JPH066231B2 - Plasma arc processing equipment - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a plasma arc processing apparatus, in which the rise of a pilot arc current is made steep to facilitate the generation of a pilot arc, and the main arc from the pilot arc is increased. The present invention provides a device capable of effectively preventing the current from being shunted through the nozzle after the shift to (4) and preventing heating and wear of the nozzle electrode.

Prior Art FIG. 3 shows a conventional example of this type of device. In the figure, reference numeral 1 designates a DC power supply, which is a known one such as one that rectifies a commercial AC power supply, a DC generator, or a battery. Reference numeral 2 is a switching element for adjusting the output of the DC power supply 1 by switching to control a predetermined current, and a transistor is usually used. Reference numeral 3 is a reactor which, together with the flywheel diode 4, constitutes a smoothing circuit for smoothing a pulsed output current from the DC power supply 1 which is chip-controlled by the switching element 2. Reference numeral 5 is a resistor for limiting the pilot current, 6 is a main electrode made of a non-consumable or non-consumable material such as tungsten, 7
Is a nozzle electrode that surrounds the main electrode 6 and forms a gas passage therebetween. 8 is a workpiece, 9 is a detector for detecting the main arc current, 10 is a reference signal source for determining the main arc current, 1
Reference numeral 1 is a comparator for comparing the output of the reference signal source 10 and the output of the detector 9 to obtain a difference signal, and 12 is a control for controlling the conduction time ratio of the switching element 2 according to the output of the comparator 11. Circuit.

When an activation command signal is applied to the control circuit 12 by an activation switch (not shown) in the device of the same drawing, the switching element 2 becomes conductive and the DC power source 1 between the main electrode 6 and the nozzle electrode 7 passes through the reactor 3 resistor 5. The output voltage is applied. At the same time, a high-frequency high voltage is applied between the main electrode 6 and the nozzle electrode 7 from a high-frequency supply device (not shown), and the working gas is supplied to the gas passage formed between the main electrode 6 and the nozzle electrode 7. To be done. A spark discharge is generated between the main electrode 6 and the nozzle electrode 7 by this high frequency voltage, and an arc discharge is induced by the supply voltage from the DC power supply 1. The current flowing at this time becomes a pilot arc current which is limited by the resistor 5 and does not damage the nozzle electrode 7. Next, when both electrodes are brought close to the workpiece 8, the main arc is generated between the main electrode 6 and the workpiece 8 by the working gas ionized by the pilot arc.
This main arc current is detected by the detector 9 and becomes a signal e _f according to the main arc current, and is compared with the output e _r of the reference signal source 10 by the comparator 11. The comparator 11 calculates the difference e _r −e _f between the two input signals and supplies it to the control circuit 12. The control circuit 12 operates so that the switching element 2 is made conductive when the output of the comparator 11 is positive, that is, e _r −e _f > 0, and is cut off when the output is negative (e _r −e _f <0). To do. As a result, the average value of the main arc current is
It will be maintained at the value set in.

Problems to be Solved by the Invention The conventional apparatus of FIG. 3 operates as described above, but has the following drawbacks.

Since the electric power of the pilot arc is supplied through the reactor 3, the rising of the current becomes gentle when the pilot arc is activated. Therefore, it is necessary to increase the voltage to be applied in order to ignite the pilot arc and to apply the high frequency voltage for a relatively long time. Since the pilot arc is ignited every time the main arc is cut off after machining is completed or interrupted, the operation becomes extremely unstable unless the pilot arc is ignited smoothly.

In order to prevent the above-mentioned drawbacks, the resistor 5 may be connected to the DC power source 1 side of the reactor 3 as shown by the dotted line in the figure. In this way, the rise of the pilot arc can be made steep, but the reactor 3 This creates new problems. That is, when the switching element 2 is cut off when the main arc is generated, the energy stored in the reactor 3 is discharged through the circuit of the workpiece 8 → the nozzle electrode 7 → the resistor 5 → the reactor 3]. 5 and the nozzle electrode 7 are overheated, and it is necessary to use a large resistor 5 and the consumption of the nozzle electrode 7 becomes large. In particular, when the nozzle electrode is brought into contact with the workpiece 8 for processing, most of the energy of the reactor 3 is released through the nozzle electrode 7, and the contact portion between the workpiece 8 and the nozzle electrode 7 is overheated. As a result, the amount of consumption increases dramatically, and in extreme cases, the two may weld together. For this reason, not only the consumption of the nozzle electrode 7 increases, but also in the case where the nozzle electrode 7 is processed by contact, the movement cannot be performed smoothly. Further, the current flowing through the nozzle electrode at this time is a current that should originally flow as a main arc between the main electrode and the workpiece, bypassing the current, and the current detector 9
Since this cannot be distinguished, a large error occurs in the actual main arc current, and the desired machining result cannot be obtained.

Means for Solving the Problems The present invention provides a method for adjusting the output of a DC power supply by a switching element to take out a power source for a pilot arc from before a reactor for output smoothing, and to provide a backflow prevention and a current control function. By supplying to the nozzle electrode via the series circuit which it has, the disadvantage of the above-mentioned conventional device is solved.

Embodiment FIG. 1 is a connection diagram showing an embodiment of the present invention, and 1 to 12 show those having the same function as that in FIG. Again 1
3 is a reverse current blocking diode, 14 is a current detector 9
It is a contact that opens when the main arc current is conducted due to the output of.
When a high frequency high voltage is applied between the main electrode 6 and the nozzle electrode 7 by a high frequency generating circuit (not shown) in the embodiment shown in the figure, a spark discharge is generated between the two, and this spark discharge causes a switching element. 2, a pilot arc is induced through the diode 13, the resistor 5 and the contact 14.
Since this pilot arc current is limited by the resistor 5, the output e _{f of the} detector 9 at this time is the reference signal source 10
Since it is sufficiently smaller than the set value e _r of the control circuit 12, the control circuit 12 keeps the switching element 2 conductive. As a result, in the pilot arc, the output of the DC power supply 1 continues to flow through the resistor 5 as it is. When the nozzle electrode 7 and the main electrode 6 are brought close to the workpiece 8 in this state, the main arc is generated between the main electrode 6 and the workpiece 8 by the ions generated by the pilot arc. When the main arc is generated, the current is only limited by the reactor 3, so the current is the time constant of the main circuit consisting of the DC power source 1, the switching element 2, the reactor 3, the workpiece 8, the main arc, and the main electrode 6. It will increase as you follow. This current is detected by the detector 9 and becomes e _f , and the comparator 11 causes the reference signal source 10
The output e _{r of} is compared. The main arc current increases and e _r <e _f
Then, the control circuit 12 shuts off the switching element 2. When the switching element 2 is cut off, the current flowing in the reactor 3 until then does not suddenly become zero, so that the current continues to flow through the diode 4 instead of the switching element 2. This current gradually decreases according to the time constant of the circuit of the reactor 3, the work piece 8, the main arc, the main electrode, and the diode 4, and when the output of the comparator 11 is reversed, the switching element 2 starts to conduct again. Return to the state of. On the other hand, when the main arc current begins to flow, the contact 14 is opened in response to this, so that the pilot arc is completely cut off. Also, during the operation delay time from the transition to the main arc until the contact 14 operates, the discharge current of the reactor due to the interruption of the switching element is blocked by the diode 13, so that it should be diverted from the workpiece 8 to the nozzle electrode 7. Disappears. The contact 14 need not be provided in particular. When the contact 14 is omitted, a slight pilot arc will be generated only when the switching element 2 is conducted, but it does not mix with the detected value of the main arc, so that actual machining is not possible. There is no misunderstanding of the current. In addition, the diode 13 for the reverse current element,
The current control resistor 5 and the contact 14 use separate parts as shown in the drawing. It is not necessary to use separate parts, as long as they have a reverse current blocking function, a current limiting function, and an opening / closing function. be able to.

FIG. 2 is a connection diagram showing an embodiment in such a case, and is the diode 13, resistor 5 and contact 14 of FIG.
Instead of the transistor, a transistor 15 is connected between the nozzle electrode 7 and the switching element 2. Further, 16 receives the output of the current detector 9 as an input and supplies a base current so that a predetermined current flows through the transistor 15 when the input signal is below a certain value, and the transistor 15 when the input signal, that is, the main arc current is above a certain value. Is a control circuit for shutting off. The operation of the embodiment shown in FIG. 9 is the same as that shown in FIG.

Further, in the embodiment shown in FIGS. 1 and 2, the generation of the main arc may be detected by a voltage drop between the main electrode 6 and the workpiece 8 instead of the current detector 9. And voltage detection may be used together.

Furthermore, instead of the DC power supply 1 and the switching element 2 in the embodiment shown in FIGS. 1 and 2, a commercial AC power supply is once rectified and then converted into a high frequency AC by an inverter circuit, and rectified again to obtain DC. But it's okay.

EFFECTS OF THE INVENTION Since the present invention operates as described above, it has the following effects.

(1) Since the current for the pilot arc is supplied without passing through the reactor, the current rises quickly at startup, and the pilot arc is established quickly. Therefore, even if the main arc is repeatedly turned on and off frequently, the pilot arc can be turned on and off sufficiently so that stable operation can be obtained.

(2) When the switching element for adjusting the output current is shut off during main arc generation, the backflow to the pilot arc circuit due to the energy stored in the series reactor is blocked, so that the nozzle electrode is not damaged and the pilot arc current limiting element Also when using a resistor, it is not necessary to increase the capacitance of the resistor.

(3) When the pilot arc circuit is cut off during the generation of the main arc, no current flows in the pilot arc circuit during the generation of the main arc, and the life of the nozzle electrode is significantly extended.

(4) Since the main arc current is not shunted to the pilot arc circuit, the true machining current can be detected and accurate control becomes possible.

[Brief description of drawings]

1 and 2 are connection diagrams showing an embodiment of the present invention, and FIG. 3 is a connection diagram showing an example of a conventional device. DESCRIPTION OF SYMBOLS 1 ... DC power supply 2 ... Switching element 3 ... Reactor 5 ... Resistor 6 ... Main electrode 7 ... Nozzle electrode 8 ... Workpiece 9 ... Current detector 13 ... Diode 14 ... Contact 15 ... Transistor

Claims (3)

[Claims] 1. A pilot arc is formed between a main electrode and a nozzle electrode which surrounds the main electrode and forms a gas passage,
In a plasma arc processing apparatus that performs processing by generating the main arc by transferring the pilot arc between the main electrode and the workpiece, a DC power supply for controlling the output current by switching, and an output of the DC power supply. A series reactor for smoothing, a main circuit for supplying main arc power from the DC power source to the main electrode and the workpiece through the series reactor, and a series circuit having a reverse current blocking function and a current limiting function. A plasma arc machining apparatus comprising: an auxiliary circuit that supplies electric power for a pilot arc between the main electrode and the nozzle electrode from the DC power source via a DC line. 2. The plasma arc machining apparatus according to claim 1, wherein the auxiliary circuit is a circuit for detecting that a main arc current has flowed in the main circuit and automatically deactivating the main arc current. 3. The plasma arc machining apparatus according to claim 1, wherein the nozzle electrode is used while being in contact with the surface of the workpiece during machining.