JPH0394982A - Plasma arc machine - Google Patents

Abstract

PURPOSE:To prevent burning of a torch for machining by detecting an instantaneous value of the voltage between a main electrode and a chip electrode being made in a short-circuited state during machining and cutting off a plasma arc by this. CONSTITUTION:The torch having the main electrode 2 and the chip electrode 3 is used with the plasma arc machine. The voltage between the main electrode 2 an the chip electrode 3 is then inputted to a short-circuit detection circuit 11 by a detector 7 during machining. It is then detected that the instantaneous value of the voltage between both electrodes 2 and 3 is made to a short circuit for the low voltage close to the short circuit. The plasma arc is cut off by a protective circuit according to an output signal of the short-circuit detection circuit 11. By this method, the replacing time of the electrodes can be known exactly.

Description

[Detailed description of the invention] Industrial application field> The present invention relates to the improvement of plasma arc machining equipment, and in particular detects when the electrode wears out over time and reaches the end of its life. This is to notify the worker by stopping the power supply, turning on a warning light, etc., to prevent torch burnout and improve processing quality.

Conventional technology> In plasma arc processing equipment, an arc is generated between the main electrode and the workpiece, and processing such as fusing and welding is performed using this arc. Prior to ignition, a small current arc called a pilot arc is ignited between the main electrode and a tip electrode provided concentrically surrounding the main electrode. This pilot arc is used to induce the main arc, and therefore the tip electrode is provided for the purpose of generating this pilot arc. This tip electrode also has a small hole called an orifice at its tip to narrowly converge the main arc, and cools the area around the arc by passing a high-speed gas flow between the main electrode and this tip electrode. By narrowing the arc and ejecting it from a narrow orifice, a plasma arc with high energy density is obtained.

When the electrode reaches the end of its life, the arc generation position becomes unstable and the arc is not converged sufficiently, resulting in a disordered state, making it impossible to perform the desired processing.

Additionally, if a series arc phenomenon occurs in which a large current for machining flows through the tip electrode, the tip electrode will burn out in a short period of time.
This will subsequently lead to damage to the cup covering the tip electrode and rapid wear of the main electrode, eventually leading to burnout of the torch.

Therefore, it is important to detect signs of electrode abnormality and immediately take measures to stop the work, but conventional detection devices do not have sufficient functionality.

FIG. 6 shows an example of a conventional device. In the figure, 1 is a power source for plasma arc processing, and a DC power source with constant current characteristics is usually used. Reference numeral 2 denotes a rod-shaped main electrode provided inside the processing torch, and a cathode member 2a made of a non-consumable material is embedded at the tip thereof, and is connected to the negative output terminal of the power source 1. Reference numeral 3 denotes a tip electrode provided so as to concentrically surround the main electrode 2, and is shown in a longitudinal section. This tip electrode 3 has a conical cylindrical shape and is provided with an orifice portion 3a at its tip. Reference numeral 4 denotes a workpiece, which is connected to the positive output terminal of the power source 1. 5 is an arc occurring between the main electrode 2 and the workpiece 4, 6 is the voltage between the main electrode 2 and the tip electrode 3, and this input voltage changes with respect to the set value. This is an abnormality detector that outputs an abnormal signal when Although not shown, plasma generating gas is supplied from above the torch between the tip electrode 3 and the main electrode 2, and is ejected from the orifice portion 3a as a high-speed gas flow.

In the figure, when a normal arc occurs, the gas flow ionized by the arc 5 passes through the orifice portion 3a, so the tip @ pole 3 that is in contact with this gas flow is at a potential equivalent to the orifice portion 3a of the arc 5. .

Problems to be Solved by the Invention In the conventional device shown in FIG. 6, even if the electrode wear progresses, the arc length between the tip electrode 3 and the main electrode 2 does not change much, so the voltage does not change immediately. does not appear, and this voltage change becomes detectable only after the electrode is extremely worn out and the arc disturbance becomes large.

Therefore, in the conventional device shown in FIG. 6, it is difficult to detect this at the end of the life of the electrode.

On the other hand, as an alternative to the device shown in Fig. 6, a system that detects changes in the voltage between the tip electrode 3 and the workpiece 4 is being considered, but the voltage between this detects the voltage even when the main arc is normal. Since the arc generation point in the workpiece changes in the thickness direction of the workpiece, the fluctuation is large and cannot be distinguished from the phenomenon that occurs at the end of the electrode's life.

Another possibility is to detect changes in the machining current, but
When a machining power source R1 with constant current characteristics is used, the current is naturally controlled to be constant regardless of the state of the arc, so the more accurate the machining power source is, the more difficult it is to detect.

Means for Solving the Problems> The present invention detects the signs of torch burnout by detecting changes in the average value and instantaneous value of the voltage between the main electrode and the tip electrode, and turns on the processing power source. This is designed to cut off the output and issue an alarm.

Prior to the present invention, as a result of a detailed study of the state in which the torch burns out due to consumption of the main electrode, it was found that just before the torch burns out, the voltage between the main electrode and the tip electrode changes as shown in Figure 7. It turns out that there are some changes to be made.

FIG. 7 shows the voltage Vc between the tip electrode 3 and the main electrode 2 when the distance between the tip electrode 3 and the workpiece 4 is kept constant.
This figure shows the changes in the over time. In the figure, when plasma arc machining is started using a new main electrode and tip electrode, the voltage VC remains at a nearly constant value Vn determined by the arc length corresponding to the distance between the main electrode and the tip electrode until time t1. is maintained.

However, at time t-ti, just before the end of the life of the main electrode or tip electrode, the average value of the voltage Vc begins to rise, and during this rising voltage, 81 to B in the figure
As shown in 4, a low pulse-like part that is much lower than the previous normal voltage Vo and drops to almost zero appears, and if the use continues as it is, the electrode wear will progress and at time t- At t2, the torch finally burns out. The reason for this is that as the machining continues, the main electrode 2 gradually wears out, and when the total machining time approaches the electrode's life time, the hard-to-consumable material 2a buried at the tip of the main electrode 2 runs out and the arc is generated directly from the constituent materials of the main electrode 2. This causes rapid wear and tear on the electrode surface, and eventually the main electrode begins to partially burn off. When this main electrode burns off, there is a molten electrode structure between the main electrode and the tip electrode. It is thought that a moment occurs when the voltage between the two electrodes drops momentarily.

On the other hand, the average value of the voltage between the main electrode and the tip electrode varies depending on the arc current, and may increase in a pulsed manner due to the occurrence of momentary arc breakage. Therefore, in order to detect the wear limit of each electrode, accurate results cannot be obtained by simply detecting the increase in voltage.

Therefore, in the present invention, when an extreme drop in the voltage between the main electrode and the tip electrode (a short circuit or a voltage drop that reaches a low voltage close to a short circuit) is detected, it is assumed that the electrode life has ended, and the power supply for machining is shut off. It was also designed to issue a warning.

Furthermore, in the second invention of the present invention, in addition to the above-mentioned first invention, an abnormality (electrode life end ), which enables more accurate detection.

Also, if the tip electrode and the workpiece become extremely close to each other and the tip electrode comes into instantaneous contact with the workpiece,
The voltage between the tip electrode and the workpiece will drop abnormally, and if this voltage drop is sudden, vibration will occur due to the delay in the current control system, and the average voltage between the tip electrode and the main electrode will decrease. This will lead to an increase in voltage. For this reason, an increase in the average value and a drop in the instantaneous value of the voltage between the main electrode and the tip electrode may occur, but in this case, it is not an electrode abnormality and should be distinguished.

Therefore, in the third invention of the present invention, in addition to the second invention, the abnormality detection is enabled only when the voltage between the tip electrode and the workpiece is above a certain level. It is designed to differentiate between

Furthermore, in order to make the above-mentioned phenomenon at the end of the electrode's life more conspicuous, an electrode is used that has a smaller cross-sectional area on the base side of the electrode than the buried position of the cathode member made of a difficult-to-expendable material at the tip. This is what I proposed to do.

Effects> The device of the present invention determines the electrode lifespan by capturing the extreme drop in voltage between the main electrode and the tip electrode that characteristically occurs at the end of the electrode lifespan, so accurate detection is possible. is possible.

Embodiment> FIG. 1 shows an embodiment of the present invention. In the same figure, numerals 1 to 4 indicate the same reference numerals and the same functions as those of the conventional device shown in FIG. 7 is a voltage detector that detects the voltage Vc between the main electrode 2 and the tip electrode 3; 8 is a comparator that compares the output Vc of the voltage detector 7 with the output Vs of the reference voltage setting circuit 9; Vc< When the voltage reaches Vs, a high level signal is output. 10 is an RS flip-flop circuit, and the output of the comparator 8 is supplied to the S input terminal. Further, the R terminal is connected to a DC power source E via a reset push button switch PB3. As a result, the RS flip-flop circuit 10 has a voltage of V
When C<VS and the output of the comparator 8 becomes high level, the output terminal Q becomes high level and operates to maintain this level until the reset pushbutton switch PB3 is pressed.

The comparison circuit 8 and the reference voltage setting circuit 9 constitute a short circuit detection circuit 11 that detects a decrease in the voltage between the main electrode 2 and the tip electrode 3. Therefore, the set value VS of the reference voltage setting circuit 9 is set to a voltage sufficiently lower than the average voltage between the main electrode 2 and the tip electrode 3 during normal operation. TRI is a transistor, which becomes conductive when the Q output of the flip-flop circuit 10 becomes high level, and excites the relay CR1. FBI is a push button switch for commanding start of machining, PB2 is a push button switch for command to stop machining, and when push button switch FBI is pressed, relay CR is activated.
2 is excited, the coil is self-held at the contact CR2a, and the contact CR2a is closed, thereby starting the processing power source 1. ALM is an abnormality alarm, which is connected to the DC power supply E in series with the contact CRla of the relay CRI.

In the device shown in Fig. 1, the push button switch F for start command is
When BI is pressed, relay CR2 is energized and contact CR2a is closed. As a result, the machining power supply 1 is activated and the workpiece 4
A positive output is supplied to the main electrode 2, and a negative output is supplied to the main electrode 2. At the same time, gas is supplied and a pilot arc is ignited between the main electrode and the tip electrode by a starting circuit (not shown).

When the gas ionized by this pilot arc reaches the workpiece 4, a machining plasma arc is generated between the main electrode 2 and the workpiece 4.

When a plasma arc occurs, the voltage detection circuit 7
The voltage Ve between the chip electrode 3 and the tip electrode 3 is detected and supplied to the comparator 8. In the comparator 8, the detected voltage Vc is the reference voltage V
s, and when VC>Vs, low level output, Vc
When <Vs, a high level output is generated. Therefore,
It is necessary to devise measures so that the output of the comparator 8 is not transmitted to the flip-flop circuit IO during the unstable period before and immediately after the plasma arc is started.

Now, as the machining continues, the main electrode 2 will gradually wear out, and when the total machining time approaches the life time, the voltage between the main electrode and the tip electrode will decrease as explained in Fig. 7. A momentary drop occurs.

When this voltage drop occurs, the comparator 8 generates a pulse-like output signal corresponding to the period of the voltage drop, and the rise of this first pulse-like output causes the flip-flop circuit 10 to
is set and the Q terminal output is inverted to high level. This energizes relay CRI, opens contact CRlb, and de-energizes relay CR2. As a result, the starting command contact CR2a of the machining power source 1 is opened, so the output is stopped and the plasma arc is cut off. On the other hand, the contact CR1a is closed by the excitation of the relay CRI, and the alarm ALM is connected to the DC power supply E.
The output is applied to notify the operator of the occurrence of an abnormality. Even if the S terminal input becomes low level, the flip-flop circuit IO keeps the Q terminal output at high level until a reset input signal is supplied to the R terminal or the control power is cut off, so the plasma arc is cut off. The alarm will continue to be issued even after the When the operator learns of the electrode abnormality (end of life) from this alarm, inspects and replaces the main electrode 2 and tip electrode 3, and then presses the reset pushbutton switch PB3, the flip-flop circuit 10 is reset. As a result, relay CRI becomes de-energized, contact CR1a opens, and contact CR1b closes, returning to the state before activation.

Figure 2 shows that at the end of the life of the electrode, as explained earlier in Figure 7, there is an extreme pulse-like drop in the voltage between the main electrode and the tip electrode, as well as an increase in its average value. Therefore, when these two phenomena occur at the same time, it is determined that the electrode is abnormal, thereby enabling more reliable detection.

In the figure, 12 is an average value calculating circuit that calculates the average value of the output VC of the voltage detection circuit 7, and calculates the average value 1 of the input signal Vc per unit time.

l3 is a reference voltage setting circuit that outputs a voltage Vr that is slightly higher than the average value of the voltage between the main electrode 2 and tip electrode 3 during normal machining, and l4 is the output Vm of the average value calculation circuit l2 and the reference voltage setting. Compare the output Vr of circuit l3 and find that V@>
Comparator that outputs a high level signal when Vr, AND
I is an AND circuit which inputs the output of the short circuit detection circuit 11 and the output of the comparator l4, and outputs a high level signal when both input signals become high level. The other circuits in FIG. 2 are the same as those in the embodiment shown in FIG. 1, so those with the same functions are given the same reference numerals and the details will be omitted.

In the embodiment shown in the figure, when the electrode wear progresses after the plasma arc is started, the average voltage between the main electrode 2 and the tip electrode 3 increases, and the output of the comparator 14 becomes high level. After that, when the wear of the electrode further progresses, partial burn-through of the main electrode 2 occurs, and the instantaneous value of the voltage between the main electrode 2 and the tip electrode 3 drops in a pulse-like manner. The output also becomes high level, and the output of the AND circuit AND1 with both input signals at high level becomes high level, and the Q output of the RS flip-flop circuit 10 is inverted to high level, detecting an electrode abnormality and using it for processing. The output of power supply 1 is cut off, and an alarm is issued by ALM.

FIG. 3 is a modification of the embodiment shown in FIG. 2 in which the electrode abnormality detection signal is made valid only when the voltage between the tip electrode 3 and the workpiece 4 is normal.

In FIG. 3, 15 is a voltage detector that detects the voltage between the tip electrode 3 and the workpiece 4, and 1B is a voltage detector that detects the voltage between the tip electrode 3 and the workpiece 4 during normal machining. is also a reference voltage setting circuit that is set to a slightly lower voltage, and l7 compares the output Vo of the reference voltage setting circuit l6 and the output Vw of the voltage detector l5, and determines that Vw > Vo
That is, it is a comparator that outputs a high level signal only when the voltage between the tip electrode 3 and the workpiece 4 is the voltage during normal machining. Components having the same functions as those in FIGS. 1 and 2 are designated by the same reference numerals and their explanations will be omitted.

In the figure, the voltage between the tip electrode 3 and the workpiece 4 is a normal voltage, and the average value of the voltage between the main electrode 2 and the tip electrode 3 is higher than the reference value. When a pulse-like drop phenomenon occurs, it is determined that an abnormality has occurred, and an abnormality signal is output.

By the way, in the embodiments shown in FIGS. 1 to 3 above, when the end of the electrode life is detected and the RS flip-flop circuit 10 is set and the relay CRI is energized to issue an alarm, the operator can The electrodes will need to be replaced. However, in these embodiments, the alarm can be canceled at any time by pressing the reset pushbutton switch PB3. For this reason, if a worker presses only the reset pushbutton PB3 without replacing the electrode even though an alarm has been issued, the machining operation can be resumed without changing the electrode, and there remains a risk of burning out the torch. . To prevent this, an interlock function may be provided so that once an alarm occurs, the reset pushbutton switch PB3 will not become effective unless the electrode replacement work is started.

FIG. 4 is a connection diagram showing an embodiment in which the above functions are added to the embodiment of FIG. 3. In the figure, 18 is a contact that opens when the tip electrode 3 is removed when replacing the electrode, and CR3,
CR4 is a relay, and each is connected as shown. Other components having the same functions as those of the embodiment shown in FIG. 3 are given the same reference numerals.

In the embodiment shown in the figure, when the life of the electrode ends, A
When the ND gate AND1 supplies a high level output to the RS flip-flop circuit lO, its Q terminal output becomes high level, thereby making the transistor TRI conductive and energizing the relay CRI. By energizing relay CRI, relay CR2 is de-energized and machining electric current ri. 1, the output is stopped, the arc is cut off, and the machining is interrupted. At the same time, the alarm ALM is activated by the contact CR1a to notify the operator that it is time to replace the electrode. At this time, contact CR1a connected in series to the coil of relay CR4 is also closed, but contact 18 remains closed until the operator starts removing the electrode, so relay CR3 is energized and contact CR3b is closed. Since it is open, relay CR4 remains de-energized. For this reason, the contact CR4a connected in series to the reset pushbutton switch PB3 is open, and in this state, the reset pushbutton switch PB3
is disabled and cannot be reset.

Next, when the operator removes the main electrode 2 and tip electrode 3 for inspection, replacement, etc., the contact 18 corresponding to this operation opens, and the relay CR3 becomes de-energized. For this reason relay CR4
is excited through contacts CRla and CR3b. As a result, the contact CR4a connected in series to the reset pushbutton switch PB3 is closed, and the pushbutton switch PB3 becomes valid, that is, resettable. Further, since the relay CR4 is self-held by the contact point 4a, the relay CR4 continues to be energized even if the main electrode 2 and the tip electrode 3 are attached to the torch body again. After the electrode replacement work is completed, and before restarting machining, press the reset pushbutton switch PB3 to reset the flip-flop circuit IO and de-energize the relay CRI. By closing the contact CRlb, the relay can be restarted. CR4 becomes de-energized and returns to the state before the alarm occurred. In the embodiment shown in Fig. 4, the alarm ALM cannot stop issuing the alarm until the electrode replacement work is performed, but the alarm is switched in series to the alarm ALM so that the worker can only stop the alarm after confirmation. Providing a switch to shut off the alarm is convenient when a sound generator such as a buzzer is used as an alarm.

The contacts 18 used in FIG. 4 include electrode inspection,
It can be opened as long as it can be opened during replacement work, such as each output contact of the tip electrode attachment detector, the main electrode attachment detector, or when using a water-cooled torch, stopping or discharging cooling water. When this is a pre-process for these operations, a contact that opens when cooling water is stopped or discharged may be used.

FIG. 5 is a partially cross-sectional view showing the structure of a torch having a detection mechanism for detecting the attachment state of the tip electrode of the torch. The tip electrode 3 is attached concentrically to the main electrode 2 by an insulating protective cap 3l screwed onto the torch body 19. A shorting conductor 181 made of a ring-shaped conductor is embedded in the protective cap 31 at the portion where it meets the torch body, and the torch body 19 has two contact conductors IHa. L82b is buried, and this contact conductor 182a. The lead wire 182b is always urged outward by a spring against the torch body l9 so as to have good electrical contact with the shorting conductor 181 when the protective cap 3l is attached.
3, it is pulled out to the outside. This short circuit conductor 181
The contact conductors 182a and 182b constitute a contact l8 that opens when replacing the electrode in FIG.

In the torch having the structure shown in FIG. 5, when the protective cap 31 is removed to remove the main electrode 2 or the tip electrode 3 from the torch body for inspection or replacement, the short circuit between the contact conductors 182a and 182b is released. Therefore, if these contact conductors 182a and 182b are used directly as contact l8 in FIG. 4, or if a relay contact operated by these contacts is used, the apparatus of the embodiment shown in FIG. 4 can protect the torch after an alarm occurs. When the cap is removed, that is, when the operator starts inspecting and replacing the electrodes, this is stored in the relay CR4 and the alarm can be reset.

Note that in the embodiments shown in FIGS. 1 to 4 above, an RS flip-flop circuit was used as a means for holding the abnormal signal, and a relay was used for each command, but these could be configured using other logic elements. Of course you can.

By the way, the above-mentioned phenomenon at the end of the electrode life in plasma arc processing equipment is that the cathode member made of a hard-to-consumable material buried at the tip of the main electrode 2 wears out over time, and the base material of the main electrode 2 This appears when a direct arc begins to occur. Therefore, immediately after the buried cathode member is exhausted, the base material of the main electrode 2 will be melted together as much as possible, resulting in a large droplet and a significant voltage drop between the tip electrode 3 and the main electrode 2. appears in

Therefore, the structure of the main electrode 2 suitable for the plasma arc processing apparatus of the present invention is such that the cross-sectional area is locally small on the base side (the side opposite to the electrode tip, that is, the part close to the power feeding point) from the buried position of the cathode member. We recommend that you use one that has a section that allows you to do so.

FIG. 8 is a diagram showing an example of such a main electrode,
4(a) is a side view of its appearance, and FIG. 2(b) is a longitudinal cross-sectional view. In the example shown in the figure, an annular groove 2b is provided in a part of the cathode member 2a buried near the tip of the main electrode 2 that is closer to the base than the deepest part, so that the cross-sectional area of this part is larger than that of other parts. It has also been made smaller.

The position g2 of this recess 2b from the tip of the main electrode 2 is determined so that g1≦g2 with respect to the buried depth g1 of the cathode member 2a.

If a groove portion with a smaller cross-sectional area is provided on the base side of the cathode member 2a as described above, even while the cathode member is present, the machining current will cause the groove portion and the electrode base material on the tip side thereof to becomes overheated. For this reason, while the cathode member 2a remains, the arc 5 is generated concentratedly from the cathode member 2a, and as shown in FIG. 9(a), wear occurs mainly in the cathode member 2a, and the electrode base material The cathode member 2a hardly wears out until it becomes extremely worn out. When the cathode member 2a wears out and disappears completely as shown in FIG. 9(b), the arc 5 comes to be generated directly from the electrode base material. At this time, as shown by diagonal lines in FIGS. 9(a) and 9(b), the concave groove part 2b, which has a smaller cross-sectional area than the other parts, and the distal end thereof were almost overheated while the cathode member 2a remained. Therefore, when the cathode member 2a is removed and the arc 5 is generated directly from the electrode base material, the temperature in the area corresponding to the shaded area in the figure will rapidly rise and the melting rate will be accelerated.
The above-mentioned phenomenon in which the voltage between the main electrode 22 and the tip electrode 3 drops significantly occurs more significantly. Therefore, it becomes possible to detect the end of the electrode life more reliably.

In order to obtain the above effect, a part with a smaller cross-sectional area is provided on the base side (feeding point side) than the cathode member 2a in the middle of the current-carrying path of the main electrode 2, and the electrode base material at the tip is formed from this part. The structure may be such that it promotes overheating.

FIGS. 10(a) to 10(C) are diagrams showing other embodiments of the structure of the main electrode 2a as described above, and FIG. 10(a) is an example in which a plurality of horizontal holes 2c are provided above the cathode member 2a. , (b) show an example in which a cavity 2d is provided in the upper part of the cathode member 2a, and (C) shows an example in which a groove 20 is partially provided. In FIGS. 10(a) to 10(C), (a) shows a longitudinal cross-sectional view, and (b) shows a cross-sectional view taken along the line xx.

Effects of the Invention> As described above, the present invention utilizes the characteristic of voltage change between the main electrode and the tip electrode at the end of the electrode life in a plasma arc processing apparatus, so that it is possible to improve the timing of electrode replacement. Since this can be accurately known, it is possible to prevent the processing torch from burning out.

[Brief explanation of drawings]

Fig. 1 is a connection diagram showing an embodiment of the present invention, Figs. 2 to 4 are connection diagrams showing other embodiments of the invention, and Fig. 5 is a connection diagram showing an embodiment of the present invention.
A diagram showing an example of the structure of a torch having contacts used in the embodiment shown in the figure, FIG. 6 is a connection diagram showing an example of a conventional device, and FIG. 7 explains the change in voltage between the main electrode and the tip electrode. Fig. 8 is a diagram showing an example of the main electrode 2 structure suitable for use in the plasma arc processing apparatus of the present invention, and Fig. 9 explains the state of wear of the electrode of the structure shown in Fig. 8. Figure for, 1st
Figure 0 is a diagram showing another example of an electrode structure suitable for use in the plasma arc processing apparatus of the present invention. 1... Power supply for processing, 2... Main electrode, 2a.
・Cathode member made of non-consumable material, 2bq 2e・・
・Concave groove, 2C...Horizontal hole, 2d...Cavity,
3... Chip electrode, 4... Workpiece,
5... Arc, 7... Voltage detector,
8... Comparator, 9... Reference voltage setting circuit,

Claims (1)

[Claims] 1. In a plasma arc processing apparatus using a torch having a main electrode and a tip electrode, the voltage between the main electrode and the tip electrode is input during processing, and the voltage between the two electrodes is changed. A plasma arc processing device comprising: a short circuit detection circuit that detects when an instantaneous value becomes a short circuit or a low voltage close to a short circuit; and a protection circuit that interrupts a plasma arc based on an output signal of the short circuit detection circuit. 2. In a plasma arc processing device using a torch having a main electrode and a tip electrode, a voltage detector detects a voltage between the main electrode and the tip electrode, and the output of the voltage detector is input as input. an average value calculation circuit that obtains the average value Vm of the voltage; a reference value setting circuit that determines the average value Vr of the voltage between the main electrode and the tip electrode during normal machining; and an output Vm of the average value calculation circuit and the a comparator that receives the output Vr of the reference value setting circuit as an input and outputs a binary logic signal corresponding to the magnitude relationship between the two input signals; a short-circuit detection circuit that generates an output when the voltage between the electrodes is short-circuited or reduced to a voltage close to a short-circuit; and a voltage between the main electrode and the tip electrode with the output of the comparator and the output of the short-circuit detection circuit as inputs. an AND circuit that outputs an abnormal signal when the average value Vm of is higher than the reference value Vr and the short circuit detection circuit detects a short circuit between the two electrodes; and a protection circuit that interrupts the plasma arc by the output of the AND circuit. Plasma arc processing equipment equipped with. 3. In a plasma arc processing apparatus using a torch having a main electrode and a tip electrode, a first voltage detector detects a voltage between the main electrode and the tip electrode, and the tip electrode and the workpiece. a second voltage detector for detecting the voltage Vw between the main electrode and the main electrode during normal machining; a first reference value setting circuit that determines an average value Vr of the voltage between the tip electrode and the tip electrode, and a second reference value setting circuit that determines the voltage Vo corresponding to the voltage between the tip electrode and the workpiece during normal machining. and a first reference value setting circuit which receives the output Vm of the average value calculation circuit and the output Vr of the first reference value setting circuit and outputs a binary logic signal according to the magnitude relationship of both input signals. a comparator circuit, which inputs the output of the second voltage detection circuit and the output of the second reference value setting circuit, and outputs two voltages according to the magnitude relationship of both input signals.
a second comparator circuit that outputs a logic signal of a value; and a short circuit that receives the output of the first voltage detection circuit and outputs a signal when the instantaneous value of the input voltage drops to a predetermined voltage that is zero or close to zero. A detection circuit receives the output of the first comparison circuit, the output of the second comparison circuit, and the output of the short circuit detection circuit, and simultaneously satisfies Vm>Vr and Vw>Vo, and detects the short circuit. an AND circuit that outputs an abnormal signal when the circuit detects a short circuit between the main electrode and the tip electrode;
A plasma arc processing device comprising: a protection circuit that interrupts the plasma arc based on the output of the AND circuit. 4. The plasma arc processing apparatus according to claim 2 or 3, further comprising an alarm circuit that is activated by the output of the AND circuit and outputs a continuous alarm until the power is shut off or manually reset. 5. The plasma arc processing apparatus according to claim 4, wherein the manual reset of the protection circuit is enabled by a contact in response to an electrode replacement operation. 6. An electrode for plasma arc machining in which a cathode member made of a non-consumable material is embedded in the center of the tip of an electrode body made of a rod-shaped conductive base material, the electrode being located closer to the base than the buried position of the cathode member. 6. The plasma arc processing apparatus according to claim 1, wherein an electrode is used in which an electrode body is provided with a portion having a small cross-sectional area.