Method and device for welding egnition for arc welding apparatus

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to arc welding, and more particularly, to a method and device for igniting and stabilizing a welding arc of an arc welding apparatus.

Welding is a vital manufacturing technology in many industries. Welding processes are energy intensive as they require the production of high energy densities in order to create and move a pool of plasma. In most common welding methods, the energy coupling efficiency between the welding tool and a workpiece is twenty to thirty percent at best, depending upon the material and welding technology selected. Thus, significant economic benefits can be obtained if the coupling efficiency can be increased. Other aspects of the welding process, such as weld quality and productivity are also of interest and can impact the economics of the welding process.

Because all of these factors are in some way dependent upon the energy density which is incident on the workpiece, much effort has been made to increase this quantity by developing novel technologies and welding tools.

Igniting a welding arc and keeping its burning stability is essential for any welding technology. Several methods of igniting a welding arc are described, for example, in the book "<i>Welding Handbook</i> (American Welding Society), vol. 2, 8th Edition, "Welding Processes", entitled "Arc initiation methods", pp.91-93, and include scratch or touch start, pilot arc start, high-frequency start, pulse start, high-voltage DC-current start, etc.

<b>Scratch or touch start:</b>

With the power supply energized, and the shielding gas flowing from a cup, the torch is lowered toward the workpiece until the electrode makes contact with the workpiece. The torch is quickly withdrawn a short distance to establish the arc, see, for example, SU Pat. No. 703265, entitled "Protective gas metal spot welding arc ignition".

The advantage of this method of arc initiation relies in its simplicity in operation for both manual and machine welding.

The disadvantage of touch starting is the tendency of the electrode to stick to the workpiece, causing electrode contamination and transfer of metal from the electrode to the workpiece.

<b>Pilot arc start:</b>

Pilot arc starting can be used with DC welding power sources. The pilot arc is maintained between the welding electrode and the torch nozzle. The pilot arc supplies the ionized gas required to establish the main welding arc.

As shown in Figure 1, a pilot arc starting circuit <b>2</b> which is used for gas tungsten arc spot welding consists of a welding power supply <b>4</b> and a pilot arc power supply <b>6</b>, which are connected with a tungsten electrode <b>8</b> and a workpiece <b>10</b>. Tungsten electrode <b>8</b> is inside a coaxial anode ring <b>12</b> and insulated gas nozzle <b>14</b>. A pilot arc <b>16</b> is powered by a small auxiliary power source, namely, pilot arc power supply <b>6</b> and is started by high-frequency initiation. See, in this respect, also European patent application EP 0753371 and SU Pat. Nos. 539703 and 1623846.

<b>High-frequency start:</b>

High-frequency starting can be used with DC or AC power source for both manual and automatic welding applications. High-frequency generators usually have a spark-gap oscillator or thyristor oscillator that superimposes a high-voltage AC output at radio frequencies in series with the welding circuit. Such a circuit is shown in Figure 2. A high frequency arc starting circuit <b>20</b> consist of a welding power supply <b>22</b> and a high frequency generator <b>24</b>, which includes a spark gap oscillator <b>26</b> and an air core transformer <b>28</b>, which are connected with an electrode <b>30</b> and a workpiece <b>32</b>. Electrode <b>30</b> is placed inside a coaxial gas nozzle <b>34</b>. The high voltage generated by high frequency generator <b>24</b> ionizes the gas present between electrode <b>30</b> and workpiece <b>32</b>, and the ionized gas then conducts a welding current that initiates the welding arc. See, in this respect JP 10166145 and SU 1613263 patent documents.

High-frequency generator <b>24</b> is usually connected to circuit <b>20</b> in series and performs a breakdown of an air gap present between electrode <b>30</b> and workpiece <b>32</b>.

A high-frequency discharge generates a conductive duct within the gap wherein, under certain conditions, there develops an arc discharge from the welding power source. Since the source open circuit voltage does not generally exceed 60 to 150 V, it is necessary to provide high conductivity of the discharge gap to generate an arc dischcharge, i.e., essentially heat the high-frequency discharge duct. Therefore, high-frequency generator <b>24</b> must be of sufficiently high power. In this case, high-frequency currents flow along the welding circuit, so the latter radiates in a wide frequency range, generating an intensive level of electromagnetic disturbances for radio appliances, electronic equipment and computers. These disturbances may be harmfull to people as well.

High-voltage pulses act in the welding circuit as long as high-frequency generator <b>24</b> is operated. Typical pulse shapes <b>36</b>, <b>38</b> thereof are shown in Figures 3 and 4.

Since radiation from high-frequency generators affects people, environment, radioelectronic appliances, control equipment and computers, the application of welding arc igniting devices based on such generators requires high quality of manufacturing the latter, as well as adequate safety measures in operation which is, in some countries, regulated by the Government. So, in the U.S. the application of such arc igniting equipment is governed by regulations of the Federal Communications Commission.

<b>Pulse start:</b>

In pulse start, application of a high-voltage pulse between a tungsten electrode and a workpiece ionizes a shielding gas and establishes a welding arc.

This method is generally used with DC power supplies in machine welding applications. In this respect, see, for example, U.S. Pat. Nos. 4,061,899; and 5,365,035, European patent applications EP 0149916; and EP 0586325, and patent documents JP 05084578; JP 09038771 and SU 1613263. The arc ignition is performed by high voltage pulses transmitted to the discharge gap by means of a high frequency transformer.

The breakdown of the discharge gap by high voltage pulses causes the appearance of high frequency oscillations in the welding circuit. This results in the appearance of disturbances, which are less than at a high frequency start but great enough to affect control, radio and electronic equipment, as well as computers.

<b>High-Voltage DC start:</b>

High-voltage can be used with a DC-power source mainly for automatic applications. See, for example, U.S. Pat. No. 4,123,646 and JP 06179076, EP 0585068 and SU 448091.

In high-voltage DC start, arc ignition is performed by connecting a high voltage, low power, DC-power source (10 to 15 kV at 0.01 to 0.5 A) to a discharge gap. When an arc discharge is formed, the voltage decreases below that of the open circuit of the welding power source and the current flows now from the welding power source. This method creates a very low level of disturbances. Its main disadvantage is the need for protecting the welding power source and the user from the high voltage of the DC-power source. Therefore, this method is chiefly used for automatic welding, at very small currents, typically between 10 and 20 mA.

Methods and devices for igniting a welding arc are known, directed at lowering the electric field resulting from radiation disturbance and, at the same time, providing a stable arc burning, which is also the aim of the present invention. Such methods and devices have been developed by "Matsushita Electric Ind. Co. Ltd.", Japan, and are described in U.S. Pat. No. 5,582,751 and 5,773,792, and in JP 05261536, JP 06063745, JP 06182547, JP 06254679, JP 07009137, JP 07051853, JP 08118013, JP 08300147 and EP 0585068.

Most of the devices described in the above documents include a second DC-power source connected to the circuit of the welding power source and generating high voltage, a pulse of which is applied between the welding electrode and the workpiece, igniting an arc.

Figure 5 shows an "arc welding unit" as described in JP 10166146 of "Matsushita Electric Ind. Co. Ltd.". This device comprises a welding circuit <b>40</b>, which includes a welding power supply-transformer <b>41</b> supplemented with a current control element <b>42</b> and a processor <b>43</b>, a DC high voltage circuit <b>44</b> supplemented with a resistor <b>45</b>, which are connected to a non-consumable electrode <b>47</b> and to workpiece <b>48</b>. An output of DC high voltage circuit <b>44</b> is superimposed in parallel with an output terminal between non-consumable electrode <b>47</b> and workpiece <b>48</b>, and a single high voltage pulse from high voltage pulse generating circuit <b>49</b> is superimposed in series over the output terminal by a transformer <b>50</b> for coupling. In this device, a non-load voltage from circuit <b>40</b> must be no less than 100 V and no more than 1,000 V. A single high voltage pulse with a peak value of no less than 1,000 V is generated by high voltage pulse generating circuit <b>49</b>. A welding current flowing from current control element <b>42</b> is detected by a current detecting circuit <b>51</b>, and the output from circuit <b>44</b>, as well as the output from high voltage pulse generating circuit <b>49</b> are stopped.

The high voltage pulse creates a conducting duct in a gap present between electrode <b>47</b> and workpiece <b>48</b>, wherein an arc discharge is developed from welding power source <b>41</b>. Simultaneously with the application of a high voltage pulse, the conductive duct receives high DC-voltage from circuit <b>44</b> to at least partially suppress high frequency disturbances.

A drawback of the described device is that the voltage pulse generator operates during the whole period of arc ignition, transmitting high voltage pulses and creating a set of pulses of high frequency continuous oscillations in the transformer winding and the entire welding circuit.

High frequency currents flow along the welding circuit during the entire period of the high frequency pulse generator and the welding circuit emits electromagnetic radiation within a wide frequency range, creating an intensive level of electromagnetic disturbances dangerous to control, radio and electronic equipment, as well as to computers. In addition, within the entire period, the electrode and workpiece receive high (up to 1000 V) voltage, so special measures must be taken to protect the user.

There is thus a widely recognized need for, and it would be highly advantageous to have, a method and device for igniting a welding arc in an arc welding apparatus, which permit to essentially reduce the time required for arc ignition and lower the electromagnetic disturbances level to a minimum.

SUMMARY OF THE INVENTION

The present invention relates to a method and device for welding arc ignition in an arc welding apparatus, effected by high-voltage discharge between a welding electrode of the welding apparatus and a workpiece.

The method of igniting a welding arc according to preferred embodiments of the present invention includes several successive steps as follows:

First, a welding electrode and a workpiece are connected with a welding power source. Then, the electrode and the workpiece are simultaneously connected to at least two additional high voltage power sources. Thereafter, a short aperiodic high voltage pulse is transmitted from the first high voltage power source to a gap present between the welding electrode and the workpiece, to break down, by an electric discharge, the air present between the welding electrode and the workpiece, and to create a current conducting duct therebetween. In this way, the current output of the high voltage power source is restricted in amplitude and rate of rise. Next, the short non-periodic high voltage pulse is superimposed by another, long, high voltage pulse from the second high voltage power source. The long pulse has a current rate of rise not exceeding that of the short pulse, and the open circuit voltage of the second high voltage power source is lower than that of the first high voltage power source. As a result, a stretched pulse appears, and the duration of the current discharge increases. As a consequence, the current conducting duct is heated, its electric resistance decreases and an arc is ignited. When the voltage in the current conducting duct decreases to a value less than that of the open circuit of the welding power source, the current starts flowing from the latter through the welding electrode and arc to the workpiece and, as a result, a welding arc develops, burning from the welding power source.

According to further features in preferred embodiments of the invention described below, the high voltage short pulse is transmitted from the first high voltage power source within 1 to 10 microseconds, at a voltage of 0.5 to 15 kV and at a current of 1 to 10 A.

According to still further features in the described preferred embodiments the long high voltage pulse is transmitted from the second high voltage power source within 0.1 to 100 microseconds, at a voltage of up to 1 kV.

According to still further features in the described preferred embodiments, the open circuit voltage of the welding power source is preferably 60 to 100 V.

According to still further features in the described preferred embodiments, the first high voltage power source is, in effect, a current pulse generator connected with a primary winding of a current transformer, while the secondary winding of the transformer is connected to the circuit of the welding power source in series.

According to still further features in the described preferred embodiments the second high voltage power source is, in effect, a current pulse generator connected with the circuit of the welding power source in parallel. According to still further features in the described preferred embodiments currents flowing along the primary and secondary windings of the current transformer exhibit:<maths id="math0001" num=""><math display="block"><mrow><mfenced open="|" close="|"><mrow><mfrac><mrow><msub><mrow><mtext mathvariant="italic">I</mtext></mrow><mrow><mtext>1(</mtext><mtext mathvariant="italic">t</mtext><mtext>)</mtext></mrow></msub><mtext>-</mtext><mtext mathvariant="italic">K</mtext><mtext>·</mtext><mtext mathvariant="italic">I</mtext><msub><mrow><mtext>​</mtext></mrow><mrow><mtext>2(</mtext><msub><mrow><mtext mathvariant="italic">t-t</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><mtext>)</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext mathvariant="italic">I</mtext></mrow><mrow><mtext>1(</mtext><mtext mathvariant="italic">m</mtext><mtext>)</mtext></mrow></msub></mrow></mfrac></mrow></mfenced><mtext>&lt;10%</mtext></mrow></math><img file="EP1016484A1_D0001.tif" /></maths> where <i>I</i>_<i>1(t)</i> and <i>I</i>_{<i>2(t-t0)</i>} are instantaneous current values in the primary and secondary transformer windings respectively; <i>K</i> is the transformation ratio of the transformer; <i>I</i>_<i>1(m)</i> is the maximum current value in the transformer primary winding; <i>t</i> is the current time; and <i>t</i>_<i>0</i> is the time lag of current pulse in the secondary winding in relation to current pulse in the primary winding.

According to the present invention, the welding arc ignition device can be designed both for an arc DC-welding apparatus and an arc AC-welding apparatus.

A welding arc ignition and stabilization device for an arc DC-welding apparatus, operating via a high voltage discharge between a welding electrode and a workpiece comprises, according to preferred embodiments of the invention, a first high voltage power source for generating a short, high voltage pulse and a second high voltage power source for generating a long, high voltage pulse.

According to still further features in the described preferred embodiments an outlet of the first power source is connected with a transformer's primary winding, whereas the transformer's secondary winding is connected in series with a circuit of a welding power source.

According to still further features in the described preferred embodiments the second high voltage power source is connected in parallel with the circuit of the welding power source.

According to still further features in the described preferred embodiments the first power source is provided with an operation synchronizer, the inlet of which being connected with an outlet of the second power source.

According to still further features in the described preferred embodiments the welding arc ignition device is designed as an integral part of an arc welding apparatus and is additionally provided with a control device connected with the outlets of the welding power source and is designed to switch on the second power source every time when the welding power source is switched on.

According to still further features in the described preferred embodiments the first power source is deigned to emit a short high voltage pulse and includes a thyristor pulse generator with a DC power source and energy-storage capacitor, and has a discharge circuit with a current limiting element connected to an output of the pulse generator between the output and the energy storage capacitor.

According to still further features in the described preferred embodiments the thyristor pulse generator comprises a charging thyristor, a high resistance charging resistor, an energy-storage capacitor, a discharge thyristor, and a current limiting element ― a discharge resistor. Preferably, the energy-storage capacitor has a capacity up to 1 microfarad, and the discharge circuit of the thyristor pulse generator includes a discharge resistor and an inductor connected in series thereto. The discharge circuit elements exhibit:<maths id="math0002" num=""><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">R</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><mtext> = </mtext><msqrt><mfrac><mrow><msub><mrow><mtext mathvariant="italic">L</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext mathvariant="italic">C</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow></mfrac></msqrt></mrow></math><img file="EP1016484A1_D0002.tif" /></maths> where <i>R</i>_<i>0</i> is the discharge circuit resistance; <i>L</i>_<i>0</i> is the inductor inductivity; and <i>C</i>_<i>0</i> is the capacity of the energy-storage capacitor.

According to still further features in the described preferred embodiments the second power source, which is designed to generate the long high voltage pulse, comprises at least one thyristor pulse generator with an energy-storage capacitor, which is connected with the welding electrode and the workpiece via a resistor limiting the discharge current.

According to still further features in the described preferred embodiments the second power source may as well comprise at least two thyristor pulse generators, the outlets thereof are connected in parallel, and the generators are provided with a control device to switch them on in an order defined by the control device.

According to still further features in the described preferred embodiments, the second power source may be designed as a voltage ―controlled current source provided with a control circuit. In this case, the open circuit voltage of this source is at least 100 V, the source outlet is connected in parallel with the outlet of the welding power source and the control circuit is connected with the control device.

According to still further features in the described preferred embodiments the second power source may alternatively be a current-controlled power source based on a transistor generating smoothed high voltage pulses. In this case, an emitter of the transistor is connected, via a resistor, to a respective outlet of the DC welding power source, a collector of the transistor is connected to the pulse generator outlet, and a transistor base of the transistor is connected to the control device, the voltage thereof specifies a current of a certain form at the pulse generator outlet, and in this case:<maths id="math0003" num=""><math display="block"><mrow><mtext mathvariant="italic">I</mtext><mtext> = </mtext><mfrac><mrow><msub><mrow><mtext mathvariant="italic">U</mtext></mrow><mrow><mtext mathvariant="italic">c</mtext></mrow></msub><mtext>-</mtext><msub><mrow><mtext mathvariant="italic">U</mtext></mrow><mrow><mtext mathvariant="italic">BE</mtext></mrow></msub></mrow><mrow><mtext mathvariant="italic">R</mtext></mrow></mfrac></mrow></math><img file="EP1016484A1_D0003.tif" /></maths> where <i>I</i> is the outlet generator pulse current; <i>U</i>_<i>c</i> is the control device voltage; <i>U</i>_<i>BE</i> is the voltage between the base of the transistor and the emitter thereof; and <i>R</i> is the resistor resistance in the circuit of the transistor emitter.

A welding arc ignition and stabilization device for an arc AC-welding apparatus, operated by a high voltage discharge between a welding electrode and a workpiece comprises, according to preferred embodiments of the present invention, a first and a second high voltage pulse generating power sources and a synchronizer for synchronizing their operation.

According to still further features in the described preferred embodiments the first power source is designed to emit short, high voltage pulses and an outlet of this power source is connected with a transformer's primary winding, whereas a secondary winding of the transformer is connected in series with a circuit of a welding power source.

According to still further features in the described preferred embodiments the second high voltage pulse power source is connected in parallel with the circuit of the welding power source and is designed to emit long high voltage pulses. Preferably, the synchronizer of the first and second power sources has an inlet connected with an outlet of the welding power source, and outlets thereof are connected with the first and second high voltage pulse power sources.

According to still further features in the described preferred embodiments the welding arc ignition device for an arc AC welding apparatus is designed as an integral part of the welding apparatus itself. It is preferably provided with a control device connected with the welding power source outlets of the arc welding apparatus and which is used to actuate the second power source every time when the welding power source is actuated.

According to still further features in the described preferred embodiments the welding arc ignition device for an arc AC-welding apparatus may alternatively be designed as a separate unit attached to the arc AC-welding apparatus and electrically connected with the outlets of the welding power source thereof via a diode connected in series with the power source circuit in current direction. In this case, the device is preferably provided with a remote control device shaped as a separate unit located close to the welding electrode on the welding power source, or in any other suitable place.

According to still further features in the described preferred embodiments the first high voltage pulse power source which is used to emit short high voltage pulses comprises two thyristor pulse generators with DC power sources and energy-storage capacitors. Preferably, each of the thyristor pulse generators has a charging circuit including an energy-storage capacitor and a charging thyristor, and is connected, via a common current limiting element common for both generators, to one pole of the DC-power source, and a discharge circuit including a discharge thyristor and the common current limiting element. The discharge circuits of the pulse generators are connected to opposite outlets of the transformer's primary winding, and the winding midpoint is connected to the free pole of the DC-power source.

According to still further features in the described preferred embodiments each of the thyristor pulse generators comprises a charging thyristor, a high resistance charging resistor, an energy-storage capacitor, a discharge thyristor and a current limiting element ― discharging resistor. In addition, each of the thyristor pulse generators preferably further comprises an energy-storing capacitor with a capacity of up to 1 microfarad, and the discharging circuit includes a discharging resistor and an inductor connected in series, the discharging circuit elements exhibit:<maths id="math0004" num=""><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">R</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><mtext> ≥ </mtext><msqrt><mfrac><mrow><msub><mrow><mtext mathvariant="italic">L</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext mathvariant="italic">C</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow></mfrac></msqrt></mrow></math><img file="EP1016484A1_D0004.tif" /></maths> where <i>R</i>_<i>0</i> is the discharge circuit resistance; <i>L</i>_<i>0</i> is the inductor inductivity; and <i>C</i>_<i>0</i> is the capacity of the energy-storage capacitor.

According to still further features in the described preferred embodiments the second power source, which is designed to emit a long high voltage pulse, comprises at least two thyristor pulse generators supplemented with energy-storage capacitors. Each of the generators is connected by the first of its outlets directly with the workpiece, and by the second outlet thereof via a resistor for limiting the discharge current and a discharge thyristor, with the welding electrode. In this case, the first of the energy-storage capacitors is connected via its second outlet via a charging thyristor and a current limiting element with the positive pole of the respective DC power source, and the second energy-storage capacitor is connected via a charging thyristor and a current limiting element to the negative pole of the respective DC power source, the free opposite poles of the DC power sources being connected together and with the workpiece.

According to still further features in the described preferred embodiments the second high voltage pulse power source, which is designed to emit a long high voltage pulse, further comprises two antiparallel voltage-controlled current sources, each of them having a control circuit. The open circuit voltage of the sources is preferably at least 100 V, the outlets of the voltage controlled current sources are connected in parallel with the outlet of the welding power source, and the control circuits are connected with the synchronizer.

According to still further features in the described preferred embodiments, the second high voltage pulse power source may further comprise two antiparallel current-controlled current sources, each having a control circuit, and the open circuit voltage of the sources is of at least 100 V. In this case, the outlets of the current-controlled current sources are connected in parallel with the outlet of the welding power source, and the control circuits with the synchronizer.

Further objects and advantages of the present invention will become apparent from the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention herein described, by way of example only, with reference to the accompanying drawings, wherein: <ul id="ul0001" list-style="none"><li>FIG. 1 shows a prior art pilot arc starting circuit used for tungsten inert gas welding (TIG);</li><li>FIG. 2 shows a prior art high frequency arc starting;</li><li>FIGs. 3 and 4 show typical shapes obtained using prior art high frequency generator pulses;</li><li>FIG. 5 shows components of a prior art arc welding unit;</li><li>FIGs. 6a-c show block diagrams of a device according to the present invention;</li><li>FIGs. 7 and 8 show a diagram of a device for welding arc ignition for an arc DC-welding apparatus according to the present invention;</li><li>FIGs. 9a-b show a diagram of a device for welding arc ignition for an arc AC-welding apparatus according to the present invention;</li><li>FIG. 10 shows a diagram of a device for welding arc ignition based on thyristors according to the present invention; and</li><li>FIGs. 11-13 show typical forms of current pulses using welding arc ignition according to the method of the present invention.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of a method and device for igniting and stabilizing a welding arc which can be used for igniting and stabilizing a welding arc of an arc welding apparatus. Specifically, the present invention can be used to provide a novel power source for arc ignition, diminishing the high frequency oscillation characterizing the prior art, and which dramatically increases the reliability of automated or semiautomated and robotically controlled welding systems.

The principles and operation of a method and device according to the present invention may be better understood with reference to the drawings and accompanying descriptions.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

Referring now to the drawings, Figures 6-10 illustrate some general features and preferred embodiments of a device for welding arc ignition and stabilization according to the present invention.

A block diagram of the device according to the present invention is shown in Figure 6. The device is built into an electric circuit <b>100</b> designed for feeding a welding electrode <b>102</b> and a workpiece <b>104</b> from a welding power source <b>106</b>.

The device shown in Figure 6 includes a first high voltage power source <b>110</b>, designed to emit a short high voltage pulse provided with a synchronizer <b>108</b>, and a second high voltage power source <b>112</b>, designed to emit a long high voltage pulse. Both high voltage power sources <b>110</b> and <b>112</b> are connected by synchronizer <b>108</b>. Both high voltage power sources <b>110</b> and <b>112</b> are connected with circuit <b>100</b>, wherein first source <b>110</b> is connected in series with welding circuit <b>100</b> via a transformer <b>116</b>, and second source <b>112</b> is connected in parallel with welding power source <b>106</b>.

The welding arc ignition device according to the present invention may be an integral part of an arc welding apparatus. However, it may alternatively be a separate unit attached to an arc welding apparatus and electrically connected with the outlets of welding power source <b>106</b> thereof, the outlets being connected with welding electrode <b>102</b> and workpiece <b>104</b>.

In the first case, the welding arc ignition device is additionally provided with a control device 113 (Figure 6a) connected with the power source outlets of the arc welding apparatus and designed to switch on, second high voltage pulse power source <b>112</b> every time when welding power source <b>106</b> is switched on.

In the second case the welding arc ignition device is a separate unit attached to an arc welding apparatus and electrically connected with the outlets of arc welding power source <b>106</b> via a diode <b>115</b> (Figure 6b), connected in series with circuit of power source 106 in current direction. The device is preferably provided with a remote control device <b>117</b>. This remote control device may be designed as a separate block located close to welding electrode <b>102</b>, on the welding power source <b>106</b> or at any suitable place circuit <b>100</b>.

According to the present invention, the welding arc ignition device may be designed both for an arc DC-welding apparatus, as specifically shown in Figures 7 ― 9, and an arc AC-welding apparatus as specifically shown in Figure 10.

Thus, a welding arc ignition device for a DC arc welding apparatus is shown in Figures 7-9 and includes a first high voltage power source <b>110</b> designed to generate a short high voltage pulse, and a second high voltage power source <b>112</b> designed to generate a long high voltage pulse. The outlet of first power source <b>110</b> is connected with a transformer's <b>116</b> primary winding, whereas the transformer's <b>116</b> secondary winding is connected in series with the circuit of a welding power source <b>106</b>. Second high voltage power source <b>112</b> is connected in parallel with that circuit. First high voltage pulse power source <b>110</b> is provided with an operation synchronizer <b>108</b>, the inlet thereof is connected with the outlet of second high voltage pulse power source <b>112</b>.

First high voltage power source <b>110</b>, which is designed to emit a short high voltage pulse, comprises a thyristor pulse generator <b>120</b> with a DC-power source <b>122</b> and an energy-storage capacitor <b>124</b>. Thyristor pulse generator <b>120</b> comprises a charging thyristor <b>125</b>, a high resistance charging resistor <b>126</b>, an energy-storage capacitor <b>124</b>, a discharge thyristor <b>128</b>, and a current limiting element ― a discharge resistor <b>130</b>. Generator <b>120</b> has a discharge circuit with a current limiting element ― discharge resistor <b>130</b> connected to the outlet of pulse generator <b>120</b> between the outlet and the energy storage capacitor <b>124</b>.

Energy-storage capacitor <b>124</b> has a capacity up to 1 microfarad, and the discharge circuit of thyristor pulse generator <b>120</b> includes a discharge resistor <b>130</b> and inductor <b>132</b> connected in series, the discharge circuit elements exhibit:<maths id="math0005" num=""><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">R</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><mtext> ≥ </mtext><msqrt><mfrac><mrow><msub><mrow><mtext mathvariant="italic">L</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext mathvariant="italic">C</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow></mfrac></msqrt></mrow></math><img file="EP1016484A1_D0005.tif" /></maths> where <i>R</i>_<i>0</i> is the discharge circuit resistance; <i>L</i>_<i>0</i> is the inductor inductivity; and <i>C</i>_<i>0</i> is the capacity of the energy-storage capacitor.

Second high voltage power source <b>112</b>, which is designed to generate a long high voltage pulse, comprises at least one thyristor pulse generator <b>140</b> with an energy-storage capacitor <b>144</b>, and a discharge thyristor <b>148</b> connected with welding electrode <b>102</b> and workpiece <b>104</b> via a resistor <b>150</b> for limiting the discharge current. Generator <b>140</b> further comprises a charging thyristor <b>152</b> and a high resistance charging resistor <b>156</b>.

Second high voltage pulse power source <b>112</b> may as well comprise at least two thyristor pulse generators <b>160</b> (shown in Figure 8) the outlets thereof are connected in parallel, and the generators <b>160</b> are provided with a control device <b>168</b> to switch them on in an order defined by the device.

Furthermore, as specifically shown in Figure 9a, second high voltage pulse power source <b>112</b> may be designed as a voltage―controlled current source controlled by a control circuit <b>240</b>. In this case, an emitter <b>20</b>2 of a transistor <b>200</b> is connected with a respective outlet of a DC-power source <b>220</b> via resistor <b>203</b>. A collector <b>204</b> of transistor <b>200</b> is connected with a circuit of welding power source <b>106</b>, whereas, a base <b>212</b> of transistor <b>200</b> is connected with a control device <b>240</b>.

In this case the open circuit voltage of source <b>112</b> is preferably at least 100 V, source's <b>112</b> outlet is connected in parallel with the outlet of welding power source <b>106</b> and the control circuit is connected with control device <b>168</b>.

Finally, as specifically shown in Figure 9b, second pulse power source <b>112</b> may alternatively be a current-controlled current source controlled by control device <b>240</b>, the open circuit voltage of this source is preferably at least 100 V. The outlet of the source is connected in parallel with the outlet of welding power source <b>106</b>, and control device <b>240</b> with control device <b>168</b>. Second power source <b>112</b> may be a current-controlled current source based on a powerful high voltage transistor <b>200</b> which serves generating smoothed high voltage pulses (Figure 9b). In this case, an emitter <b>202</b> of transistor <b>200</b> is connected to a respective outlet of a DC-power source <b>220</b>, a collector <b>204</b> of transistor <b>200</b> is connected to the circuit of welding power source <b>106</b>, and a base <b>212</b> of transistor <b>200</b> is connected with a base of a low voltage transistor <b>230</b>, the emitter thereof, via a resistor <b>210</b>, is connected with the outlet of source <b>110</b>, and collector <b>204</b> is connected to the inlet of a control device <b>240</b>, the voltage thereof defining a current of a certain form at the pulse generator outlet, and in this case:<maths id="math0006" num=""><math display="block"><mrow><mtext mathvariant="italic">I</mtext><mtext> = </mtext><mfrac><mrow><msub><mrow><mtext mathvariant="italic">U</mtext></mrow><mrow><mtext mathvariant="italic">c</mtext></mrow></msub><mtext>-</mtext><msub><mrow><mtext mathvariant="italic">U</mtext></mrow><mrow><mtext mathvariant="italic">BE</mtext></mrow></msub></mrow><mrow><mtext mathvariant="italic">R</mtext></mrow></mfrac></mrow></math><img file="EP1016484A1_D0006.tif" /></maths> where <i>I</i> is the outlet pulse generator current; <i>U</i>_<i>c</i> is the control device voltage; <i>U</i>_<i>BE</i> is the voltage between the base of the transistor and the emitter thereof; and <i>R</i> is the resistor resistance in the circuit of the transistor emitter.

A transistor <b>250</b> designed to amplify the control signal of current transfer ratio is connected at base <b>252</b> thereof with the inlet of control device <b>240</b>, by an emitter <b>254</b> thereof to the bases of transistors <b>200</b> and <b>230</b>, and by a collector <b>256</b> thereof to DC-power source <b>220</b>.

Figure 10 shows a welding arc ignition and stabilization device for an arc AC-welding apparatus according to the present invention. The device comprises a first and a second high voltage pulse power sources <b>310</b> and <b>320</b>, respectively, and a synchronizer <b>330</b> for synchronizing their operation.

First high voltage pulse power source <b>310</b> is designed to emit short high voltage pulses. The outlet of power source <b>310</b> is connected with a transformer^'s <b>116</b> primary winding, whereas the secondary winding of transformer <b>116</b> is connected in series with the circuit of welding power source <b>106</b>. A second high voltage pulse power source <b>320</b> is connected in parallel with the circuit of welding power source <b>106</b> and is designed to emit long high voltage pulses. The operation synchronizer <b>330</b> of first <b>310</b> and second <b>320</b> pulse power sources has an inlet connected with the outlet of welding power source <b>106</b>, and the outlets thereof are connected with first <b>310</b> and second <b>320</b> high voltage pulse power sources.

First high voltage pulse power source <b>310</b>, which is used to emit short high voltage pulses, comprises two thyristor pulse generators <b>342</b> and <b>344</b> respectively, with a DC-power source <b>340</b> and energy-storage capacitors <b>346</b> and <b>348</b>. Each of thyristor pulse generators <b>342</b> and <b>344</b> has a charging circuit including an energy-storage capacitor, <b>346</b> and <b>348</b>, respectively, and a charging thyristor, <b>352</b> and <b>354</b>, respectively. Generators <b>342</b> and <b>344</b> are connected, via a current limiting element <b>350</b>, which is common for both generators <b>342</b> and <b>344</b>, to one pole of a DC-power source <b>340</b>. Each of thyristor pulse generators <b>342</b> and <b>344</b> further has a discharge circuit including a discharge thyristor <b>356</b> and <b>358</b>, respectively, and a current limiting element <b>360</b> and <b>362</b>. The discharge circuits of pulse generators <b>342</b> and <b>344</b> are connected to opposite outlets of transformer's <b>116</b> primary winding, and the winding's midpoint is connected to the free pole of DC-power source <b>340</b>. Each of thyristor pulse generators <b>342</b> and <b>344</b> further comprises an energy-storage capacitor, <b>346</b> and <b>348</b>, respectively, with a capacity of up to 1 microfarad. The discharge circuits include resistors <b>360</b> and <b>362</b> and inductors <b>364</b> and <b>366</b>, the discharge circuit elements exhibit:<maths id="math0007" num=""><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">R</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><mtext> ≥ </mtext><msqrt><mfrac><mrow><msub><mrow><mtext mathvariant="italic">L</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext mathvariant="italic">C</mtext></mrow><mrow><mtext>0</mtext></mrow></msub></mrow></mfrac></msqrt></mrow></math><img file="EP1016484A1_D0007.tif" /></maths> where <i>R</i>_<i>0</i> is the discharge circuit resistance; <i>L</i>_<i>0</i> is the inductor inductivity; and <i>C</i>_<i>0</i> is the capacity of the energy-storage capacitor.

Second high voltage pulse power source <b>320</b>, which is designed to emit a long high voltage pulse, comprises at least two thyristor pulse generators <b>370</b> and <b>372</b> with energy-storage capacitors <b>374</b> and <b>376</b>. Each of generators <b>370</b> and <b>372</b> is connected by one of its outlets directly with workpiece <b>104</b>, and by the second outlet, via a resistor which serves for limiting the discharge current, <b>378</b> and <b>380</b>, respectively, and further via a discharge thyristor, <b>382</b> and <b>384</b>, respectively, with welding electrode <b>102</b>. In this case, the first of energy-storage capacitor <b>374</b> is connected by its second outlet, via a charging thyristor <b>386</b>, and a current limiting element <b>390</b>, with the positive pole of a respective DC-power source <b>394</b>. The second energy-storage capacitor <b>376</b> is connected via a charging thyristor <b>388</b> and a current limiting element ― resistor <b>392</b>, to the negative pole of a respective DC power source <b>396</b>, whereas the free opposite poles of DC-power sources <b>394</b> and <b>396</b> are connected together and with workpiece <b>104</b>.

Second high voltage pulse power source <b>320</b>, which is designed to emit a long high voltage pulse, comprises two antiparallel voltage-controlled current sources, each of them is controlled by the outlet of synchronizer <b>330</b>, which therefore forms a control circuit (see Figure 10). The open circuit voltage of the sources is at least 100 V, the outlets of the voltage controlled current sources are connected in parallel with the outlet of welding power source <b>106</b>, and the control circuits are connected with synchronizer <b>330</b>.

Furthermore, second high voltage pulse power source <b>320</b>, which is designed to emit a long high voltage pulse, may further comprise two antiparallel current-controlled current sources, each is controlled by an outlet of synchronizer <b>330</b>, which therefore forms a control circuit (see Figure 10). The open circuit voltage of the sources is at least 100 V. In this case, the outlets of the current-controlled current sources are connected in parallel with the outlet of welding power source <b>106</b>, and the control circuits with synchronizer <b>330</b>.

According to the method of the present invention welding arc ignition by a high voltage electric breakdown of a discharge gap present between welding electrode <b>102</b> and workpiece <b>104</b> is performed in several steps.

When welding electrode <b>102</b> and workpiece <b>104</b> are connected to welding power source <b>106</b>, they are simultaneously connected to two additional high voltage pulse power sources <b>110</b> and <b>112</b>. Then, a required gap is set between welding electrode <b>102</b> and workpiece <b>104</b> and a short aperiodic high voltage pulse is transmitted from first high voltage pulse power source <b>110</b> to welding electrode <b>102</b>. Since the voltage of source <b>110</b> is beyond doubt greater than the breakdown voltage, the discharge gap between the welding electrode <b>102</b> and workpiece <b>104</b> is broken down and a spark discharge occurs, the duration of which is limited by the capacity and inductivity of the current conductors connecting welding power source <b>106</b> with electrode <b>102</b> and workpiece <b>104</b>. As a result, a narrow current conducting duct is formed.

Then, the short high voltage pulse is superimposed by another, long high voltage pulse from second high voltage a pulse power source <b>112</b> which is necessary to heat up the current conductive duct, reducing its electric resistance and to thereby ignite an arc. When the resistance in the conducting duct is reduced to a value less than the open circuit voltage of welding power source <b>106</b>, the power from the latter is delivered to welding electrode <b>102</b> and workpiece <b>104</b>, and an arc discharge is developed and bums.

Recall that first high voltage pulse source <b>110</b> is connected to the circuit of welding power source <b>106</b> via current transformer <b>116</b>, and second high voltage pulse power source <b>112</b> is connected thereto in parallel. Transformer <b>116</b> transmits current from the primary circuit to the secondary circuit with small distortions. Therefore, having defined restrictions in the current rate of rise an amplitude in the transformer primary circuit, one shall receive an adequate current pulse without essential distortions in the secondary circuit to thereby ensure a reduction of disturbances from pulse current flowing in the welding circuit.

The short high voltage pulse is emitted from first high voltage pulse power source <b>110</b> during 1 to 10 microseconds at a voltage of 0.5 to 15 kV and current of 1 to 10 A. The long high voltage pulse is emitted from second high voltage pulse power source <b>112</b> during 0.1 to 100 microseconds at voltage of up to 1 kV. The open circuit voltage of welding power source <b>106</b> is 60 to 100 V.

Welding arc ignition by high voltage discharge between welding electrode <b>102</b> and workpiece <b>104</b> in the device shown in Figure 7 is performed likewise.

First, welding electrode <b>102</b> and workpiece <b>104</b> are connected with welding power source <b>106</b>. Then, welding electrode <b>102</b> and workpiece <b>104</b> are simultaneously connected with high voltage pulse power sources <b>110</b> and <b>112</b>. Power from DC-source <b>122</b> is delivered to energy-storage capacitor <b>124</b> via high resistance charging resistor <b>126</b> and the capacitor is therefore charged. Voltage is delivered to energy-storage capacitor <b>144</b> via charging resistor <b>156</b> for charging thyristor <b>152</b>.

For arc ignition, first, synchronizer <b>108</b> actuates thyristor <b>152</b> and then, after charging capacitor <b>144</b>, synchronizer <b>108</b> actuates thyristor <b>128</b> and the previously charged capacitor <b>124</b> which discharges through the primary winding of transformer <b>116</b>. In this case, a voltage pulse occurs in the secondary winding of transformer <b>116</b>, and after the discharge gap is broken down, current flows through the gap, the amplitude and rate of change of the current is defined by a discharge circuit which includes resistor <b>130</b> and inductivity <b>132</b>. Simultaneously, thyristor <b>148</b> is actuated and capacitor <b>144</b> is connected with the discharge gap via resistor <b>150</b>. The discharge current of capacitor <b>144</b> heats up and maintains the discharge burning when the discharge pulse of capacitor <b>124</b> is over. As soon as the voltage in the current conducting duct has reduced to a value less than the open circuit voltage of welding power source <b>106</b> (e.g., 60 to 100 V), current starts flowing from source <b>106</b> to electrode <b>102</b> and workpiece <b>104</b> and the arc ignites.

A short high voltage pulse is emitted from first thyristor pulse generator <b>120</b> during 1 to 10 microseconds at a voltage of 0.5 to 15 kV and a current of 1 to 10 A. The long high voltage pulse from second thyristor pulse generator <b>140</b> has an open circuit voltage of up to 1 kV and duration from 0.1 to 100 microseconds.

The long high voltage pulse of second high voltage pulse power source <b>112</b> is delivered to the conducting duct simultaneously with the short pulse of first high voltage pulse power source <b>110</b> or even slightly earlier, so that the high voltage long pulse may pick up the discharge initiated by the short high voltage pulse. This pulse heats up the discharge conducting duct and creates conductivity sufficient to ignite a discharge from welding power source <b>106</b> having a low open circuit voltage.

The operation of thyristor pulse generators <b>120</b> and <b>140</b> with energy-storage capacitors, <b>124</b> and <b>144</b>, respectively, differs in that capacitor <b>124</b> in generator <b>120</b> is charged from source <b>122</b> via thyristor <b>125</b>, high resistor <b>126</b> and discharges via thyristor <b>128</b>, resistor <b>130</b>, which serves for limiting the discharge current, and inductor <b>132</b>, whereas, in the generator <b>140</b>, capacitor <b>144</b> is charged via charging thyristor <b>152</b> and high resistance charging resistor <b>156</b>, and is discharged via a discharge thyristor <b>148</b> and discharge current limiting resistor <b>150</b>. Resistors <b>130</b> and <b>150</b> serve in this case to provide an aperiodic mode and eliminate oscillations of the high voltage discharge.

The operation of high voltage pulse power source <b>112</b> which includes at least two thyristor pulse generators <b>160</b> (Figure 8) is performed likewise. Control device <b>168</b> defines the order of actuating generators <b>160</b> and so maintains the specified current level substantially during the entire specified pulse duration of second high voltage power source <b>112</b>.

The operation of the pulse generators based on transistors <b>200</b> (Figures 9a-b) is as follows.

When there is no control, currents from device <b>240</b>, transistors <b>200</b>, <b>230</b> and <b>250</b> are off. When control device <b>240</b> is switched on, current from the latter flows through transistor <b>250</b>, is amplified and delivered to the bases of transistors <b>200</b> and <b>230</b>. The current ratio of transistors <b>200</b> and <b>230</b> is defined both by the ratio of transistor current densities according to the known Ebersa-Molla equation and by additional shift on base <b>212</b> of transistor <b>200</b>, which is defined by resistor^'s <b>210</b> emitter circuit of transistor <b>230</b> (voltage reduction of 60 millivolts increases the current of transistor <b>200</b> tenfold). Hence, using resistor <b>210</b> it is possible to change the current transfer rate from the control circuit of control device <b>240</b> to the collector circuit of transistor <b>200</b> in a wide range.

As shown in Figure 11, the application of transistor <b>200</b> permits to create smoothed high voltage pulses <b>400</b> featuring a flat apex <b>410</b>, as distinguished from thyristor pulse generators <b>120</b> and <b>140</b> the pulses <b>420</b> thereof have, as shown in Figure 12, acute angle peaks <b>422</b>.

In general, the pulse of the welding arc ignition shown in Figure 13 includes stages of breakdown voltage peak (Up), charge voltage (Ud) and voltage of an arc ignition and stabilization (Ua).

The operation of the device for welding arc ignition and stabilization for an arc welding AC apparatus is performed likewise.

The application of the method and device according to the present invention offers an advantage because a single high voltage pulse is emitted, and the current in the discharge circuit is limited, since the ratio of current rate of rise di/dt (current time gradient) is reduced. In addition, the long pulse has a limited current rate of rise (by 2 to 10 fold lower than the short pulse) and practically creates no disturbances. However its energy is sufficient to heat up the conducting duct to a level on which the welding arc can burn directly from the welding power source.

The novel method and device disclosed herein are based on the concept of reduction of the time gradient of the ignition current, due to the successive decreasing of the arc ignition current amplitude and increasing of the arc ignition pulse duration. Initiation and arc propagation is performed in two stages. During the first stage, the intermediate gas discharge develops due to the application of the high voltage and low power single pulse. Then, the above intermediate discharge is transferred to arc discharge by applying the low voltage relatively long aperiodical current pulse. As a result, the arc current time-gradient decreased. Therefore, the level of electromagnetic radiation (interference) dramatically decreased also.

All existing power supplies perform ignition and main arc discharge establishment via the use of high frequency oscillating electrical fields, which results in unacceptable electromagnetic interference level. The conventional power supplies frequently cannot be readily integrated in modern complicated automated systems, which employ intelligent controllers (robots, lasers, PC based control systems, etc.). To be integrated, such power supplies require special, complicated and expensive protective methods, such as filters, suppressors, screening, etc. However, these methods do not guarantee the required reliability and consistency of the weld.

Especially difficult problems are experienced while using standard power supply when welding aluminum alloys in AC mode and ignition of arc current during the entire welding process. In addition, the high level of electromagnetic radiation increases the risk of developing health problems.

The method and device according to the present invention solves all of the above listed problems associated with the prior art, due to a dramatically lower level of electromagnetic interference, resulting in simple and reliable power supply for different welding applications.

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included just for the sole purpose of increasing intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.