for welding even with the small electrical currents and high contact resistance present in the aluminum oxide layer. Furthermore, adhesive at the weld point can lead to contact problems during welding. In addition, the ever-shorter cycle times of automated manufacturing must be met.

[0011] Another problem arises when welding sheet metal combinations of different thicknesses, for example, thick / thin. It is important to note that the welding gun must always be held against the sheet metal combination in such a way that the electrode, which heats up more, is applied to the thicker sheet. The electrode that heats up more corresponds to the positive terminal. Therefore, positioning the welding gun on components for sequential welds is often time-consuming.

[0012] To meet the required cycle times, two robots are often used, each operating a welding gun. This increases the space required and the setup and operating costs of the resistance welding equipment.

[0013] Furthermore, when welding aluminum, undesirable burn-off at the welding electrodes or material migration can occur, depending on the welding current direction. Reasons for this include different alloys used for the welded sheets and / or different sheet thickness combinations and / or the Peltier effect.

[0014] Therefore, the object of the present invention is to provide a welding transformer, a welding device, and a welding method for welding at least one component, with which the aforementioned problems can be solved. In particular, a welding transformer, a welding device, and a welding method for welding at least one component are to be provided in which, during a welding process involving small electrical currents and high contact resistances, a sufficient current flow for welding is ensured with a small installation space and high efficiency for the welding transformer, and cycle times for welding in automated production can be kept short while maintaining high quality and low production costs.

[0015] This problem is solved by a welding transformer for a welding device for welding at least one component according to claim 1. The welding transformer has a primary winding for connection to a power supply, a first secondary winding inductively coupled to the primary winding for connection to a first welding electrode of a welding tool, a second secondary winding and a third secondary winding, each inductively coupled to the primary winding and connected to the first secondary winding and provided for connection to a second welding electrode of the welding tool, in which the at least one component is to be contacted with the welding electrodes for welding, and a first rectification branch for rectifying an electric current flowing between the first secondary winding and the first welding electrode.a second rectifying branch for rectifying an electric current flowing between the second secondary winding and the second welding electrode, and a current-limiting element connected to the third secondary winding to limit an electric current through the third secondary winding to a predetermined maximum value.

[0016] The additional auxiliary winding or inductance, which can be implemented as an air-core coil, is particularly advantageous in welding processes involving low electrical currents and high contact resistances. Such conditions exist especially during welding, particularly resistance welding, of at least one component with a surface coated with aluminum oxide and / or adhesive. Despite the low electrical currents and high contact resistances and / or contact difficulties of such a component, the described welding transformer can ensure a sufficient current flow through at least one welding electrode.

[0017] Due to its design, the welding transformer, in combination with the rectifier, is capable of very fast commutation. This results in higher power output on the secondary side of the welding transformer compared to conventional welding transformers combined with a diode rectifier.

[0018] The design of the claimed welding transformer allows for a lower mains power requirement compared to a conventional welding transformer combined with a diode rectifier. This lower mains power requirement encompasses active power, reactive power, and phase current. Consequently, the welding transformer also achieves CO2 savings during operation.

[0019] Furthermore, the welding transformer only requires a smaller inverter and smaller power supply components, such as main switches, etc. This results in cost advantages in the manufacture and operation of the welding transformer.

[0020] Furthermore, the cooling requirements for the welding transformer during operation are reduced. This also results in cost advantages in the manufacture and operation of the welding transformer.

[0021] Overall, the welding transformer makes a significant contribution to keeping welding cycle times short in automated production, ensuring high quality and low manufacturing costs. The welding transformer enables faster power disconnection after the welding process compared to conventional welding equipment. This significantly reduces the time required to create a weld. Consequently, the specified cycle times can be met even if only a single welding robot is used. This results in substantial advantages in terms of space requirements and the setup and operating costs of the welding equipment.

[0022] The welding transformer claimed in the claims has lower losses and a smaller installation space, yet higher performance than the previously described thyristor solution. Furthermore, the welding transformer claimed in the claims has similar performance and a similar installation space, but lower losses than a prior art MF-DC transformer with a diode rectifier. These properties are a significant advantage with regard to the efficient use of resources. Moreover, the improved operating characteristics result in lower costs for the operator of the resistance welding equipment.

[0023] Therefore, the welding transformer used in the requirements, which is suitable for resistance welding, offers a very advantageous solution due to the often prevailing limited space conditions and for cost reasons.

[0024] Advantageous further embodiments of the welding transformer are specified in the dependent claims.

[0025] The first and second rectifier branches may each have a series connection of two transistors connected between the welding tool and an output of the welding transformer, with the polarity of one transistor in the series connection reversed relative to the polarity of the other transistor in the series connection.

[0026] The two transistors can be metal oxide semiconductor field-effect transistors, with the transistor with the reversed polarity connected to the welding tool.

[0027] Additionally, a third rectification branch may be provided for rectifying an electric current flowing between the first secondary winding and the first welding electrode. In one embodiment, the third rectification branch has a series connection of two transistors connected between the welding tool and an output of the welding transformer, wherein the polarity of one transistor in the series connection is reversed relative to the polarity of the other transistor in the series connection.

[0028] In one embodiment, the current-limiting element is a controllable inductance and / or a resistor that is connected between the second secondary winding and the second welding electrode.

[0029] In another embodiment, the current-limiting element is a semiconductor switch connected between the second secondary winding and the second welding electrode. The semiconductor switch can be a bipolar transistor or a metal oxide field-effect transistor. Alternatively, the semiconductor switch can be a thyristor, optionally with an antiparallel thyristor connected to it.

[0030] In yet another embodiment, the current-limiting element is a series circuit consisting of a first tunnel diode, to which a second tunnel diode is connected antiparallel, and a semiconductor switch, wherein the series circuit is connected between the second secondary winding and the second welding electrode, and wherein the semiconductor switch is a bipolar transistor or a metal oxide field-effect transistor or a thyristor.

[0031] At least one welding transformer as previously described can be part of a welding device for welding at least one component. The welding device also has a welding tool designed as a welding clamp with two welding electrodes, between which the at least one component is positioned during welding. The at least one welding transformer can serve to supply an electric welding current to the welding tool during the welding of the at least one component.

[0032] The welding device can also include a control unit for switching the series connection of two transistors in the rectifier branch during a converter-controlled welding time in a first operating mode, and for switching the series connection of two transistors in the rectifier branch at the end of the converter-controlled welding time for a predetermined duration in a second operating mode that differs from the first operating mode. In this second mode, the first operating mode applies a polarity-reversible welding voltage and a polarity-reversible welding current to the welding transformer, while the second operating mode accelerates the decay of the welding current generated in the first operating mode.

[0033] The welding device described above can be part of a system designed for processing objects. In this context, the resistance welding device can be configured for resistance welding at least one component for at least one of the objects. Additionally or alternatively, the system can be configured for manufacturing vehicle bodies, radiators, or chains as objects. Additionally or alternatively, the system can be configured for manufacturing objects from at least one aluminum component, which may have adhesive present.

[0034] The problem is further solved by a resistance welding process for resistance welding at least one component according to claim 15. In this process, a welding tool is used which is electrically connected to a welding transformer, which has a primary winding for connection to a power supply, a first secondary winding which is inductively coupled to the primary winding for connection to a first welding electrode of the welding tool, a second secondary winding and a third secondary winding, each of which is inductively coupled to the primary winding and connected to the first secondary winding and is provided for connection to a second welding electrode of the welding tool.The resistance welding process comprises the following steps: contacting the at least one component with the first and second welding electrodes of the welding tool; rectifying, with a first rectifying branch, an electric current flowing between the first secondary winding and the first welding electrode; rectifying, with a second rectifying branch, an electric current flowing between the second secondary winding and the second welding electrode; and limiting, with a current-limiting element connected to the third secondary winding, an electric current through the third secondary winding to a predetermined maximum value.

[0035] The welding process achieves the same advantages as previously mentioned in relation to the welding transformer and the welding device.

[0036] Other possible implementations of the invention also include combinations of features or embodiments described previously or subsequently with regard to the exemplary embodiments, even if not explicitly mentioned. In such cases, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the invention.

[0037] The invention is described in more detail below with reference to the accompanying drawing and by means of exemplary embodiments. The drawing shows: Fig. 1 a block diagram of a system with a welding device according to a first embodiment; Fig. 2 a time-course diagram of a welding current generated during welding with the welding device according to the first embodiment and which decays after the welding process, compared to a welding current profile in a conventional operating mode of the welding device; Fig. 3 a block diagram of a welding device according to a second embodiment; and Fig. 4 a block diagram of a welding device according to a third embodiment.

[0038] In the figures, identical or functionally equivalent elements are provided with the same reference symbols unless otherwise specified.

[0039] Fig. Figure 1 schematically shows a system 1 with a welding device 2, which is in particular a resistance welding device 2. The system 1 can, for example, be a production plant for items 4, such as vehicles, furniture, radiators, etc.

[0040] In the production plant 1, metallic components 5, 6 can be joined by welding, in particular resistance welding, such that a welded joint 7 is produced. For this purpose, the welding device 2 has at least one welding tool 10 in the form of a welding gun, a control unit 20, a converter 25, a welding transformer 30, a current-limiting element designed as a controlled inductor 36, a rectifier circuit 40, and a device 50 for guiding a welding tool 10. The converter 25, and thus the welding transformer 30, is supplied with electrical energy from a power supply network 27.

[0041] The welding tool 10 has, in the example of Fig. 1 two welding electrodes 11, 12.

[0042] The welding transformer 30 has a primary winding 31 on its primary side. On its secondary side, the welding transformer 30 has a first secondary winding 32, a second secondary winding 33, and an auxiliary winding 34, which is a third secondary winding. The welding transformer 30 has four outputs, between which the windings 31 to 34 are arranged. The transformer 30 with the downstream rectifier circuit 40 is a medium-frequency DC transformer, which can also be referred to as an MF-DC transformer. The welding transformer 30 also has the auxiliary winding 34 as a third secondary winding, as already mentioned.

[0043] The rectifier circuit 40 has a first rectifier branch 41, a second rectifier branch 42, and a third rectifier branch 43. The first rectifier branch 41 is configured as a series connection of a first transistor and a second transistor. The second rectifier branch 42 is also configured as a series connection of a first transistor and a second transistor. The transistors of branches 41 and 42 are, in particular, metal-oxide-semiconductor field-effect transistors, which can also be abbreviated as MOSFETs. The third rectifier branch 43 has, for example, at least one transistor. In particular, the third rectifier branch 43 has a series connection of transistors, especially MOSFETs, which can be operated in synchronous mode by the control device 20.

[0044] The welding device 2 can create a weld joint 7 with the welding tool 10 under the control of the control unit 20. For this purpose, at least one of the components 5, 6 is arranged between the two welding electrodes 11, 12, as described above and as, for example, in Fig. Figure 1 shows that, in particular, components 5 and 6 may have special combinations, especially sheet metal combinations. Such a special combination is, in particular, a combination of sheets with different material thicknesses, a combination of special shapes of the components, a combination with an aluminum component and / or aluminum sheet, etc. For welding aluminum, the resistance welding device 2 is operated by means of the control unit 20 such that the respective aluminum oxide layer(s) on the relevant surface of the aluminum component is penetrated.

[0045] Furthermore, a weld joint 7 can also be produced with the welding device 2 under the control of the control unit 20 if adhesive is present at the point where the weld joint 7 is to be made. This will be described in more detail below.

[0046] It is possible, for example, that two edges of a single component 5 are joined together by resistance welding with one or more welds 7. Regardless of how many components 5, 6 are joined together with a weld 7, the weld(s) 7 can be a spot weld, a seam weld, or a combination thereof.

[0047] In normal welding operation, the welding transformer 30 converts a primary voltage U1 into the first to third secondary voltages U 21 , U 22 , U 23 Here, the sum of the secondary voltages U is 21 , U 22 , U 23smaller than the value of the primary voltage U1. Furthermore, the welding transformer 30 converts a primary current I1 on the primary side of the welding transformer 30 into the secondary current I2 on the secondary side of the welding transformer 30. The secondary current I2, which can also be referred to as the welding current, has a higher value than the primary current I1.

[0048] Thus, for welding, a first secondary voltage U is applied to the secondary side of the welding transformer 30 at the welding device 2. 21 at the first secondary winding 32 of the welding transformer 30. The secondary voltage U 21 is present between the first and second outputs of the welding transformer 30. The second secondary voltage U is also present. 22 between the second and third outputs of the welding transformer 30. In addition, the third secondary voltage U is present. 23between the second and fourth outputs of the welding transformer 30 and results in an electric current I3 through the winding 34. The first secondary voltage U 21 , the second secondary voltage U 22 and the third secondary voltage U 22 form a welding voltage U 21 , U 22 , U 23 , which results in the welding current I2.

[0049] The first rectifier branch 41 is connected to the first secondary winding 32 of the welding transformer 30. In other words, the first rectifier branch 41 is connected to the first output of the welding transformer 30. The first rectifier branch 41 is connected between the welding transformer 30 and the welding tool 10. More precisely, the first rectifier branch 41 is connected between the welding transformer 30 and the first welding electrode 11.

[0050] The second welding electrode 12 is directly connected to the tap between the first and second auxiliary windings 32, 33. The tap between the first and second auxiliary windings 32, 33 is the second output of the welding transformer 30.

[0051] The second rectifier branch 42 is connected between the welding transformer 30 and the first welding electrode 11. The third output of the welding transformer 30 is connected to one side of the controlled inductor 36, which is connected on the other side to the second rectifier branch 42. In other words, the second rectifier branch 42 is connected to the third output of the welding transformer 30. The second rectifier branch 42 is connected between the welding transformer 30 and the welding tool 10. More precisely, the second rectifier branch 42 is connected between the welding transformer 30 and the first welding electrode 11.

[0052] The auxiliary winding 34 is connected at one end to the second output of the welding transformer 30, or to the tap between the first and second secondary windings 32, 33. The other end of the auxiliary winding 34 is connected to the controlled inductor 36. Furthermore, the controlled inductor 36 is connected to the third rectifier branch 43. In other words, the third rectifier branch 43 is connected to the third output of the welding transformer 30. The third rectifier branch 43 is connected between the welding transformer 30 and the welding tool 10. More precisely, the third rectifier branch 43 is connected between the welding transformer 30 and the first welding electrode 11.

[0053] In normal welding operation, as previously described, the control unit 20 controls the transformer 30 such that the transformer 30 does not saturate when the auxiliary winding 34 is operated in one direction. The control unit 20 controls the controllable inductance 36 such that, at a predetermined current in the primary winding 31 of the transformer 30, the electric current I3 in the auxiliary winding 34 is limited. The current I3 flows through the rectifier branch 43 to the first welding electrode 11. The third rectifier branch 43 has a corresponding series connection of transistors, for example, field-effect transistors, in particular MOSFETs, to rectify the current from the auxiliary winding 34. The series connection of transistors, for example, field-effect transistors, in particular MOSFETs, can be operated in synchronous mode, as described below for the first and second rectifier branches 41, 42.

[0054] Thus, when controlled by the control unit 20, the controllable inductance 36 limits the current of the auxiliary winding 34 and its components. Such components are present in the device 2 of Fig. 1. The controllable inductor 36 and the third rectifier branch 43. The current limiting ensures that the auxiliary winding 34 and its components are not damaged by an excessively high current I3. Alternatively, the third rectifier branch 43 is implemented as a diode.

[0055] The auxiliary winding 34 on the secondary side of the transformer 30 allows the output voltage of the transformer 30 to be increased under no-load conditions and at low welding currents I2. This ensures that, in the event of contact problems at the weld point, for example due to adhesive at the weld, the welding current I2 can be driven more effectively at the beginning of the welding process.

[0056] The control described above by the control unit 20 corresponds to a transformer variant with rectification operation or synchronous switching of the transistors in the rectification branches 41, 42, 43.

[0057] Alternatively, the control unit 20 can have two operating modes B1, B2 or B1, B 20 for controlling the rectifier branches 41, 42. The effect of operating modes B1, B2 as well as operating modes B1, B 20 the welding current I2 is in Fig. Figure 2 illustrates this. In addition, the control unit 20 controls the third rectifier branch 43, as described previously.

[0058] For a first operating mode B1, in which welding or the creation of a welded joint 7 is performed, energy is actively transferred from the primary side of the welding transformer 30 to the secondary side of the welding transformer 30 to generate the welding direct current, in other words, the welding current I2. For such energy transfer from the primary side to the secondary side of the welding transformer 30, the control unit 20 is designed to switch the polarity of the transistors of the rectifier branches 41, 42 as desired. For this purpose, the control unit 20 is designed to switch on one transistor of the rectifier branches 41, 42 depending on the output voltage of the welding transformer 30 and the polarity selection. In synchronous operation, the series-connected transistor of the rectifier branches 41, 42 is then switched on in a negative conducting state when current is applied.

[0059] For example, the control unit 20 switches on the first transistor of the rectifier branch 41 depending on the output voltage and the polarity selection. The second transistor 42, connected in series, is then switched on in a negative conducting state during synchronous operation when the welding current I2 is generated.

[0060] This means that in each rectifier branch 41, 42 of the rectifier circuit 40, the two transistors are then switched on in series.

[0061] In this way, a polarity-switchable welding voltage U can be applied to the welding transformer 30. 21 , U 22 and a polarity-switchable welding current I2 can be implemented.

[0062] This allows for reversible polarity of the welding transformer 30 and current direction of the welding current I2. Consequently, the previously described magnetization effects are avoided. Furthermore, in the case of a component 5, 6 with an aluminum layer, the aluminum oxide layer can be reliably broken down and / or the problem caused by the presence of adhesive at the weld point is resolved.

[0063] After a time t0, which corresponds to the end of the welding time T1 during which a welding process is performed, the control unit 20 switches to a second operating mode B2. In the second operating mode B2, the control unit 20 performs an active change of the rectification operating mode on the secondary side, as described below.

[0064] For the second operating mode B2, the control unit 20 controls the transistors of the rectifier branches 41, 42 to achieve a rapid decay of the welding current I2, as with the aid of Fig. 2 illustrated.

[0065] In Fig. Figure 2 illustrates the welding current I2 over time t with a dashed curve, as it occurs during standard rectification with diodes (not shown). In such standard rectification, the energy from the secondary circuit of the welding transformer 30 is dissipated after the end of the welding time T1 via ohmic losses in the diodes. The welding current I2 has decayed by time t1.

[0066] In contrast, in the present embodiment, in the second operating mode B2, the current drop or decay of the welding current I2 is accelerated at the end of the normal inverter-controlled welding time T1 for the control of the transistors of the rectifier branches 41, 42. For this purpose, the relevant transistors in the rectifier branches 41, 42, whose pn junctions each correspond to two diodes connected in series, are taken out of synchronous operation, and the decaying welding current I2 is instead routed through parallel diodes of the series-connected transistors of the rectifier branches 41, 42.

[0067] As a result, the welding current I2 has already decayed at time t2 or after a time period T2, and not, as before, only after time t1, as in Fig. 2 illustrated.

[0068] Thus, the control unit 20 switches the respective series circuit of two transistors of the rectifier branches 41, 42 at the end of the inverter-controlled welding time T1 for a predetermined duration T2 = t2 - t0 in the second operating mode B2. The first and second operating modes B1, B2 therefore differ from each other.

[0069] Consequently, in the second operating mode B2, the forced switching of rectifier elements 41, 42 generates additional energy through the secondary-side current switching in the rectifier branches, resulting in a remagnetization of the welding transformer 30 on the secondary side. This causes magnetization losses on the secondary side of the welding transformer 30, which reduce the energy of the secondary circuit and thus lead to a faster decay of the welding current I2.

[0070] The control device 20 is therefore designed for an active change of the rectification operating mode on the secondary side, which change results in magnetization losses on the secondary side of the welding transformer 30.

[0071] It is optional, but not absolutely necessary due to the current limiting of current I3 described above, for the control unit 20 to also switch the transistors of the rectifier branch 43 in the second operating mode as described above for the transistors of the rectifier branches 41, 42.

[0072] The previously described design of the transformer 30, in combination with the rectifier branches 41, 42 with the transistors and their control by the control unit 20, also has the effect that the welding transformer 30 can have a lower turns ratio than a conventional welding transformer operated with diodes in the rectifier branches 41, 42, for the same output power. In other words, if the previously described welding transformer 30 has the same turns ratio as a conventional welding transformer of the same output power, the previously described welding transformer 30 requires less input power than the conventional welding transformer 30. Therefore, the welding transformer 30 with the downstream rectifier circuit 40 consumes less power supplied by the inverter 25 than a conventional welding transformer of the same output power.

[0073] The reason for this is that the diodes in the rectifier branches 41, 42 of a conventional welding transformer are always running, even between welding operations. As a result, the diodes also experience power loss during free-running, which ultimately leads to high internal losses in the conventional welding transformer.

[0074] For example, a welding transformer 30 with the previously described configuration, including the auxiliary winding 34, the controlled inductor 36, and the combined rectifier circuit 40, can have a turns ratio of approximately 60:1. Such a welding transformer 30 in synchronous operation has approximately the same rated current of 6.5 kA as a conventional welding transformer according to the industry standard DIN EN ISO 22829, which has a turns ratio of 55:1. Nevertheless, such a welding transformer 30 with a turns ratio of 60:1 can drive a welding current I2 = 25 kA at a secondary resistance of 200 µΩ, as required by the industry standard DIN EN ISO 22829 for the welding transformer with a turns ratio of 55:1.

[0075] In comparison to the state of the art, the welding transformer 30 thus has Fig. 1. a smaller size, lower weight and a cost advantage with regard to the inverter 25 and the main switch for switching off the inverter 25 or feed components of the welding device 2.

[0076] The welding device 2 can be used particularly advantageously for sheet metal combinations where undesirable burn-off of the welding electrodes 11, 12 or material migration occurs when using a welding gun, and / or when welding aluminum, and / or where adhesive is present at the intended welding position, and / or where fast cycle times are required when using the device 2. Furthermore, the formation of electrode caps on the welding electrodes 11, 12 can be avoided. Additionally or alternatively, the welding device 2 can be used particularly advantageously for welding chain links and for welding radiators.

[0077] According to a modification of the preceding control of the control unit 20 during welding, the control unit 20 can proceed as follows. In this case, the control unit 20 switches the welding current I2 in the second operating mode B2 alternately back and forth via the transistors of the rectifier branches 41, 42 into the two secondary branches of the transformer 30 in order to achieve an even faster decay of the welding current I2 or to generate a faster current drop of the welding current I2.

[0078] In the second operating mode B2, the changeover can occur more quickly due to the higher primary voltage of transformer 30. This allows for the generation of greater magnetization losses. Consequently, the decay of the welding current I2 occurs even earlier, between time t0 and time t2. Thus, the welding current I2 decays even faster than in the first embodiment.

[0079] The advantages mentioned previously in relation to the first embodiment can also be achieved in this way.

[0080] Fig. Figure 3 shows a welding device 2A with a welding transformer 30A and a rectifier circuit 40A according to a second embodiment.

[0081] In contrast to the welding transformer 30 according to the preceding embodiment, the welding transformer 30A according to the present embodiment is designed for a transformer variant with pole switching.

[0082] The 30A welding transformer has an electrical resistor as a current-limiting element 37. The resistor is optionally formed, at least partially, in or through the auxiliary winding 34.

[0083] The third rectifier branch 43 has a series connection of transistors, in particular MOSFETs, which are controlled by polarity-dependent control of the control device 20. In addition, the rectifier branch 43 can be operated in synchronous mode, as described with reference to the preceding embodiment.

[0084] According to a modification of the second embodiment, the current-limiting element 37 of the welding transformer 30A is a semiconductor switch that switches off the auxiliary winding 34 from a predetermined current value I3. The semiconductor switch can be a diode. The diode is combined with the rectifier branch 43, which consists of a series connection of transistors, which, as described above, are in particular MOSFETs controlled by polarity-dependent control of the control device 20. Furthermore, the rectifier branch 43 can be operated in synchronous mode.

[0085] The in Fig. The circuit of the welding device 2A shown in Figure 3 can be switched in all variants for the current-limiting element 37 by the control device 20 in a manner as described with reference to one of the preceding embodiments.

[0086] The energy savings compared to a conventional welding transformer operated with diodes in the rectification branches 41, 42 are higher in the transformer variant with the two MOSFETs for rectification (synchronous circuit) in one rectification branch 41, 42 than in the transformer variant with pole switching.

[0087] The welding device 2A can be used in place of the welding device 2 according to the preceding embodiment in the system 1 according to the preceding embodiment.

[0088] Fig. Figure 4 shows a welding device 2B with a welding transformer 30B and a rectifier circuit 40B according to a third embodiment.

[0089] In contrast to the welding transformers 30, 30A according to the preceding embodiments, the welding transformer 30B according to the present embodiment is designed for a transformer variant with pole switching.

[0090] The welding transformer 30B has two antiparallel tunnel diodes connected as a current-limiting element 38, to which a semiconductor switch is connected in series. The semiconductor switch is, for example, a MOSFET, a bipolar transistor, or a thyristor. The tunnel diode limits the current I3 in the auxiliary winding 34 to the predetermined current value.

[0091] In this case, no additional electrical component is required in the rectifier circuit 40 to rectify the current I3.

[0092] The in Fig. The circuit of the welding device 2B shown in Figure 4 can be switched by the control unit 20 in a manner as described in relation to one of the preceding embodiments.

[0093] Welding device 2B can be used in place of welding device 2 according to one of the preceding embodiments in Annex 1 according to the preceding embodiments.

[0094] All previously described configurations of the system 1, the welding devices 2, 2A, 2B, the control unit 20, the welding transformer 30, 30A, 30B, the rectifier circuit 40, and the resistance welding process can be used individually or in any possible combination. In particular, it is possible to combine all features and / or functions of the previously described embodiments as desired. The following modifications are also conceivable.

[0095] The parts shown in the figures are schematic and may differ in their exact design from the forms shown in the figures, as long as their previously described functions are guaranteed.

[0096] The transistors of the rectifier branches 41, 42, 43 are alternatively bipolar transistors, but the design as metal-oxide-semiconductor field-effect transistors (MOS-FET) is preferred.

[0097] The welding transformer 30, 30A, 30B can alternatively be constructed from a parallel connection of two transformers.

[0098] The control unit 20 may be able to control more than one welding tool 10 at least temporarily and / or partially simultaneously with at least one other welding tool 10.

Claims

[1] Welding transformer (30; 30A; 30B) for welding at least one component (5, 6), with a primary winding (31) for connection to a power supply (27), a first secondary winding (32) which is inductively coupled to the primary winding (31) for connection to a first welding electrode (11) of a welding tool (10), a second secondary winding (33) and a third secondary winding (34), each inductively coupled to the primary winding (31) and connected to the first secondary winding (32) and provided for connection to a second welding electrode (12) of the welding tool (10), in which at least one component (5, 6) is to be contacted with the welding electrodes (11, 12) for welding, a first rectifying branch (41) for rectifying an electric current (I2) flowing between the first secondary winding (32) and the first welding electrode (11), a second rectifying branch (42) for rectifying an electric current (I2) flowing between the second secondary winding (33) and the second welding electrode (12), and a current-limiting element (36, 37) which is connected to the third secondary winding (34) to limit an electric current through the third secondary winding (34) to a predetermined maximum value. [2] Welding transformer (30; 30A; 30B) according to claim 1, wherein the first and second rectifying branch (41, 42) each have a series connection of two transistors which are connected between the welding tool (10) and an output of the welding transformer (30; 30A; 30B), wherein the polarity of one transistor of the series connection is reversed relative to the polarity of the other transistor of the series connection. [3] Welding transformer (30; 30A; 30B) according to claim 2, where the two transistors are metal-oxide-semiconductor field-effect transistors, and wherein the transistor with the reversed polarity is connected to the welding tool (10). [4] Welding transformer (30; 30A; 30B) according to one of claims 1 to 3, further comprising a third rectifying branch (43) for rectifying an electric current (I3) flowing between the first secondary winding (32, 33, 34) and the first welding electrode (11, 12). [5] Welding transformer (30; 30A) according to claim 4, wherein the third rectification branch (43) has a series connection of two transistors which are connected between the welding tool (10) and an output of the welding transformer (30; 30A), and wherein the polarity of one transistor (42; 44) of the series circuit is reversed relative to the polarity of the other transistor (41; 43) of the series circuit. [6] Welding transformer (30) according to any one of claims 1 to 5, wherein the current limiting element (36) is a controllable inductance and / or a resistor connected between the second secondary winding (33) and the second welding electrode (12). [7] Welding transformer (30A) according to one of claims 1 to 5, wherein the current limiting element (38) is a semiconductor switch connected between the second secondary winding (33) and the second welding electrode (12). [8] Welding transformer (30A) according to claim 7, wherein the semiconductor switch is a bipolar transistor or a metal oxide field-effect transistor. [9] Welding transformer (30A) according to claim 7, wherein the semiconductor switch is a thyristor to which an antiparallel thyristor is optionally connected. [10] Welding transformer (30B) according to one of claims 1 to 3, wherein the current-limiting element (38) is a series circuit consisting of a first tunnel diode, to which a second tunnel diode is connected antiparallel, and a semiconductor switch, wherein the series connection is connected between the second secondary winding (33) and the second welding electrode (12), and where the semiconductor switch is a bipolar transistor, a metal oxide field-effect transistor, or a thyristor. [11] Welding device (2; 2A, 2B) for welding at least one component (5, 6), with a welding tool (10) designed as a welding tong with two welding electrodes (11, 12) between which the at least one component (5, 6) is arranged during welding, at least one welding transformer (30; 30A; 30B) according to one of the preceding claims and for supplying an electric welding current (I2) to the welding tool (10) when welding the at least one component (5, 6), [12] Welding device (2; 2A, 2B) according to claim 11 depending on claim 2 or 3, further comprising a control device (20) for switching the series connection of two transistors in the rectifier branch (41; 42) during an inverter-controlled welding time (T1) in a first operating mode (B1) and for switching the series connection of two transistors in the rectifier branch (41; 42) at the end of the inverter-controlled welding time (T1) for a predetermined time period (T2) in a second operating mode (B2) which differs from the first operating mode (B1). [13] Welding device (2; 2A; 2B) according to claim 12, wherein the first operating mode (B1) applies a polarity-switchable welding voltage (U) to the welding transformer (30; 30A, 30B). 21 , U 22 , U 23) and a polarity-switchable welding current (I2) is realized, wherein the second operating mode (B2) accelerates a current decay of the welding current (I2) generated in the first operating mode (B1). [14] Plant (1) for the treatment of objects (4), comprising a welding device (2; 2A; 2B) according to any one of claims 11 to 13, wherein the welding device (2; 2A; 2B) is provided for resistance welding of at least one component (5, 6) for at least one of the objects (4), wherein the system (1) is optionally designed for the manufacture of vehicle body shells or radiators or chains as items (4), and / or wherein the system (1) is optionally designed for the manufacture of articles (4) from at least one component (5, 6) made of aluminium, on which adhesive may be present. [15] Welding method for welding at least one component (5, 6) with a welding tool (10) electrically connected to a welding transformer (30) having a primary winding (31) for connection to a power supply (27), a first secondary winding (32) inductively coupled to the primary winding (31) for connection to a first welding electrode (11) of the welding tool (10), a second secondary winding (33) and a third secondary winding (34) each inductively coupled to the primary winding (31) and connected to the first secondary winding (32) and provided for connection to a second welding electrode (12) of the welding tool (10), and wherein the welding method comprises the steps Contacting at least one component (5, 6) with the first and second welding electrodes (11, 12) of the welding tool (10), Rectification, with a first rectifying branch (41), of an electric current (I2) flowing between the first secondary winding (32) and the first welding electrode (11), Rectifying, with a second rectifying branch (42), an electric current (I2) flowing between the second secondary winding (33) and the second welding electrode (12), and limiting, with a current-limiting element (36, 37) connected to the third secondary winding (34), an electric current through the third secondary winding (34) to a predetermined maximum value.

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