FI89248B - Method for striking a secondary arc in plasma arc welding and auxiliary power source for use in the method - Google Patents

Abstract

Method and auxiliary power source in plasma arc welding in order to strike a stable pilot arc. An electrical voltage is fed from an auxiliary power source 10 between the poles A, B of the pilot arc of a plasma arc torch. The voltage of the auxiliary power source consists of a pulsating direct current voltage Uk, the frequency of which is selected from within the audio frequency range 800 Hz B 6 kHz. The arc current Ik that is generated by the auxiliary power source and the arc voltage are set so low that the method is also suitable for micro-plasma welding. In the auxiliary power source 10 there is a rectifier unit 13, a transformer 19 and semiconductor couplings 16a, 16b. Coupled to the secondary coil 19c of the said transformer 19 is a rectifier unit 20, to the coils of which an LC circuit is coupled, which serves to actively maintain the pilot current, the circuit comprising a shunt capacitor 23 and a series coil 24 as filter capacitors. <IMAGE>

Description

89248

The invention relates to a method for producing auxiliary arc in a plasma.

The invention further relates to an auxiliary power supply for plasma welding, in particular microplasma-15 welding, comprising input terminals, a housing and output terminals connectable to the auxiliary arc targets of the plasma welding device, the auxiliary power supply comprising a rectifier unit, a transformer and semiconductor switches, 20 rectifier units are connected to the secondary windings of the transformer.

The previously known plasma welding is an arc welding method in which the plas--. a gas such as argon or a mixture of 2-7% hydrogen and hydrogen flows through the nozzle of the welding torch towards the workpiece. The tungsten electrode connected to the negative terminal of the welding current source emits an arc through the nozzle, the constricted arc. main arc to the workpiece. In the main arc, a part of the flowing plasma gas is excited into plasma, which, when it encounters the workpiece, releases its high energy centrally to the place to be welded. In the atmosphere, the plasma lifetime is very short, usually less than 10 1 ms, so the plasma-maintaining effect of the main arc is a basic prerequisite for plasma welding.

As is known in the art, an auxiliary arc, i.e. a pilot arc, is maintained to ignite the main arc between a tungsten electrode connected to the negative terminal of a separate auxiliary current source and its nozzle connected to a plasma torch connected to its positive terminal. In micro and medium plasma torches, the pilot arc current is in the range 1-10 A.

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There are many ways to ignite the pilot arc between the nozzle and the center electrode. The most common of these is a high-voltage high-voltage ignition circuit (~ 3 kV, -2 MHz) in which the high voltage generated between the electrode and the nozzle ignites the pilot arc over the gap. A high frequency of 5 is used to keep the system safe. If the design of the plasma torch allows, the pilot arc can be ignited without high voltage by providing contact between the nozzle and the electrode. Particularly in such a case, the object of the present invention is to facilitate the ignition of the arc.

10

The plasma generated by the pilot arc makes the gap between the tungsten electrode and the workpiece electrically conductive and the main arc ignites when the main welding voltage is switched on. During welding, a pilot arc is only required when using a very small welding current, usually less than 5 A.

15

Due to the nature of plasma, pilot arcing is not commonly used in butt welding. In microplasma welding, a pilot arc is necessary to stabilize the main arc, but its current is kept to a minimum because an unnecessarily large pilot current unnecessarily heats and consumes a small nozzle or forces the nozzle to be otherwise too large for welding and the weld environment to overheat, causing hydrogenation.

The current life of the plasma nozzle is inversely proportional to the combined current of the pilot and 25 main arcs. The durability of the sharpened tip of the tungsten electrode also depends on the current used. The severity of the pilot arc, in turn, depends on the current in the pilot arc, which, when reduced, becomes unstable to the pilot arc and eventually shuts off and is only difficult to re-ignite.

30

A pilot power supply with a single-phase mains voltage of 50 Hz is already known. However, the disadvantage is the high voltage and current required and the poor ignition of the auxiliary arc, as well as the frequent shutdowns. The instability and extinction of the arc are primarily due to the effect of the zero points of the AC voltage 35. This known auxiliary power supply also causes a rather high noise level.

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Mains-frequency three-phase pilot power supplies are also known, which operate mainly satisfactorily, but have the disadvantage of their expensive structure and the limited availability of three-phase electricity, which limits the usability of the plasma welding machine.

In addition, various audio frequency power supplies are already known, but they have been used in plasma welding equipment only as a main arc power supply at higher powers, in the order of several kW.

10

Various high-frequency power sources with a frequency range of the order of 10-30 kHz are also known in the art. The disadvantage is radio interference, and these known power sources are not economically viable as pilot power sources for plasma welding.

15

To unbalance the pilot arc, the unloaded voltage of the power supply can be increased, but 120 V DC is the maximum permissible voltage for safety reasons. The severity of the pilot arc is highly dependent on the direct current used. A 1-phase mains-frequency rectified pilot 20 requires a current of 7-10 A at a voltage of 110 V and a 3-phase mains-frequency rectified pilot arc is stable at a current of 2 A and a voltage of approx. 70 V. The disadvantage of 3-phase electricity is the high cost of its implementation and the limited availability.

The object of the present invention is to provide a new method for obtaining a pilot arc for plasma welding, as well as a new power supply applying the method, so that the disadvantages discussed above can be largely avoided.

It is a particular object of the invention to provide such a new method and power supply which can produce a stable pilot arc from single-phase AC power which is also easily ignitable.

It is a particular object of the invention to provide a method and a pilot current source in which the pilot arc returns stably at an even lower current so that the pilot arc does not heat the plasma welding machine or workpiece unnecessarily. This is a particularly important goal in microplasma welding, where the main arc voltage is typically in the range of 18 to 28 V and the welding current is typically in the range of 0.1 to 10 A. In arc welding, the arc voltage range is typically 5 to 20 to 23 V and the idle voltage is generally in the range of 30. ..80 V, typically -60 V.

It is a particular object of the invention to provide such a method and power supply which do not cause serious radio interference or audio frequency noise disturbing the working environment.

It is a non-essential additional object of the invention to provide a method and pilot power supply that operates in a power range in which inexpensive standard transformers can be used.

15

It is a further object of the invention to provide a method of plasma welding and a pilot power supply applying it which can use a relatively low no-load voltage, whereby the device is safe to use.

20

In order to achieve the above and later objects, the method of the invention is mainly characterized in that the auxiliary power supply voltage is a pulsating DC voltage, the frequency of which is selected from the audio frequency range 800 Hz to 6 kHz, and that the auxiliary current source is so low. also suitable for microplasma welding.

The pilot or auxiliary power supply according to the invention, in turn, is mainly characterized in that an LC circuit acting as an active pilot current maintainer is connected to the terminals of the latter rectifier unit 30, which comprises a parallel capacitor and a series coil as a filter capacitor.

From the filter capacitor of said LC circuit, a dose of energy is obtained through the choke to the plasma torch. The filtration capacitor, together with the energy stored in the inductance of the series coil of the LC circuit, tends to keep 5 89248 current constant in the circuit formed when the pilot arc burns. If the arc breaks, the energy stored in the inductance will increase the voltage until a voltage discharge occurs between the electrode and the nozzle. Thus, the capacitor and choke of that LC circuit form a “kick-5 rinM that tends to keep the pilot in the combustion chamber.

The invention provides a stable auxiliary or pilot arc with relatively low power utilizing suitable waveform modification. Despite the audio frequency of the auxiliary power supply, the use of the method and power supply 10 does not cause annoying noise, which is a problem in higher power audio frequency devices, because at the audio frequencies used in the invention, radio interference can also be controlled by simple filters and enclosures.

The method and power supply of the invention can produce a stable pilot arc from single-phase AC electricity that is highly flammable without the need to use high frequencies that cause radio interference.

The method and device according to the invention are particularly advantageous for microplasma welding, since it operates stably at low current so that often a small workpiece and the plasma torch do not heat up unnecessarily. In addition, when a relatively low idle voltage can be used, the device is safe to use.

In the following, the invention will be described in detail with reference to. . to some examples of the application of the invention shown in the figures of the accompanying drawing, to the details of which the invention is in no way narrowly limited.

Figure A schematically shows prior art plasma welding power supplies and their interconnection.

Fig. 1 schematically shows a prior art torch used in plasma welding and its connection to, e.g., its power supplies according to Fig. A.

Λ 89248 p

Figure 2 shows a circuit diagram of a power supply according to the invention, partly in block diagram form.

Figures 3A, 3B and 3C show preferred waveforms of current and voltage of the pilot power supply 5 according to the invention.

According to Fig. A, the power supply used in plasma welding comprises a main arc power supply 9A, the positive terminal C of which is connected to the workpiece 2 according to Fig. 1. The negative terminal of the main power supply 9A is connected to the terminal A of the tungsten electrode 1 of the plasma torch via a high-power circuit 9B. The pilot power supply 10A is connected between the negative terminal of the main power supply and the terminal B of the plasma torch nozzle 5.

The so-called welding arc used in a plasma arc torch the main arc 3 burns between the tungsten electrode 1 of the torch 15 and the workpiece 2. The nozzle part of the bulb consists of two nested chambers 4 and 6, of which a tungsten electrode 1 is located in the middle of the inner chamber 6 and a hole 8 at its tip. A shielding gas surrounding the arc 3 is supplied to the outer chamber 4.

Since the arc 3 of the plasma torch burns in the gas between the workpiece 2 and the tungsten electrode 1, it must be ionized before igniting the main arc 3 in order for the gas to conduct electricity. The ionization 25 takes place between the nozzle 5 formed by the inner chamber 6 and the tungsten electrode 1 by means of a burning pilot arc 7. The pilot arc 7 ionizes the plasma gas, whereby an electrically conductive ionic bridge is formed between the workpiece 2 and the tungsten electrode 1 and the main arc 3 can ignite.

The main arc 3 should only burn between the electrode 1 and the workpiece 2, because a high-power arc burning between the electrode 1 and the nozzle 5 would quickly destroy the nozzle 5. The cooling of the nozzle 5 and the electric and magnetic forces in the burner prevent the main light arc 35 from igniting the electrode 1 and between the nozzle 5.

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The pilot power supply 10 according to the present invention shown in Fig. 2 is housed in a housing 11 which is grounded at its terminal 11a. The housing 11 prevents the spread of interference together with the line filter 13a located on the inlet side of the power supply. The input terminals 12 of the pilot power supply 10 are connected to a 220 V and 50 Hz AC mains or equivalent AC output. The alternating current supplied from the terminals 12 is rectified as is known per se in the unit 13, from which a direct voltage of 300 V is obtained. This DC voltage is converted by the inverter to a pulsating DC voltage, the waveform Ut of which is a preferred example in Figure 3A.

10

According to Figure 2, the inverter comprises a converter 14, a phase control unit 15 and, as power switches, e.g. HEXFET semiconductor switches 16a and 16b, which are connected to voltage protectors 17a and 17b. The switches 16a, 16b are connected via a choke 18 to the primary windings 15 19a and 19b of the transformer 19. The central output of the windings 19a and 19b is connected to a 300 V DC voltage from the rectifier unit 13. A rectifier unit 20 is connected to the secondary winding 19c of the transformer 19, which is connected to the output voltage filter 13b via current limiting resistors 21 (Rp). On the output voltage side, there is a pilot current Ik control resistor 22 (Rs) 20 and a parallel capacitor 23 (C) and a coil 24 (L). The output voltage Uk and current Ik are obtained from the terminals 25 of the power supply 10. As shown in Figure 1, terminals 25 are connected to terminals A and B of a plasma welding machine as a pilot arc power supply.

The essential components of the present invention are a parallel capacitor 23 on the output side of the pilot power supply 10 and a series coil 24, which together form the above-mentioned "kick circuit". In addition, as a side effect, the LC circuit rounds the current peaks of the output voltage of the power supply 10 and thus prevents high frequency transients. Capacitance-30 si C and inductance L are preferably selected in the range C - 3 ... 10 μι, L - 1 ... 10 mH. On this basis, the pilot arc 7 is obtained -. burn steadily.

The voltage Ut from the pilot power supply 10 is a pulsating DC voltage. 35 When loaded, the basic voltage U0 is usually in the range U0 - 13 ... 23 V, peak; low voltage Umax - U0 + 3 V and minimum voltage Umin - U0 - 3 V. Figure 3A

8 According to 89248, the period time T of the voltage Ut is about 0.25 ms and thus the pulse frequency of the direct current fdc - 1 / T - 4 kHz. In this case, the inverter frequency is -2 kHz.

According to Fig. 3A, the voltage Ut is characterized by voltage spikes between portions of DC voltage U0. The duration of the peaks T0 is chosen in relation to the period T so that T / T0 is about 10, usually T / T0 - 5 ... 13. The voltage form Uc corresponding to the voltage form Ut in Fig. 3A at the terminals of the capacitor 23 is shown in Fig. 3B. Although the capacitor 23 removes the voltage spikes upwards and the coil 24 in turn smooths the voltage spikes downwards and the pilot arc voltage Uk thus obtained is very uniform and stable, variations in the voltage Ut have a substantial effect on the combustion of the pilot arc. Variations in voltage Ut appear as variations in pilot current as shown in Figure 3C. This variation-15 la has an arc-reducing and stabilizing effect due to self-induction.

In general, the voltage Uk of the pilot power supply 10 in the present invention is audio frequency and its frequency f is in the range of 800 Hz to 6 kHz, preferably in the range of 2 to 4 kHz. The no-load voltage is generally relatively low and in the range 50-80 V, preferably about 70 V. The arc Ik of the pilot current source arc 7 is arranged to be regulated by a control resistor 22 in the range 0.5-10 A, preferably in the range 1-7 A with the loaded arc voltage Uk in the range 10 -30 V, preferably in the range 13-20 V.

When using the waveform of the voltage Uk shown in Fig. 3 and the relatively small power of the pilot power supply 10, the device does not cause interfering noise despite the audio frequency used. In addition, radio interference can be controlled by the device housing 10 and the line filter 13a, 13b. It is also an advantage that standard transformers can be used in the pilot power supply 10 because they operate at an inverter frequency below 3 kHz.

Many different variations can be made to the above embodiment of the invention. For example, the current limiting resistors 21 and the control resistor 22 can be replaced by a feedback circuit 35 known per se, which automatically drops the primary voltage i of the transformer 19.

9 89248 300 V immediately after the main arc 3 is lit and returns to the desired level.

In the following, the claims are set out, within the scope of the inventive idea defined by them, the various details of the invention may vary and differ from those set forth above by way of example only.

Claims (11)

1. A method of plasma welding to achieve a stable pilot light arc, a method of supplying an electrical voltage from an auxiliary current source (10) between the poles (A, B) of the pilot arc of a plasma burner, characterized in that the voltage pA the auxiliary current source consists of a pulsating direct current (Uk), the frequency of which is selected within the audio frequency range 800 Hz - 6 kHz and that the low current (Ik) provided in the auxiliary current source and the low voltage is selected so that the process is also suitable for the process. Method according to claim 1, characterized in that the audio frequency pA of the pulsating direct voltage (Uk) of the auxiliary current source (10) is selected in the range 2-4 kHz. 15 3. A method according to claim 1 or 2, characterized in that the idle voltage of the auxiliary power source is selected within the range 50-80 V, that the back current (Ik) of the auxiliary power source is arranged to be controllable within the range 0.5-10 A, 10-30 V, most preferably 13-20 V. 4. A method according to any one of claims 1-3, characterized in that the electricity to be measured at the auxiliary power source (10) is taken from a single-phase AC power network or similar other electrical outlet. 5. A method according to any one of claims 1-4, characterized in that the waveform (Out) of the voltage of the auxiliary current source is selected in such a way that between the units with a uniform base voltage (U0) there are • relatively short voltage spikes both up and down and that lasts - the density (T0) of the voltage spikes is chosen in such a way that the ratio between the time of the total period (T) and the durations (T0) of said voltage spikes is in the range T / T0 «5 ... 13. Auxiliary power source (10) for plasma welding, especially for microplas welding, which includes input coils (12), a case (11), and output coils (25) which can be coupled in connection with the poles (AB) of the high beam light of the plasma welding device, which Auxiliary power source (10) comprises a rectifier unit (13), a transformer (19) and semiconductor couplings (16a, 16b), which components are connected in the form of an inverter, a rectifier unit (20) being connected to the secondary coil of the transformer ( 19), characterized in that an LC circuit that functions as an active maintainer of the pilot current has been coupled to the poles of the latter rectifier (20), which circuit includes a parallel capacitor (23) and a series coil (24) as filter capacitors. 7. A power supply according to claim 6, characterized in that said inverter comprises a receiving unit (15) which controls the semiconductor couplings (16a, 16b), that said semiconductor couplings (16a, 16b) are connected by supplying throttle coils (18) to the extreme outlets of the primary coils (19a, 19b) of said transformer (19) while the central outlet of the prirrr coil is connected to said rectifier unit (13), whereby a voltage of class 300 V is obtained. 8. A power supply according to claim 6 or 7, characterized in that the poles of the rectifier unit (20) connected to the secondary coils (19c) of the transformer (19) are connected by the power supply (21) and a control resistor resistor (22) of the pilot. (1¾). 25 9. A power source according to any one of claims 6-8, characterized in that both the input coils (12) and the output coils of the power source are coupled by mediating line filters (13a, 13b). 10. A pilot circuit according to any one of claims 6-9, kdnne - drawn claim, that the capacitance C of the parallel capacitor (23) of the LC circuit of the output of the current source is selected in the range C - 3 ... 10 μ ¥ and the inductance L of the coil (24) of said LC circuit selected in the range L - 1 ... 10 mH. 35 is 8924 8 Power source according to any of claims 6-10, characterized in that the receiving control of said semiconductor couplings (16a, 16b) or the like is arranged such that the frequency of the inverter is in the range 400-3000 Hz, that the supply electricity of the power source 5 ( 10) is arranged to be capable of being taken from a single phase AC network, that the idle voltage of the current source (10) is arranged within the range 50-80 V and that the beak current (Ik) of the power source is arranged to be adjustable within the range 0.5-10 A the loaded beak voltage is 10-30 V, most preferably 13-20 V. 10