Arc welding current supply

An a.c. operated arc welding current supply unit comprises a frequency converter of the series capacitor type and operating with a half period which is substantially less than the average duration of the current and voltage transients caused by short circuits through droplets of weld material during welding. The frequency converter is connected to welding electrodes through a transformer in series with a rectifier to provide direct current for the welding electrodes, and the frequency converter is associated with a control device for maintaining the operating frequency of the converter constant, thereby maintaining the arc power substantially unchanged irrespective of changes in load caused by the welding operation.

The present invention relates to an arc welding current supply unit which 
is arranged to be fed with alternating current and to provide direct 
current for welding electrodes. 
An object of the invention is to provide a novel and useful welding current 
supply unit which will facilitate welding operations so that an acceptable 
weld can be made by relatively unskilled persons and which will also 
enable more satisfactory welding operations to be carried out than was 
hitherto possible, with the use of conventional welding current units 
operating at main frequencies. 
To this end it is suggested in accordance with the invention that an arc 
welding current supply unit of the aforementioned type includes a 
controlled frequency converter operating with a half-period which is less 
than the average duration of the current and voltage transients caused by 
short circuits through droplets of the weld material, e.g. less than 3 
milliseconds and preferably less than 1.5 milliseconds, and adapted to be 
connected to the welding electrodes through a transformer in series with a 
rectifier, and also includes a control device which is adapted to control 
the converter in a manner such that the arc power remains substantially 
unchanged irrespective of changes in load caused by the welding operation, 
wherein said converter is of the series-capacitor type, i.e., the primary 
winding of the transformer is supplied from a d.c. voltage intermediate 
stage by controlled alternating discharge of one or more capacitors 
connected in series with said primary winding, and wherein the control 
device is adapted to maintain the operating frequency of the converter 
constant. 
With such a welding current supply unit there is obtained a particularly 
stable and quiescent arc, irrespective of small variations in the distance 
between electrode and work piece. In addition, in the event of a short 
circuit caused by droplets of welding material, the arc will be smoothly 
re-ignited with small dynamic effect on the molten material.

The current supply unit shown in FIG. 1 is connected at 10 to a 3-phase 
a.c. network. The input current is rectified to a six-element full-wave 
rectifier 11, the rectified output voltage on lines 12, 13 being smoothed 
by a buffer capacitor 14 and applied to a frequency converter having--as a 
consequence of the shown arrangement of elements 11 to 14--a low input 
impedance. 
With the illustrated embodiment the switching elements of the frequency 
converter comprise thyristors 15, 16 which are controlled so as to be 
alternately energized. The frequency converter is associated with a 
transformer generally shown at 17 and the primary winding 18 of which is 
connected in series with load capacitors 19, 20 forming parts of the 
frequency converter. The secondary winding 21 of the transformer 17 is 
connected via a bridge rectifier 22 and a choke 29 to welding electrode 
terminals 24, 25, which terminals are capable of being connected to a 
welding electrode holder and a work piece to be welded. With the 
illustrated embodiment there is connected between the terminals 24, 25 a 
capacitor 26 which is used to maintain a desired open-circuit voltage. A 
shunt 27 may also be arranged for measuring the load current. 
In FIG. 2, there is shown a control device which is adapted to control the 
frequency converter in a manner such that the arc power remains 
substantially constant irrespective of changes in load caused by the 
welding operation. This control device includes a power adjustment device 
which comprises a potentiometer 33 which is connected between a fixed 
negative voltage source and earth and which potentiometer is adapted to 
control an oscillator 34. The oscillator 34 includes an integrating 
circuit comprising an amplifier 340, resistors 341 and 342, and a 
capacitor 343. The oscillator also includes a level discriminator 344 with 
associated comparison resistors 345, 346, the resistor 345 being connected 
to the output of the integrating circuit and the resistor 346 being 
connected to a voltage source having a fixed negative voltage. The 
amplifier 340 is controlled in the positive sense by a control signal from 
the potentiometer 33 and in the negative sense by signals from the 
Q-output of a monostable flip-flop 50 hereinafter described. A voltage 
adjuster 35 in the form of a potentiometer is provided for adjusting the 
desired maximum output voltage from the current supply unit, which voltage 
adjuster 35 is connected via comparison resistors 36 to the terminal 25 of 
the current supply unit and to an amplifier 37 which serves as a level 
discriminator. 
Control of the state of the thyristors 15, 16 is effected by means of a 
sensing circuit comprising a transformer 38, the primary side of which is 
connected via diodes 39, 40 to the anodes A.sub.1, A.sub.2 and cathodes 
K.sub.1, K.sub.2 of the thyristors 15, 16. One end of the secondary 
winding of the transformer 38 is earthed and the other end is connected to 
a comparison circuit which comprises two resistors 41, 42, the resistor 42 
having one end thereof connected to a constant negative voltage. The 
junction between the resistors 41, 42 is further connected to an amplifier 
43 which serves as a level discriminator and the change-over point of 
which is determined by the resistors 41, 42 and the aforementioned 
constant negative voltage. Each of the amplifiers 344, 37 and 43 is 
connected to a respective input 45, 46, 47 of an AND gate 48, which 
operates in a known manner. Thus, in order for the AND gate 48 to emit an 
output signal, it is necessary for the output signal obtained from the 
amplifier 344 to be positive. Correspondingly it is necessary for the 
output signal from the amplifier 37 to be positive, i.e. for the load 
voltage on the terminal 25 to be less than the value for which the 
adjuster 35 is set. Finally, the output signal from the amplifier 43 must 
be positive, which means that the anode voltage of one of the thyristors 
15 or 16 is negative in relation to the cathode, which in turn means that 
both the thyristors 15 and 16 are de-energized. 
The output of the AND gate 48 is connected to the input of the monostable 
flip-flop 50 which has a pre-determined pulse period corresponding to the 
recovery time of the thyristors 15, 16, for example 30 us. Thus, on the 
output Q of the flip-flop 50 there is obtained a positive voltage pulse 
for a period of time corresponding to the recovery time of the thyristors 
15, 16. This pulse is transmitted through the resistor 342 to the 
amplifier 340 and is also transmitted to the trigger input T of a JK 
flip-flop 51, so that the JK flip-flop 51 changes the status of its 
outputs at the end of the pulse obtained from the flip-flop 50. The 
outputs Q, Q of the JK flip-flop 51 are connected to the base electrodes 
of respective transistors 54, 55 via capacitors 52, 53. The emitters of 
the transistors 54, 55 are connected to earth while their collectors are 
connected to the primary windings of respective ignition transformers 56, 
57 for the thyristors 15, 16. The other ends of the primary windings are 
connected to a terminal having a pre-determined positive potential, and 
the ends of the secondary windings are connected to the cathode K.sub.1 or 
K.sub.2 and the gate G.sub.1 or G.sub.2 of a respective thyristor 15 or 
16. The signals from the outputs of the JK flip-flop 51 will alternatively 
render the transistors 54, 55 conductive via the capacitors 52, 53 for a 
short period of time determined by the capacitors 52, 53, so that the 
thyristors 15, 16 alternately receive a short ignition pulse and thereby 
alternately supply current to the primary winding 18 of the transformer 17 
to produce an alternating current whose frequency is determined by the 
signals on the inputs 45, 46, 47 of the AND gate 48. With this arrangement 
a desired frequency of the oscillator 34 and hence a desired arc power can 
be set solely by a corresponding adjustment of the potentiometer 33. 
FIG. 3 shows the voltages U.sub.60, U.sub.61 at points 60, 61 and the 
voltage U.sub.18 across the primary winding 18 of the transformer 17 with 
normal load on the output of the current supply unit shown in FIGS. 1 and 
2. FIG. 3 also shows the output current I.sub.25 through the terminal 25 
and the output currents I.sub.15 and I.sub.16 (the latter shown in dash 
lines) from the thyristors 15, 16. In FIG. 3 the reference t.sub.1 
illustrates the point of time when the thyristor 15 is ignited, t.sub.2 
the point of time when the thyristor 15 is de-energized and obtains a 
negative voltage between the anode A.sub.1 and cathode K.sub.1 as a result 
of the resonant circuit formed by the primary winding 18 of the 
transformer 17 and the capacitors 19, 20, t.sub.3 the point of time when 
the thyristor 16 is ignited, and t.sub.4 the point of time when the 
thyristor 16 is de-energized and obtains a negative anode voltage as a 
result of said resonant circuit 18, 19, 20. The reference t.sub.5 shows 
the point of time at which the thyristor 15 is re-ignited, whereupon the 
sequence is repeated provided that the load remains substantially 
unchanged. 
Referring to FIGS. 1, 2 and 3 together, it can be seen that the circuit 
shown provides constant power to the weld by alternately switching on 
thyristors 15 and 16 at a constant frequency determined by the setting of 
the potentiometer 33. Each time a thyristor is turned on, a voltage equal 
to the d.c. voltage on line 12 is supplied to one of the capacitors 19 and 
20. Thus, there is a fixed or constant amount of energy stored on each 
capacitor at the end of each half cycle. Since each capacitor is fully 
discharged, as explained below, prior to each charging half cycle, there 
is a constant amount of energy supplied through the transformer during 
each half cycle. Increasing or decreasing the frequency, therefore, will 
increase the total power to the weld. 
Of the three inputs to the AND gate 48, the input 45 from oscillator 34 
determines the switching frequency for the thyristors 15 and 16. Input 46 
is normally positive and will have no effect on the switching frequency. 
However, if the voltage to the weld at terminal 25 goes above a 
predetermined maximum, set by the potentiometer 35 and resistor 36, the 
input at terminal 46 will become negative thereby preventing further 
ignition of either thyristor until the voltage applied to the weld drops 
below the predetermined maximum. The input to terminal 47 also does not 
control the frequency of ignition of the thyristors. It simply insures 
that neither thyristor will be turned on until both thyristors are fully 
turned off. This is accomplished by connecting the two primaries of 
transformer 38, as shown, between the cathode and anode of the respective 
thyristors. As can be seen from FIG. 3, after the current through the 
conducting thyristor fully charges one of the capacitors 19 and 20, and 
concomitantly fully discharges the other capacitor, a small reverse bias 
is applied across the thyristor, as explained below. This fully turns off 
the formerly conducting thyristor. When the latter occurs the input 47 to 
AND gate 48 becomes positive and permits the next positive pulse from the 
oscillator 34 to initiate the ignition of the other thyristor. 
The charging and discharging of capacitors 19 and 20 can be best understood 
by reference to the waveforms shown in FIG. 3. Assume that capacitor 20 is 
fully charged and that the time is just prior to t.sub.3. The voltages at 
terminals 60 and 61 are equal and slightly higher than the voltages on 
line 12. A reverse bias appears across thyristor 15 and therefore 
thyristor 15 is turned off and no current is being conducted. At time 
t.sub.3 the Q output of flip-flop 51 becomes positive, turning on 
transistor 55 for a short period of time determined by the capacitor 53. 
Current flows through the primary of coil 57 thereby providing an ignition 
pulse to the thyristor 16 for a short period of time sufficient to turn on 
thyristor 16. The impedance of thyristor 16 is substantially zero when it 
is conducting. Thus the voltage U.sub.60 at terminal 60 immediately drops 
to ground. The voltage at terminal 61, however, cannot immediately drop to 
ground level because of the presence of inductance in the primary coil 18 
and the capacitors 19 and 20. Capacitor 20 begins discharging through coil 
18 and the thyristor 16. Also, capacitor 19 begins charging negatively at 
terminal 61. As seen from the waveforms, the voltage at terminal 61 
reaches zero volts thereby indicating that the capacitor 20 is fully 
discharged. When capacitor 20 fully discharges, the voltage at terminal 61 
is at ground. However, because of the inductance present in the 
transformer 17, there is energy stored therein, and the current cannot 
cease immediately. Thus, the current continues to flow for a very short 
period of time until the energy in the inductance is dissipated and 
transferred to the capacitor 20. This time is represented by the overswing 
of the voltage V.sub.18 across the primary coil from the instant the 
capacitor 20 is discharged until time T.sub.4. The continued flow of 
current during this short period actually puts an insubstantial reverse 
charge on capacitor 20 as can be seen from the slight negative voltage 
V.sub.61 from T.sub.4 to T.sub.5. Thus, from T.sub.4 to T.sub.5, terminal 
61 will have a slight negative voltage, the voltage V.sub.18 across the 
primary will be zero, and a reverse bias will appear across the thyristor 
16. It will be noted that the thyristor 16 is not cut off at any time 
during the discharging of the capacitor 20, but only after capacitor 20 
has been fully discharged. The operation of the circuit when thyristor 15 
is turned on is exactly the same except the functions of capacitors 19 and 
20 are reversed and the voltage polarities are reversed from that 
previously described. Thus, it can be seen that since the capacitors 19 
and 20 are alternately charged to the maximum voltage and fully discharged 
during each cycle, the amount of energy supplied during each cycle is 
constant and therefore a variation of the frequency varies the total 
amount of energy supplied and concomitantly varies the power output. 
Conversely, by maintaining the frequency of ignition of the thyristors 
constant, constant power is supplied to the weld without a complicated 
feedback control system. 
The invention is not limited to the frequency converter illustrated and 
described, but may be used in conjunction with other frequency converters, 
for example frequency converters having forced commutation or d.c. 
controlled intermediate stages.