IMPROVED POWER SOURCE FOR METAL TRANSFER PROCESSES

Abstract: A power source for an electric arc metal transfer process, such as arc welding, including a transformer (T1) for providing a pulsed current source (S1, S3, B1) and a background current source (S4, S5, S6, B2) for maintaining the arc between current pulses, said power source including control circuitry (P1, C0) for independently varying the amplitude of the current pulses (P1), the level of the background current (S4, S5, S6), the frequency of the current pulses (Fs) and the amount of each current pulse supplied to the arc (Ph, Fs, Cd, SCR1 SCR2), The amplitude of the current pulses and the level of the background current are preferably varied by tapping and switching the primary windings (P1, P2, P3) and the background current secondary windings (S4, S5, S6). The frequency of the current pulses is varied by means of a frequency selection circuit (Fs) which allows suitable variation of the frequency depending on the metal to be transferred. Control of each current pulse is achieved in the preferred embodiment by means of silicon control rectifiers (SCR1, SCR2), the conduction angles of which are controlled by means of a phase shifting circuit (Ph).

Description: IMPROVED ' POWER SOURCE FOR .METAL ' TRANSFER PROCESSES This invention relates to improvements in power sources for metal transfer processes, such as Gas Metal Arc (GMA) welding and electroplating processes, and more particularly to improvements in so-called pulsed-current power sources of the type used in such metal transfer processes. Pulsed-current power sources are well known and are relatively widely used in Gas Metal Arc welding. One particular type of pulsed-current source is described in Australian Patent No.272,070. The specification of this patent describes in some detail the theory of pulsed- current metal transfer and this description is incorpor¬ ated into the present specification by cross-reference. The term "transition current" as used below is in accord- ance with the definition contained in the above specifi¬ cation. In most of the presently available commercial forms of pulsed-current power sources, the welding para¬ meters of the power source are relatively fixed so that only certain metals can be welded using any particular machine. Such power sources may be divided into two categories: a low current power source which is used for the welding of non-ferrous alloys such as bronze, and high current power sources which are suitable for the welding of aluminium and steel. Since such power sources are designed for a relatively narrow range of metals, the adjustability of the welding parameters is somewhat lim¬ ited. Thus, at the present time a fully versatile pulsed- current power source for welding and other metal transfer processes is not available. It is accordingly the object of the present inven¬ tion to provide a pulsed-current power source for metal transfer processes in which the welding parameters are independently variable to enable the power source to be used for the welding of most weldable metals. The invention therefore provides a power source for electric arc metal transfer processes comprising a transformer including one or more primary windings and a plurality of secondary windings providing a background current for maintaining the arc used in the metal trans¬ fer process and for producing current pulses which exc¬ eed the "transition current" , said power source being characterised by control circuitry for independently varying the frequency of said current pulses and the amount of each current pulse supplied to said arc. In a preferred form of the invention, the power source is also characterised by means for independently varying the amplitude of said current pulses and the level of said background current. The independent variability of each of the above parameters enables the power source to be adjusted for use with most types of metals. In this way the power source is far more versatile than the prior art power sources without any loss of performance characteristics. The variability of each parameter may be achieved in a number of different ways. In one preferred embodi¬ ment, the variability of the amplitude of the current pulses is obtained by providing various switched taps by means of which the primary current and therefore the secondary current may be raised or lowered. Similarly, the background current may be varied in the same manner. Variability of the frequency of the pulses may be obtain¬ ed by the use of a suitable electronic frequency selec¬ tion circuit while the variation in the amount of each current pulse transmitted to the arc may be achieved by means of a controlled switching circuit operated by means for selecting the phase angle at which the switch is opened. In one preferred form of the invention, the switching . circuit is in the form of an SCR circuit having its conduction angle determined by a phase shift circuit. o? > WW Where the power source is used for arc welding, the control circuitry includes a starting current facil¬ ity which overrides the frequency and conduction angle control referred to above for a predetermined short period during which a higher welding current is supplied to establish the arc. Other features of the preferred embodiment of the invention will be described in greater detail with reference to the accompanying drawings in which: Figure-1 is a circuit diagram of a power source embodying the invention; Figure 2 is a circuit diagram of the control circuitry used to control the pulsed-current output of the power source, and Figures 3 and 4 are plan and elevational views of a transformer suitable for use in conjection with the power source of Figure 1. Referring firstly to Figure 1 of the drawings, the power source embodying the invention includes a three-phase transformer TI having primary windings PI, P2 and P3, tapped and switched in the manner shown to enable variation of the primary current input to the transformer TI, secondary windings SI, S2 and S3, which operate to supply pulsed current via windings SI and S3 or constant potential current via windings SI, S2 and S3, and background supply secondary windings S4, S5 and S6, which are tapped and switched asymmetrically in the manner shown to provide for variation of the background current. An additional single phase transformer T2 is provided to supply control current for the control cir¬ cuitry of Figure 2 and for a wire feeder and any other auxiliary circuits of the welding machine associated with the power source. The secondary windings SI, S2 and S3 are connec- ted to a hybrid full-wave rectifier bridge circuit Bl, comprising diodes Dl, D2, D3 and D4.and silicon control- rectifiers SCR1 and SCR2 together with high frequency protection capacitors connected in the known manner shown. A diode protection circuit Dp is connected across the bridge Bl as shown. The background supply windings S4, S5 and S6 are connected to a full-wave rectifier bridge B2 which is in turn connected to the bridge Bl via a droop res¬ istor Rd f which provides a drooping current supply which maintains the welding current between pulse cycles, and a maximum current limiting resistor Rm. A commutating inductance II interconnects the background and pulse supplies to the pulsed welding current output P+. A welding inductance 12 is also connected to the positive side of bridges Bl and B2 and terminates in a terminal M+ providing constant potential welding current. The power source is also provided with a "start current" facility, the operation of which will be descr¬ ibed below, and this facility, together with the pulsed current and constant potential welding modes, are selec¬ ted by means of contactors Cl, C2 and C3. Contactor Cl is the main welding contactor which connects the three phase supply to the primary windings Pi, P2 and P3. Contactor C2 is the pulsed current contactor which allows the selection of the pulsed current mode. In the const¬ ant potential mode, the contactors Cl and C2 are energ- ised and the secondary winding S2 is in circuit while the background secondary windings S4 , S5 and S6 are switched out of circuit. As will be described below,the gates of SCRl and SCR2 are. supplied with current to en¬ sure full conduction whereby the bridge Bl supplies con- stant potential current to terminal M+. In the pulsed current mode, contactor C2 is de-energised thereby dis¬ connecting winding S2 from the main rectifier bridge Dl. The background supply windings S4, S5 and S6 are connec¬ ted in circuit and SCRl and SCR2 are caused to conduct as controlled by the control circuitry Co, which will be described in more detail with reference to Figure 2 of the drawings. Contactor C3 is used to activate the starting current facility which will be described in more detail below. Referring now to Figure 2 of the drawings, the control circuit includes a plurality of sub-circuits which will be described in more detail below, and which receive 10 VAC and 30 VAC from the transformer T2 (fig¬ ure 1) . A power supply circuit Ps produces the indicated DC voltages, the symbols C D F and R respectively indic¬ ating clipped, unfiltered, filtered and reference voltages respectively. The 18 V C output from the power supply sub- circuit Ps passes through a zero-crossing detector Zc comprising an operational amplifier IC3 which senses the zero-crossing periods of the control voltage. Since this voltage is in phase with the pulse supply output, • the output pulses from this sub-circuit form the basis for the timing of the remainder of the circuitry. - In order to stabilise the output from the zero- crossing sub-circuit Zc, a pulse shaping sub-circuit Sh is connected to the output from the zero-crossing detec¬ tor Zc, and comprises an NE555 timer IC4 which functions to provide zero-crossing pulses of a constant width in time as well as providing "clean" input pulses to the frequency selection sub-circuit Fs to be described below. To overcome any problems that may be associated with trailing edge ringing, and to trigger timer IC4, a diff¬ erentiator circuit comprising R2, R3, and C2 is arranged between the zero-crossing detector. Zc and the pulse shaper Sh. A ramp generator sub-circuit Rg comprises resis¬ tors R4 and R5, capacitor C3 and transistor Q5 supplied with 12 VDC reference voltage from the power supply circuit Ps to ensure that the ramp wayform remains const¬ ant in periods of changing mains supply voltage. At each zero-crossing point, the transistor Q5 conducts and the capacitor C3 discharges to provide the desired ramp voltage. The span of the ramp voltage is preset by means of fixed resistors R4 and R5. In the phase shift sub-circuit Ph, an operation¬ al amplifier IC5 is used to compare the ramp voltage with an adjustable DC voltage from potentiometer PTl in order to determine the turn-on point of the silicon control- rectifiers SCRl and SCR2. The 18 VDC filtered supply from which the potentiometer voltage is derived is sub¬ ject to some drift resulting from mains voltage variat- ions and this has been exploited in the following way. Assuming a fall in mains supply, the 18 VDC supply and hence the potentiometer wiper voltage also fall. This results in increasing the width of the output pulse of IC5, and the appropriate one of SCRl and SCR2 is turned on earlier in the half cycle. In this way a degree of compensation (without feedback) against mains voltage variation is achieved. The SCR control sub-circuit Sc comprises trans¬ istors Q6 and Q7 forming a Darlington pair and which control the firing of the SCRs. The firing of the SCRs are triggered as an "and" function of the output of the phase shift circuit Ph (pin 6 IC5) and the frequency selection sub-circuit Fs (pin 3 IC6) to be described below. When both of these outputs are high, the light emitting diode of an optical coupler IC7 is illuminated and the coupled transistor conducts. Transistors Q6 and Q7 cease conducting and gate current flows to SCRl and SCR2 through resistor R6 and via diodes to pins 17 and 19 in Figure 1 of the drawings whereby the appropriate SCR is triggered to its On' state. The frequency selection sub-circuit Fs will be seen to include a CD4017 CMOS counter/divider IC6. The counter has 10 outputs, 0 to 9 , and as the count pro¬ ceeds, each output in turn goes high. Frequency selec- tion switches connect a chosen output to the reset terminal of the device and if the device is switched to reset say at pulse number 4, it will count 0, 1, 2, 3, 0, 1, 2, 3, 0 etc. Output Q, at pin 3, of the counter divider is connected to transistor Q8 in the manner shown and the combined effect is such that when output 0 is high, transistor Q8 is turned on so that when pin 6 5 of IC5 goes high the light emitting diode is illuminated as described above. When output 0 is low, the SCRs are prevented from going into conduction and in this way 4 function is provided. When desired, the counter can be switched to divide by any number between 1 and 9 in- ° elusive. Based on a 100 pulse input signal, the frequ¬ ency selection switches may be actuated to derive 11.1, 12.5, 14.3, 16.7, 20, 33.3, 50 and 100 pulses per second. The hundred pulse mode "of operation is achieved by hold¬ ing the counter/divider permanently reset. 5 As mentioned above , the power supply includes a starting current facility controlled by contactor C3 and which operates as follows. At the commencement of each period of pulse welding, SCRl and SCR2 are prevented from conducting and the arc is initiated by a supply 0 derived from the background current rectifier bridge B2 , but with the droop resistor Rd shorted out of circuit. The limiting resistor Rm remains in circuit to limit the maximum current which may be drawn from this supply. A current detection circuit Cd detects the point in time 5 t which the electrode wire first contacts the workpiece by detecting the voltage drop- across resistor Rm. An adjustable timer circuit Tm is initiated at this point to set the length of the starting current period, at the elapse of which droop resistor Rd is re-inserted 0 into the background current bridge B2 and SCRl and SCR2 commence operation as controlled by the control circuit whereby pulsed current welding ensues. The timer Tm is adjusted by potentiometer PT2 to achieve optimum results for each particular set of welding conditions 5 and the manner in which the control circuit functions to achieve the above will now be described in greater detail. In the quiescent state, pin 6 of IC1 is low, Ql is not conducting, Q2 has no collector supply, and Q3 is conducting holding Cl in a discharged state. As the voltage at pin 3 of IC2 is less than that at pin 2 , pin 6 is low, Q4 is non-conducting and RA/2 is de-energised. . Contact RA/1 is normally closed, so shorting the gate to cathode junctions of SCRl and SCR2 thereby preventing triggering into conduction. Contact RA/2 is normally closed, and the starting current contactor Cl is energ¬ ised whereby the resistor Rd is shorted out. When the operator's gun switch is closed, the auxiliary switch AUX of the welding contactor Cl closes providing a supply to transistor Q2 which therefore con- ducts but initiates no other 'changes of state. At the time the welding electrode first touches the workpiece, a voltage appears across Rd and is detected by ICl where¬ by pin 6 of ICl goes high. Transistor Ql conducts and Q2 and Q3 become non-conducting. This allows Cl to charge at a rate as controlled by the setting of timing potentiometer PTl. When the charge on Cl exceeds the voltage drop across Rl, pin 6 of IC2 goes high, Q4 conducts and contactor RA/2 is energized. Hence SCRl and SCR2 are permitted to conduct and Rd is re-inserted in circuit. The circuit will remain in this state until the gun switch is released. Referring now to Figures 3 and 4 of the drawings, a particularly preferred form of welding transformer is shown. In this arrangement, the transformers Tl and T2 are provided on a single toroidal transformer of the type shown in Figures 3 and 4. The transformer comprises a pair of toroidal laminated magnetic cores 3 interconnect¬ ed by three laminated legs L on which the primary wind¬ ings Pi, P2 and P3 and the secondary windings SI to S5 are wound. The cores 3 and the legs L are held in assembly by clamping cross bars 6 held in place by through bolts 7. The legs L are arranged at 90° to each other rather than at the usual 120° spacing to make room for the main leg 4 and flux return leg 4a of the control transformer T2. This transformer arrangement introduces • a significant cost saving and reduces the overall size and weight of the welding transformer. It will be appreciated that the power supply in- corporating the control circuitry described above incl¬ udes four independently variable means of controlling j the power or heat input to the work being welded. The ; first means of control is provided by the switched taps on the primary windings which enable variation of the input current and consequential variation of the ampli¬ tude of the output pulses and the general level of the background supply. The second means of control is pro¬ vided by the switched taps on the background supply sec¬ ondary windings which enable the overall level of the background supply to be independently varied. The third means of control is provided by controlling the firing angle of SCRl and SCR2 by means of the phase shift sub- circuit Pn of the control circuitry. As will be evident to the non-skilled in the art, this determines the amount of each pulse which is allowed to pass to the workpiece thereby controlling the amount of heat input to the work¬ piece via this source. The fourth means of control is provided by the frequency selection sub-circuit by means of which the frequency of the current pulses may be varied to in turn vary the heat input to the work. The provision of these independently variable controls in the power source facilitates adjustment of the power source to enable welding of a multiplicity of different types of weldable metal. The power source is therefore substantially versatile when compared with the prior art pulsed current power sources which were specifically designed for narrow ranges of metals depending on whether the required current was high or low. Thus the power source embodying the invention offers quite sig- nificant advantages over the prior art pulsed current power sources.

Claims: WHAT WE CLAIM IS: 1. A power source for electric arc metal transfer processes comprising a transformer including one or more primary windings and a plurality of secondary windings for producing current pulses which exceed the "transition current" and for providing a background current for main¬ taining the arc used in the metal transfer process between pulses , said power source being characterised by control circuitry for independently varying the frequency of said current pulses and the amount of each current pulse supplied to said arc. 2. The power source of claim 1, characterised in that means are provided for independently varying the ampli¬ tude of said current pulses and the level of said back¬ ground current. 3. The power source of claim 1 or 2 , characterised in that the frequency of said current pulses is. varied by means of a frequency selection circuit (Fs) . 4. The power source of claims 1, 2 or 3, characterised in that the amount of each current pulse supplied to said electric arc is varied by means of a controlled switching circuit (SCRl, SCR2, Sc) operated by means (Ph) for selecting the phase angle at which said switching circuit is actuated. 5. The power source of claim 4, characterised in that said switching circuit includes silicon control-rectif¬ iers (SCRl, SCR2) , the conduction angles of which are determined by a phase shift circuit (Ph) connected to said silicon control-rectifiers. 6. The power source of claim 5, further including a starting current means which connects the background current to the arc electrode and disconnects the current pulse producing means for a pre-determined short period to establish said arc, following which said current pulse producing means is reconnected. OMPI 7. The power source of claims 1 to 6, further comprising a droop resistor (.Rd) connected to said background cur¬ rent supply for maintaining the level of current between pulses, and a maximum current limiting resistor (Rm) , said starting current means including switch means for shorting said droop resistor (Rd) during operation of said starting current means. 8. The power source of claim 7, further comprising ah " additional transformer supplying control current to said control circuitry, said transformer and said additional transformer being included .in a single substantially toroidal transformer including a pair of toroidal lamina¬ ted magnetic cores interconnected by three laminated legs on which said primary and secondary windings are wound and a main leg on which said additional transformer is wound and a flux return leg associated therewith. 9. The power source of any one of the preceding claims, characterised in that said current pulse producing means includes a rectifier bridge connected to said secondary windings, said bridge including silicon control- rectifiers controlled by said control circuitry to det¬ ermine the amount of each current pulse supplied to said arc. " J RE OMPI

Inventors: BROUGHAM W, CHEESMAN G
Applicants: BROUGHAM W, CHEESMAN G, WELDING IND OF AUSTRALIA
Application year: 1980
Main classification: B23K9
IPC codes: B23K9