DE10122104C1 - Arc welding device with HF blocking choke and additional secondary winding - Google Patents

Abstract

The invention relates to an arc welding device comprising an inverter current source with a rectifier (1) supplied with line voltage, an intermediate circuit (2), a current transformer clocked on the primary side, a rectifier (5) arranged on the secondary side of the current transformer (4), and an inverter powered by the inverter current source the output of which is connected to the welding process, a high-voltage pulse generator (19) being connected in parallel with the inverter output to ignite the arc and which is decoupled from the inverter output via a blocking choke (14). In order to prevent non-linearities in the reversal of the welding current even when using blocking chokes with a non-linear magnetic characteristic, the invention is characterized in that the blocking choke (14) formed with a core made of magnetically saturable material forms the primary side of a transformer (20), which has at least one turn on the secondary side, which can be short-circuited via controlled switching means (22, 23) during the arc-burning phase.

Description

The invention relates to an arc welding device comprising an inverter current source with a rectifier ( 1 ) supplied with mains voltage, an intermediate circuit ( 2 ), a current transformer clocked on the primary side, a rectifier ( 5 ) arranged on the secondary side of the current transformer ( 4 ), and an inverter powered by the inverter current source Output of the welding process is connected, a high-voltage pulse generator ( 19 ) being connected in parallel with the inverter output to ignite the arc and which is decoupled from the inverter output via a blocking choke ( 14 ).

An arc welding device of this type is known from the prior art of DE 41 28 175. In this arc welding device, a high-voltage pulse generator is connected to the output of the inverter, which emits a high-voltage pulse at the start of the welding process and after each zero crossing of the welding current when the arc has extinguished (see Fig. 1, 2). Since the amplitude of the high-voltage pulses is of the order of magnitude of approx. 10 KV, a blocking choke is connected in series in order to avoid high-voltage effects on the upstream inverter current source circuit. In such so-called non-contact high-voltage ignition devices such blocking chokes are typically used, which typically consist of a few turns, for. B. 10 to 20 exist and have a magnetic core. For example, ferrite cores are common here. On the one hand, this results in the desired blocking effect against the reaction of the high voltage and, on the other hand, a choke of this type does not have a disadvantageous effect on the welding behavior due to its small number of turns in continuous current operation.

However, the non-linearity of the magnetic characteristic of a blocking choke with ferrite core, especially with AC welding, that's why disadvantageous, because the reversal of the Welding current in the area of the zero crossing results in non-linear behavior of the current characteristic. This is due to the fact that the inductance of a Blocking choke with ferrite core in the area outside the magnetic saturation, especially for core designs without air gap, increases sharply.

From US 4,484,059 an infrared sensor for Arc welding known for direct observation of the Welding arc during arc welding.

Proceeding from this, the object of the invention is in an arc welding device of the type mentioned Kind even when using blocking reactors with a nonlinear magnetic characteristic curve nonlinearities to prevent when the welding current is reversed.

This object is achieved in that the one with a core made of magnetically saturable material trained blocking choke the primary side of a Transformer forms, which at least on the secondary side has a turn which is controlled  Switching means during the arc burning phase can be short-circuited.  

The invention is characterized in that on the one hand the effect of the blocking choke for decoupling the High voltage from the inverter output is retained and that, on the other hand, the above non-linear ones Properties of the throttle can be avoided. Through the "Extension" of the blocking choke to a transformer with it has a turn that can be short-circuited on the secondary side then when its core is in the magnetically unsaturated State located, the effect of a transformative Short circuit. This means that the throttle when it is in the magnetically saturated state, the maintains appropriate inductive properties and in magnetically unsaturated state de facto one Represents bridging. This solution shows the difference to the solution that is based on the prior art obvious way, namely that there existing RF blocking choke after the ignition of the Arc over a switch to bridge one special advantage: With a bridging switch the total welding current during the period of Welding process are carried out, so that at least Current values of typically 400 A permanently over a those usually implemented as semiconductor switches Switches must flow. In contrast, the solution according to the invention with a saturated core low inductance of the blocking choke practically not out. When the current drops to a value below the Core saturation is due to the change in flow in the Transformer core induces a voltage on the secondary side. Therefore, on the secondary side of the transformer only briefly made a current load, namely exclusively in the area of the zero crossing of the Welding current when the choke is in the unsaturated Condition. If the current increases again, so that the  Core becomes saturated again, the secondary turn loses of the transmitter again their influence.

This results in a conceivable circuit effective way of effectively suppressing the Non-linearity of the ferrite core blocking inductor Non-linearity at zero crossing of the welding current.

The turn on the secondary side is according to the invention during the arc burning phase is always closed, whereby only if the arc is to be ignited so the amount of welding current is zero, must be open. There is a detector for this provided, which according to the invention to different Can be configured, namely either as Current detector means for determining whether the amount of Welding current deviates from zero when Voltage detector means or as an optical detector means observing the arc.

Further advantageous embodiments of the invention result from the subclaims.

The invention is described below with reference to drawings explained in more detail. Show

Fig. 1 shows an embodiment of an arc welding machine, as is known from the prior art:

Fig. 2 is a graph of the welding current over time in the case of current reversal, as is common in the prior art;

FIG. 3 shows the design of the blocking inductor in accordance with embodiments of the present invention;

FIG. 4 shows the time course for the reversal of the welding current in accordance with the present invention;

Fig. 5 shows the time course of the secondary short-circuit current according to the present invention.

Fig. 1 shows a supplied by a three-phase network L1, L2, L3 rectifier 1 which feeds a an intermediate circuit forming capacitor 2, which provides a downstream seienrseits him DC converter 4. The current transformer 4 is clocked on the primary side via semiconductor switches 3 a, 3 b, such that a clock frequency in the medium frequency range of, for example, 25-100 kHz is produced on the primary side.

On the secondary side, the current transformer 4 is connected to a rectifier diode 5 , the free-wheeling diode 6 being connected in parallel with the series circuit comprising the secondary winding of the current transformer 4 and the rectifier diode 5 .

The circuit described so far represents a so-called Inverter power source, which is the following descriptive circuit part supplied with direct current.

To smooth the output current of the inverter current source, a smoothing choke 7 is provided, which is followed by an inverter. The inverter is formed by a bridge circuit made up of four semiconductor switches 12 a to 12 d, which can be designed, for example, as transistors. Each of the semiconductor switches 12 a to 2 d, a free-wheeling diode 13 a to 13 d is connected in parallel. At the output of the inverter, the welding process is connected, which is shown in the drawing by the welding electrode 15 and the ground connection 16 . A high-voltage pulse generator 19 is connected in parallel with the inverter output and emits a high-voltage pulse at the start of the welding process and after each zero crossing of the welding current. A blocking choke 14 is connected in series to decouple the high-voltage pulses from the inverter output.

The function of the circuit shown in FIG. 1 is explained in more detail below:
To supply the welding process 15 , 16 with an alternating current I _{S of} the desired alternating frequency, for example 50 to 500 Hz, the semiconductor switches 12 a to 12 d are clocked accordingly at their base connections so that in a first switch position the ones from the semiconductor switches 12 a and 12 d formed diagonal of the inverter is live and the diagonal formed from the semiconductor switches 12 b and 12 c is currentless. In this phase, the current flows from the inverter current source via the smoothing inductor 7 , the semiconductor switch 12 a, the inductance 14 , the welding process 15 , 16 and via the semiconductor switch 12 d.

During the second half-wave of the alternating current, the bridge diagonal formed by the semiconductor switches 12 b and 12 c is live and the bridge diagonal formed by the semiconductor switches 12 a and 12 d is currentless. Therefore, current flows in the second half-wave from the inverter current source via the smoothing inductor 7 , the semiconductor switch 12 b, the welding process 16 , 15 , the inductor 14 and the semiconductor switch 12 c.

The commutation, i.e. the transition from one bridge diagonal to the other, is now carried out in such a way that the previously closed semiconductor switches 12 a, 12 d and 12 b, 12 c are opened and after the commutation time has elapsed, the semiconductor switches of the bridge diagonal that is then live are closed ,

Fig. 2 shows the course of the welding current I _S at the transition from the value + I _S to -I _S and vice versa. After the start of the current reversal, the current I _S initially drops linearly until the upper saturation point is reached, ie up to which the core of the RF blocking choke 14 is still in saturation. If the welding current drops further, the magnetic characteristic of the ferrite core of the blocking choke changes, as a result of which the current drops much more slowly than in the area of saturation. As a result, there is a kink in the current profile curve according to FIG. 2, which extends to the lower saturation point, below which the ferrite core in turn saturates. The same applies to the transition from -I _S to + I _S (second section of the course in FIG. 2).

According to the solution according to the invention, the inductance is modified according to FIG. 3. A transformer 20 is provided, the primary winding of which forms the blocking choke 14 through which the welding current I _S flows and which has a turn 21 on the secondary side, which can be short-circuited by a switch 22 , so that the short-circuit current I _{K is} conducted in it. The transmission ratio of the transmitter 20 is preferably 10: 1.

The switch 22 is controlled by means of a detector 23 which detects whether the arc is ignited or not. When it is ignited, switch 22 is closed. The actual value detection device 24 for the welding current shown in FIG. 3 serves as the control means for the detector 23 . As an alternative to this, the arc voltage can also serve as an input variable or an optical signal with which it is determined whether the arc is burning or not.

The circuit variant according to the invention works as follows (see FIGS. 4 and 5):
After the arc is ignited, the switch 22 is closed by the detector device 23 . In the case of a high amount of welding current, the choke 14 works on the primary side of the transformer 20 in the area of the core saturation, ie it typically has a value of approximately 10 μH. If the current I _{S is} now reduced ( FIG. 4), the winding 21 of the transformer 20 which is short-circuited on the secondary side does not have an effect until there is no change in flow within the ferrite core, ie as long as it is still in the saturated state. However, if it gets out of the saturation state, a voltage is induced on the secondary side which drives a short-circuit current I _K through the winding 21 , the magnitude of which depends on the winding ratio of the primary current, that is to say the welding current. At the moment when the core leaves saturation, the current peak of the short-circuit current IK shown in FIG. 5 arises. If the current continues to decrease, the short-circuit current also decreases linearly in accordance with the curve shown, until the lower saturation point is reached, at which the short-circuit current I _{K reaches} its maximum negative value. After saturation is reached and the flux change in the ferrite core is brought to zero, the short-circuit current jumps back to zero. A corresponding course results when the transition from -I _S to + I _S , as is also shown in FIGS. 4 and 5.

According to the solution proposed according to the invention, the inductance thus acts as a transformer short-circuited on the secondary side, the turns ratio of which is optimally designed if the number of turns 1 on the secondary side and the number of turns on the primary side is approximately 10.

Compared to the common solution, which is the blocking choke during the burning phase of the arc to bridge over a semiconductor switch the solution according to the invention, apart from the above advantages described, also by simple circuitry measures without additional high voltage resistant components would be required.

Claims (9)

1. Arc welding device consisting of an inverter current source with a rectifier ( 1 ) supplied with mains voltage, an intermediate circuit ( 2 ), a current transformer clocked on the primary side, a rectifier ( 5 ) arranged on the secondary side of the current transformer ( 4 ), an inverter ( 13 a- 13 ) fed by the inverter current source d), at the output of which the welding process ( 15 , 16 ) is connected, a high-voltage pulse generator ( 19 ) being connected in parallel with the inverter output to ignite the arc and which is decoupled from the inverter output via a blocking inductor ( 14 ), characterized in that the A core made of magnetically saturable material, the blocking choke ( 14 ) forms the primary side of a transformer ( 20 ), which has at least one turn on the secondary side, which can be short-circuited via controlled switching means ( 22 , 23 ) during the arc-burning phase. 2. Arc welding device according to claim 1, characterized in that the core is a ferrite core. 3. Arc welding device according to claim 1 or 2, characterized in that the turns ratio between the primary side and secondary side of the transformer ( 20 ) is at least 5: 1, preferably 10: 1. 4. Arc welding device according to one of the preceding claims, characterized in that the switch ( 22 ) is a semiconductor switch. 5. Arc welding device according to claim 4, characterized in that the Semiconductor switch a thyristor, a transistor, a Is IGBT or an FET. 6. Arc welding device according to one of the preceding claims, characterized in that the switch ( 22 ) can be controlled by a detector device ( 23 ), which has as a control condition for the switch ( 22 ) that the amount of the welding current (I _S ) is greater than zero is. 7. Arc welding device according to claim 6, characterized in that the detector device ( 23 ) is assigned an actual current detection ( 24 ). 8. Arc welding device according to one of the preceding claims, characterized in that the switch ( 22 ) can be controlled via a detection device for the arc voltage. 9. Arc welding device according to one of the preceding claims, characterized in that the switch ( 22 ) can be controlled via an optical detection means.