Method and circuits for igniting and stabilizing a welding arc

Abstract

The disclosure relates to a circuit and a method for igniting and stabilizing a welding arc in a welding apparatus. A pulse generating circuit (15) comprises a pulse transformer (12), a DC power source (45) coupled to at least one primary winding of the pulse transformer (12) and at least one switch (44) for enabling and disabling current from flowing from the DC power source (45) to the at least one primary winding. A secondary winding of the pulse transformer (12) is configured to provide an ignition voltage pulse to the welding torch (20) upon opening the respective at least one switch (44) and further a stabilizing current after the ignition voltage pulse.

Description

[0001] METHOD AND CIRCUITS FOR IGNITING AND STABILIZING A

[0002] WELDING ARC

[0003] FIELD OF THE DISCLOSURE

[0004] The present disclosure relates to the field of electrical engineering, and particularly to electrical circuits for welding. The present disclosure further concerns a method and circuitries for igniting and stabilizing a welding arc.

[0005] BACKGROUND OF THE DISCLOSURE

[0006] Tungsten Inert Gas (TIG) welding, also known as Gas tungsten arc welding (GTAW), is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. In context of TIG welding, the arc may be referred to as a TIG arc. The weld area and electrode are protected from oxidation or other atmospheric contamination by an inert shielding gas (argon or helium). A constant-current welding power supply produces electrical energy, which is conducted across the arc through a column of highly ionized gas and metal vapours known as a plasma.

[0007] In TIG welding, two ignition modes are available: High Frequency (HF) ignition and Lift TIG ignition. The HF ignition uses a voltage pulse to initiate the arc, and the Lift TIG ignition needs a physical contact between the electrode and the work piece. In HF ignition, a press on the welding torch trigger produces a high voltage pulse that creates a spark to ignite the arc. This high voltage pulse is referred herein as a ignition voltage pulse. In Lift TIG ignition you briefly touch the work piece with the electrode, then press the trigger and lift the electrode to a small distance from the work piece. The Lift TIG ignition must be activated in the control panel. Also known as "Touch ignition" or "Contact ignition". The present disclosure concerns HF ignition of a TIG arc.

[0008] Document W0200128730 discloses a method and devices for excitation (igniting) of an electric arc. Although the disclosure is based on traditional electronic components rather than semiconductors, this basic solution is applied in many standard TIG welding apparatuses currently available in the market. Document DE2706102C2 discloses a TIG ignition circuit that aims in limiting the ignition voltage, and diodes are used to separate the ignition transformer from the welding circuit.

[0009] Patent application EP0667205 Al discloses a synchronized pulse arch starter and stabilizer.

[0010] Patent application US 2021 / 0260682 Al discloses an arc starting / stabilizing circuit with a plurality of transformers to avoid handling very high voltages with a single transformer.

[0011] Patent US 6,156,999 discloses a method and device for welding arc ignition for arc welding apparatus.

[0012] BRIEF DESCRIPTION OF THE DISCLOSURE

[0013] An object of the present disclosure is to provide a method and circuitries so as to solve the above problem.

[0014] The object of the disclosure is achieved by a method and circuitries which are characterized by what is stated in the independent claims. Some embodiments of the disclosure are disclosed in the dependent claims.

[0015] According to a first aspect, method for igniting and stabilizing a welding arc in a welding apparatus is provided. The welding apparatus comprises a power supply configured to provide a welding current, a welding torch configured to receive the welding current for its electrode from the power supply, a grounding clamp configured to be coupled with a work piece and to a ground contact of the power supply, and a pulse generating circuit comprising a pulse transformer comprising at least one primary winding and a secondary winding. The method comprises providing, from the secondary winding of the pulse transformer coupled in series between the power supply and the welding torch, an ignition voltage pulse to the electrode of the welding torch. The ignition voltage pulse is configured to ignite the welding arc upon initiating the welding current. The method comprises switching on a switch coupled to the primary winding of the pulse transformer to feed a current from a DC power source into the primary winding and generating the ignition voltage pulse in response to switching off the switch. The method comprises further providing, from the secondary winding of the pulse transformer, a stabilizing current after the ignition voltage pulse. The stabilizing current combines with the current provided by the power supply and thus facilitates maintaining the welding arc until the power supply provides a sufficient welding current to maintain the welding arc.

[0016] According to some embodiments, the welding current is an AC welding current, and the method comprises stabilizing the welding arc by intermittently generating by the pulse generating circuit the ignition voltage pulse and the stabilizing current after zero crossings of the AC welding current.

[0017] According to some embodiments, the switch is switched off upon or after determining that a zero crossing of the welding current has occurred, and the power supply is active.

[0018] According to some embodiments, during ongoing welding, the switch is configured to be switched on upon determining that welding current is zero, and the power supply is on. The switch is configured to be switched off upon determining that the current in the primary winding has reached a predetermined current threshold value.

[0019] According to some embodiments, the switch is configured to be switched on in response to receiving a start command from a user, and the switch is configured to be switched off upon determining that the current in the primary winding has reached a predetermined current threshold value.

[0020] According to some embodiments, the pulse generating circuitry comprises two switches, the pulse transformer has two primary windings and one secondary winding, and each switch is coupled to a first end of a different one of the two primary windings of the pulse transformer, and the DC power source is coupled to a second end of each of said two primary windings, said second ends being mutually coupled. The method comprises operating said two switches alternately, wherein each switch is closed and opened intermittently in dependence to one of the two polarities of zero crossings of an AC welding current provided by the power supply.

[0021] According to a second aspect, a circuitry for igniting and stabilizing a welding arc in a welding apparatus is provided. The welding apparatus comprises a power supply configured to provide a welding current, a welding torch configured to receive the welding current for its electrode from the power supply, a grounding clamp configured to be coupled with a work piece and to a ground contact of the power supply, and a pulse generating circuit comprising a pulse transformer, wherein the pulse transformer comprises at least one primary winding and a secondary winding. The secondary winding is coupled in series between the welding torch and an output of the power supply. The pulse generating circuit comprises a DC power source coupled to the at least one primary winding, and at least one switch for enabling and disabling a current from flowing from the DC power source to the at least one primary winding. The secondary winding is configured to provide an ignition voltage pulse to the welding torch upon opening the respective at least one switch. The secondary winding of the pulse transformer is further configured to provide a stabilizing current after the ignition voltage pulse. The stabilizing current combined with the current provided by the power supply facilitates maintaining the welding arc until the power supply provides a sufficient welding current to maintain the welding arc.

[0022] According to some embodiments, the switch is a transistor, and the switch is coupled to the respective end of the primary winding via a diode for disabling negative voltage peaks from the transformer to reach the switch.

[0023] According to some embodiments, the switch is a transistor, and the switch is coupled in parallel with a snubber capacitor for protecting the switch from a voltage spike caused by opening the switch. According to some embodiments, said at least one switch is two switches, the pulse transformer includes two primary windings and one secondary winding, wherein each switch is coupled to a first end of a different one of the two primary windings of the pulse transformer, and the DC power source is coupled to a second end of each of said two primary windings, said second ends being mutually coupled.

[0024] According to some embodiments, said two switches are configured to be operated alternately, and each switch is closed and opened intermittently in dependence to one of the two polarities of zero crossings of an AC welding current provided by the power supply.

[0025] The disclosure is based on the idea of a combined circuitry that generates an ignition voltage pulse for generating a spark to ignite the welding arc and also stabilizes the welding arc by providing sufficient amount of stabilizing current for a short period after the ignition voltage pulse, until the power supply provides sufficient output current.

[0026] An advantage of the method and circuitries of the disclosure is that they are simple and reliable and enable producing a stable welding arc also when using AC current. Preferred embodiments are also cost efficient, since standard high- voltage switches are applicable, but no exceptionally high voltage tolerant switches are required.

[0027] Although the invention has been primarily designed in view of and tested with a TIG welding apparatus, the same solution is applicable for many other welding methods, including but not limited to metal inert-gas (MIG), metal active gas (MAG) and manual metal arc (MMA) welding.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In the following the disclosure will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which: Figure 1 illustrates schematically principle of an electrical circuit for spark igniting a TIG arc as known in the art.

[0030] Figure 2 illustrates an ignition circuitry in a welding apparatus capable of applying AC and / or DC current for welding as known in the art.

[0031] Figure 3 illustrates a operation principle of a pulse generating circuit.

[0032] Figure 4 illustrates a welding circuitry with a pulse generating circuit.

[0033] Figure 5 illustrates another welding circuitry with a pulse generating circuit.

[0034] Figure 6 shows a schematic of a pulse generating circuit with a pulse transformer.

[0035] Figure 7A illustrates current and figure 7B illustrates voltage over a switch in the pulse generating circuit.

[0036] Figure 8 illustrates schematically a welding current, output voltage provided towards the welding torch, and switch current over a switch of the pulse generating circuit, as well as timing of some steps of the method.

[0037] Figure 9 illustrates measured output voltage, welding current and current in a primary winding of a welding transformer.

[0038] DETAILED DESCRIPTION OF THE DISCLOSURE

[0039] The disclosure relates to electrical circuitry for a welding apparatus, in particular electrical circuitry for generating an ignition voltage pulse for igniting a welding arc, such as a Tungsten Inert Gas (TIG) arc as well as for generating a stabilizing current for stabilizing the welding arc after ignition, if needed. Like in TIG welding, a welding arc is applied also in metal inert-gas (MIG), metal active gas (MAG) and manual metal arc (MMA) welding. The disclosed solution is applicable to any of these and other like welding methods where ignition and possible reignition of the welding arc may be needed.

[0040] Figure 1 illustrates basic principle of an electrical circuit that produces a spark for igniting a TIG arc as known in the art. The TIG welding apparatus comprises a power supply 10, in this example providing a DC current output, a welding torch 20, also known as a welding torch, and a grounding clamp 21 by which the work piece 25 is coupled to form a common electric circuit with the TIG welding apparatus. A pulse transformer 12 is coupled in series with the welding torch 20. The pulse transformer 12 receives energy from a pulse generator 11 and provides an ignition voltage pulse or burst to the welding torch 20, in end of an ignition electrode rod thereof. A spark is generated between the electrode rod of the welding torch 20 and the work piece 25 that ignites an electrically conductive channel between the electrode rod and the work piece 25 to enable welding current to flow and ignite the TIG arc. Welding with DC current is herein referred as DC current welding.

[0041] Capacitor C is provided for preventing a spark current from entering the power supply 10. Capacitor C appears as a short circuit on spark frequencies. This is needed, because the power supply 10 is too slow to react to very quick electrical signals caused by spark generation, and the power peak caused by spark could be harmful for the power supply 10.

[0042] In a practical application, the ignition voltage pulse is a single, short, typically about 1 s, while the pulse voltage of the ignition voltage pulse may be about 10 kV. A pulse generator 11 has been known since early days of TIG welding. Traditional pulse generators 11 have been implemented using a high voltage transformer, a capacitor and a spark-gap; the high voltage transformer charges the capacitor, and the charge is released over the spark-gap into a primary winding of the pulse transformer 12. After the 1970's, power semiconductors, such as thyristors, Insulated-Gate Bipolar Transistors (IGBT) etc. have been used in the implementation, but the basic operation principle of the pulse generators 11 remains the same.

[0043] Figure 2 illustrates an ignition circuitry in a welding apparatus configured to use a DC current or an AC current for welding as known in the art. This circuitry is applicable to TIG welding, but also to other welding types including but not limited to metal inert-gas (MIG), metal active gas (MAG) and manual metal arc (MMA) welding. In welding with AC current, which is referred in short as AC current welding, welding current switches direction (polarity) intermittently so that between the pulse values, when the AC current changes polarity, the voltage over the welding arc crosses zero, which tends to cut off the welding arc. Thus, the welding arc may need to be reignited to ensure stable, continuous operation. Reignition is achieved by producing a reignition voltage pulse, also referred to as an stabilizing current. Reignition occurs with short intervals and reigniting the welding arc does not require as high voltage as the initial ignition. The reignition voltage however needs to exceed the normal supply voltage available from the power supply 10.

[0044] To ensure reignition of the welding arc, prior art systems are provided with an auxiliary pulse generator 13, which feeds a reignition voltage pulse, referred herein as an auxiliary pulse, when the AC current voltage switches polarity. A typical auxiliary pulse has a voltage of about 300 V and a duration of about 100 ps. Ideally, the auxiliary pulse is provided immediately upon zero crossing of the AC current, but in practice there may be a short delay after the welding current reaches zero, before the auxiliary pule takes place. One possible reason for delaying the auxiliary pulse is to allow the welding current provided from the power supply 10 to increase to a level sufficient for maintaining the welding arc before the auxiliary pulse is provide, thus ensuring stability and, if needed, reignition of the welding arc. Any inductance comprised in the electrical circuit feeding the welding current from the power supply to the welding arc causes some delay for rising of the welding current to the intended level. For providing such auxiliary pulse that fulfils requirements set by welding technology and electrical engineering as well as electrical safety regulations, the auxiliary pulse generator comprises complex circuitry which is not only expensive but also error prone. The circuitry in the Figure 2 can also be used for DC current welding. The auxiliary pulse generator 13 may be inactivated during DC current welding since it is not required for stabilizing the welding arc.

[0045] Figure 3 illustrates a theoretical operation principle of the invention, in which generation of the ignition pulse as well as a stabilizing current needed for stabilizing the welding arc are combined into a single circuitry.

[0046] When the power supply 10 is on, an open circuit voltage is provided, typically in range between 50 V and 110V. When the switch 44 is closed, a current starts to flow in the inductor 16, increasing linearly. When the switch 44 is opened, the current from the inductor 16 generates a high voltage pulse to the welding torch 20 to generate an ignition voltage pulse having like voltage level as known prior art ignition voltage pulse solutions. The same principle is also applicable for generating a high enough ignition voltage for igniting the welding arc.

[0047] Suitable dimensioning of the inductor 16, and load voltage causes the stabilizing current to be provided from the inductor 16 after ignition of the welding arc. Because of the stabilizing current, no separate auxiliary voltage is needed. The stabilizing current is high enough stabilizing the welding arc in AC current welding after a zero crossing of the AC welding current, when current provided by the power supply is not yet sufficient to maintain the ignited welding arc. The stabilizing current is provided from the inductor 16 also when igniting DC current welding. Even though the stabilizing current may not be necessary after zero crossing in DC current welding, it does not cause problems either. Thus, the solution principle is equally applicable in AC and DC welding.

[0048] A practical problem in the theoretical circuitry illustrating the operation principle in the Figure 3 is the high voltage the switch 44 is subject to; generation of a spark that ignites the TIG arc requires a high voltage in level of several kilovolts, and the switch 44 should be tolerant to being subject to such high voltages repeatedly. A high voltage tolerance requires special high- voltage switches, which are expensive, although available. Although the stabilizing current is not required for DC current welding, the operation principle of the circuitry shown in the Figure 3 is applicable to both AC and DC TIG welding as well as for AC and DC MIG, MAG, and MMA welding. As a nonlimiting example, AC current frequency of a power supply 10 used for TIG welding is preferably adjustable within a frequency range, which is typically from 50 Hz to 250 Hz.

[0049] In the following figures 4 to 6, circuitries are disclosed which are applicable to both AC and DC TIG welding as well as for AC and DC MIG, MAG, and MMA welding and like. Specific characteristics of the applied welding technology may affect currents, voltages and durations of different steps of the ignition process, and thus design parameters of circuit elements of the pulse generating circuit, but the same basic operation principle and circuitry design are applicable. Figure 4 illustrates a welding circuitry with a pulse generating circuit 15 according to some embodiments, solving the problem of expensive high- voltage switches needed in the solution shown in the Figure 3 by replacing the inductor 16 with a pulse transformer 12 as a part of a circuitry referred herein as a pulse generating circuit 15. The pulse transformer 12 may be configured to have a transformer ratio of about 1: 10. The magnetization inductance of the pulse transformer 12 shall be dimensioned so that it can store enough energy for first generating the ignition voltage pulse for generating the igniting spark and further providing a sufficiently large stabilizing current very quickly upon igniting the welding arc for stabilizing purpose. An exemplary pulse transformer 12 used for example in tests made for confirming the concept according to the disclosure has a zinc manganese ferrite transformer core having a cross- sectional area of about 5 cm2, and optionally an air gap of 0.1 to 0.5 mm. Transform ratio is typically 1: 10, with primary winding having just a single turn and the secondary winding having about 10 turns. This kind of pulse transformers is also used in some commercially available welding devices that use traditional spark generation methods.

[0050] Generation of the welding arc igniting spark by the pulse generating circuit 15 coupled to the welding circuit can be considered as a stochastic event, depending on electrical characteristics of the entire welding circuit, and therefore, optimal dimensioning of circuit elements is preferably determined experimentally. Tests with existing welding apparatus have indicated that suitable dimensioning can be determined by a person skilled in the art without undue burden, so that requirements concerning stability of the welding arc are achieved. Some of the parameters may be adjustable by the operator of the welding machine to accommodate individual preferences.

[0051] The transformer ratio enables reducing a pulse voltage tolerance requirement of the switch 44 by about the same ratio as the transformer ratio, which brings the voltage tolerance requirement to a level at which reasonably cheap semiconductor switches are available. The primary winding of the pulse transformer 12 is coupled to a DC power source 45 of the pulse generating circuit 15. The DC power source 45 may be a dedicated power source, or DC power may be provided by the power supply 10. The secondary winding of the pulse transformer 12 is coupled in series with the welding torch 20. The secondary winding operates in equivalent manner to the inductor 16 shown in the figure 3, but energy needed to produce the ignition voltage pulse is fed to the secondary winding by means of the primary finding (instead of the power supply 10), and the DC power source 45 provides power for purpose of generating both the ignition voltage pulse and the stabilizing current until current provided by the power supply reaches sufficient level to maintain the welding arc. The applicable voltage of the DC power source 45 is determined based on the desired peak voltage of the igniting voltage pulse and electrical characteristics of the welding circuit and the pulse transformer 12. DC voltage of the DC power source 45 can be between 20 V and 50 V, in comparison to a traditional auxiliary pulse generator's supply voltage that is typically between 100 V and 300 V. In one exemplary test circuitry, DC voltage provided by the DC power source 45 was 24 V.

[0052] Operation of the pulse generating circuit 15 can be described as follows. It is assumed that the DC power source 45 is active. When the switch 44 is closed, a current starts to flow in the primary winding of the pulse transformer 12. Rise time of the current depends primarily on design and structure of the pulse transformer 12. Parameters of the pulse transformer 12 are designed using normal electrical design rules as known to a skilled person. Upon opening the switch 44, a high voltage is induced in the secondary winding of the pulse transformer 12, thus generating an ignition voltage pulse for generating the spark between the electrode rod of the welding torch 20 and the work piece 25 ant thus igniting the welding arc. At this phase, there is still energy stored in the magnetic circuit of the pulse transformer 12, which is released in form of electric current, referred herein as the stabilizing current, through the electric channel generated by the spark and forming the welding arc, which is thereafter maintained by the power supply 10. The energy remaining in the magnetic circuit after generation of the spark is released from the pulse transformer 12 after the initial ignition voltage pulse, thus forming the stabilizing current following the ignition voltage pulse for stabilizing the welding arc, if needed. The welding circuit should not comprise circuit elements that would disable generation of the stabilizing current.

[0053] The pulse generating circuit 15 thus operates both as a generator of the ignition voltage pulse for spark generation for reigniting upon zero crossings, and the pulse generating circuit 15 further provides the stabilizing current after each ignition / reignition voltage pulse, thereby stabilizing the welding arc. The need for the dedicated auxiliary pulse generator is thus avoided. In DC current welding, the stabilizing current is typically not required after ignition / reignition of the welding arc, but in AC current welding, feeding of stabilizing current may be needed. On the other hand, the stabilizing current may be applied in DC current welding for replacing known circuit arrangements for increasing open circuit voltage, which may also be needed.

[0054] The circuitry disclosed in Figure 4 is applicable both in DC current welding and AC current welding. However, with the circuitry shown in the figure 4, in the AC current welding the ignition voltage pulse and also the stabilizing current can only be provided only with one polarity. In a practical product implementation, the welding arc typically reignites spontaneously without requiring any stabilizing current, but to ensure reliable ignition and reignition in all situations in AC welding, both the ignition voltage pulse and the stabilizing current are preferably provided with both, alternating polarities of AC welding.

[0055] Figure 5 illustrates a welding circuitry with a pulse generating circuit 15 according to some embodiments, in which the pulse generating circuit 15 is further improved to provide ignition voltage pulses and stabilizing currents with two alternating polarizations, in dependence of present polarization of the AC supply voltage from the power supply 10. This is achieved by coupling the DC power source 45 between two primary windings of the pulse transformer 12 and providing the pulse generating circuit 15 with two switches 44a and 44b coupled to opposite ends of the two primary windings of the pulse transformer 12. The two switches 44a, 44b thus enable control of coupling the DC power source 45 to either of the primary windings, such that the current coupled to these two primary windings have opposite polarities. By alternately switching the two switches 44a and 44b according to changes of polarity of the AC voltage provided by the power supply 10, the pulse generating circuit 15 thus can provide both the ignition voltage pulse for spark ignition, and the stabilizing current for stabilizing the welding arc, with alternating polarities, so that the stabilizing current is provided after each zero crossing of the AC current of the power supplylO. Timing of the stabilizing current is not determined by any dedicated timing arrangement or signal, but it occurs spontaneously after the ignition voltage pulse, and timing and duration of the stabilizing current are determined by structure of the pulse transformer, welding current and welding voltage. A voltage-time area of the stabilizing current is constant: the higher the voltage during the stabilizing current, the shorter the stabilizing current. Triggering generation of the ignition voltage pulse is timed by actively detecting welding current and indicating a change of a half-cycle (zero crossing) of the AC current, which is used for triggering generation of the next ignition voltage pulse and the stabilizing current. This control operation is active and may be controlled by a controller device comprised in the welding machine. The controller preferably provides signals for controlling the operation of the switch 44 or switches 44a / 44b.

[0056] Figure 6 illustrates schematically a pulse generating circuit 15 according to an exemplary implementation. Switches 44a and 44b are implemented with switches VI and V2, which may be implemented for example using Insulated Gate Bipolar Transistors (IGBT) or Field effect transistors (FET). IGBTs, as shown in the Figure 6, are more economical than FET's. Only fairly modern IGBTs made available in market since about change of millennium have sufficiently good performance to be applicable. Diodes V3 and V4 ensure that no negative voltage from the primary winding can reach collectors of transistor switches VI, V2 when the other one of the switches VI, V2 is used. Snubber capacitors Cl and C2 protect transistor switches from being destroyed by voltage spikes caused by opening the respective transistor switch VI or V2.

[0057] A like diode and snubber capacitor arrangement is applicable to protect a switch of a one-sided pulse generating circuit illustrated in the figure 4. Figure 7A illustrates current over one of the switches (VI or V2) and the associated snubber capacitor (Cl or C2) during one operation period thereof. After switching the transistor switch (VI, V2) on (closing the switch) at time instant tl, current in the transistor switch (VI, V2) increases according to equation (1) when fed from the DC power source 45. dI / dt=E / Lp (1)

[0058] Wherein E is the DC voltage provided by the DC power source 45, and Lp is a magnetizing inductance of the respective primary winding of the pulse transformer 12. At time instant t2 the transistor switch (44a or 44b) is switched off (the switch is opened), whereafter current flows only through the respective snubber capacitor (Cl or C2) and quickly returns to zero.

[0059] Figure 7B illustrates voltage over one of the transistor switches VI and V2 during one operation period. Peak voltage of the generated over the respective transistor switch VI or V2 is determined by equation (2).

[0060] Vcep=Icp* lp / Cx (2) wherein Icp is peak current of the switch (44a, 44b) at the time t2 of opening the respective transistor switch, Lp is magnetizing inductance of the primary winding and Cx is capacitance value of the respective snubber capacitor Cl or C2.

[0061] The ignition voltage peak 85 occurs at time t3. The exact timing of the ignition (t3) cannot be explicitly controlled because it is based on a stochastic process. Ignition by the spark occurs when dielectric strength between the electrode and the workpiece is exceeded. Rise time of the ignition voltage pulse to its peak value depends on the structure of the pulse transformer, the snubber capacitor (Cl or C2) and breaking current of the switch. When the spark is generated at time t3, peak voltage in level of few kilovolts quickly decreases to less than 100 V, and current in the welding circuit quickly increases to a high enough level to maintain the welding arc due to the stabilizing current provided by the pulse generating circuit. A current provided to the welding torch further rises to the desired welding current provided by the power supply with some delay. The ignition process and the welding current are dependent on many factors, such as design and characteristics of the welding torch and its electrode, workpiece, distance between the electrode and the workpiece, as well as arc voltage. Therefore, dimensioning for electrical parts of the pulse generating circuit to achieve the desired, stable welding arc is preferably determined individually / experimentally for each type of welding apparatus, and fine tuning may further be made by enabling the user to adjust various welding parameters to best fit to the present welding task at hand.

[0062] Figure 8 illustrates schematically welding current and voltage, switch current, as well as mutual timing of steps of the method in case of a polarity change of an AC welding current. Time, current and voltage scales in the figure 8 are arbitrary to enable illustrating different steps of the ignition process that involve quantities having phenomena with both narrow and wide value ranges and both slow and very fast occurring steps. In real life, polarity change of the welding current (to to t3) occurs in a very short time period, for example less than 100 ps, arc voltage falls from ignition voltage pulse of several kilovolts to an arc voltage value of about 20 V typically in less than 100 ns, whereas length of an operation period of the AC welding current may vary within range of about 5 to 20 ms. The stabilizing current is provided from the pulse transformer for a short period in a range from one to a few microseconds after the ignition voltage pulse at t3, referred herein as the stabilizing current period 90. Figure 9 illustrates measured ignition voltage pulse and stabilizing current of an exemplary pulse generation circuit with actual measured timings.

[0063] Figure 8 illustrates a polarity change from negative to positive welding current. Similar steps are performed for a polarity change from positive to negative welding current, but with opposite polarity. Upper graph illustrates welding current and middle graph illustrates voltage between output connectors of the power supply 10. Bottom graph illustrates switch current over one of the switches of the pulse generating circuit 15. The pulse generating process illustrated in the figure 8 is also applicable to initial ignition of the welding arc, in which case the process can be considered to start from tl, i.e. that the welding current and the power supply is active. The power supply is active when its output voltage is not zero. When the pulse generating circuit 15 is applied for spark generation upon initiating a welding process, the applicable switch (44, 44a, 44b) is preferably switched on in response to a start command received by the welding apparatus from a user via a user interface, and the switch (44, 44a, 44b) is switched off upon determining that the current in the primary winding has reached a predetermined current threshold value. A start command may be received from the user interface for example by using a start button or equivalent in a user interface of the welding apparatus. In this context, in response to the start command means either immediately or after a small delay. A small delay may be for example due to a short period of time for allowing welding gas to flow before igniting the welding arc, which is known in the art as pregas.

[0064] The power supply 10 feeds desired welding current to the welding arc until time tO. At time tO, welding current fed from the power supply 10 towards the welding torch begins to decrease. Any known type of power supply 10 suitable for feeding a welding current is applicable. A typical characteristic of the power supply 10 is that when polarity of the output current is changed, there is a short period (in level of tens of microseconds) of no output current, and the output current decreases and increases with some delay. A characteristic feature of a practical welding apparatus involving inductance is that current in the welding circuit does not immediately drop to zero even if the power supply is switched off. Likewise, full output current is not immediately available after switching the power source on again, and a short period of no output current may exist upon switching the power supply on again with the opposite polarity. For example, welding apparatuses using an inverter for producing the welding current behave like that.

[0065] The speed of decreasing of the welding current in dependence of time constant of the welding circuit. The welding circuit comprises the power supply, the electrode of the welding torch, the welding arc, the work piece, the secondary winding of the pulse transformer, as well as welding cables. In this context, welding cables refer to cables between the welding machine and the welding torch and between the welding machine and the workpiece. Only after the welding current is decreased down to a predetermined threshold value, polarity of the power supply is changed. As a result of changing the polarity of the power supply, the welding current becomes quickly zeroed. In response to zeroing the welding current, the power supply is restarted with an opposite polarity, and one of the switches 44a, 44b of the pulse generating circuit 15 is switched on (tl), so that current starts to flow in the pulse generating circuit including but not limited to the primary winding of the pulse transformer, in case of two primary windings, in the relevant primary winding of the pulse transformer with desired polarity as shown in the figure 7A. When the current in the primary winding of the pulse transformer has reached a desired current threshold value at time t2, the respective switch 44a, 44b is switched off. In tests performed with an existing welding apparatus, a feasible current threshold value has been found between 100 A and 300 A. This causes voltage in the secondary winding of the pulse transformer 12 to peak up to a level of a few kilovolts at t3, depending on dimensioning of the pulse transformer, which causes a breakdown in the electrode of the welding torch and generates a conductive channel between the electrode welding torch and the workpiece, which causes a spark, but also a sudden voltage drop.

[0066] The primary current of the pulse transformer 12 commutes into the welding arc via the secondary winding, thus providing the stabilizing current, its voltage value being reduced by the conversion ratio of the pulse transformer. This short period of stabilizing current is seen as a short period of constant value of the welding current before the welding current from the power supply increases to a level that is sufficient to maintain the welding arc. This stabilizing current thus enables stabilizing the welding current until the power supply can provide the desired welding current that further stabilizes the welding arc. In addition to AC welding, a polarity change of the welding current may occur also when the welding is first started using one polarity and thereafter, polarity of the welding current is reversed. The same steps apply when initiating welding, the only difference being that there is no power provided from the power supply before tl. Generation of the ignition voltage peak can be initiated upon first activating the power supply (tl). Another possible igniting model is that welding is initiated with the welding current having a first polarity and polarity of the welding current is switched right after ignition.

[0067] Figure 9 illustrates measured values of ignition voltage pulse and stabilizing current in an exemplary welding apparatus with a welding torch acting as load. Output is from a real oscilloscope, and each curve has its own magnitude scale. The time scale is common to all curves, 2 ps per division, with 0 marked at the time t3 of the spark igniting the welding arc.

[0068] Curve 91 illustrates output voltage. The power supply is switched on at time tl and the igniting voltage pulse occurs at time t3. The peak value of the igniting voltage pulse in level of several kilovolts is outside the shown voltage scale the graph, which is 50 V per division for the output voltage curve 91, Immediately after the igniting voltage pulse, the output voltage falls to an arc voltage level, which is about 25V. Fluctuation shown in the measured output voltage after the ignition voltage pulse at t3 is caused by measurement circuitry rather than any actual fluctuation of the output voltage.

[0069] Curve 93 illustrates current in the primary winding of the pulse generating circuit. Scale is 100A per division. The stabilizing current is commuted to the secondary winding of the pulse transformer over the stabilizing current period 90, here lasting until about 2 ps has passed since the igniting voltage pulse at t3. By the end of the stabilizing current period 90, all available energy from the secondary winding of the pulse transformer has been combined with output current provided form the power supply to increase the welding current. After end of the stabilizing current period 90 onwards at time t4, and the power supply produces sufficient welding current.

[0070] Curve 92 illustrates resulting welding current. Amplitude scale of the welding current is 10 A per division. To the left of time t3, the welding current is zero even after the power supply is switched on at tl. The welding current starts to flow in response to igniting the welding arc at t3. At this time, output current from the power supply is not yet in sufficient level for stabilizing the welding arc, but additional stabilizing current is provided from the secondary winding of the pulse transformer of the pulse generating circuit for about 2 ps, after which the welding current from the power supply is high enough to maintain the welding arc. After end of the stabilizing current period 90 (t4), it can be noted that the welding current still rises towards its intended value. Thus, the pulse generating circuit enables the welding current to rise very quickly after the igniting voltage pulse to a level that is sufficient to stabilize the welding arc, and no separate auxiliary voltage pulse is needed for reigniting the welding arc as in the prior art solutions.

Claims

CLAIMS1. A method for igniting and stabilizing a welding arc in a welding apparatus comprising:- a power supply configured to provide a welding current,- a welding torch configured to receive the welding current for its electrode from the power supply,- a grounding clamp configured to be coupled with a work piece and to a ground contact of the power supply, and- a pulse generating circuit comprising a pulse transformer comprising at least one primary winding and a secondary winding, the method comprises:- providing, from the secondary winding of the pulse transformer coupled in series between the power supply and the welding torch, an ignition voltage pulse to the electrode of the welding torch, wherein the ignition voltage pulse is configured to ignite the welding arc upon initiating the welding current,- switching on a switch coupled to the primary winding of the pulse transformer to feed a current from a DC power source into the primary winding, and- generating the ignition voltage pulse in response to switching off the switch, and- further providing, from the secondary winding of the pulse transformer, a stabilizing current after the ignition voltage pulse, wherein the stabilizing current combines with the current provided by the power supply and thus facilitates maintaining the welding arc until the power supply provides a sufficient welding current to maintain the welding arc,- characterized in that the method comprises:- switching off the switch upon or after determining that a zero crossing of the welding current has occurred, and the power supply is active.

2. The method according to claim 1, wherein the welding current is an AC welding current, and the method comprises stabilizing the welding arc by intermittently generating by the pulse generating circuit the ignitionvoltage pulse and the stabilizing current after zero crossings of the AC welding current.

3. The method according to claim 1 or 2, wherein during ongoing welding, the switch is configured to be switched on upon determining that the welding current is zero, and the power supply is on, and wherein the switch is configured to be switched off upon determining, that the current in the primary winding has reached a predetermined current threshold value.

4. The method according to claim 1 or 2, wherein the switch is configured to be switched on in response to receiving a start command from a user, and the switch is configured to be switched off upon determining that the current in the primary winding has reached a predetermined current threshold value.

5. The method according to any one of claims 1 to 4, pulse generating circuit comprises two switches, the pulse transformer has two primary windings and one secondary winding, and each switch is coupled to a first end of a different one of the two primary windings of the pulse transformer, and the DC power source is coupled to a second end of each of said two primary windings, said second ends being mutually coupled, and the method comprises:- operating said two switches alternately, wherein each switch is closed and opened intermittently in dependence to one of the two polarities of zero crossings of an AC welding current provided by the power supply.

6. A circuitry for igniting and stabilizing a welding arc in a welding apparatus, the welding apparatus comprising:- a power supply configured to provide a welding current,- a welding torch configured to receive the welding current for its electrode from the power supply,- a grounding clamp configured to be coupled with a work piece and to a ground contact of the power supply, and- a pulse generating circuit comprising a pulse transformer, wherein the pulse transformer comprises at least one primary winding and a secondary winding, wherein the secondary winding is coupled in series between the welding torch and an output of the power supply, wherein the pulse generating circuit comprises:- a DC power source coupled to the at least one primary winding,- at least one switch for enabling and disabling current from flowing from the DC power source to the at least one primary winding, wherein the secondary winding is configured to provide an ignition voltage pulse to the welding torch upon opening the respective at least one switch, and wherein the secondary winding of the pulse transformer is further configured to provide a stabilizing current after the ignition voltage pulse, wherein the stabilizing current combined with the current provided by the power supply facilitates maintaining the welding arc until the power supply provides a sufficient welding current to maintain the welding arc, characterized in that the respective at least one switch is configured to be opened upon or after determining that a zero crossing of the welding current has occurred, and the power supply is active.

7. The circuitry according to claim 6, wherein the switch is a transistor, and the switch is coupled to the respective end of the primary winding via a diode for disabling negative voltage peaks from the transformer to reach the switch.

8. The circuitry according to claim 6 or 7, wherein the switch is a transistor, and the switch is coupled in parallel with a snubber capacitor for protecting the switch from a voltage spike caused by opening the switch.

9. The circuitry according to any one of claims 6 to 8, wherein said at least one switch is two switches, the pulse transformer includes two primary windings and one secondary winding, wherein each switch is coupled to a first end of a different one of the two primary windings of the pulsetransformer, and the DC power source is coupled to a second end of each of said two primary windings, said second ends being mutually coupled.

10. The circuitry according to claim 9, wherein said two switches are configured to be operated alternately, and each switch is closed and opened intermittently in dependence to one of the two polarities of zero crossings of an AC welding current provided by the power supply.

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