Switching Arrangement For The Stabilization And Ignition Of Welding Arcs And The Like

Abstract

A switching system for the stabilization of alternating-current welding arcs and for the ignition of alternating-current or direct-current welding arcs in which the ignition or stabilization current pulse between electrode and workpiece or between two electrodes is transmitted through a capacitor and at least one semiconductive controlled rectifier (SCR or thyristor) is provided in the discharging circuit of the capacitor. The gate of the controlled rectifier is triggered by a control circuit synchronized with the current source and including a voltage-responsive switching element in circuit with a control capacitor. The voltage-responsive switching element is a DIAC-type trigger diode whose output is connected directly i.e., via only ohmic impedance) with the control electrode or gate of the controlled rectifier.

Parent Case Text

CROSS-REFERENCE TO COPENDING APPLICATION

This application is a continuation-in-part of our application Ser. No. 741,065, filed 28 June 1968, now U.S. Pat. No. 3,551,741 and entitled ARC-STRIKING OR STABILIZATION NETWORK HAVING A DETECTING TRANSFORMER AND CAPACITOR CONNECTED TO THE ELECTRODE.

FIELD OF THE INVENTION

Description

Our present invention relates to a switching arrangement or system for the stabilization of alternating-current arcs and/or for the ignition of alternating-current and direct-current arcs of the type which are produced between two electrodes of a twin-electrode arc-welding or plasma installation or between one electrode and the workpiece of an arc-welding system.

BACKGROUND OF THE INVENTION

In the aforementioned commonly assigned copending application there is described and claimed a method of and a circuit for the stabilization of electric arcs and for the ignition of welding or plasma arcs in which a zero passage-detecting transformer is connected across the alternating-current (line current) input to the arc-electrode system and supplies a storage capacitor; the latter discharges upon conduction of a breakdown device to operate a solid-state semiconductor switch (thyristor) to discharge the main storage capacitor and apply the resulting pulse to the arc gap just after the occurrence of a zero passage in the alternating welding current or just before the maximum of the supply current is reached for ignition.

The term "zero passage" refers to the alternating-current (generally sinusoidal) characteristic which increases to a maximum of positive polarity, has an inflection thereafter, passes through a zero value at a transition between positive polarity and negative polarity, reaches a negative maximum and, via a further inflection, returns to zero in the course of each cycle, the zero passage being the moment, in terms of the cycle, at which the amplitude of the current approaches, passes through and proceeds beyond the zero value. The "maximum" of the current can be the positive or negative maximum of the welding-current characteristic.

In the above-identified application, we have pointed out that it has been known prior to the developments described therein to stabilize alternating-current welding arcs, as may be generated between an arc-welding electrode and the workpiece, or between two electrodes when a twin-electrode cutting arc, brazing arc or filler-rod deposition is involved, by superimposing high-frequency alternating current upon the substantially lower frequency welding current to prevent extinction of the welding arc as the welding-current amplitude passes through its null or zero value in each alternating-current cycle (i.e., during zero passage).

It appears that such high-frequency voltages superimposed on the welding current ensure a rapid reignition of the arc as the current passes through the zero value (zero passage) so that recycling of the arc is unnecessary and the arc is stabilized over the entire period of the alternating current cycle and, therefore, over the set of such cycles constituting the welding period. It has also been proposed to use high-frequency alternating current in superimposition upon direct-current welding systems to promote ignition of the arc at the initial striking or in place of such striking. An important advantage of the latter systems is that the electrode may lie within 2 to 4 mm. of the workpiece and yet ignite the arc, i.e., arc ignition occurs without direct contact of the electrode with the workpiece. The high-frequency current for this purpose has been generated heretofore by circuits and systems which are not free from disadvantages which have precluded their widespread use. For example, a problem frequently arising in these systems is that of interference of the high-frequency generator with radio communication. Attempts have been made to overcome this particular disadvantage by generating one or more pulses only at the precise instants at which the pulse or pulses are necessary, i.e., as the welding-current amplitude passes through its null value, and thereafter terminating the generation of these pulses when the principal welding current is again effective to maintain the arc. The switching arrangements to this end are relatively complex and, in addition, have consumed energy which otherwise might have been used, or would otherwise be available, for the welding operation itself. When the arrangements have made use of sparks or arcs to control the pulse generator, the discharge itself produces ozone and nitrogen oxides which are noxious and adversely affect the metal parts of the apparatus unless these parts are protected against oxidation by special precautions. As a whole, therefore, prior art techniques have not satisfactorily dealt with the problem of stabilizing alternating-current arcs or of igniting alternating-current or direct-current welding arcs of the character described.

As noted earlier, the system of application Ser. No. 741,065 was able to eliminate these disadvantages by providing a circuit arrangement for the stabilization of alternating-current welding and plasma arcs, for the ignition of such arcs, and for the ignition of direct-current arcs which manifests minimum dissipation of the available current and is of relatively simple construction but can be used without substantial modification for the several purposes indicated and various welding modes as will be apparent hereinafter.

The circuit arrangement includes a primary storage capacitor or other electrical energy storage means (i.e., a charge-storage impedance) connected in a discharge circuit with a solid-state controlled rectifier or any other semiconductor switching device of similarly sharp switching characteristics, which is triggered to provide an ignition and/or stabilization current pulse with a duration of 0.8 to 20 microseconds, preferably 1 to 10 microseconds, across the arc gap; the triggering signal is advantageously derived from the secondary (control) storage capacitor whose discharge, via a voltage-responsive breakdown device, is applied to the control element (e.g., gate) of the solid-state switch, preferably a thyristor.

An important feature of the discharge circuit is a step-up transformer whose primary winding is connected in series with the thyristor and the primary storage capacitor while the secondary winding is connected across the electrode and the workpiece so that the voltage level of the pulse is augmented without the necessity of dimensioning the controlled rectifier or the primary storage capacitor to yield outputs of similarly high parameters.

This method differs from the prior art systems in that the output pulse applied between the workpiece and the electrode may have an energy level well above that of the source because the capacitor is charged for the balance of the cycle of the basic welding current source, i.e., for the period in which the controlled rectifier is quenched. Furthermore, the discharge is independent of the welding-current source and does not materially dissipate the welding current. As will be apparent hereinafter, the system also ensures proper triggering of the discharge without affecting the welding operation itself and, indeed, the only power drain is the insignificant drain required to operate the triggering circuit. The significance of this will be appreciated when it is recognized that conventional pulse-producing devices commonly draw 30 kw. or so from the welding source whereas this system requires only about 1 watt.

The triggering circuit of the system includes a detecting transformer responsive to the zero passage of the waveform of the welding source; the transformer has its primary winding connected across the welding source together with an isolating capacitor and a choke. The secondary winding of the detecting transformer is connected in series with a current-limiting variable charging resistor across the secondary capacitor, thereby forming the charging circuit therefor.

The discharging circuit of this control capacitor is formed by a time-constant network (RC or resistance-capacitance network) whose capacitor is bridged by a bleeding resistor and is interposed between the secondary storage capacitor and a breakdown device. The latter is rendered conductive as the voltage applied across the gap rises upon quenching of the arc and discharges the secondary storage capacitor through a further current-limiting resistor to form the triggering pulse. The breakdown device of this system may be constituted by a glow tube, a nonlinear diode such as a zener diode or the like.

The triggering-pulse output of this device is applied to the primary winding of a pulse transformer whose output side feeds a rectifier which, in turn, is connected to the gate of the thyristor. Where a number of thyristors are provided in oppositely poled orientation or a TRIAC is used, the rectifier output is applied to the gates thereof although a number of individual rectifiers may be employed, each of which can be connected to the individual gate of a thyristor. The primary capacitor has a charging circuit which includes an isolating transformer whose primary winding is connected to the line current source between a switch and a transformer serving as this source. At the output transformer of the discharge circuit of the primary capacitor, a diode is placed in shunt across the primary winding and is poled oppositely to the semiconductor switch.

OBJECTS OF THE INVENTION

It is the main object of the present invention to extend the principles set forth in our application Ser. No. 741,065.

It is another object of this invention to provide an improved system for igniting alternating-current electric arcs for arc-welding installations and the like.

It is another object of the invention to provide improved switching circuitry for the stabilization of alternating-current welding arcs.

A further object of the invention is the provision of an improved system for igniting direct-current welding arcs and the like.

SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter are attained, in accordance with the present invention, by providing a switching arrangement for igniting or stabilizing the arc generated between a pair of electrodes or between an electrode and the workpiece wherein a controllable semiconductive rectifier, e.g., a silicon controlled rectifier (SCR or thyristor) has its principal electrodes connected in circuit with the electrode or electrodes in the firing circuit (directly or via a pulse transformer) and a voltage-responsive switching element is provided for triggering this semiconductor switch, the voltage-responsive switching element consisting of a trigger diode, for example a DIAC, whose output side is connected directly with the control electrode of the controlled rectifier, i.e., the gate thereof.

A trigger diode may be described as a solid-state element which functions similarly to a neon-lamp trigger but offers the advantages of reduced requirements for peak-voltage firing, higher pulse-current capability and longer life.

The solid-state bidirectional trigger diodes are often referred to as silicon bidirectional DIAC's and are usually three-layer symmetrical avalanche-type devices which break over in the negative-resistance region whenever a particular voltage, termed the breakover voltage, is exceeded in either voltage polarity. In these devices, there usually is the maximum limit to the voltage imposed upon the symmetry between positive and negative breakover voltages (voltage symmetry). A slight current offset in the characteristic before the voltage breakover point is leakage current which may be of the order of microamperes. The conductive state continues until the current through the trigger DIAC falls below a threshold level.

In accordance with the principle of the present invention, the input electrode of the DIAC is connected to a resistor in an RC (resistance-capacitance) circuit such that, when the voltage at the input terminal reaches the breakover voltage, the DIAC becomes partially conductive to permit a capacitor to discharge partially through the DIAC into the gate of the controlled rectifier, thereby triggering the SCR into a conduction mode for the remainder of the half cycle, i.e., until the SCR is quenched, and yielding the ignition and/or stabilization current pulse.

In more general terms, a trigger diode can be described as a semiconductor which may be traversed by electric current in opposite directions upon exceeding the relatively high ignition voltage of 20 to 30 volts at which current throughflow is permitted, i.e., the DIAC breaks down and become conductive.

Upon a drop in the current below a threshold corresponding to high-current throughflow, the trigger diode is rendered blocking. As a practical matter, a separate triggering circuit for the bidirectional trigger diode is not required and the device itself is an effective trigger (trigger DIAC).

Aside from this advantage, the use of a trigger diode in the triggering circuit of a silicon-controlled rectifier which is switched thereby to stabilize an alternating-current welding arc or ignite either an alternating current or direct current arc, has surprising effectiveness which will be apparent hereinafter. These advantages arise from the inherent characteristics of the bidirectional trigger diodes which have been found to be precisely those which are required for the triggering of the main switching device, namely, the controlled rectifier mentioned earlier.

It is thus possible, in accordance with the present invention, to simply derive a substantially constant voltage from the voltage source via an isolating transformer and a full-wave rectifier bridge and to connect the output of this bridge via a potentiometer or voltage divider network to the output of the welding current source to form a voltage which, as applied to the DIAC, triggers through the latter the primary switch. When the output voltage of the welding-current source reaches a predetermined level, therefore, the trigger diode breaks down and applies a control pulse from a control-circuit capacitor to the gate of the controlled rectifier, thereby directly opening or unblocking the controlled rectifier as well. The pulses applied to the gap, in turn, are always of the same polarity.

According to a more specific feature of this invention, the blocking of the trigger diode is effected automatically by the discharge of the control capacitor, the discharge rate of which is determined by a resistor in the discharge circuit. Hence the duration of the control pulse is precisely determined by the magnitude of the capacitance and resistance in the discharge circuit and can readily be regulated by varying one or the other. A current taper to a level beyond the limiting current of the trigger diode, serves as the cutoff.

Immediately thereafter, the control capacitor commences its recharge phase which is maintained until the capacitor has reached maximum charge or the reignition of the trigger diode is required whereupon the previously described breakdown occurs again and the repetition is substantially periodic. When the spacing of the electrode and the workpiece is limited as noted earlier, the welding current is normal alternating current and no unusual conditions arise at the gap, the control pulses have a duration of a small fraction of a period of the welding current.

An important advantage of the present invention, deriving from the use of a trigger diode, is that there is always a single adjustment or setting of the potentiometer arrangement of the charging circuit of the control capacitor to establish current pulses sufficient only to occupy the fraction of the period at which the welding arc is extinguished, i.e., the period of zero passage. To this end it is only necessary to set the voltage at the trigger diode such that the open-circuit voltage of the welding-current source exceeds the breakdown voltage of the trigger diode while the operating voltage (during welding) of the source lies below the breakdown voltage of the trigger diode. With this setting, control pulses arise in a uniform sequence only as long as the electrodes are positioned for welding but the arc has not yet been struck. As soon as the arc strikes, the open-circuit voltage of the welding-current source falls to the welding-voltage level and the voltage at the input side of the trigger diode no longer suffices to maintain the trigger diode conductive so that, as the welding current approaches its next zero passage, the control pulse is extinguished. The firing and quenching of the trigger diode is then repeated to stabilize an alternating-current welding arc.

With direct-current welding, the circuit operates in a similar manner, although only for ignition of the arc, inasmuch as the existence of a continuous constant-current and constant-voltage arc during the rest of the welding period, maintains the trigger diode in a blocking state, thereby excluding further pulses during the period in which the arc remains ignited.

The switching arrangement, as described above, produces a current pulse between electrode and workpiece as long as the arc extinguished. In general, only a single control pulse is required for the ignition of the arc, provided that the control pulse is generated precisely at the instant the ignition pulse is required, i.e., just as the welding-current amplitude reaches a maximum. The potentiometer arrangement also described above allows the correct timing to be established without further control devices. However, it is frequency desired to vary the ignition point within limits selected in accordance with welding requirements and/or to control other parameters of the current pulse. This can easily be achieved in accordance with the present invention, by restricting the control pulse to a single one per half cycle of the welding-current waveform.

According to still another feature of this invention, the control circuit comprises, in parallel with the control capacitor, a solid-state controlled rectifier which may lie in series with a resistor so as to constitute a shunt network across the control capacitor. The resistor then functions as a drain when the controlled rectifier is conductive. The control electrode or gate of this latter SCR or thyristor is, in turn, connected to the output of the trigger diode in series with a switch so that, depending upon the position of this switch, either a single pulse (for arc ignition) or a train of pulses is produced, e.g., to continue until ignition of the arc. When the switch is open, the shunt thyristor is in a blocking mode and the trigger diode operates as previously described, i.e., the output of the potentiometer network charges the capacitor, which, when the breakdown voltage of the trigger diode is exceeded, drains through this diode to trigger the power switch. When the current drain through the trigger diode diminishes below the cutoff threshold, the trigger diode becomes nonconductive and voltage again builds up in the capacitor for repetition of the sequence. When the control switch is rendered conductive by appropriate feedback of the output of the trigger diode to the control electrode of this thyristor in a closed condition of the selector switch, a continuous drain is maintained across the capacitor and repetition of the voltage buildup thereat is prevented. The trigger diode is rapidly brought to the quenched condition and blocked so that its reignition voltage, prior to a further zero passage of the welding current or prior to the decay of the half cycle of the welding current in progress, cannot be reached.

Only when the current flow in the welding circuit disappears, for example during zero passage of the alternating welding current, is the main or power thyristor blocked so that the trigger diode sees an increase in the voltage at its input side and is capable of reignition and breakdown. Thus when the selector switch is closed, only a single-current pulse is provided per half cycle of the alternating welding current and only a single ignition pulse is provided when a direct welding current is switched on.

According to an important feature of this invention, the charging circuit of the control capacitor of the present invention, i.e., the capacitor whose potential determines the breakdown of the trigger diode, comprises an isolating capacitor for the quenching of the current pulse released by the controlled rectifier, a choke (inductive impedance) and the primary winding of a pulse transformer whose secondary winding is connected in circuit with the electrode and the workpiece in the discharge network. This primary winding is, moreover, provided with a diode shunt to prevent oscillation at the primary, e.g., as described in our copending application mentioned earlier. The energy required at the control capacitor is drawn via an isolating transformer whose primary side or winding is connected to an alternating-current source, e.g., the welding-current source (preferably across the electrode and workpiece) while the secondary is provided with rectifier means connected with the potentiometer arrangement and thence with the control capacitor.

The switching arrangement described immediately above generates ignition and/or stabilization current pulses of a single polarity since the voltage-responsive switching element, i.e., the trigger diode, is activated during only one half of the alternating-current cycle. It has been found that, in many cases, it is desirable to provide two polarity current pulses at the welding gap and the principles associated with this concept have been set forth in the prior application.

It is, according to another aspect of this invention, possible to modify the basic switching arrangement as described above so that the electrode and workpiece see current pulses of alternating polarity, e.g., corresponding to the polarity of the alternating-current source. To this end, the discharging circuit of the power capacitor, i.e., the capacitor connected in series with the controlled rectifier switch means and the primary winding of the power transformer (whose secondary winding is connected across the electrode and workpiece), includes two paths, each of which is effective during one-half cycle of the supply-current waveform.

According to this aspect of the invention, two control circuits are provided, each of which is designed to trigger a respective solid-state controlled rectifier or thyristor, the thyristors being connected in antiparallel relationship. An important point in connection with this embodiment is that the two control circuits may exclude rectifier diodes between the isolating transformer and the charging circuit of the control capacitor if desired. Again, it is significant that each of the control circuits comprises a respective trigger diode, the output side of which is tied directly to the gap of the respective power thyristor while its control capacitor is charged through the isolating transformer during a corresponding half cycle of the alternating current input. In general, the switching circuit of this embodiment is equivalent to the basic switching circuit set forth above, i.e., the discharge circuit of the power capacitor includes an isolating or blocking capacitor, a choke and a primary winding of the pulse transformer whose secondary is connected across the electrode and workpiece. The energy of the power capacitor is delivered to the welding gap without the intermediary of a rectifier as would be desirable otherwise to shunt half the reverse half of each cycle. We have also discovered that it is of importance to optimum operation of a system to maintain the energy stored in the capacitor and the magnetic energy of the isolating transformer in the charging circuit of the power capacitor in a ratio of 1:1 to 1:3.

It will be appreciated that it is not always possible and may not be desirable for reasons of circuit design and convenience to provide the discharge circuit of the power capacitor with a pulse transformer which is interposed between the SCR switch and the arc gap. Such an occasion may arise when the power pulse may have low voltages. In this case, the discharge circuit may be coupled galvanically or capacitively to the electrode and the workpiece.

When the charging circuit of the power capacitor is connected to a well-defined alternating-current source and pulses of identical polarity are applied to the arc gap, there is the further advantage of the present system over prior art circuits that the control circuit draws no energy from the welding current circuit to produce the trigger pulses.

DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the present invention will be more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a circuit diagram of a switching arrangement for the generation of current pulses of one polarity between electrode and workpiece;

FIG. 2 is a circuit diagram of another embodiment of this invention in which the main current pulses are of opposite polarities; and

FIG. 3 is a circuit diagram representing a modification.

SPECIFIC DESCRIPTION

In FIG. 1, a welding-current source 1 is represented as a transformer which is connected with the high power alternating-current line 2 and has its output side 1' connected across the welding load represented by a welding electrode 3 and the workpiece 4.

It will be apparent that, upon the striking of an arc across the gap G, the source 1 continues to supply welding current of alternating or sinusoidal waveform such that the zero passage, the amplitude of the welding current in the gap G falls to substantially zero and it is possible that the arc will extinguish. For stabilization of the arc, in accordance with the principles set forth in our prior application, pulses may be superimposed upon the gap in the cadence of the welding current to ensure continuous current flow and preclude arc quenching during the zero passage. In the present embodiment, the pulse constitutes a reignition pulse for each of the half cycles of the current in one mode as noted below.

To facilitate an understanding of the operation of the circuit of the present invention, the output of the source 1 will be referred to as an alternating welding voltage while the current through the system 3, G, 4, will be designated the alternating welding current insofar as the current derives from the source 1.

The switching arrangement provides a current pulse which bridges the electrode 3 and the workpiece 4 following each zero passage of the alternating welding current and thereby assuring reignition of the arc during the subsequent half cycle. The same arrangement may also serve to trigger the direct-current arc, in which case the current pulse is applied when a switch 2a is closed to connect the transformer 2b and the rectifier 2c across the electrode 3 and the workpiece 4. A selector switch 2e may be operated to choose direct or alternating current welding. For reasons which will become apparent hereinafter, the adding of current through the electrode system 3, G, 4, when a direct-current welding arrangement is employed, occurs only when the switch 2a is closed to initiate the arc and at any time the DC arc may quench by inadvertence. In the welding-current circuit, there is provided an iron-core choke 5 to block dissipation of the triggering pulses which are applied across the electrode 3 and the workpiece 4 in the arc-welding source 1.

The energy required for the pulse source, according to the invention, may be supplied from a separate alternating-current line 6 via an isolating transformer 7, the output of which is connected across a full-wave rectifier bridge 8. Instead of a separate source 6, the isolating transformer can be tied to the welding source 2 or the welding source 1 can be connected to the line 6. Furthermore, the isolating transformer 7 may be eliminated and the rectifier 8 tied to the source 1 or the line 2.

At the output side, the rectifier bridge 8 is connected in the charging circuit 9 of the power capacitor 10 delivering the current required for the ignition or stabilization pulse or pulse train. Here the capacitor 10 is tied directly across the rectifier bridge 8. A variable resistance may be provided in series with the capacitor 10 to enable the charging rate of the latter to be controlled if desired.

The discharging circuit 11 of the power capacitor 10 comprises an isolating or blocking condenser 12, a surge-suppressing choke 13, the primary winding 14a of a pulse transformer 14 and a thyristor 15 which, in the open condition, blocks discharge of the capacitor 10 through the primary winding 14a of the pulse transformer.

The isolating condenser 12 is bridged by a discharge resistor 16 whose ohmic resistance, in combination with the ohmic impedances of the other elements in series with the power capacitor 10, determines the rate of discharge of the power capacitor. The primary winding 14a of the pulse transformer 14 is shunted by a diode 17 which serves to dissipate any reverse-polarity surges in the discharge network. Since either the capacitor 12 or the choke 13 may serve as a thyristor-quenching impedance, one or the other may be omitted from the circuit, as desired.

The thyristor-control circuit 18 (trigger-pulse circuit) is likewise provided with a storage capacitor, here represented at 19. The control capacitor 19 is provided at its discharge side 18 with a trigger diode of the DIAC type, as represented at 20, the input and output terminals being represented at 20a and 20b respectively. The DIAC has the electrical characteristics discussed above.

The output terminal 20b of the DIAC 20 is tied directly to the gate 15g of the thyristor 15, i.e., without intervening inductive or reactive impedances, the direct connection here being shown to include a variable current-limiting resistor 21. Both branches of the charging circuit 27 and the discharging or control circuit 18 are bridged by a load resistor 22 to enable the gate-control potential for the thyristor 15 to appear thereacross. Parallel to the control capacitor 19 we provide a short-circuiting thyristor 23 in series with a resistor 24 controlling the drain rate of the capacitor 19 when the thyristor 23 is conductive. The control electrode of thyristor 23 is connected via a current-limiting resistor 25 and a series switch 26 to the output terminal 20b of DIAC 20.

The charging circuit 27 of the control capacitor 19 includes a potentiometer arrangement designated generally at 28 and includes a first potentiometer 28a bridged across the output terminals of a full-wave rectifier bridge 29 as a voltage divider, and a series potentiometer 28b whose input terminal is connected to the wiper 28c of potentiometer 28a. The output side of the variable resistor 28b is tied to the input terminal 20a which coincides with the high-voltage side of the capacitor 19.

The rectifier bridge 29 has its input tied across the secondary winding of an isolating transformer 30 whose primary winding is connected across both branches of the welding-current circuit, i.e., across the output of the welding-current source 1 and across the electrode 3 and workpiece 4. The pulse circuit 33 for the electrode 3 and the workpiece 4 includes the secondary winding 14b of the pulse transformer 14 and, in series therewith an isolating capacitor 31 and a current limiting resistor 32. It will be understood that the isolating transformer 30 is not always required and that the rectifier bridge 29, for example, can be connected directly across the electrode 3 and workpiece 4, or to the welding source as described above.

The welding current source 1 and the isolating transformer 30 are connected with the sources 2 and 6, respectively via appropriate circuit breakers and switching devices not illustrated. Both are switched simultaneously so that, immediately upon switching, capacitor 10 charges via the rectifier bridge 8. Once the gate of the control rectifier 15 receives a control pulse, the control rectifier opens and the power capacitor 10 is permitted to discharge through the primary winding of the pulse transformer 14. An ignition and/or stabilization current pulse is thereby induced in a pulse circuit 33 which traverses the electrode 3 and workpiece 4 in the form of a current pulse if a discharge is in progress. In a quenched state of the gap G a potential is permitted to building up across the electrode 3 and the workpiece 4 to a maximum value determined by the turn ratio of the transformer 14 and may be between 300 and 5,000 volts. The peak amplitude of the current pulse lies between 5 and 20 amperes and its duration 1 to 10 microseconds. In any event, the maximum potential is selected to be sufficient to break down the gap. It is sufficient to note that when the current pulse can have a maximum of 1,000 volts, the pulse transformer 14 can be eliminated and the pulse circuit 33 connected galvanically or capacitively to the discharge circuit 11 of the pulse capacitor 10.

Thyristor 15 remains conductive until the current builds up in the discharge circuit 11, whereupon capacitor 12 builds up a charge while capacitor 10 is drained, thereby reversing the polarity across the principal electrodes of the thyristor and transforming the latter into its blocking mode. Each subsequent pulse is similarly applied to the electrode 3 and workpiece 4.

The control pulse is, of course, produced synchronously with the alternating welding current. Simultaneously with the growth of the voltage in the welding current circuit 33, there is a corresponding increase in the potential developed across the capacitor 19 and, therefore, at the terminal 28 of the DIAC 20.

The rate of increase in voltage at the DIAC input can be set with the potentiometer arrangement 28 such that the moment at which the potential at input 20a reaches the ignition potential of the DIAC corresponds to the point in which ignition of the arc across electrode 3 and workpiece 4 is to be initiated. Upon an increase in the potential at input 28 of the DIAC 20 to a level above its breakdown potential, the DIAC is rendered conductive and the control capacitor 19 discharges through the control circuit 18, i.e., via the gate of thyristor 15. The thyristor 15 is thereby rendered conductive to apply the ignition and/or stabilization current pulse through the pulse transformer 14 to the pulse circuit 33, i.e., to the electrode 3 and the workpiece 4.

Resistor 21 and resistor 22 determine the rate at which capacitor 19 may drain in the conductive state of the DIAC 20, thereby establishing the duration of the control pulse which may be adjusted by varying the resistance of the resistor 21. Depending upon this resistance, therefore, the potential at the DIAC input 20a may fall at a more rapid or at a slower rate. When the potential at the input 20a drops to a level such that the current flow in the control circuit 18 is below the avalanche threshold of the DIAC 20, this trigger diode is rendered nonconductive and the first control pulse is terminated.

Insofar as the initial current pulse ignites the electric arc, as is usually the case, no further control pulse is generated during the half cycle in progress since the reduction of potential in the welding-current circuit corresponds to a reduction in potential at the input 20a of the DIAC 20, whereby the DIAC remains nonconductive. The relationship between the potential buildup in the welding-current circuit and at the input 20a of the DIAC 20 is, of course, established by the potentiometer arrangement 28.

In the event the arc has not been ignited by the first ignition pulse applied by the circuits 11 and 33 to the electrode 3 and workpiece 4, the open circuit voltage in the welding-current circuit increases with the corresponding increase of the potential across the DIAC 20 to produce a subsequent control pulse which is again applied to the gate of the thyristor 15 as previously described. In the absence of ignition, a train of such control pulses is produced and the thyristor rendered conductive periodically until breakdown occurs in the gap. Similarly, during each zero passage of an alternating welding current during the actual welding operation, a corresponding buildup of the potential across the gap and a corresponding increase in the potential across the DIAC results in breakdown of this trigger diode, the formation of the control pulse, the triggering of the thyristor 15 and the application of an ignition pulse in the pulse circuit 33. In the case of direct-current arc welding, the sequence as described occurs whenever the arc is quenched and reignition is automatically effected by the successive breakdown of the trigger diode, triggering of the power thyristor and generation of a triggering pulse in circuit 33.

In practice, it is found that the arc ignites during the inflection of the first half cycle of the welding-current pulse, especially when the potentiometer arrangement 28 is set to cause breakdown of the DIAC 20 at a point at which the open circuit voltage due to the welding-current source is approximately at its maximum value. It is advantageous to avoid electrical noise and interference with radio and television transmissions and to this end, only a single-current pulse should be applied per half cycle of the alternating welding current such that only one ignition pulse is generated at the electrode 3 and workpiece 4 for each half cycle.

Accordingly, the thyristor 23 may be placed in the circuit by closure of switch 26. When switch 26 is closed, the development of the first control pulse causes conductivity of the thyristor 15 while the same pulse opens thyristor 23 and short-circuits the input terminal 20a to the opposite terminal of the system and provides a shunt for the discharge current of capacitor 19 through the resistor 24; the value of the latter determines the rate of which the current through the DIAC 20 is reduced and hence the point at which the DIAC closes. As long as the thyristor 23 remains conductive, the buildup of a potential at input 20a cannot commence and the subsequent formation of a control pulse is prevented. Only after the alternating welding current goes through a zero passage, is the potential across the principal electrodes of thyristor 23 reversed so that this thyristor is quenched and voltage buildup permitted at 20a. Only a single control pulse can, therefore, occur during the half cycle. When the switch 26 is open, however, a train of control pulses is produced until arc ignition at a frequency determined by resistors 21 and 22 and the capacitance of capacitor 19 which form an RC time constant network.

When the switching arrangement is intended solely for the production of a current pulse per half cycle of the alternating welding current, the DIAC 20 and the thyristor 23 can be replaced by a bilateral four-layer diode which, once opened or conductive, remains so until a zero passage.

As noted earlier, the circuit of FIG. 1 yields ignition or stabilization current pulses always of one polarity. In FIG. 2, however, we have shown a circuit in which ignition and stabilization current pulses of opposite polarities, corresponding to the polarities of the alternating welding current are generated. To the extent that similar elements are present in the circuits of FIG. 1 and 2, similar reference numerals have been employed.

In FIG. 2, we provide a circuit in which the isolating transformer 7 has its primary winding 7a connected to the lines 1' of the welding current source 1 while the secondary winding 7b contains no rectifier of the type shown at 8 in the charging circuit 9 of rectifier 10. In this embodiment, however, the charging circuit 9a includes only the secondary winding 7b of transformer 7, the terminals of which are applied directly across the power capacitor 10. In the discharge circuit 11a of the latter, two thyristors 34, 35 are provided in antiparallel relationship in series between the capacitor 10, the primary winding 14a of pulse transformer 14, the choke 13, and the parallel network of isolating capacitor 12 and resistor 16. The antiparallel thyristors 34 and 35 can, of course, be replaced by a bidirectional triode thyristor of the TRIAC type.

The gates of the thyristors 34 and 35 are each provided with a respective control circuit 36, 37 of approximately similar construction, each circuit being effective during only one half cycle of the alternating welding current.

The control circuits 36 and 37 each comprise a respective DIAC 38, 39, the input terminals 38a, 39a of which are connected to one side of the respective control capacitor 40 or 41. Load resistances 44 and 45 are connected across the series circuits represented by the DIAC 38 and the control capacitor 40 and by the DIAC 39 and the control capacitor 41, while current-limiting and DIAC-conductivity-setting variable resistors 42 and 43 are connected between the output terminals 38b and 38c and the gates 34g and 35g of the thyristors 34 and 35.

The charging circuits 46 and 47 of the control capacitors 40 and 41, respectively, each include a potentiometer arrangement 48 or 49 in the form of a single voltage-dividing potentiometer connected across the respective secondary winding 50a, 51a of isolating transformers 50 and 51 in series with oppositely poled rectifiers 52 and 53. The isolating transformers 50 and 51 have their primary winding 50b and 51b connected in series across the welding-current source 1. The isolating transformers 50, 51 and the rectifier diodes 52, 53 (each of which constitutes a half-wave rectifier) is so arranged as to permit each charging circuit 46 or 47 to be effective during only one half cycle of the alternating-current input.

The energy requirement of the power-pulse capacitor 10 is in this embodiment derived from the welding-current circuit. It is also feasible to constitute the isolating transformers 50 and 51 as a single-transformer arrangement, in which case only one primary winding may be connected across the welding-current source 1 and member 7b, 50a and 51a will be constituted as individual windings of the single transformer.

When the alternating welding current is turned on in the arrangement of FIG. 2 (via a switch or circuit breaker not otherwise illustrated), current pulses are generated between electrodes 3 and 4 by alternate conductivity of the thyristors 34 and 35 corresponding to the polarity of the successive half-cycles of the alternating welding current. The trigger pulses are thus synchronized therewith.

The control circuits 36 and 37, which are effective alternately to trigger the thyristors 34 and 35 into conductive states, here operate in a manner similar to that described for the control circuit 18 of FIG. 1. A further switching arrangement such as that shown at 23, 25 and 26 in FIG. 1, to restrict the number of control pulses to a single-control pulse per half-cycle of the alternating welding current in each of the trigger circuits 36, 37, can be provided in an analogous manner. Also, the circuit of FIG. 2 may be operated with a direct-current source using only a single thyristor 34 or 35 or with a pair of oppositely poled direct-current sources, each in series with a respective one of these thyristors.

It will be understood that the switching arrangement of the present invention can be used in substantially any arc-ignition or arc-stabilization system and for alternating-current as well as direct-current arcs. Preferably, the system is used in a welding arrangement or in an arc-cutting system. It is also contemplated to provide the circuit in other arrangements in which arcs are to be ignited or should be sustained. The switching device has been found to be highly flexible especially since it permits ready control of the number, timing and duration of the control pulses and, therefore, variation of the same parameters of the ignition or stabilization current pulses as previously described. In practice, solid-state units embodying the invention have operated trouble-free for prolonged periods.

In FIG. 3, we have shown another arrangement, according to the present invention, embodying some of the modifications discussed earlier. For example, we have previously observed that a TRIAC may be substituted for a pair of antiparallel thyristors in the ignition or stabilization weld-pulse circuit, that the pulse transformer may be omitted, that the trigger source may be connected directly across the electrodes and that, in place of a trigger DIAC and a thyristor shunt, a four-layer bilateral diode may be provided. While each of these modifications may be employed individually in either the system of FIG. 1 or the system of FIG. 2, they have been shown collectively in FIG. 3. Hence it is contemplated in the present invention to employ the four-layer bilateral diode together with a thyristor switch, to use the TRIAC switch and a trigger DIAC trigger circuit without a pulse transformer, to provide one trigger DIAC and one four-layer directional diode in respective trigger circuits for operation of respective power-pulse thyristors, to use either or any of the modifications set forth above with direct coupling of the trigger circuit to the electrodes (i.e., galvanic coupling without an isolating or detecting transformer) or to provide capacitive coupling in any of the compounds consistent with the foregoing.

In FIG. 3, for example, the welding-current source is shown at 140 to be connectable via a double-pole, double-throw switch 142 to either the twin electrodes 143 of an arc-cutting torch or other two-electrode arc-producing device or to the electrode 144 and a workpiece 145 for submerged-arc deposition or other arc-welding of this workpiece. The source 140 is here represented as a alternating-current source although a separate direct-current source may be provided in series with the switch means 146 and the electrodes while only the trigger circuit is energized from an alternating-current or clock-pulse source.

In this embodiment, however, the switching arrangement 146 for the ignition and stabilization current pulses is connected by an isolating transformer 147 to the source 140 and is seen to include a bidirectional TRIAC 148, the function of which is analogous to the antiparallel thyristors of FIG. 2. Whereas ordinary thyristors (SCR's) are reverse-blocking and unidirectional, the TRIAC is a bidirectional switching device. The output of the TRIAC switch is applied directly in the form of current pulses to the electrode system 142, 143 or the electrode system 144, 145, i.e., without the intermediary of a pulse transformer. The block 146 thus includes a network corresponding to charging circuit 9, power capacitor 10, R-C network 12, 16, choke 13, etc., as described with respect to FIGS. 1 and 2, except that the TRIAC 148 is substituted for the three-electrode switches of these circuits.

Tapped directly across the electrode, in either electrode system, is the control or trigger circuit, generally designated at 149 and consisting of a pair of networks 149a and 149b, each effective during a half cycle of the welding current waveform as determined by the oppositely poled rectifiers 150 and 151. The rectifiers can be connected directly to the electrode network (galvanic connection) in one position of a switch 153 or may be capacitively coupled to the electrode gap via a condenser 152 in the other position of this switch. When capacitive connection is used, of course, the networks 149a and 149b reverse with respect to the polarity of the alternating current waveform during which they are effective.

The rectifier diodes 150 and 151 are each connected in series with a charging resistor 154, 155 and to the trigger capacitor 156, 157, the capacitors being each connected in series with a four-layer bidirectional diode 158, 159 across a common load resistor 160 whose one grounded terminal is connected to the gate of the TRIAC 148 via a variable resistor 161 functioning in a manner similar to that of the resistor 21.

The circuit of FIG. 3 operates in a manner corresponding to the circuit of FIG. 2 with the modification that no thyristor shunt is required to restrict the number of ignition or stabilization pulses per half cycle of the alternating welding current since the bidirectional four-layer diodes 158 and 159, once brought to breakdown by the potential across the respective capacitor, remain conductive until the zero passage of the alternating current applied to the electrode system whereupon the four-layer diode becomes nonconductive.

The improvement described and illustrated is believed to admit of many modifications within the ability of persons skilled in the art, all such modifications being considered within the spirit and scope of the invention except as limited by the appended claims.

Claims

We claim:

1. A circuit arrangement operable for the stabilization of an alternating-current arc and for the ignition of an alternating-current arc or direct-current arc to be generated at an electrode system, said circuit arrangement comprising:

an electrode system;

a current-pulse generator connected to said electrode system and including:

a storage capacitor,

a source of electric current connected with said storage capacitor for electrically charging same, and

a primary discharge circuit between said storage capacitor and said electrode system including at least one solid-state triggerable switch having a control electrode energizable to discharge said storage capacitor through said discharge circuit and said switch and apply a current pulse to said electrode system; and

a triggering circuit connected with said control electrode for electrically energizing same to render said switch conductive, said triggering circuit including:

a control capacitor adapted to receive an electric charge,

a control discharge circuit operatively connected between said control capacitor and said control electrode including a trigger diode in series with said control electrode and said control capacitor, and

a silicon-controlled rectifier connected in shunt across said control capacitor and having a gate connected to the output side of said trigger diode for bypassing discharge current from said control capacitor away from said trigger diode.

2. The circuit arrangement defined in claim 1 wherein said trigger diode is a DIAC.

3. The circuit arrangement defined in claim 1 wherein said trigger diode is a bidirectional four-layer diode.

4. The circuit arrangement defined in claim 1 wherein said triggerable switch is a unidirectional silicon-controlled rectifier.

5. The circuit arrangement defined in claim 1 wherein said solid-state triggerable switch is a TRIAC.

6. The circuit arrangement defined in claim 1 wherein said triggering circuit comprises a charging circuit connected across said control capacitor for charging same, said charging circuit including a source of alternating current and a rectifier connected between said source of alternating current and said control capacitor.

7. The circuit arrangement defined in claim 6 wherein an alternating welding current source connected across said electrode system for applying welding current thereto synchronized with control pulses passed through said trigger diode, said source of alternating current of said triggering circuit including an isolating transformer having a primary winding connected across said welding current source and a secondary winding in circuit with said rectifier.

8. The circuit arrangement defined in claim 1 wherein said primary discharge circuit comprises at least one quenching impedance connected in series with said storage capacitor and said solid-state triggerable switch, and a pulse transformer having a primary winding in series with said impedance, with said solid-state triggerable switch and with said storage capacitor, said pulse transformer having a secondary winding connected to said electrode system.

9. The circuit arrangement defined in claim 8 wherein said quenching impedance is a condenser, said solid-state triggerable switch is a thyristor and said primary discharge circuit further includes a diode shunting said primary winding, and a resistor shunting said condenser.

10. The circuit arrangement defined in claim 8 wherein said solid-state triggerable switch is a thyristor, said quenching impedance is an inductor, and said primary discharge circuit includes a diode connected in shunt across said primary winding.

11. A circuit arrangement operable for the stabilization of an alternating-current arc and for the ignition of an alternating-current arc or direct-current arc to be generated at an electrode system, said circuit arrangement comprising:

an electrode system;

a current-pulse generator connected to said electrode system and including:

a storage capacitor,

a source of electric current connected with said storage capacitor for electrically charging same, and

a primary discharge circuit between said storage capacitor and said electrode system including at least one solid-state triggerable switch having a control electrode energizable to discharge said storage capacitor through said discharge circuit and said switch and apply a current pulse to said electrode system, said switch including at least a pair of thyristors poled in opposite directions in series with said storage capacitor; and

a triggering circuit connected with said control electrode for electrically energizing same to render said switch conductive, said triggering circuit including:

a control capacitor adapted to receive an electric charge,

a control discharge circuit operatively connected between said control capacitor and said control electrode including a trigger diode in series with said control electrode and said control capacitor,

means connecting the first-mentioned control capacitor to the gate of one of said thyristors through said triggerable diode,

a second control capacitor,

a second triggerable diode connected between said second control capacitor and the gate of the other of said thyristors; an alternating current charging source, and

respective rectifier diodes connecting said charging source with each of said control capacitors for energizing same during respective half cycles of the alternating current output of said charging source.

12. The circuit arrangement defined in claim 11 wherein said primary discharge circuit comprises a pulse transformer having a secondary winding connected to said electrode system and a primary winding connected in series with said thyristors and said storage capacitor; a resistor-bridged isolating condenser in series with said storage capacitor, said thyristors, and said primary winding; and an inductor connected in series with said condenser, said storage capacitor, said primary winding and said thyristors.

13. The circuit arrangement defined in claim 1 wherein said solid-state triggerable switch is connected directly with said electrode system.

14. The circuit arrangement defined in claim 1 wherein said primary discharge circuit includes a capacitive coupling between said electrode system and said solid-state triggerable switch.

15. The circuit arrangement defined in claim 1 wherein said source of electric current includes an isolating transformer having a primary winding connected across a further alternating current source; a secondary winding connected with said storage capacitor; and rectifier means interposed between said secondary winding and said storage capacitor.

16. The circuit arrangement defined in claim 1 wherein said source of electric current connected with said storage capacitor includes a transformer having a primary winding connected to an alternating current line and a secondary winding connected to said storage capacitor for charging same, the storage energy of said storage capacitor and the magnetic energy of the primary side of said transformer being substantially in the ratio of 1:1 to 1:3.

17. The circuit arrangement defined in claim 19

wherein an alternating welding current source is connected across said electrode system, said arrangement further comprising a choke connected in series with said alternating welding current source and said electrode system;

said source of electric current of said current-pulse generator including:

a first isolating transformer having a primary winding connected to an AC line and a secondary winding connected to said storage capacitor, and

a first rectifier connected between said secondary winding and said storage capacitor;

said primary discharge circuit including:

at least one thyristor constituting said solid-state triggerable switch and having its principal electrodes connected in series with said storage capacitor,

a resistor-bridged isolating capacitor connected in series with said storage capacitor and said thyristor,

an inductor connected in series with said resistance-bridged condenser and said thyristor, and

a pulse transformer having a primary winding in series with said principal electrodes and a secondary winding connected across said electrode system;

said triggering circuit further comprising:

a second isolating transformer having a primary winding connected across said alternating welding current source and a secondary winding, a second rectifier connected to said secondary winding of said second transformer, and a potentiometer arrangement connected across said second rectifier and diode to said control capacitor for charging same at a rate determined by said potentiometer arrangement; and

said control discharge circuit further comprising:

a variable resistor in series with said trigger diode and the gate of said thyristor; said trigger diode being constituted as a DIAC and said control capacitor being provided with a resistor and thyristor shunt triggerable upon conduction of said DIAC.

18. The circuit arrangement defined in claim 19,

wherein an alternating welding current source connected across said electrode system, said arrangement further comprising a choke in series with said alternating welding current source and said electrode system;

said source of electric current of said current-pulse generator including a first isolating transformer having a primary winding connected across said alternating welding current source and a secondary winding connected across said storage capacitor;

said primary discharge circuit including:

at least one thyristor constituting said solid-state triggerable switch;

a pulse transformer having a primary winding connected in series with said thyristor and with said storage capacitor,

a resistor-shunted isolating condenser in series with said storage capacitor, and

an inductor in series with said storage capacitor, with said primary winding of said pulse transformer and with said thyristor, said pulse transformer having a secondary winding connected across said electrode system;

said triggering circuit further comprising:

a second transformer having a primary winding connected across said alternating welding current source and a secondary winding connected to said control capacitor,

a second rectifier between said secondary winding of said second transformer and said control capacitor, and

a potentiometer bridged across said rectifier and connected to said control capacitor for charging same at a rate determined by the second of said potentiometer;

said trigger diode being constituted as a DIAC and said control discharge circuit including a load resistor bridged across the series combination of said DIAC and said control capacitor; said arrangement also including a variable resistor connecting said DIAC with the gate of said thyristor.

19. A circuit arrangement operable for the stabilization of an alternating-current arc and for the ignition of an alternating-current arc or direct-current arc to be generated at an electrode system, said circuit arrangement comprising:

an electrode system;

a current-pulse generator connected to said electrode system and including:

a storage capacitor,

a source of electric current connected with said storage capacitor for electrically charging same, and

a primary discharge circuit between said storage capacitor and said electrode system including at least one solid-state triggerable switch having a control electrode energizable to discharge said storage capacitor through said discharge circuit and said switch and apply a current pulse to said electrode system; and

a triggering circuit connected with said control electrode for electrically energizing same to render said switch conductive, said triggering circuit including:

a control capacitor adapted to receive an electric charge, and

a control discharge circuit operatively connected between said control capacitor and said control electrode including a self-breakdown two-electrode DIAC trigger diode connected in series with said control electrode and said control capacitor along a path containing only ohmic impedance and a network for charging said control capacitor connected thereacross.