Direct current arc power supply

Abstract

An arc power supply system having a three phase transformer and a full wave rectifier which includes silicon controlled rectifiers. A feedback control system includes first and second cascaded operational amplifiers. The first amplifier sums (1) a reference set point signal, (2) an adjustable voltage feedback signal and (3) an adjustable current feedback signal. The first amplifier provides low pass filtering to smooth the otherwise high ripple content of the two feedback signals. The second amplifier provides both integral and proportional control and drives a firing circuit. By the use of a single slope adjustment control, the current and voltage feedback signals may be adjusted to provide infinite and continuous control of the voltampere characteristic of the power supply between a constant current characteristic and a constant potential characteristic. An adjustable start circuit provides control of the average output voltage, and hence the initial arc energy, at the start of operation to more easily establish the arc.

Description

BACKGROUND OF THE INVENTION

l. Field of the Invention 1.

This invention relates to the field of art of direct current arc power supplies having feedback control systems.

1. Prior Art

Direct current power supplies are known in which an arc is established and maintained between a pair of electrodes. The incoming line voltage is reduced in potential by a transformer, the output of which is full wave rectified for establishing a desired direct current voltage and current output. Such an arc welding supply is disclosed in U.S. Pat. No. 3,549,978 in which a polyphase transformer-rectifier system includes silicon controlled rectifiers and has both current and voltage feedback control. This feedback control is effective to establish the desired phasing or firing of the silicon controlled rectifiers. In this patent, the feedback control system comprises a pair of summing amplifiers, the first of which sums (1) a reference set voltage and (2) the current feedback. The second amplifier sums (1) an output of the first amplifier and (2) the voltage feedback. A separate slope adjustment circuit is provided in the current feedback to establish substantially zero slope even though the resistance in the output line tends to establish a drooping characteristic. Such prior power supplies have left much to be desired in that the range of slope control in the constant potential mode is limited for stable operation. As a result, such prior supplies have been limited primarily to the short arc or spray transfer mode of welding where either a constant potential or a slightly drooping volt-ampere characteristic is desired.

SUMMARY OF THE INVENTION

In order to overcome the limitations of prior art devices, there is disclosed herein a direct current arc power supply for controlling the volt-ampere characteristic of direct current applied to establish and maintain an arc between a pair of electrodes. A set reference signal is produced related to a desired value of direct current. The arc current is sensed to provide an adjustable current feedback signal. In addition, the voltage across the electrodes is sensed to provide an adjustable voltage feedback signal. A feedback control system sums the set reference signal, the adjustable current feedback signal and the adjustable voltage feedback signal and applies a resultant control signal to a power circuit to control the volt-ampere slope characteristic. There is simultaneously adjusted both the adjustable current sensing means and the adjustable voltage sensing means to provide infinite and continuous control of the volt-ampere characteristic of the power supply between a constant current characteristic and a constant potential characteristic.

Further in accordance with the invention, the simultaneous adjustment is provided by a single slope adjustment control in the form of a mechanical gang between a current slope potentiometer and a voltage slope potentiometer. These two potentiometers are interconnected so that with the single slope adjustment control set to one extreme, the current feedback signal is maximum and the voltage feedback signal is zero thereby providing a true constant current volt-ampere characteristic. With the single slope adjustment control set to the other extreme, the current feedback signal is zero and the voltage feedback signal is maximum, thereby providing a true constant potential volt-ampere characteristic. This single slope adjustment control may be adjusted continuously between the two extremes to yield an infinitely adjustable volt-ampere slope characteristic.

In this manner, the arc power supply may be used in applications without the adverse stability characteristics inherent in the prior art supplies. Thus, the invention may be used not only in the short arc or spray transfer modes of welding (Gas Metal Arc Welding) but may be used equally well for automatic or manual Gas Tungsten Arc Welding and Shielded Metal Arc Welding. The arc power supply may use solid state construction with high gain feedback operational amplifiers to provide a highly reliable and long life system in addition to compensating for ambient temperature conditions in the circuitry of the feedback control system. As semiconductor devices increase in temperature, they tend to decrease in resistance in direct opposition to the increase in resistance associated with increase in temperature of resistors in the circuit. Thus in accordance with the invention, there is provided highly desirable continuously adjustable slope and setpoint control with a stable output which is essentially independent of load, line and temperature conditions.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a direct current arc power supply in accordance with the invention;

FIGS. 2A-B are volt-ampere characteristics helpful in explaining the operation of the arc power supply of FIG. 1; and

FIG. 3 is a schematic circuit diagram showing details of the block diagram of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown an arc power supply system 10 in which a three phase power source 11 supplies a polyphase welding transformer 12. Transformer 12 provides an alternating current input to a full wave polyphase rectifier assembly 14. Rectifier 14 has a pair of output leads 15 and 16 with output lead or load line 16 indicated as the negative lead and output lead or load line 15 indicated as the positive lead.

In the illustrated embodiment of the supply system of FIG. 1, output negative lead 16 is connected to an electrode 20 while output positive lead 15 is connected by way of a shunt 22 to a work member 23. When supply system 10 is energized, an arc 26 is established and maintained between electrode 20 and work member 23.

The foregoing connections of leads 15 and 16 are useful for both the manual and automatic modes of the Gas Tungsten Arc Welding ("GTAW" or "TIG") process for certain modes of the Shielded Metal Arc Welding process (dependent on the particular type of shielded electrode 20 used). For the remaining modes of the Shielded Metal Arc Welding process and for both the short arc and spray transfer modes of the Gas Metal Arc Welding process, lead 15 is connected to electrode 20 while lead 16 is connected to work member 23.

For proper arc welding in the manual mode of the Gas Tungsten Arc Welding process, the volt-ampere characteristic of system 10, as shown in FIG. 2A, is preset so that a desired straight line characteristic 30 is substantially tangent to an ideal constant power characteristic (K) 31. Voltage V is the voltage across arc 26 taken between leads 15 and 16 while current I is the actual current through arc 26. The tangency is provided at a desired nominal operating point 33. Ideal constant power characteristic 31 is a known characteristic in the Gas Tungsten Arc Welding process.

With the volt-ampere characteristic of system 10 set in this manner, automatic compensation for variations in changes of electrode 20 with respect to work member 23 may be achieved. For example, if electrode 20 is moved away from member 23, the voltage across arc 26 increases from its nominal value along characteristic 30 to a point 35, for example. Thus the voltage increases from V.sub.1 to V.sub.2. This in turn results in a decrease in the arc current I.sub.1 to I.sub.2, thereby tending to maintain the power input to arc 26 approximately constant.

In the automatic mode of the Gas Tungsten Arc Welding process, a true constant current volt-ampere characteristic is generally desired. In the automatic mode, arc voltage is normally maintained constant by means of a separate automatic arc voltage controller which controls electrode 26, as described for example in U.S. Pat. No. 2,516,777 For Control Apparatus for Automatic Welding Heads.

In the spray transfer mode of the Gas Metal Arc Welding process, the volt-ampere characteristic of system 10 is preset normally to either a true constant potential characteristic or a slightly drooping (negative slope) characteristic. In the short arc mode of the Gas Metal Arc Welding process, the volt-ampere characteristic of system 10 is preset normally to a higher magnitude negative slope characteristic than that in the spray transfer mode. On the other hand, in the Shielded Metal Arc Welding mode, the volt-ampere characteristic is set the same as in the manual mode of the Gas Tungsten Arc Welding process.

Referring again to supply system 10, a reference set point voltage input 40 (set in a manner to be described) is applied to one input 42a of a summing filter amplifier 42. A second input 42c of amplifier 42 is connected to a current feedback unit 45 while a third input 42b to amplifier 42 is taken from a voltage feedback unit 46. In this manner, amplifier 42 is effective to sum the set reference signal, an adjustable current feedback signal and an adjustable voltage feedback signal.

Current feedback unit 45 obtains its current input from a current sensor or shunt 22 which provides a signal proportional to the current through lead 15 (arc current I). Load resistors 17 are connected between leads 15 and 16 and voltage feedback unit 46 is connected across load resistors 17 to provide a signal proportional to the voltage across arc 26. Units 45 and 46 comprise potentiometers which are mechanically ganged by gang 47. The potentiometers are ganged so that with a setting at one extreme (as for example, clockwise) the voltage feedback signal at voltage feedback conductor 44 is of zero value and the current feedback signal at current feedback conductor 43 is of maximum magnitude. Accordingly, at the output 50 of amplifier 42 there is produced a voltage signal proportional to the setting of reference 40 which at that time establishes the desired set point of current.

On the other hand, with gang 47 set at the other extreme (as for example counterclockwise), the current feedback signal at conductor 43 is of zero magnitude while the voltage feedback signal at conductor 44 is at a maximum value.

Accordingly, the output of amplifier 42 provides a voltage signal proportional to the setting of reference 40 which establishes the desired set point of voltage.

For intermediate settings of gang 47, output 50 produces modified proportional set point signals related to the slope characteristic established by the current and voltage feedback signals. One of the slope characteristics is shown in FIG. 2A as characteristic 30 while another is shown as characteristic 37.

Amplifier 42 also provides low pass filtering characteristic which is effective to smooth the otherwise high ripple content of the current feedback applied to input 42c and the voltage feedback applied to input 42b.

Output 50 of amplifier 42 is connected to an input of amplifier of 52 which provides both integral plus proportional control of the feedback for system 10. The output of amplifier 52 is effective to control the firing networks of firing unit 55. Firing unit 55 is adapted to provide properly timed and spaced firing pulses to a bank of triggered silicon controlled rectifiers in rectifier 14. With this proper phasing of rectifier 14, there is established and maintained the desired voltage and current characteristics at output leads 15 and 16 and therefore to arc 26.

In order to assist in the start of supply system 10, a start circuit 53 is effective to provide controlled arc energy at the start of operation to more easily establish arc 26. When the weld current increases sufficiently, a relay is energized thereby removing the start weld setting so that the weld current assumes its normal desired value as established by reference 40.

It will now be understood that current feedback unit 45 comprises a separate potentiometric slope adjustment connected between shunt 22 and input 42c while voltage feedback unit 46 comprises a separate potentiometric slope adjustment connected between load resistors 17 and input 42b. The current slope potentiometer is mechanically ganged to the voltage slope potentiometer.

With the single slope adjustment control (gang 47) set to one extreme, the current feedback signal is maximum and the voltage feedback signal is zero, thereby providing a true constant current volt-ampere characteristic. Specifically with gang 47 in its extreme clockwise position, the current feedback signal at conductor 43 is at a maximum and the current through arc 26 is maintained substantially constant at a value determined by the setting of reference. For example as shown in FIG. 2B depending upon the setting of reference 40, the current I through arc 26 may be 100 amps at one setting, 500 amps at another setting, etc. Each constant current characteristic varies between a voltage value approaching zero volts (as for example, 5 volts) to the maximum voltage of supply system 10.

When the single slope adjustment control (gang 47) is set to the other extreme, the current feedback signal is zero and the voltage feedback signal is maximum, thereby providing a true constant potential volt-ampere characteristic. Specifically, with gang 47 and its extreme counterclockwise position, the voltage feedback at conductor 44 is maximum and the voltage across arc 26 is maintained substantially constant at a voltage value determined by the setting of reference 40. For example as shown in FIG. 2b, the voltage across arc 26 may be 20 volts at one setting, 40 volts at another setting, etc. with current varying from a minimum to a maximum value.

The single slope adjustment control (gang 47) may be adjusted continuously between the two extremes to yield an infinitely adjustable volt-ampere slope characteristic.

Referring now to FIG. 3, there is shown a detailed drawing of supply system 10 in which transformer 12 comprises a three phase delta primary 60 and a six phase star secondary 62. The windings of primary 60 and secondary 62 are suitably coupled on a common core (not shown) to provide a constant potential transformer. Power source 11 is coupled to primary 60. The common 64 or star point of secondary 62 is connected by way of lead or load line 15 through current sensing shunt 22 to work member 23. The outer end of each phase winding of secondary 62 is connected through respective silicon controlled rectifiers (SCR) 70a-b, 71a-b, 72a-b and then through a smoothing and stabilizing reactor 67 and lead or load line 16 to electrode 20. Reactor 67 is provided with taps so that different values may be selected for different modes of welding.

It will be understood that diametrically opposite phase windings of the star connected secondary 62 are connected to simultaneously conducting SCR's 70a-b, 71a-b, and 72a-b. Thus phase 62a is connected to a pair of SCR's 70a-b and then through reactor 67 to line 16. The adjacent phase 62b is connected to a similar pair of SCR's 71a-b and the final phase 62c is connected to a final pair of SCR's 72a-b. Each pair of SCR's 70a-b, 71a-b, and 72a-b is connected to a firing unit 55 for simultaneous pulsing with the three pairs being pulsed in proper sequence. In this manner, the portion of the half-wave of each winding applied across leads 15-16 is controlled by the phased firing of the respective SCR's.

In the Gas Tungsten Arc Welding process, it is desirable to utilize a background supply comprising auxiliary secondary windings 66a-c in the main power transformer 60. Windings 66a-c are connected in three phase delta with the output thereof coupled to a background rectifier 69. Rectifier 69 may be connected as a three phase bridge rectifier. The output of the rectifier 69 is coupled through a switch 69a to leads 15 and 16. This circuit operates to help establish a more stable arc by smoothing the ripple produced by the arc.

Firing unit 55 may be any one of the firing units well known in the art to provide the required phase control and firing of SCR's. For example unit 55 may be a firing unit Part No. R613F372 manufactured by Firing Circuit, Inc., Norwalk, Conn.

Firing unit 55 is actuated by an input conductor 74 which is coupled by way of a gain control potentiometer 75 to output 77 of amplifier 52. The signal at conductor 74 determines the particular time in each half cycle at which a firing pulse is applied to a particular SCR and thereby determine the particular time in the phase that SCR conducts the output applied to it from secondary 62. Since power source 11 is also applied to firing unit 55, this power input is synchronized with the control voltage applied to an SCR. The firing of the SCR is modified by the input signal at conductor 74 which reflects the current feedback produced by unit 45 through summation amplifier 42 thereby to establish a desired voltage and current slope characteristic.

Shunt 22 comprises a millivolt shunt connected in series with lead 15. The upper end 22b of shunt 22 is coupled by way of common lead 80 to the junction of fixed contacts of current and voltage feedback potentiometers 82, 83 respectively. Common 80 is the common for the entire electronic feedback circuit. The lower end 22a of shunt 22 is coupled by way of conductor 84 to the other fixed contact of current potentiometer 82. In this manner, potentiometer 82 is coupled across shunt 22 and the current feedback signal is taken from moveable arm 82a of potentiometer 82 with respect to common 80. That current feedback signal is applied to input 42c of summing filter amplifier 42.

Within amplifier 42, input 42c is coupled to an input circuit 85 comprising resistors 80a-c in series and capacitors 97a-b. One end of the series resistance circuit is connected to input 42c and the other end is connected to a summing junction 92 of an operational amplifier 100 which comprises the amplifying device of summing filter amplifier 42. The feedback network 102 of operational amplifier 100 comprises resistors 103-106 and capacitors 108-111. The values of these resistance and capacitance feedback components are selected in conjunction with the values of resistors 90a-c and capacitors 97a-b to provide adequate filtering of the current feedback millivolt signal. These components are further selected to provide proper voltage gain to make the current feedback millivolt signal compatible with the reference set point voltage applied to input 42a and the voltage feedback signal applied to input 42b. The voltage levels applied to inputs 42a-b may for example each be adjusted to approximately a 10 volt maximum level.

For sensing voltage feedback, a series circuit of a potentiometer 17b and a fixed resistor 17a (load resistors 17) is connected between the other fixed contact of potentiometer 83 and load line 16. In this manner, the potential between load lines 15 and 16 is developed across the series circuit combination of resistors 17a-b and 83 with the voltage feedback signal taken from arm 83a of potentiometer 83 with respect to common. By suitably moving arm 83a, the voltage feedback signal is adjusted to a level compatible with the set point reference voltage 40. The voltage feedback signal is applied by way of conductor 44 to input 42b. Within amplifier 42, input 42b is connected to input circuit 86 comprising resistors 93a- c in series circuit coupled to junction 92 and capacitors 98a-b. The values of the components of this resistance-capacitance network are selected in conjunction with the feedback resistance-capacitance network 102 to provide adequate, unity gain filtering of the voltage feedback signal.

For the reference set point voltage, reference circuit 40 comprises a potentiometer 95 having its arm connected to input 42a. One fixed contact of the potentiometer is connected to common while the other fixed contact is connected through a potentiometer 96 to a positive supply. Input 42a is coupled within amplifier 42 to an input circuit 87 comprising resistors 94a-c in series circuit and capacitors 99a-b. The values of these components are selected in conjunction with the values of the components of the resistance-capacitance feedback network 102 to provide a transfer function for the reference input which is compatible with the current feedback to input 42c and the voltage feedback to input 42b.

The ends of input circuits 85-87 remote from inputs 42c, 42b and 42a, respectively are summed at junction 92 which is coupled to the negative input of operational amplifier 100. Input circuits 85-87 and feedback network 102 effectively define a Butterworth filter network.

An additional input to junction 92 may be traced by way of a resistor 116 and then to an arm of a potentiometer 115. One fixed contact of potentiometer 115 is connected by way of a resistor 114 to a positive supply and the other fixed contact is connected to common. Potentiometer 115 and resistor 114 form an adjustable voltage divider network used to calibrate the low end of the reference potentiometer readout in amperes when the slope is adjusted to the constant current mode. Potentiometer 96, previously described, is used to calibrate the high end of reference potentiometer 95 readout in amperes when the slope is adjusted to the constant current mode.

It will be understood that the arms 82a and 83a are ganged together by gang 47 to provide a single slope adjustment control as previously described. With gang 47 at its extreme right position (corresponding to the extreme clockwise position) it will be seen that arm 82a is at its furthest position from common while arm 83a is at its closest position. Accordingly, the current feedback is at a maximum and the voltage feedback is at a minimum. On the other hand, with gang 47 in its extreme lefthand position (corresponding to the extreme counterclockwise position) arm 82a is at its closest position to common while arm 83a is at its furthest position. Accordingly, the current feedback is zero and the voltage feedback is maximum.

The output of operational amplifier 100 is applied by way of a potentiometer 120 and a resistor 121 to the negative input of an operational amplifier 125; the positive input of which is connected by way of a resistor 123 to common. The output of amplifier 125 is connected by way of an integrator capacitor 130 and a resistor 131 to the negative input. In this manner, operational amplifier 125 operates as an augmented integrator, or integral plus proportional amplifier. In the steady state if a perturbation occurs in the error signal at the output of operational amplifier 100, then capacitor 130 charges in such a direction as to drive the output of amplifier 125 and hence the firing circuit 55 to that level necessary to reduce the error signal to zero. Potentiometer 120 may be adjusted to set the closed loop gain of the feedback circuit.

Start circuit 53 is provided to assist in the start of supply system 10. It will be understood that in both manual and automatic welding that it is desired to have higher weld currents at the start of operation in order to more easily establish arc 26. However, once the arc has been established, it is necessary that the weld current be decreased to its normal desired value as established by reference 40.

In order to provide this start level, current feedback by way of the conductors 84 and 129 is applied through a resistor 132 to a negative input 135a of an operational amplifier 135. The current feedback produces an effective negative potential at input 135a. Input 135a is also connected by way of a voltage divider network 136 to a positive supply. The positive input of amplifier 135 is connected by way of a resistor 136 to common. When the value of the negative potential produced by the current feedback reaches a level greater than the positive potential developed by network 136, then the output of amplifier 135 changes polarity thereby to turn on a switching transistor 140 as shown. The collector of transistor 140 is coupled to a relay 142, the normally closed contacts 142a of which are coupled between an arm of a potentiometer 144 and a diode 145. As shown, this circuit is connected between the input and output of amplifier 125, thereby providing an output clamp on this amplifier.

Accordingly, at start up, relay 142 is deenergized and the normally closed contact 142a is effective to apply the start weld level setting of potentiometer 144 to the input of amplifier 125. This level setting is applied to firing unit 55 for the higher value start up current. When the weld current I increases sufficiently to produce a potential at input 135a of amplifier 135 greater than the setting of voltage divider 136 than transistor 140 switches thereby energizing relay 142 and opening the contact 142a. With contact 142a open, the start weld setting is removed and amplifier 52 operates normally.

Having thus described my invention what is desired to be secured by Letters Patent of the United States is set forth in the appended claims.

Claims

I claim:

1. An arc power supply for controlling the volt-ampere characteristic of the direct current applied by way of lead means to a pair of electrodes which establishes and maintains an arc between said electrodes, comprising:

power means operable for providing through said lead means direct current to said electrodes,

reference setpoint means to produce a set reference signal related to a desired value of said direct current,

adjustable means connected to said lead means for sensing said arc current and providing an adjustable current feedback signal,

adjustable means connected to said lead means for sensing the voltage across said electrodes and providing an adjustable voltage feedback signal,

feedback control means coupled to said power means for summing said set reference signal, said adjustable current feedback signal and said adjustable voltage feedback signal for applying a control signal to said power means to control said volt-ampere characteristic, and

means for simultaneously adjusting both said adjustable current sensing means and said adjustable voltage sensing means to provide infinite and continuous control of said volt-ampere characteristic of said direct current between and including a constant current characteristic and a constant potential characteristic.

2. The arc power supply of claim 1 in which said simultaneously adjusting means comprises means ganging said adjustable current sensing means and said adjustable voltage sensing means to provide a single slope adjustment control which when set (1) to one extreme said current feedback signal is maximum and said voltage feedback signal is zero thereby providing said constant current volt-ampere characteristic and (2) to another extreme said current feedback signal is zero and said voltage feedback signal is maximum thereby providing said constant potential volt-ampere characteristic.

3. The arc power supply of claim 1 in which said adjustable current sensing means comprises a current sensor coupled to said lead means and voltage potentiometric slope adjustment means coupled to said current sensor for providing said adjustable current feedback signal proportional to the current flow through said lead means, and in which said adjustable voltage sensing means comprises load resistor means coupled to said lead means and voltage potentiometric slope adjustment means coupled to said load resistor means for providing said adjustable voltage feedback signal proportional to said voltage across said electrodes.

4. The arc power supply of claim 3 in which said simultaneously adjusting means comprises means for ganging said current and voltage potentiometric means so that (1) at one extreme setting of said ganging means the current feedback signal is maximum and the voltage feedback signal is of zero value to produce a control signal proportional to said set reference signal for establishing a desired setpoint of current, and (2) at another extreme setting of said ganging means and voltage feedback signal is maximum and the current feedback signal is of zero value to provide a control signal proportional to the set reference signal for establishing a desired setpoint of voltage; with intermediate settings of said ganging means producing proportional control signals related to the slope characteristics established by the current and voltage feedback signals.

5. The arc power supply of claim 1 in which said feedback control means includes first amplifier means for summing said set reference, said adjustable current and adjustable voltage feedback signals and for providing a low pass filtering characteristic for smoothing the high ripple content of said feedback signals.

6. The arc power supply of claim 5 in which said feedback control means includes second amplifier means connected to the output of said first amplifier means for providing integral plus proportional control of the first amplifier output and for applying the resultant signal as said control signal to said power means.

7. The arc power supply of claim 6 in which said power means comprises rectifying means having silicon controlled rectifiers for rectifying a source of alternating current for providing said direct current and firing means adapted to provide properly timed and spaced firing pulses to rectifiers of said rectifier means under the control of said control signal.

8. The arc power supply of claim 5 in which said first amplifier means comprises first, second, and third input circuits connected respectively to said reference setpoint means, said adjustable current sensing means and said adjustable voltage sensing means, a first operational amplifier having an input connected to a common junction coupling said first, second, and third filter input circuits, a resistance-capacitance network providing feedback for said first operational amplifier, and said first, second, and third input citcuits and said feedback network forming a Butterworth network.

9. The arc power supply of claim 8 in which said second input circuit is a resistance-capacitance network which together with said resistance-capacitance feedback circuit provide (1) adequate filtering of said current feedback signal and (2) proper voltage gain so that said current feedback signal is compatible with said set reference signal and said voltage feedback signal.

10. The arc power supply of claim 9 in which said third input circuit is a resistance-capacitance network which in conjunction with said resistance-capacitance feedback circuit provides adequate unity gain filtering of said voltage feedback signal.

11. The arc power supply of claim 10 in which said first input circuit is a resistance-capacitance circuit having values selected in conjunction with the values of said resistance-capacitance feedback circuit provide a transfer function of the said set reference signal which is compatible with the current feedback signal and said voltage feedback signal.

12. The arc power supply of claim 6 in which said second amplifier means includes a second operational amplifier having a feedback integrator capacitor to integrate the applied signal to affect zero error in the steady state.

13. The arc power supply of claim 1 in which there is provided start means coupled to said feedback control means for varying said control signal by a start level setting at the start of operation to provide said direct current of value higher than said desired value established by said set reference signal.

14. The arc power supply of claim 13 in which said start means comprises start reference means for establishing a start reference signal, means connected to said adjustable current sensing means for comparing a nonadjustable current feedback signal with said start reference signal for (1) applying at the start of operation said start level setting and (2) returning said control signal to its normal value when the arc current increases sufficiently to increase the nonadjustable current feedback signal to a predetermined value with respect to said start reference signal so that said direct current decreases to its desired value established by said set reference signal.

15. The arc power supply of claim 14 in which said comparing means comprises a third operational amplifier and a relay connected to an output of said third operational amplifier, said relay having normally closed contacts connected in circuit with said feedback control means for applying said start level setting when said contacts are closed and for removing said start level setting when said contacts are opened upon actuation of said relay by said third operational amplifier.

16. A direct current arc power supply for establishing and maintaining an arc between a pair of electrodes in which upon start of operation a higher than normal valued startup direct current is applied to said electrodes, comprising

power means operable for providing direct current to said electrodes,

reference setpoint means to produce a set reference signal related to a desired value of said direct current,

means for sensing said arc current and providing a current feedback signal,

means for sensing the voltage across said electrodes and providing a voltage feedback signal,

feedback control means coupled to said power means for summing said set reference signal, said current feedback signal and said voltage feedback signal for applying a control signal to said power means to control the volt-ampere characteristic of said direct current,

start reference means to produce a start reference signal, and

start means for comparing said current feedback signal with said start reference signal for varying said control signal upon startup to produce said direct current of value higher than said desired value which value decreases to said desired value when said current feedback signal increases to a predetermined value with respect to said start reference signal.

17. The arc power supply of claim 16 in which said start means includes an operational amplifier having one input coupled to said current sensing means, relay means connected to an output of said operational amplifier and having a normally closed relay contact, start level setting means connected to said normally closed relay contact and to said feedback control means for varying said control signal by a start level setting until said current feedback signal increases to said predetermined value at which time said relay is actuated.

18. The arc power supply of claim 17 in which said feedback control means includes an operational amplifier having an integrator capacitor connected in a feedback loop therewith and providing at an output said control signal, said start level setting means coupled in another feedback loop whereby upon startup a start level setting is applied to the input of said operational amplifier until said current feedback signal increases to said predetermined value.

19. A method of controlling the volt-ampere characteristic of direct current applied to establish and maintain an arc between a pair of electrodes which comprises

producing a set reference signal related to a desired value of said direct current,

sensing the arc current and providing an adjustable current feedback signal related to the value of the arc current,

sensing the voltage across the electrodes and providing an adjustable voltage feedback signal related to the value of the electrode voltage,

summing the set reference signal, the adjustable current feedback signal and the adjustable voltage feedback signal and controlling the volt-ampere characteristic as a function of these signals, and

simultaneously varying both the adjustable current feedback signal and the adjustable voltage feedback signal to provide an infinite and continuous control of the volt-ampere slope characteristic of the direct current between and including a constant current mode and a constant potential mode.

20. The method of claim 19 in which the simultaneously varying step includes simultaneously varying the signals so that (1) at one extreme the adjustable current feedback signal is maximum and the adjustable voltage feedback signal is zero thereby providing a constant current volt-ampere characteristic and (2) at another extreme the adjustable current feedback signal is zero and the adjustable voltage feedback signal is maximum thereby providing a constant potential volt-ampere characteristic.

21. The method of claim 19 in which the summing step includes providing a low pass filtering characteristic for smoothing the high ripple content of said adjustable current and voltage feedback signals and then providing integral plus proportional control.

22. The method of claim 19 in which the summing step includes varying the controlling of the volt-ampere characteristic by a start level setting at the start of operation to provide the direct current of value higher than said desired value established by said set reference signal.