JPS6232693Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an improvement of an AC arc welding machine, particularly an AC arc welding machine suitable for TIG arc welding using a tungsten electrode and an inert gas.

When TIG welding aluminum or its alloys, alternating current is used because it is necessary to take advantage of the so-called cleaning effect, which destroys the strong oxide film existing on the surface of aluminum with a reverse polarity current in which the tungsten electrode has a positive potential. However, when alternating current is used in TIG arc welding of aluminum, etc., due to the difference in electron emission ability between the tungsten electrode and the material to be welded, such as aluminum, the tungsten electrode has a negative potential, resulting in a reverse polarity during the positive half-wave. There is a so-called rectification effect in which a larger current flows than in a half wave, which causes an unbalance in the alternating current waveform, reducing the cleaning effect and causing a current containing a direct current to flow through the welding transformer, causing the iron core to Unbalanced magnetic saturation caused the primary current to become excessive, causing the windings to overheat and sometimes burn out.

In order to eliminate the above problems, various attempts have been made to reduce the unbalanced current by connecting a resistor, reactor, capacitor, current, etc. in series to the secondary output side of the transformer. Among these, one known as one that is superior in terms of power loss and cost is one that combines a reactor and a diode to balance both forward and reverse currents, as shown in FIG.

To explain the device shown in Fig. 1, numeral 1 is a welding transformer with adjustable output, such as a moving core type leakage transformer, which is supplied with alternating current power from input terminals 2a and 2b, and arc welded to secondary terminals 3a and 3b. Outputs low voltage and large current suitable for 5 is a current limiting reactor, and 6 and 7 are diodes connected with opposite polarities. 4a and 4b are output terminals, and when the polarities of the diodes 6 and 7 are determined as shown in the figure, a tungsten electrode 8 is connected to the terminal 4a, and a welding material 9 is connected to the terminal 4b, and a welding arc is generated by a high-frequency voltage superimposing device (not shown) or the like. Start up and perform TIG welding. With the configuration shown in the figure, the positive polarity current is limited by the reactor 5, and the reverse polarity current is not limited in any way because it passes through the diode 7. The rectifying effect that occurs between the current and the object is canceled out, and the positive and reverse polarity currents become balanced.

By the way, if we examine in detail the current flowing through each diode in the conventional device shown in FIG. It turns out that it can also be reached. That is, when the secondary terminal 3b of the welding transformer 1 has a positive polarity, the diode 6 becomes forward biased, and a current flowing from the diode 6 toward the secondary terminal 3a of the welding transformer 1 flows through the reactor 5. , and flows with a phase lag determined by the power factor of the entire circuit including the reactor 5 and the welding arc section. When this current reaches its peak point, the energy stored in the coil due to the current flowing through the reactor 5 causes the reactor 5 to e=-L due to changes in its inductance L and current i.
An electromotive force of di/dt is generated, and a circulating current I _l flows in the same direction as before through the diode 7 and the diode 6.

When the phase of the power supply voltage is reversed and the output terminal 3a of the welding transformer 1 becomes positive, the polarity is reversed and after the power factor angle described above, the diode 7 also receives a reverse polarity current _Ir supplied from the welding transformer 1. As a result, when the polarity is reversed, a large current of (I _l +I _r ), which is the sum of both, will flow. Furthermore, this circulating current I _l is a transient current that takes the current I _s at the time of positive polarity as its maximum value and attenuates with a time constant L/R (R is the DC resistance of the circuit through which the circulating current I _l flows). The reactor 5 is normally designed to have a small DC resistance in order to minimize resistance loss, and the circuit in which the aforementioned current I _l circulates also has a large resistance value. Therefore, the above-mentioned time constant L/R is quite long, and the circulating current I _l remains at a large value for a quite long time. Therefore, a current having a peak value approximately twice as high as the output current of the welding machine flows through the diode 7 during a half-wave of opposite polarity. In addition, in the half-wave of positive polarity, a current I _s which is the output current of the welding machine flows through the diode 6, and in the half-wave of reverse polarity, a circulating current I _l having a value almost equal to the current I _s flows, and as a result, the diode 6 6, the welding machine output current or a current close to this always continues to flow. In this way, due to the presence of the reactor 5, although the peak value of the current flowing through the diode 6 does not increase, its conduction period nearly doubles, and since the peak value of the current flowing through the diode 7 nearly doubles, the average current is also 2. Almost double. For this reason, it is necessary to use diodes 6 and 7 with a much larger capacity than the diode required for half-wave rectification corresponding to the output current of the welding machine, which not only makes the device large and expensive, but also requires the use of diodes. Heat generation due to forward voltage drop was also large (approximately 4 times), making it uneconomical.

The present invention further adds a third diode to the conventional device shown in FIG. 1 and connects it in parallel with the reactor 5.
By making the polarity the same as that of the second diode 7, the energy stored in the reactor 5 when the polarity is positive is released through the added third diode. All diodes can be of small capacity.

FIG. 2 is a connection diagram showing an example of the AC arc welding machine of the present invention, in which a third diode 10 is added compared to FIG. 1. Considering the case of positive polarity in the figure, the current flows from the tungsten electrode 8 to the first diode 6, as in FIG.
It flows through the reactor 5 and does not flow anywhere else. Next, when the polarity becomes reversed, the current flowing in the reactor 5 is transferred to the first circuit around the second diode 7 and the first diode 6, and the third diode 1.
The current will be shunted into a second circuit that only passes through 0, but in reality, this first circuit passes through two diodes 6 and 7, so the circuit resistance is much larger than that of the second circuit. . Therefore, most of the discharge current I _l from the reactor 5 flows into the second circuit. Therefore, when the polarity is reversed, almost no current flows through the diode 6, only the output current I _r flows through the diode 7, and only the discharge current I _l of the reactor 5 flows through the diode 10. As a result, a current with a peak value equal to each half-wave of the welding current flows through each diode through half-waves of both positive and reverse polarities only every other half-wave, which is not necessary in the conventional example shown in Fig. 1. This means that the capacity of the diode is half that of the diode. The price of diodes increases dramatically as their capacity increases, so even if three low-capacity diodes are required in this invention, the cost will be less than half that of the original two large-capacity diodes. . Of course, the connection positions and connection directions of the diode and reactor are not limited to the example shown in Figure 2, and it is also possible to add a high-frequency voltage superimposition device to the same figure to start and re-ignite the welding arc. It does not depart from the spirit of the present invention to facilitate this.

As described above, according to the present invention, the capacity of the first and second diodes can be reduced to about half of the conventional capacity by simply adding one small-capacity diode, making the device small and inexpensive. Also, the current flowing through the diode is halved, so heat is generated.
It has many practical effects, such as being 1/4 that much easier to cool.

[Brief explanation of the drawing]

FIG. 1 is a connection diagram showing a conventional device, and FIG. 2 is a connection diagram showing an AC arc welding machine of the present invention. 1... Welding transformer, 5... Reactor, 6...
First diode, 7...Second diode, 1
0...Third diode.

Claims (1)

[Claims for Utility Model Registration] 1. A welding transformer, a reactor connected between a secondary terminal and an output terminal of the welding transformer, and a first
a series circuit with a diode, a second diode connected in parallel with the series circuit and with a polarity opposite to that of the first diode, and a second diode connected in parallel with the reactor and with the same polarity as the second diode. AC arc welding machine comprising a third diode. 2. The AC arc welding machine according to claim 1, wherein the welding transformer is a leakage transformer whose output current is adjusted by a movable iron core. 3. The AC arc welding machine according to claim 1, wherein the first diode and the third diode are diodes with approximately the same current capacity.