JPH078435B2 - AC arc welding equipment - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AC arc welding apparatus used for welding metal such as aluminum, and more particularly to a secondary side for exchanging a DC current with an AC current close to a square wave by switching a transistor. The present invention relates to an inverter control type AC arc welding device.

2. Description of the Related Art In a secondary-side inverter-controlled AC arc welding apparatus, when reversing the polarity of current, a large electromotive force is generated due to the inductance of a cable connected to the outside of the welding apparatus. In the conventional AC arc welding apparatus of this type, the following device has been attempted in order to prevent the transistor from being destroyed by the electromotive force due to the inductance of the cable as described above.

As a first method, a transistor 1 as shown in FIG.
There is a method of providing a snubber circuit between the collector and the emitter of the. According to this method, an electromotive force due to the inductance 4 of the external output cable 6 generated by the general connection state of the conventional AC arc welding apparatus 5 as shown in FIG. It is intended to be suppressed by the circuit.

As a second method, a method of controlling the output waveform of the conventional AC arc welding apparatus as shown in FIG. 5 has been tried. In this method, when reversing the polarity of the current, the polarity is reversed only through a small output current value I ′ for a certain period Tw. In this case, since the change in the welding output current I ₀ is small, the electromotive force due to the inductance 4 of the external cable is naturally small.

Problems to be Solved by the Invention In the conventional configuration as described above, in order to suppress the electromotive force generated by the cable 6 of the AC arc welding apparatus 5 of FIG. 4, the transistor 1 as shown in FIG. The snubber circuit has a problem in that it requires a very large capacity for the resistor 2 and the capacitor 3 and is not economical, and the power loss due to the snubber circuit cannot be ignored. further,
The method of controlling the output waveform of the AC arc welding apparatus as shown in FIG. 5 is suitable for actual welding because the output current value is reduced to I ′ for a certain period Tw during polarity reversal. There is a problem in that the effect of trying to obtain an AC output current close to a square wave by using a transistor without damaging the waveform is impaired.

The present invention solves the above conventional problems, without causing a large power loss due to the electromotive force due to the inductance of the cable routed to the outside, and without impairing the waveform of the welding output. An object of the present invention is to provide an AC arc welding device capable of preventing the transistor from being destroyed by an electromotive force.

Means for Solving the Problems In order to solve the above problems, the AC arc welding apparatus of the present invention is an AC arc welding apparatus that obtains an AC output to a welding load by switching a transistor connected to a DC power supply. A resistor connected in parallel to the collector and emitter of the transistor, with a cable connecting the welding torch and the DC power supply to the switching transistor.
A series circuit of elements (hereinafter referred to as varistor) whose resistance value suddenly drops when the voltage exceeds a certain value, a comparison circuit that compares the voltage across the resistance with a predetermined voltage, and an output signal from the comparison circuit. And a drive circuit that supplies a drive signal for controlling conduction of the transistor to the transistor, and is configured to stop the drive signal of the drive circuit by the output signal of the cutoff circuit. is there.

Action Due to the inductance of the output cable due to the above configuration,
The electromotive force generated when the polarity is reversed is absorbed by the resistor and the varistor connected in series to this to suppress the electromotive force applied to the transistor, and when the electromotive force due to the inductance of the output cable becomes extremely large The comparison circuit determines whether the voltage across the resistor exceeds the allowable value, and the cutoff circuit stops the output of the transistor. Therefore, it does not require a large resistance and capacitor to absorb the electromotive force, does not cause a large power loss, and the transistor is destroyed even when the output cable is used for a long time outside. Things will disappear.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic block diagram of an output side transistor driving portion of an AC arc welding apparatus according to an embodiment of the present invention. First
In the figure, 11 is a transistor, 12 is a resistor, 13 is a voltage-dependent element such as a varistor or zener diode, 14 is a comparison circuit for comparing the voltage across the resistor 12 with a predetermined voltage, and 15
Is a cutoff circuit which operates by receiving the output signal from the comparison circuit 14, 16 is a drive circuit, which supplies a drive signal for controlling the conduction of the transistor 11 to the transistor 11
Switch 11 to apply AC output to welding load.

In FIG. 1, the electromotive force generated by the inductance 19 of the cable 18 of the torch 17 is temporarily absorbed by the resistor 12 and the varistor 13 connected between the collector and the emitter of the transistor 11 to prevent the transistor 11 from collapsing. . However, an extremely large electromotive force may be generated depending on the routed state of the cable 18 on the output side. In addition, in order to absorb this electromotive force in the resistor 12 and the varistor 13 for a long time, the resistor 12 and the varistor 13 with large allowable loss are required, and when trying to protect the transistor 11 from destruction in any output state, A large-capacity resistor 12 and varistor 13 will be required.

Therefore, in order to prevent transistor destruction in an abnormal use state such as when an extremely large electromotive voltage is generated,
When an electromotive force more than that which can be absorbed by the resistor 12 and the varistor 13 is applied between the collector and the emitter of the transistor,
The voltage across the resistor 12 is compared with a predetermined value in the comparison circuit 14, and if the voltage across the resistor 12 exceeds the allowable value, the cutoff circuit
It is set to send an operation signal to 15. Cutoff circuit 15
Then, when the signal from the comparison circuit 14 is received, the drive circuit 16
A signal for stopping the drive signal of the transistor 11 is sent to the.

FIG. 2 shows the output current waveform in FIG. 1, and a square wave suitable for welding, which is different from the conventional FIG. 5, can be obtained.

As described above, according to the present invention, the series circuit of the resistor and the varistor is connected in parallel between the collector of the transistor and the emitter, and the output from the comparison circuit for comparing the voltage across the resistor with a predetermined voltage. Since the drive signal of the drive circuit of the transistor is stopped by the signal through the cutoff circuit, a large resistance and capacitor are not required to absorb the electromotive force generated when the polarity is reversed due to the inductance of the output cable. In addition, it is possible to provide an industrially inexpensive AC arc welding device without causing a large power loss and preventing the transistor from being broken no matter how the output cable is routed. Further, the output current waveform can be a square wave suitable for welding, and excellent welding performance can be maintained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram of an AC arc welding apparatus showing an embodiment of the present invention, FIG. 2 is an output current waveform diagram of the AC arc welding apparatus, and FIG. 3 is a conventional AC. An output side circuit diagram of the arc welding apparatus, FIG. 4 is a diagram showing a connection state of the conventional AC arc welding apparatus, and FIG. 5 is an output current waveform diagram of the AC arc welding apparatus showing another example of the conventional art. 11 …… transistor, 12 …… resistor, 13 …… voltage dependent element, 14 …… comparator circuit, 15 …… breaker circuit, 16 …… drive circuit, 17 …… torch, 18 …… cable, 19 …… inductance .

Claims (1)

[Claims] 1. An AC arc welding apparatus for obtaining an AC output to a welding load by switching a transistor connected to a DC power supply, wherein a switching torch is connected to a cable connecting the welding torch and the DC power supply. A series circuit of a resistor connected in parallel to the collector and the emitter of the resistor, an element whose resistance value suddenly drops when a certain voltage value is exceeded, a comparison circuit for comparing the voltage across the resistance with a predetermined voltage, and the comparison circuit. And a drive circuit that supplies a drive signal for controlling the conduction of the transistor to the transistor, and stops the drive signal of the drive circuit by the output signal of the cutoff circuit. AC arc welding equipment configured as above.