Method for contactlessly striking an arc and welding current source for carrying out a striking process

Abstract

The invention relates to a method for contactless ignition an arc (L) between an electrode (3) and a workpiece (4) which is to be welded, for carrying out a welding process, wherein a welding current (I) and a welding voltage (U) are provided at an output (2) of a welding current source (1), wherein the welding current source (1) contains a resonance converter (5) for generating a periodically varying, preferably substantially sawtooth-shaped, open circuit welding voltage (U.sub.LL) with voltage maxima (U.sub.LL,max) which recur periodically with a repetition rate (f.sub.w) and a welding current source (1) for carrying out the igniting process. In order to achieve reliable contactless ignition of the arc (L) without complicated circuitry, the resonance converter (5) is formed by a series-parallel resonant converter, and temporally synchronous high-frequency pulses (U.sub.I,HF) are superimposed on the open circuit welding voltage (U.sub.LL) in the region of at least some of the periodically recurring voltage maxima (U.sub.LL,max) of the open circuit welding voltage (U.sub.LL).

Claims

1. A method for contactless ignition of an arc between an electrode and a workpiece which is to be welded, for carrying out a welding process, the method comprising: providing a welding current and a welding voltage at an output of a welding current source; generating using a resonant converter in the welding current source a periodically varying, substantially sawtooth-shaped, open circuit welding voltage with voltage maxima which recur periodically with a repetition rate, wherein the resonant converter comprises a series-parallel resonant converter; and superimposing temporally synchronous high-frequency pulses on the open circuit welding voltage in a region of at least some of the periodically recurring voltage maxima of the open circuit welding voltage.

2. The method according to claim 1, wherein temporally synchronous high-frequency pulses are superimposed on the open circuit welding voltage in a region of every nth voltage maximum of the open circuit welding voltage, where n is a positive whole number greater than or equal to 1.

3. The method according to claim 1, wherein the welding voltage is measured and the exceeding of a defined voltage value is detected, and the high-frequency pulses are superimposed time-synchronously on at least some detected defined voltage values.

4. The method according to claim 1, wherein the high-frequency pulses are superimposed by a specified period of time before or after the occurrence of the voltage maximum.

5. The method according to claim 1, wherein an open circuit welding voltage is made available with a repetition rate between 10 Hz and 100 Hz.

6. The method according to claim 1, wherein high-frequency pulses are superimposed with a frequency between 100 kHz and 10 MHz.

7. A welding current source for providing a welding current and a welding voltage at an output for carrying out a welding process with an arc between an electrode and a workpiece to be welded, comprising: a resonant converter for generating a periodically varying, substantially saw-tooth-shaped, open circuit welding voltage with voltage maxima, which recur periodically with a repetition rate, wherein the resonant converter comprises a series-parallel resonant converter; and a circuit for generating high-frequency pulses, wherein the circuit, in order to ignite the arc contactlessly, is designed to time-synchronously superpose the high-frequency pulses onto the open circuit welding voltage in a region of at least some periodically recurring voltage maxima of the open circuit welding voltage.

8. The welding current source according to claim 7, wherein the circuit for generating the high-frequency pulses is designed to superimpose the high-frequency pulses of the open circuit welding voltage time-synchronously in a region of every nth voltage maximum of the open circuit welding voltage, where n is a positive whole number greater than or equal to 1.

9. The welding current source according to claim 7, wherein further comprising a measuring device comprising a voltage detector for detecting the welding voltage; and a control device comprising a controller or control circuit; said measuring device being connected via the control device to the circuit for generating the high-frequency pulses so that the high-frequency pulses can be time-synchronously superimposed with detected defined voltage values.

10. The welding current source according to claim 7, wherein the circuit for generating the high-frequency pulses is designed to superimpose the high-frequency pulses temporally offset by a specified period of time before or after the occurrence of the voltage maximum.

11. The welding current source according to claim 7, wherein the resonant converter is designed for generating the periodically varying open circuit voltage with a repetition rate of between 10 Hz and 100 Hz, in particular 33 Hz.

12. The welding current source according to claim 7, wherein the circuit is designed for generating high-frequency pulses between 100 kHz and 10 MHz.

13. The welding current source according to claim 7, wherein the electrode comprises a non-melting electrode.

14. The method according to claim 1, wherein the high-frequency pulses are superimposed by up to 5 ms before or after the occurrence of the voltage maximum.

15. The welding current source according to claim 7, wherein the circuit for generating the high-frequency pulses is designed to superimpose the high-frequency pulses temporally offset by up to 5 ms before or after the occurrence of the voltage maximum.

16. The method according to claim 5, wherein the repetition rate is 33 Hz.

17. The welding current source according to claim 7, wherein the repetition rate is 33 Hz.

18. The welding current source according to claim 7, wherein the electrode comprises a tungsten electrode.

Description

(1) The present invention will be explained in further detail by reference to the attached drawings. Shown are:

(2) FIG. 1 is a block circuit diagram of a welding current source of the present invention;

(3) FIG. 2 is a block circuit diagram of a further design variant of a welding current source according to the present invention;

(4) FIG. 3 shows the temporal waveform of the open circuit welding voltage of a welding current source with a resonance converter;

(5) FIG. 4 shows the temporal waveform of the open circuit welding voltage on the application of the igniting process according to the invention;

(6) FIG. 5 shows the temporal waveform of the open circuit welding voltage in a variant of the igniting process according to the invention; and

(7) FIG. 6 shows the temporal waveform of the open circuit welding voltage in another variant of the igniting process.

(8) FIG. 1 shows a block circuit diagram of a welding current source 1 in accordance with the present invention. The welding current source 1 is used to provide a welding current I and a welding voltage U at an output 2, for carrying out a welding process with an arc L between an electrode 3 and a workpiece 4 to be welded. The electrode 3 can be a non-melting electrode, in particular a tungsten electrode, but also a melting electrode. The welding current source 1 includes a resonance converter 5 for generating the welding voltage U. The resonance converter 5 may be formed, in particular, by a series-parallel resonance converter. Such circuits are particularly simple in design and are used in a range of welding current sources 1. The resonance converter 5 is connected via appropriate upstream circuits, such as rectifiers or the like, to the supply network.

(9) For contactless ignition of the arc L between the electrode 3 and workpiece 4, a circuit 6 for generating high-frequency pulses U.sub.I,HF is provided, which high frequency pulses U.sub.I,HF are superimposed on the open circuit welding voltage U.sub.LL at the output 2 of the welding current source 1, specifically in a time-synchronous manner, in the region of at least some of the periodically recurring voltage maxima U.sub.LL,max of the open circuit welding voltage U.sub.LL. Under the assumption of fixed timing parameters, the times of the periodically recurring voltage maxima U.sub.LL,max can be determined with sufficient accuracy and the superposition of the high-frequency pulses U.sub.I,HF is carried out with sufficient accuracy. The circuit 6 for generating high-frequency pulses U.sub.I,HF can also be arranged in parallel with the output 2 or arc L (not shown).

(10) In order to improve the temporal synchronicity of the superposition a control device 8 can be provided, which controls the circuit 6 for generating high-frequency pulses accordingly. Optionally, a measuring device 7 for detecting the welding voltage U can be provided, which measuring device 7 is connected to the control device 8, so that the high-frequency pulses U.sub.I,HF can be superimposed on the output 2 time-synchronously with detected defined voltage values U.sub.LL,def.

(11) Because the high-frequency pulses U.sub.I,HF are superimposed in the region of the voltage maxima U.sub.LL,max of the open circuit voltage U.sub.LL, the igniting voltage can be increased and thus a reliable igniting of the arc L can be guaranteed. The additional circuit complexity is minimal, and therefore the welding current source 1 does not need to be designed substantially larger and more expensive compared to conventional welding current sources. The high-frequency pulses U.sub.I,HF can be superimposed on the maximum of the open circuit voltage U.sub.LL,max shortly before or shortly after it occurs. The high-frequency pulses U.sub.I,HF do not need to be superimposed on each voltage maximum U.sub.LL,max, but only on some of the voltage maxima U.sub.LL,max, for example, only every second or every third voltage maximum U.sub.LL,max.

(12) FIG. 2 shows an extended block circuit diagram compared to FIG. 1 of a welding current source 1, in which the resonance converter 5 for generating the periodically varying open circuit welding voltage U.sub.LL is formed by a series-parallel resonant converter with an inductor L.sub.R, a capacitor C.sub.R and a capacitor C.sub.p. In addition, the resonance converter 5 has a transformer T. The switches S1, S2, S3 and S4 generate voltage pulses at the input U.sub.E of the resonant circuit, which excite the resonant circuit L.sub.R, C.sub.R and C.sub.p. The capacitors shown parallel to the switches S1 to S4 are their parasitic capacitances and have no influence on the resonant circuit, because they are many times smaller than the capacitance C.sub.R. The series-parallel resonance converter illustrated also has the property that in the open circuit condition (i.e. without a connected load) the welding voltage U is increased as a result of the resonant circuit formed with C.sub.p, in such a way that a control of the resonance converter 5 is also required in the open circuit condition. To this end, the resonance converter 5 is operated in pulsed mode in the open circuit condition. At the input U.sub.E of the resonance converter 5, voltage pulses are applied for a specific period of time. An additional circuit 9 to maintain the open circuit welding voltage U.sub.LL, consisting of the diode D.sub.L, the resistor R.sub.L and the capacitor C.sub.L, represents one implementation option for operating the series-parallel resonance converter in pulsed mode in the open circuit condition. The oscillation generated by the resonance converter 5 charges the smoothing capacitor C.sub.L via the diode D.sub.L on the secondary side. In the time in which no voltage pulses are applied, the smoothing capacitor C.sub.L discharges via resistor R.sub.L. At the output of the resonance converter 5 therefore, a periodically varying, preferably substantially sawtooth-shaped open circuit welding voltage U.sub.LL with voltage maxima U.sub.LL,max periodically recurring at a frequency f.sub.w is produced (see FIG. 3).

(13) FIG. 3 shows the temporal waveform of the open circuit welding voltage U.sub.LL of a welding current source 1 with a resonance converter. Accordingly, at the output 2 of the welding current source 1 an open circuit welding voltage U.sub.LL results, which has a periodically varying, substantially sawtooth-shaped open circuit welding voltage U.sub.LL with recurring voltage maxima U.sub.LL,max at a repetition rate f.sub.w.

(14) FIG. 4 shows the temporal waveform of the open circuit welding voltage U.sub.LL on the application of the igniting process according to the invention. In the diagram, on some (here on each) of the occurring voltage maxima U.sub.LL,max, in the region of the voltage maximum U.sub.LL,max a high-frequency pulse U.sub.I,HF is superimposed. This facilitates the igniting of the arc L without the maximum average voltage at the output 2 of the welding current source 1 exceeding prescribed limits. It is important that the energy content of the high-frequency pulses U.sub.I,HF over time does not exceed regulatory limits.

(15) FIG. 5 shows the temporal waveform of the open circuit welding voltage U.sub.LL at the output 2 of the welding current source 1 in a variant of the igniting process relative to FIG. 3. In this case, the high-frequency pulses U.sub.I,HF are superimposed on only every second maximum of the open circuit voltage U.sub.LL,max. Accordingly, there is more time available for recharging storage components of the circuit 6 for generating the high-frequency pulses U.sub.I,HF, and over time a lower amount of energy is transmitted via the output 2.

(16) FIG. 6 shows an extract of the temporal waveform of the open circuit voltage U.sub.LL at the output 2 of a welding current source 1, wherein the open circuit welding voltage U.sub.LL is continuously measured and compared with a defined voltage value U.sub.LL,def. The defined voltage U.sub.LL,def is slightly below the expected or adjusted maximum open circuit welding voltage U.sub.LL,max, so that the detection in the region of the voltage maximum U.sub.LL,max can be reliably guaranteed. After the detection of the defined voltage value U.sub.LL,def, at least in some cases a high-frequency pulse U.sub.I,HF is superimposed, which facilitates or enables the contactless ignition of the arc L. The temporal delay between the occurrence of the defined voltage value U.sub.LL,def and the time of the superposition of the high-frequency pulse U.sub.I,HF is offset by a predefined time period Δt, wherein the length of the period Δt can be, for example, between 0 and 5 ms.