JPH0346228B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a power supply device for TIG welding, and particularly to initial current control at arc start. [Background of the Invention] In recent years, TIG welding has been developed using transistor inverters to provide constant current characteristics by performing current feedback control at a control frequency of several kHz to several tens of kHz (number of control repetitions per second). Power supplies have been put into practical use. In this type of TIG welding power supply, the output current reaches a steady value in a few ms to about 10 ms due to the high control frequency. Current feedback control was used to maintain a steady current value from . However, current feedback control systems usually have a second-order delay element, and because the damping coefficient ξ<1 is set to speed up the response, the current waveform oscillates transiently, especially in the small current range (about 5 to 20 A). If the current waveform oscillated when starting the arc, the current value could drop to zero, making the arc more likely to break. [Object of the Invention] The object of the present invention is to provide a good arc start performance that has good current control responsiveness and does not cause arc breakage even when starting an arc in a small current range.
The purpose of the present invention is to provide a power supply device for TIG welding. [Summary of the Invention] The present invention provides a TIG with constant current characteristics through current feedback control with a control frequency of 1 kHz or more.
In a welding power supply device, at arc start, the sum of steady current setting bias and initial current setting bias is applied as a reference input to the error amplifier in the current control circuit, and after detecting the initial current at arc start, the heat of the tungsten electrode is measured. By providing an initial current control means that cancels the initial current setting bias after a predetermined time required for cathodization and shifts the reference input in a slope shape to a value determined only by the steady current setting bias, it is possible to A current higher than the steady current value (about 30 to 50 A) is output for a predetermined period of time (about 5 to 15 ms) at the time of arc start at a certain temperature (about 5 to 15 ms), and after the predetermined time has elapsed, the output current is shifted to the steady value in a slope shape. It is something. [Embodiment of the Invention] An embodiment of the present invention will be described below with reference to FIGS. 1 and 2. FIG. 1a shows the main circuit section. The main circuit consists of a rectifier circuit 1 that performs full-wave rectification of three-phase AC input, and a transistor
Rectifier circuit 1 is activated by the switching operation of Tr ₁ to Tr ₄ .
an inverter circuit 2 that converts the output of this inverter circuit to alternating current, a transformer 3 that steps down the output of this inverter circuit to the voltage necessary for welding, and diodes D ₁ and D that convert the secondary output of this transformer to direct current again and output it. ₂ , a smoothing DC reactor 4, and a current feedback shunt 5 connected thereto. This welding power output causes the tungsten electrode 6 of the welding torch to
An arc 8 is generated between the base material 7 and the base material 7. FIG. 1b shows the current control circuit. operational amplifier
IC ₁ , IC ₂ , comparator IC ₃ , IC ₄ , analog switch
IC ₅ , inverting amplifier IC ₆ , capacitors C ₁ to C ₄ , resistors
The current control circuit including R ₁ to _{R 14} constitutes a feedback control system to obtain constant current characteristics together with the shunt 5 and inverter circuit 2 shown in Fig. 1a, and in a steady state, the output of the shunt 5 is calculated. The output of IC ₁ is amplified by amplifier IC ₁ and is a current feedback signal.
The operational amplifier IC ₂ compares v ₃ with the reference input given by the steady current setting bias v ₁ to amplify the error. The output v ₀ of IC ₂ is sent to the sawtooth wave generating circuit 9 by the comparator IC ₄ .
The resulting pulse width control signal is passed through distribution circuit 10 to the sawtooth waveform shown in FIG.
is supplied to the bases of transistors TR ₁ to TR ₄ . As a result, current feedback control is performed at a control frequency (for example, 20 kHz) determined by the oscillation frequency of the sawtooth wave generating circuit 9, and the output necessary for welding is obtained. Initial current control at arc start is performed by comparator IC ₃ ,
This is done by analog switch IC ₅ , inverting amplifier IC ₆ , capacitors C ₁ , C ₄ , resistors R ₅ , R ₁₂ , R ₁₃ , R ₁₄ and initial current setting bias v ₂ . The operation sequence is shown in FIG. At arc start, before the welding current flows, analog switch IC ₅ is closed, so the initial current setting bias v ₂ is added to the steady current setting bias v ₁ through analog switch IC ₅ , and (v ₁
+v ₂ ) becomes the (1) input (reference input) of the operational amplifier IC ₂ , and the welding power supply device is on standby with the output higher than in the steady state. When the welding current flows, after a response time of the feedback control system (set to 1 to 3 ms), the welding current reaches the initial current value set by (v ₁ +v ₂ ). At this time, the output v ₃ of the operational amplifier IC ₁ , which is the (10) input of the comparator IC ₃ , is larger than the (1) input obtained by dividing the power supply voltage (10) E by the resistors R ₁₂ and R ₁₃ . The output of ₃ goes high in steps, and then for a predetermined time determined by the time constant of approximately (R ₁₄ × C ₄ ).
After _t1 has elapsed, the output of the inverting amplifier IC ₆ becomes LOW, thereby opening the analog switch IC ₅ .
Therefore, the initial current setting bias v ₂ is canceled,
Due to the discharge of capacitor C ₁ , the (1) of operational amplifier IC ₂
The input transitions in a slope shape with a time constant of approximately (R ₅ ×C ₁ ) to a value determined only by the steady-state current setting bias v ₁ . Therefore, as shown in FIG. 2, an initial current determined by (v ₁ +v ₂ ) flows during the time t ₁ at the start of the arc, and thereafter it shifts in a slope to a steady current determined only by v ₁ . According to welding experiments, the initial current is 30~50A, time
When t ₁ is set to about 5 to 15 ms, the arc will not break due to vibration of the current waveform even when starting an arc in a small current range of 5 to 20 A, and the tungsten electrode can quickly become a hot cathode, improving arc starting performance. It was confirmed that there was no burn-through of the base metal or melting of the tungsten electrode tip due to the initial current. Next, the relationship between the steady current setting bias v ₁ and the initial current setting bias v ₂ will be explained. In order to simplify the calculation of the circuit of the operational amplifier IC ₂ , considering only the gain, the following relationship holds true during time _t1 . v ₂ −v ₄ /R ₅ +v ₁ −v ₄ /R ₆ =v ₄ −v ₀ /R ₈ …(1) v ₃ −v ₄ /R ₇ =v ₄ /R ₁₀ …(2) Here, v ₄ is the input terminal voltage of operational amplifier IC ₂ . Eliminating v ₄ from equations (1) and (2) and rearranging as R ₆ = R ₇ and R ₈ = R ₁₀ , we get R ₈・{v ₃ /R ₆ +R ₈・v ₄ /R ₅・(R ₆ +R ₈ )}−R ₈・(v ₂ /R ₅ +v ₁ /R ₆ )=v ₀ …(3) As shown in the second term on the left side of equation (3), the effect of v ₁ and v ₂ is It is determined by the values of R ₅ , R ₆ , and R ₈ . In other words, R ₅ becomes R ₆ ,
If it is set smaller than R ₈ , the effect of the initial current setting bias v ₂ becomes greater. Furthermore, when R ₅ =R ₆ is set, the effect of v ₂ gradually decreases if v ₂ <<v ₁ . In other words, when v ₁ ≦ v ₂ , the initial current setting bias v ₂ is effective, but the welding current increases (v ₂
The effect of v ₂ decreases as ≪v ₁ ). Note that the operational amplifier has an exponential slope in the transition from the initial current to the steady state current.
This is to prevent the welding current from becoming zero due to overshoot if the reference input of IC ₂ is suddenly changed and the damping coefficient of the feedback control system is small. [Effects of the Invention] According to the present invention, in a TIG welding power supply device that has constant current characteristics through current feedback control with a control frequency of 1 kHz or more, it is possible to reduce the size by simply adding a simple circuit as an initial current control means. It is possible to eliminate arc breakage due to vibration of the current waveform when starting an arc in the current range, and to accelerate the conversion of the tungsten electrode into a cathode, thereby improving arc starting performance. Table 1 shows the experimental results when welding current (steady current) was 5A and 20A.

[Welding conditions]

Electrode: 1.6mmφ (contains 2% thorium) Shielding gas: Argon, 5 (/min) Arc length: 3 (mm) Number of tests: 100 times)

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, in which a is a main circuit configuration diagram, b is a control circuit configuration diagram, and FIG. 2 is a diagram showing an operation sequence of this embodiment. 2... Inverter circuit for current control, 5... Current feedback shunt, 6... Tungsten electrode,
IC ₂ ...Operation amplifier for error amplification, IC ₃ ...Comparator for initial current detection, IC ₅ ...Analog switch for initial current setting bias addition/subtraction, C ₁ , R ₅ ...Capacitor and resistor for slope control, C ₄ , R ₁₄ ... Delay capacitor and resistor, v ₁ ... Steady current setting bias, v ₂ ... Initial current setting bias.

Claims (1)

[Claims] 1 a first rectifier circuit that rectifies a first AC input and converts it into a first DC output; an inverter circuit that converts the first DC output from the first rectifier circuit into a second AC output; A transformer that steps down the second AC from the inverter circuit to a predetermined voltage, a second rectifier circuit that converts the secondary output of the transformer into a second DC, and an output from the second rectifier circuit. a shunt for detecting the welding current to be applied, a first operational amplifier for amplifying the output of the shunt, and amplifying the error by comparing the output of the first operational amplifier with a predetermined steady-state current setting bias. a second operational amplifier;
a sawtooth wave generation circuit that generates a sawtooth wave signal at a predetermined oscillation frequency; and a first comparator that compares the output from the second operational amplifier and the sawtooth wave signal from the sawtooth wave generation circuit. and a distribution circuit that generates a control signal for controlling the inverter circuit according to the output from the first comparator, and compares the output of the first operational amplifier with a predetermined comparison voltage, and compares the output of the first operational amplifier with a predetermined comparison voltage. a second comparator whose output changes stepwise in accordance with the output from the shunt, and a time constant circuit connected to the output of the second comparator;
An inverting amplifier connected to the time constant circuit and an inverting amplifier that is controlled to open and close by the inverting amplifier, and a predetermined initial current setting bias that is input to the second operational amplifier, and which is controlled to open and close by the inverting amplifier. A power supply device for TIG welding, characterized in that it is equipped with an analog switch that adds to or disconnects from a steady current setting bias.