JP2663535B2 - Power supply for arc machining - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a power supply device for arc machining in which the output polarity can be switched between AC, positive and negative.

[Conventional technology]

FIG. 5 shows an example of a conventional arc machining power supply device. In FIG. 1, reference numeral 1 denotes an AC power supply, which is usually a single-phase or three-phase commercial AC power supply. A rectifier circuit 2 includes a simple smoothing circuit. Reference numeral 3 denotes an inverter circuit for converting the DC output of the rectifier circuit 2 to high-frequency AC, and reference numeral 4 denotes a high-frequency transformer for converting the output of the inverter circuit 3 to a voltage suitable for processing. 5 is a rectifier circuit, 6a to 6d are bridge-connected switching transistors, 7a to 7d are freewheeling diodes connected in anti-parallel to the switching transistors 6a to 6d, 8 is an electrode for arc machining, and 9 is a shielded electrode. Workpiece, 10 is a switching transistor 6a to 6d
6a and 6b or 6c and 6d as a set, and a polarity switching control circuit for supplying a signal for alternately turning on / off each transistor of each set, 11 is an output current setting circuit, 12 is an output current detector , 13 comparators to obtain a comparison difference signal and the output I _f of the output I _r of the output current setting circuit output current detector _{12 (I r -I f),} 14 in response to the output of the comparator 13 inverter This is a PWM control type inverter control circuit for adjusting the output pulse width of the circuit 3. Further, C shown in the figure indicates the stray capacitance of the circuit, and L indicates the inductance of the power cable from the output terminal to the electrode 8 and the workpiece 9.

In the apparatus shown in FIG. 5, the output of the AC power supply 1 is converted to DC by the rectifier circuit 2 and then converted back to high-frequency AC by the inverter circuit 3. The output I _f of the output I _r of the output current setting unit 11 in the inverter circuit 3 the output current detector 12
And are compared in the comparator 13, the inverter control circuit 14 determines the output pulse width in response to the difference signal △ I = I _r -I _f, are fed back controlled so as to maintain the output current to the set value. The output of the inverter circuit 3 is converted by a transformer 4 into a voltage suitable for arc machining, and then converted again by a rectifier circuit 5 into direct current. This DC output is supplied to a DC terminal of a bridge circuit composed of transistors 6a to 6d, and an AC terminal of the bridge circuit is connected to output terminals (a) and (b) to connect the electrode 8, the workpiece 9 and the power cable. Connected. The transistors 6a to 6d are controlled by a signal from the polarity switching control circuit 10 so that the transistors 6a and 6b and the transistors 6c and 6d form one set, and the transistors in each set are alternately turned on. As a result, the electrode 8
And a polarity change control circuit 10
By adjusting the output, a rectangular wave AC having an arbitrary frequency from positive and negative DC to high frequency AC can be supplied.

[Problems to be solved by the invention]

In the above-described conventional device, the switching of the output polarity and the control of the output current are performed completely independently, and thus have the following problems.

It is now assumed that the transistors 6a and 6b are conducting, and the current _Io having the polarity of the electrode 8 is flowing. When the base current to the transistors 6a and 6b is cut off and the base current is simultaneously supplied to the transistors 6c and 6d in order to switch the polarity of the output current from this state, all of the transistors 6a to 6d are turned on for a certain fiber time. Become. At this time, the output current of the rectifier circuit 5 tends to increase rapidly, but the inverter circuit 3 operates in a direction to suppress this because the constant current control is performed as described above. The supply of current from the rectifier circuit 5 to the electrode 8 and the workpiece 9 is stopped, and the electromagnetic energy (1/2 · LI _o ² ) stored in the inductance L of the power cable causes the electrode 8 and the workpiece 9 to move in the same direction as before. Electric current continues to flow and the arc continues. This current Flows around the circuit. When they completely shut off after the transition time of transistors 6a, 6b, the current Flows around the circuit. Therefore, the circulating current due to the stored energy of the inductance L charges the stray capacitance C together with the output of the rectifier circuit 5. After the oscillation period defined by the inductance L and the stray capacitance C, the current stops when this circulating current passes through the zero point, and at this time, the rectification circuit 5 receives the conduction signal from the rectifier circuit 5 by the transistors 6c and 6d. A voltage in the opposite direction is applied to the electrode 8 and the workpiece 9 in a direction in which a current flows in a direction in which the electrode 8 in the opposite direction has a negative potential. As a result, arc discharge is regenerated between the electrode 8 and the workpiece 9, and the current starts flowing again.

Since the polarity switching of the apparatus shown in FIG. 5 follows the above-described course, the stray capacitance C is charged to a considerably high voltage immediately before the reverse arc regeneration. This charging voltage is effective for destroying the developing insulation between the electrode 8 and the workpiece 9 at the time of arc reproduction, while the transistor immediately after the complete interruption (the transistor 6a in the above case) , 6b) are also applied in parallel with the processing part, and if this voltage exceeds the forward breakdown voltage of the transistors 6a, 6b, the transistors will be destroyed. Moreover, since this voltage is also applied to the rectifier elements constituting the rectifier circuit 5 connected in parallel, these may be destroyed.

The charging voltage of the stray capacitance generated at the time of the polarity switching is
It increases in proportion to the electromagnetic energy stored in the power cable. Therefore, when the processing current is large, or when the power cable is long even with a relatively small current, the generated voltage is much higher than the voltage required for arc regeneration, and the elements constituting the circuit may be destroyed. There is.

In order to prevent the generation of such a high voltage, it is conceivable to increase the stray capacitance in the figure positively. For this purpose, a capacitor should be connected between the output terminals of the rectifier circuit.However, if a capacitor with an excessively large capacity is connected, charging and discharging of this capacitor will result in poor arc regeneration and delay in controlling the output current. This will have the opposite effect. Therefore, in practice, there is no other choice but to use a device whose withstand voltage of each element is several times higher than the circuit voltage, resulting in an expensive device.

[Means for solving the problem]

The present invention has been developed in order to solve the above-described problems of the conventional device.
The reverse current is reduced immediately before the polarity switching, and then the polarity is switched in a relatively low current state.

[Action]

In the present invention, since the current is reduced when the polarity is switched, the generated voltage is prevented from becoming unnecessarily high by suppressing the electromagnetic energy stored in the inductance of the power cable to a small value at the time of switching. Things.

〔Example〕

FIG. 1 shows an embodiment of the present invention. In the figure, reference numerals 1 to 9, 12 to 14 denote elements having the same functions as those of the conventional apparatus shown in FIG. 15 is a reference signal generating circuit, level immediately before the polarity reversal, as shown in Figure 2 outputs a reference signal i _r for positive and negative alternating waveform decreases.
16 outputs the DC power current setting signal I _r of the output i _r pulsates with both waves rectified and the reference signal generating circuit 15 a reference signal rectifier circuit for full-wave rectifying the output of the reference signal generating circuit 15. The output current setting signal I _r is compared in the comparator 13 and the output I _f of the output current detector 12, the difference signal △ I = I _r -I _f is supplied to the inverter control circuit 14 the pulse width of the inverter circuit 3 Is adjusted so that the output current value becomes a waveform corresponding to the output waveform of the reference signal rectifier circuit 16. Reference numerals 17 and 18 denote polarity determining diodes, reference numeral 19 denotes an inverted amplifier, reference numeral 20 denotes an inverted amplifier, and reference numeral 21 denotes transistors 6a and 6 according to the output of the amplifier 19.
b is turned on, and the transistors 6c and 6d are turned on by the output of the amplifier 20.
Is a drive circuit that conducts.

In the embodiment shown in FIG. 1, the polarity switching timing is obtained from the reference signal generating circuit 15 and the transistor 6a, 6b or 6
c, to determine whether to conduct any 6d each set of, and the output current during the polarity will be determined connexion by an output i _r of a reference signal generating circuit 15 to the full-wave rectified signal I _r. Because of this signal
If the output of the reference signal generating circuit 15 is set so that the level decreases at the end of each polarity period of i _r, the output current I _o accordingly becomes a low level current immediately before the polarity switching. . Therefore, when the polarity is switched, the electromagnetic energy stored in the inductance L of the power cable also has a low value, so that the voltage charged in the stray capacitance C does not increase so much.

FIG. 2 shows waveforms at various points in the apparatus of the present invention shown in FIG. 1, and FIG.
Of shows the waveform of the output i _r, (b) the waveform of the output I _r of the reference signal rectifier circuit 16 which full-wave rectifies the signal i _r, (c) the transistors 6a, the output waveform of the drive circuit 21 for 6b, ( d) is the output waveform of the drive circuit 21 for the transistors 6c and 6d,
(E) shows the waveform of the machining current.

Note that the level of the output _ir of the reference signal generation circuit at the end of each polarity period should be as low as possible as long as the voltage generated at each polarity inversion is greater than or equal to the value required to reproduce an arc of the opposite polarity. And can be determined independently of the level in other periods. When the voltage required for arc regeneration differs between when the polarity of the electrode 8 is inverted from a positive potential to a negative potential, such as when a tungsten electrode is used as the material of the electrode 8, and when the polarity is reversed, the signal i _r The signal level at the end of each polarity period may be different.

The reference signal _ir is not limited to the signal shown in FIG. 2A, but may be processed as long as its level decreases at the end of each polarity period as shown in FIG. 3A or 3B. The various waveforms required for are available.

Also, in the embodiment shown in FIG.
A DC power supply unit composed of an inverter circuit 3, a transformer 4, and a rectifier circuit 5 is, besides the example shown in the figure, for converting a commercial AC power supply to a required voltage by a transformer and then performing phase control rectification by a thyristor. And the series regulator type, in which output current is adjusted by a series transistor after rectification by a diode, is slightly reduced in response speed and accuracy, and the equipment is large because a commercial frequency transformer is used. The present invention can be practiced except for the following. Further, the polarity switching circuit is not the bridge type switching circuit as shown in FIG. 1, but the secondary winding of the transformer is provided with a center tap, and both outputs are respectively diode-connected to positive and negative DC systems. Outputs may be used, and both outputs may be turned on / off by series transistors to switch the polarity.

FIG. 4 shows another embodiment of the present invention. In the figure, a transformer 4 has a center tap on a secondary winding, and diodes 22a to 22d constitute two positive and negative two-wave rectifier circuits, respectively. The transistors 23a and 23b, which are connected and disconnected alternately in series, are led to the electrode 8 and the workpiece 9 to obtain the same effect as the embodiment of FIG.
Components other than those shown in FIG. 2 having the same functions as those in FIG.

〔The invention's effect〕

Since the device of the present invention operates as described above, the output current is reduced when the polarity is switched, and no excessive voltage is generated. Therefore, it is not necessary to provide a special surge voltage absorbing circuit. Further, since the generation of the surge voltage is not suppressed by the surge absorbing circuit such as the capacitor, the arc regeneration at the time of polarity switching and the rising speed of the current are not impaired. Further, since the current at the time of the polarity switching is small, the power loss in the polarity switching transistor is reduced, and a device with a low withstand voltage can be used due to a reduction in the generated voltage, and the device can be inexpensive.

[Brief description of the drawings]

FIG. 1 is a connection diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing waveforms of respective parts in the embodiment of FIG. 1, FIG. 3 is a diagram showing various examples of reference signals, FIG. 4 is a connection diagram showing another embodiment of the present invention, and FIG. 5 is a connection diagram showing an example of a conventional apparatus. 2 ... Rectifier circuit, 3 ... Inverter circuit, 4 ... Transformer, 5 ... Rectifier circuit, 6a to 6d, 23a, 23b ... Switching transistors, 7a to 7d, 24a, 24b ... Diode, 12 ... ... Output current detector, 13 ... Comparator, 14 ... Inverter control circuit, 15 ... Reference signal generation circuit, 21 ... Drive circuit, 22a to 22d ... Diode, L ... Power cable inductance, C ... ... Stray capacitance.

Claims (1)

(57) [Claims] 1. A DC power supply, a switching circuit for switching the output of the DC power supply between positive and negative polarities by a switching element and supplying the output to a load, and a cycle corresponding to a cycle of positive / negative polarity switching and at least positive and negative polarities A reference signal generation circuit that outputs an AC signal having a characteristic that a level decreases at the end of a period, and an output signal of the reference signal generation circuit as an input, and outputs the positive and negative outputs of the switching circuit in synchronization with the inversion of the polarity of the input signal. Switching elements alternately
A polarity switching control circuit for interrupting, an output current detector, a reference signal rectification circuit for outputting a pulsating DC signal by rectifying the output of the reference signal generation circuit in both waves, an output of the reference signal rectification circuit, and the output A power supply device for arc machining, comprising: a DC power supply control circuit that compares an output of a current detector and adjusts an output of the DC power supply in a direction in which a difference decreases.