arc welding machine

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Automation India Welding Technology (P) Limited

Central Marketing Office:

‘Shanti Nivas’, M-69, LGF, Greater Kailash Part-II, New Delhi-110048

Tel: +91(0)11-292221519, Fax: +91(0)11-29212930 Email: [email protected]

PREFACE

Welding is the process of joining two or more similar or dis-similar materials with or without the

application of heat and /or Pressure with or without using the filler materials.

Electric Arc welding is the most technologically advanced flexible process, which is accepted for joining

all most all the materials in the sphere of life.

In Electric Arc welding, an electric Power source is employed to convert the input Electrical energy into

Heat energy for melting the materials to be joined and the filler wire.

Figure 1. – Basic Elements of an Arc Welding Power Source

The development of Welding power sources took place through the following steps :-

AC Tapped Primary transformers followed by Moving Core transformer were used. With the

availabilities of Ferrite core transformer material, Moving Coils Transformers are also being used

for AC output for welding.

The demand of Radiographic quality, Zero defect welding generates more & more requirement of

using DC welding current.

Three Phase Induction Motor Generators having higher maintenance due to rotary parts were used

to produce Ripple free DC output for welding.

With the invention of Straight & Reserve polarity diodes, Static machines using transformers and

rectifier stack are used for the DC output. These Magnetic Amplified Rectifiers using DC Shunt

coils controls are heavier in construction and call for higher maintenance.

The control of DC output was fine tuned with the innovation of Power Thyristors. Use of high

frequency Poly-phase Transformers with Mono-block Thyristors reduce the size of the welding

machine and increased the efficiency, which are also being used today.

Computer hardware also assisted to explore the possibilities of using pre-programmed Chips for

data memory storage and also render assistances in controlling the ultimate welding parameters.

High Frequency Ferrite Core Transformers were used with increased efficiency & negligible

Power loss. With the advent of Solid state electronics, more & more advanced switching devices

having faster dynamic response such as BJT, MOSFET and IGBT were also used with PWM

controls, which gave birth of Solid State Programmable Welding Power Sources.

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PRINCIPLES OF OPERATION Electric Arc welding involves low-voltage, high-current arcs between an electrode and the work piece. The means of reducing power system voltage in Figure 1 by means of a transformer, rectifier, or Motor Generator

A. Welding Transformer:-

Figure 2 shows the basic elements of a welding transformer and associated components. For a transformer, the significant relationships between winding turns and input and output voltages and currents are as follows:- N1 E1 I2 ----- = ---- = ---- ……………………………………..…………………….. Equation -1 N2 E2 I1 Where N1 = the number of turns on the primary winding of the transformer. N2 = the number of turns on the secondary winding E1 = the input voltage E2 = the output voltage I1 = the input current I2 = the output (load) current.

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Figure 2 – Principal Electrical Elements of a Transformer Power Supply

Fig 2:- Principal Electrical element of a Welding Transformer

Taps in the transformer secondary winding may be used to change the number of turns in the secondary, as shown in Figure 3, to vary the open circuit (no-load) output voltage. Tapped transformer permits the selection of the number of turns, N2, in secondary winding of transformer.

Fig 3 – Welding Transformer with Tapped Secondary Winding

When the number of turns is decreased on the secondary, output voltage is lowered because a smaller proportion of the transformer secondary winding is in use. The tap selection, therefore, controls open circuit voltage. As shown by the equation No 1, the primary-secondary current ratio

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is inversely proportional to the primary-secondary voltage ratio. Thus, large secondary (welding) currents can be obtained from relatively low line currents. A transformer may be designed so that the tap selection will directly adjust the output volt-ampere slope characteristics of a proper welding condition. More often, however, an impedance source is inserted in series with the transformer secondary windings to provide this characteristic, as shown in Figure 4. Some types of power sources use a combination of these arrangements, with the taps adjusting the open circuit (or no-load) voltage of the welding machine and the impedance providing the desired volt-ampere slope characteristics.

Fig 4 – Typical Series Impedance Control of Output Current

In constant-current power supplies, the voltage drop, Ex, across the impedance shown in Figure 4, increases greatly as the load current is increased. The increase in voltage drop, Ex, causes a large reduction in the arc voltage, EA. Adjustment of the value of the series impedance controls its voltage drop and the relation of load current to load voltage. This is called current control or, in some cases, slope control. Voltage Eo essentially equals the no-load (open-circuit) voltage of the power supply. In constant-voltage power sources, the output voltage is very close to that required by the arc. The voltage drop Ex, across the impedance (reactor) increases only slightly as the load current increases. The reduction in load voltage is small. Adjustment in the value of reactance gives slight control of the relation of load current to load voltage. This method of slope control with simple reactors also serves a s a method to control with simple reactors also serves as a method to control voltage with saturable reactors or magnetic amplifiers.

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Fig 5 – Ideal Vector Relationship of the Alternating Voltage Output Using reactor Control

Figure 5 shows an ideal vector relationship of the alternating voltages for the circuit of Figure 4 when a reactor is used as an impedance device. The voltage drop across the impedance plus the load voltage equals the no-load voltage only when added vectorially. For the example, the open circuit voltage of the transformer is 80V; the voltage drop across the reactor is about 69V when the load (equivalent to a resistor) voltage is 40V. The vectorial addition is necessary because the alternating load and impedance voltages are not in time phase. The voltage drop across series impedance in an AC circuit is added vectorially to the load voltage to equal the transformer secondary voltage. By varying the voltage drop across the impedance, the load voltage may be changed. This peculiar characteristic (vectorial addition) of impedance voltage in AC circuits is related to the fact that both reactors and resistances may be used to produce a drooping voltage characteristic. An advantage of a reactor is that it consumes little or no power, even though current flows through it and a voltage can be measured across it. When resistors are used, power is lost and temperature rises. In a resistive circuit (no reactance), the voltage drop across the resistor could be added arithmetically to the load voltage to equal the output voltage of the transformer. For example, a welding machine with an constant-current characteristic having 80V open circuit voltage and a power of 25V, 200A would need to dissipate (80-25 = 55) 55V x 200A or 11000 watts (W), in the resistor to supply (25x100 =5000) 5000 W to the arc due to the reason that in the resistive circuit, the voltage and current are in the phase. In the reactor circuit, phase shift accounts for the greatly reduced power loss. In the reactor circuit, there are only the iron and copper losses, which are very small in comparison. Variable inductive reactance or variable mutual inductance may be used to control the volt-ampere characteristics in typical transformer or transformer-rectifier type arc welding power sources. The equivalent impedance of a variable inductive reactance or mutual inductance is located in the AC circuit of the power source in series with the secondary circuit of the transformer, as shown in Figure 4. Another major advantage of inductive reactance is that the phase shift produced in the alternating current by the reactor improves AC arc stability for a given open circuit voltage. This is an advantage with GTAW (Gas Tungsten Arc Welding) and SMAW (Shielded metal arc welding) processes.

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The reactance of a reactor can be varied by many ways. First way is by changing taps on a coil or by other electrical/mechanical schemes. Varying the reactance alters the voltage drop across the reactor. Thus, for any given value of inductive reactance, a specific volt-ampere curve can be plotted. This is the main control feature of such power supplies. In addition to adjusting reactance, mutual inductance between the primary and secondary coils can also be adjusted. This can be done by moving the coils relative to each other or by using a movable shunt that can be inserted or withdrawn from the transformer. These methods change the magnetic coupling of the coils to produce adjustable mutual inductance. The transformer-rectifier type of arc welding power supply usually incorporates a stabilizing inductance or choke, located in the DC welding circuit, to improve arc stability. AC STATIC WELDING POWER SOURCES Alternating current power sources normally are single-phase transformers that connect to AC input power lines and transform the input voltage and amperage to levels suitable for arc welding. Because various welding applications have different welding power requirements, means for control of welding current the welding transformer power source. The methods commonly used to control the welding circuit output are as follows:- MOVABLE-COIL CONTROL A movable coil transformer consists essentially of an elongated core on which are located primary and secondary coils. Either the primary coil or the secondary coil may be movable, while the other one is fixed in position. Most AC transformers of this design have a fixed-position secondary coil. The primary coils are normally attached to a lead screw and, as the screw is turned, the coil moves closer to or farther from the secondary coil. The varying distance between the two coils regulates the inductive coupling of the magnetic lines of force between them. The more the distance between two coils, the more vertical is the volt-ampere output curve and the lower the maximum short-circuit current value. Similarly, when the two coils are closer together, the maximum short-circuit current is higher and the slope of the volt-ampere output curve is less steep.

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Fig 7 – Movable-Coil AC Power Source

Figure 7 shows one form of a movable-coil transformer with the coils far apart for minimum output and a steep slope of the volt-ampere curve. The first figure shows the coils as close together as possible. The volt-ampere curve is indicated at maximum output with less slope than the curve of second figure. Another form of movable coil employs a pivot motion. When the two coils are at a right angle to each other, output is at a minimum. When the coils are aligned with one coil nested inside the other, output is at maximum.

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MOVABLE-SHUT CONTROL In this design, the primary coils and the secondary coils are fixed in position. Control is obtained with a laminated iron core shunt that is moved between the primary and secondary coils. It is made of the same material as that used for the transformer core.

Fig 8 – Movable-Shunt AC Power Source As the shunt is moved into position between the primary and secondary coils, as shown in Figure 8, some magnetic lines of force are diverted through the iron shunt rather than to the secondary coils. With the iron shunt between the primary and secondary coils, the slope of the volt-ampere curve increases and the available welding current is decreased. Minimum current output is obtained when the shunt is fully in place. As illustrated above, the arrangement of the magnetic lines of force, or magnetic flux, is unobstructed when the iron shunt is separated from the primary and secondary coils. Here the output current is at its maximum.

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TAPPED SECONDARY COIL CONTROL A tapped secondary coil may be used for control of the volt-ampere output of a transformer as shown in Figure 9. They are the least expensive and most universally used of all welding power supplies. Basic construction is some-what similar to the movable-shunt type, except that the shunt is permanently located inside the main core and the secondary coils are tapped to permit adjustment of the number of turns. Decreasing secondary turns reduces open circuit voltage and, also, the inductance of the transformer, causing welding current to increase. Fig- 9, Tapped Secondary Control MOVABLE-CORE REACTOR The movable core reactor type of AC welding machine consists of a constant-voltage transformer and a reactor in series. The inductance of the reactor is varied by mechanically moving a section of its iron core.

Fig 10 – Movable Core Reactor Type AC Power Source

The machine is diagramed in Figure 10. When the movable section of the core is in a withdrawn position, the permeability of the magnetic path is very low due to the air gap. The result is a low inductive reactance that permits a large welding current to flow. When the movable-core section is advanced into the stationary core, as shown by broken lines in Figure 10, the increase in permeability causes an increase in inductive reactance and, thus, welding current is reduced.

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SATURABLE REACTOR CONTROL A saturable reactor control is an electrical control which uses a low voltage, low amperage DC circuit to change the effective magnetic characteristics of reactor cores. A self-saturating saturable reactor is referred to as a magnetic amplifier because a relatively small control power change will produce a sizeable output power change. This type of control circuit makes remote control of output from the power source relatively easy, and it normally requires less maintenance than the mechanical controls. The volt-ampere characteristics are determined by the transformer and the saturable-reactor configurations. The DC control circuit to the reactor system allows adjusting the output volt-ampere curve from minimum to maximum.

Fig 11 – Saturable Reactor Type AC Welding Power Source

A simple, saturable-reactor power source is diagramed in Figure 11. The reactor coils are connected in opposition to the DC control coils. If this were not done, transformer action would cause high circulating currents to be present in the control circuit. With the opposing connection, the instantaneous voltages and currents tend to cancel out. Saturable reactors tend to cause severe distortion of the sine wave supplied by the transformer. This is not desirable for gas tungsten arc welding because the wave form for that process is important. One method of reducing this distortion is by introducing an air gap in the reactor core. Another is to insert a large choke in the DC control circuit. Either method, or a combination of both, will produce desirable results.

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The amount of current adjustment in a saturable reactor is based on the ampere-turns of the various coils. The term ampere-turns is defined as the number of turns in the coil multiplied by the current in amperes flowing through the coil. In the basic saturable reactor, the law of equal ampere-turns applies. To increase output in the welding circuit, a current must be made to flow in the control circuit. The amount of change can be approximated with the following equation:- Ic.Nc Iw = --------- ……………………………………… Equation 2 Nw where Iw = change in welding current, A Ic = change in current, A, in the control circuit Nc = number of turns in the control circuit Nw = number of turns in the welding current circuit The minimum current of the power source is established by the number of turns in the welding current reactor coils and the amount of iron in the reactor core. For a low minimum current, either a large amount of iron or a relatively large number of turns, or both, are required. If a large number of turns are used, then either a large number of control turns or a high control current, or both, are necessary. To reduce the requirement for large control coils, large amount of iron, or high control currents, the saturable reactors often employ taps on the welding current coils, creating multi-range machines. The higher ranges would have fewer turns in these windings and, thus, correspondingly higher minimum currents.

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B. WELDING RECTIFIERS

1. Rotary DC Welding Alternators

Rotating machinery is also used as a source of power for arc welding. These machines are divided into two types; generators which produce direct current and alternators which produce alternating current. The mechanical power may be obtained from various sources, such as an internal combustion engine or an electric motor. A DC generator consists of a rotor and stator. The rotor assembly is comprised of (1) a through shaft, (2) two end bearings to support the rotor and shaft load, (3) an armature which includes the laminated armature iron core and the current-carrying armature coils, and (4) a commutator. It is in the armature coils that welding power is generated. The stator is the stationary portion of the generator within which the rotor assembly turns. It holds the magnetic field coils of the generator. The magnetic field coils conduct a small amount of DC to maintain the necessary continuous magnetic field required for power generation. The DC amperage is normally no more than 10 to 15 A and very often is less. In the DC generator, it is the armature that is the current-carrying conductor. The magnetic field coils are located in the stator. The armature turns within the stator and its magnetic field system, and welding current is generated. The no-load output voltage of a DC generator may be controlled with a relatively small variable current in the main or shunt field winding. This current controls the output of the DC generator series or bucking field winding that supplies the welding current. Polarity can be reversed by changing the interconnection between the exciter and the main field. An inductor or filter reactor is not usually needed to improve arc stability with this type of welding equipment. Instead, the several turns of series winding on the field poles of the rotating generator provide more than enough inductance to ensure satisfactory arc stability. These machines differ from standard DC generators in that the alternator rotor assembly contains the magnetic field coils (Figure 12) instead of the stator coils as in other generators (Figure 13). Slip rings are used to conduct low DC power into the rotating member to produce a rotating magnetic field. This configuration precludes the necessity of the commutator and the brushes used with DC output generators. The stator (stationary portion) has the welding current coils wound in slots in the iron core. The rotation of the field generates AC welding power in these coils.

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Fig 12 – Schematic view of an Alternator showing the magnetic field contained in the Rotor Assembly.

Fig 13 – Schematic view of a Generator showing the magnetic field contained in the stator assembly The frequency of the output welding current is controlled by the speed of rotation of the rotor assembly and by the number of poles in the alternator design. A two-pole alternator must operate at 3600 rpm to produce60Hz current, whereas a four-pole alternator design must operate at 1800 rpm to produce 50 Hz current. Saturable reactors and moving-core reactors may be used for output control of these units. However, the normal method is to provide a tapped reactor for broad control of current ranges, in combination with control of the alternator magnetic field to produce fine control within these ranges. These controls are shown in Figure 14.

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Fig 14 – Schematic view of an alternator type power supply showing a tapped reactor for coarse current control and adjustable magnetic field ampere for fine output current control

The armature conductors of a welding generator are relatively heavy because they carry the welding current. The commutator is located at one end of the armature. It is a group of conducting bars arranged parallel to the rotating shaft to make switching contact with asset of stationary carbon brushes. These bars are connected to the armature conductors. The whole arrangement is constructed in proper synchronization with the magnetic field so that, as the armature rotates, the commutator performs the function of mechanical rectification. An alternator power source is very similar, except that generally the magnetic field coils are wound on the rotor, and the heavy welding current winding is wound into the stator. These machines are also called revolving or rotating field machines. The AC voltage produced by the armature coils moving through the magnetic field of the stator is carried to copper commutator bars through electrical conductors from the armature coils. The conductors are soft-soldered to the individual commutator bars. The latter may be considered as terminals, or “collector bars”, for the alternating current generated from the armature. The commutator is a system of copper bars mounted on the rotor shaft. Each copper bar has a machined and polished top surface. Contact brushes ride on that top surface to pick up each half-cycle of the generated alternating current. The purpose of the commutator is to carry both half-cycles of the generated AC sine wave, but on separate copper commutator bars. Each of the copper commutator bars is insulated from all the other copper bars.

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The carbon contact brushes pick up each half-cycle of generated alternating current and direct it into a conductor as direct current. It may be said that the brush-commutator arrangement is a type of mechanical rectifier since it does change the generated alternating current (AC) to direct current (DC). Most of the brushes used are an alloy of carbon, graphics, and small copper flakes. Placing the heavy conductors in the stator eliminates the need for carbon brushes and a commutator to carry high current. The output, however, is AC, which requires external rectification for DC application. Rectification is usually done with a bridge, using silicon diodes. An alternator usually has brushes and slip rings to provide the low DC power to the field coils. It is not usual practice in alternators to feed back a portion of the welding current to the field circuit. Both single and three-phase alternators are available to supply AC to the necessary rectifier systems. The DC characteristics are similar to those of single and three phase transformer-rectifier units. An alternator or generator may be either self-excited or separately-excited, depending on the source of the field power. Either may use a small auxiliary alternator or generator, with the rotor on the same shaft as the main rotor, to provide exciting power. On many engine-driven units, a portion of exciter field power is available to operate tools or lights necessary to the welding operation. In the case of a generator, this auxiliary power is usually 115 V DC. With an alternator-type power source, 120 or 120/240 V AC is usually available. Voltage frequency depends on the engine speed. A rheostat or other control is usually placed in the field circuit to adjust the internal magnetic field strength for fine adjustment of power output. The fine adjustment, because it regulates the strength of the magnetic field, will also change the open circuit voltage. When adjusted near the bottom of the range, the open circuit voltage will normally be substantially lower than at the high end of the range.

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Fig 15 – Volt Ampere Relationship for a typical constant current rotating type power source. Figure 15 shows a family of volt-ampere curve characteristics for either an alternator or generator-type power supply. The basic machine does not often have the dynamic response required for welding and hence the same are obtained from taps on a reactor in the AC portion of the circuit. Thus, a suitable inductor is generally inserted in series connection in one leg of the DC output from the rectifier. Welding generators do not normally require an inductor. There is a limited range of overlap normally associated with rotating equipment where the desired welding current can be obtained over a range of open circuit voltages. With lower open circuit voltage, the slope of the curve is less. This allows the welder to regulate the welder to regulate the welding current to some degree by varying the arc length. This can assist in weld-pool control, particularly for out-of-position work. Some welding generators carry this feature beyond the limited steps described above. Generators that are compound would with separate and continuous current and voltage controls can provide the operator with a selection of volt-ampere curves at nearly any amperage capability within the total range of machine. Thus, the welder can set the desired arc voltage with one control and the arc current with another. This adjusts the generator power source to provide a static volt-ampere characteristic that can be “tailored” to the job throughout most of its range.

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The volt-ampere curves that result when each control is changed independently are shown in Figure 16 and 17.

Fig 16 - Effect of current control variations on Generator Output

Fig 17 – Effect of voltage control variations on Generator Output

Welding power sources are available that produce both constant current and constant voltage. These units are used for field applications where both are needed at the job site and electrical power supply is not available.

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2. Static Magnetic Amplifier Control Rectifier

Technically, the magnetic amplifier is a self-saturating saturable reactor. It is called a magnetic amplifier because it uses the output current of the power supply to provide additional magnetization of the reactors. In this way, the control currents can be reduced and control coils can be smaller. While magnetic amplifier machines often are multi-range, the ranges of control can be much broader than those possible with an ordinary saturable reactor control. In Figure 19, it can be seen that by using a different connection for the welding current coils and rectifying diodes in series with the coils, the load ampere-turns are used to assist the control ampere-turns in magnetizing the cores. A smaller amount of control ampere-turns will cause a correspondingly larger welding current to flow because the welding current will essentially “turn itself on”. The control windings are polarity sensitive.

Fig 19 – Magnetic Amplifier Welding Current Control.

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3. SOLID STATE CONTROL RECTIFIERS:-

Solid state physics - the science of the crystalline solid is being used in manufacturing these devices and hence known as Solid State Control Devices. Methods had been developed for treating certain materials such as silicon to modify their electrical properties. Transformer-rectifier or DC Alternator power sources uses on rectifiers to convert AC to DC. Early welding machines used selenium rectifiers. Today, most rectifiers are made of silicon for reasons of economy, current-carrying capacity, reliability and efficiency. A. DIODE :- A single rectifying element is called a diode, which is a one-way electrical valve. When placed in an electric circuit, a diode allows current to flow in one direction only, when the anode of the diode is positive with respect to the cathode. Using a proper arrangement of diodes, it is possible to convert AC to DC. The resistance to current flow through a diode results in a voltage drop across the component and generates heat within the diode. Unless this heat is dissipated, the diode temperature can increase enough to cause failure. Therefore, diodes are normally mounted on heat sinks (aluminum plates) to remove that heat. Diodes have limits as to the amount of voltage they can block in the reverse direction (anode negative and cathode positive). This is expressed as the voltage rating of the device. Welding power source diodes are usually selected with a blocking rating at least twice the open circuit voltage in order to provide a safe operating margin. A diode can accommodate current peaks well beyond its normal steady state rating, but a high, reverse-voltage transient could damage it. Most rectifier power sources have a resistor, capacitor, or other electronic device to suppress voltage transients that could damage the rectifiers. B. SILICON CONTROLLED RECTIFIER (SCR) – THRYISTOR Solid-state devices with special characteristics are also used to directly control welding power by altering the welding current or voltage wave form. Such solid-state devices have now replaced saturable reactors, moving shunts, moving coils, etc., formerly used to control the output of welding transformers. One of the most important of these devices is the silicon controlled rectifier (SCR) also called a Thyristor. The SCR is a diode variation with a trigger called a gate, as shown in Figure 16. An SCR is non-conducting until a positive electrical signal is applied to the gate. When this happens, the device becomes a diode, and will conduct current as long as the anode is positive with respect to the cathode. However, once it conducts, the current cannot be turned off by a signal to the gate. Conduction will stop only if the voltage applied to the anode becomes negative with respect to the

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cathode. Conduction will not take place again until a positive voltage is applied to the anode and another gate signal is received.

Fig 20 – Silicon Controlled Rectifier

The main use of SCR is in phase control mode with transformers. Using the action of a gate signal to selectively turn on the SCR, the output of welding power source can be controlled. A typical phase control SCR circuit is shown in Figure 20. In Figure 21, during the time that point B is positive with respective to point E, no current will flow until both SCR 1 and SCR 4 receive a gate signal to turn on. At that instant, current will flow through the load. At the end of that half-cycle, when the polarity of B and E reverses, a negative voltage will be impressed across SCR 1 and SCR 4, and they will turn off. With point E positive with respect to point B, a gate signal applied to SCR 2 and SCR 3 by the control will cause these two to conduct, again applying power to the load circuit. To adjust power in the load, it is necessary to precisely time where, in any given half-cycle, conduction is to initiate.

Fig 21 – Single Phase DC Power Source Using an SCR Bridge for Control

When high power is required, conduction is started early in the half-cycle and in case of low power requirement; conduction is delayed until later in a half-cycle. This is known as phase control. The result is shown in Figure 22. The resulting power is supplied in pulses to the load, and is proportional to the shaded area under the wave form envelope. A significant interval may exist

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when no power is supplied to the load. This can stop the electric arc, especially at low-power levels. Therefore, wave filtering is required.

Fig 22 – Phase Control Using an SCR Bridge In Figure 21 a large inductance, Z was shown in the load circuit. For a single phase circuit to operate over a significant range of control, Z must be very large to smooth out the pulses enough to increase the conductance times. If, however SCRs are used in a three phase circuit, the non-conducting intervals would be reduced significantly. The inductance (Z) would be sized accordingly. For this reason three-phase SCR systems are more practical for welding power sources. Timing of the gate signals and feedback, is necessary must be precisely controlled for welding service. The nature of the feedback depends on the parameter to be controlled and the degree of control required. To provide constant-voltage characteristics, the feedback must consist of some signal that is proportional to arc voltage. That signal controls the precise arc voltage at any instant so that the control can properly time and sequence the initiation of the SCR to hold the preset voltage. The same effect is achieved with constant current by using a current reference. Most of SCR phase controlled welding power sources are three-phase machines. Such power sources have distinct features because the output characteristics are controlled electronically. For example, automatic line-voltage compensation is very easily accomplished, allowing welding power to be held precisely as set, even if the input line voltage varies. Volt-ampere curves can be shaped & tailored for a particular welding process or its application. Such machines can adapt their state characteristic to any welding process.

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Other capabilities are pulsing, controlled current with respect to arc voltage, controlled arc voltage with respect to current, and a high initial current or voltage pulse at the start of the weld.

Fig 23 – Three Phase Bridge Using Six SCRs (Full Wave Control)

Several SCR configurations can be used for arc welding. Figure 23 shows a three-phase bridge with six SCR devices. With a 50 Hz main frequency this arrangement produces a 360 Hz ripple frequency under load. It also provides precise control and quick response; in fact, each half-cycle of the three-phase output is controlled separately. Dynamic response is enhanced because it reduces the size of the inductor needed to smooth out the welding current. Fig 24 – Three Phase Hybrid Using Three SCRs and Four Diodes (Half-Wave Control)

Figure 24 diagrams a three-phase bridge rectifier with three diodes and three SCRs. Because of greater current ripple this configuration requires a larger inductor than the six SCR units. For that reason it has a slower dynamic response. A fourth diode, called a freewheeling diode, can be added to re-circulate the inductive currents from the inductor so that the SCRs will turn off, i.e., commutate. This offers greater economy over the six SCR units because it uses fewer SCRs and a lower cost control unit.

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C. TRANSISTORS The transistor is another solid-state device which is used in welding power supplies. Due to economic reason, the use of transistors is limited to power supplies requiring precise control of a number of variables. The transistor differs from the SCR in several ways. One is that conduction through the device is proportional to the control signal applied. With no signal, there is no conduction. When a small signal is applied, there is a corresponding small conduction; with a large signal, there is a correspondingly large conduction. Unlike the SCR, the control can turn off the device without waiting for polarity reversal or an “off” time. Since transistors do not have the current-carrying capacity of SCRs, several may be required to yield the output of one SCR. Having the optimal characteristics, use of IGBT as a switching device for inverter circuit has found a new dimension in manufacturing of Welding Power Sources. Figure 26 and 27 illustrate the details of IGBT. Figure 25 illustrate the characteristic differences between the switching devices.

IGBT

In the case of welding power

sources the high switching

frequency is necessary so that,

apart from the noise

consideration, the volume of

the passive components can be

kept as low as possible. This

can be done with new power

semiconductor devices called

IGBT modules, which are

notable for both their switching

performance and their high

current-carrying capacity.

The concept IGBT represents

further advancement of the

power MOSFET with the goal

of reducing the turn-on

resistance. IGBT stands for

Isolated Gate Bipolar Transistor

Fig -26

Fig-25: Characteristics of Power devices commonly used for

manufacturing of inverter power sources

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Frequency modulation and Pulse width modulation methods are being used to take advantage of transistors in welding power sources. With frequency modulation, the welding current is controlled by varying the frequency supplied to the main transformer. Since the frequency is changing, the response time varies also. The size of the transformer and inductor must be more for the lowest operating frequency. In Pulse Width Modulation, welding output is controlled by varying the conduction time of the switching device. Since the frequency & the response time are constant, the size of the transformer and inductor will be much less for the highest operating frequency. D. SOLID-STATE INVERTER What is an Inverter? Inverter circuitry has been used for quite sometime in drive systems, battery charges, aircraft industries, controls and recreation vehicles. With the advent of Solid State Electronics, the high power, reliable, cost effective switching devices were made available which also make the technology adoptable for welding power sources. An inverter welding power source changes the primary power line 415/220 Volt, 1 Ph or 3 Ph, 50 Hz into direct current (DC) power usable for welding. In inverter, the primary power goes immediately to a rectifier that changes the AC to DC. The DC then goes into the high power, very fast switching devices that convert the DC back to AC, but a very high frequency (between 20 to 100 KHz). The high frequency AC then goes through a transformer to lower the voltage. The lower voltage high frequency AC goes to a second rectifier and is changed to DC. This goes through a filter and becomes the DC output. Sensing and control circuits monitor the unit output, compare the output to input commands and cause the output to match the input by controlling the inverter. In early 80’s high power Thyristors were employed in manufacturing the inverters, but with the improved availabilities of solid state switching devices such as BJT, MOSFET and IGBT, the switching speed increased with the adopt abilities for higher voltage and currents which are essential requirements for welding power sources. The primary elements to weight to mass ratio in any welding power source are the magnetic (main transformer and filter inductor). Various attempts have been made to reduce their weight and size,

Fig-27: Silicon cross-section of an

IGBT with its equivalent circuit and

symbol (N-channel, Enhancement

mode). The terminal called collector is,

actually, the emitter of the PNP. In

spite of its similarity to the cross-

section of a power MOSFET, operation

of the two transistors is fundamentally

different, the IGBT being a minority

carrier device.

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for example, the substitution of aluminum windings for copper. The use of an inverter circuit can produce significant reduction in size and weight of these components as well as decrease their electrical losses. An inverter-based power source is smaller, more compact, requires less electricity than conventional welding power sources, and offers a faster response time.

Fig 28 – Simplified Diagram of an Inverter Circuit used to demonstrate the principle of time ratio control.

An inverter is a circuit which uses solid-state devices (SCR’s or transistors) to convert DC into high-frequency AC, usually in the range of 1 KHz to 100 KHz. Conventional welding power sources use transformers operating from a line frequency of 50 Hz. Since transformer size is inversely proportional to applied frequency and hence the reductions upto 75 percent in power source size and weight are possible using inverter circuits. Inverter circuits control the output power using the principle of time ratio control (TRC). The solid-state devices (semiconductors) in an inverter act as switches; they are either switched “on” and conducting, or they are switched “off” and blocking. This operation of switching “on” and “off” is sometimes referred to as Switch Mode Operation. TRC is the regulation of the “on” and “off” times of the switches to control the output. Figure 28 illustrates a simple TRC circuit which controls the output to a load such as a welding arc. When switch is on, the voltage out (V out) equals voltage in (Vin); when switch is off, Vout = 0. The average value of Vout is as follows: Vout = ton . Vin + O . toff or Vin . ton

ton + toff ton + toff Thus : Vout = Vin . ton …………………………………………..Equation 3 tp where ton = one time (conducting) toff = off time (blocking) tp = ton + toff (or time of 1 cycle)

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Vout t is controlled by regulating the time ratio ton / tp. Since the on/off cycle is repeated for every tp interval, the frequency (f) of the on/off cycles is defined as: l f = ---- tp Thus, the TRC formula can now be written as :- Vout = Vin . ton . f The TRC formula leads two methods of controlling an inverter welding power source. By varying ton the inverter uses pulse-width modulated TRC. Another method of inverter control called frequency modulation TRC varies f. Both frequency modulation and pulse width modulation have been used in commercially available Arc welding inverters.

Fig 29 – Inverter diagram showing power supply sections and voltage wave forms

Figure 28 is a block diagram of an inverter used for DC welding. Incoming three-phase or single-phase 50 Hz power is converted to DC by a full wave rectifier. This DC is applied to the inverter which, using semiconductor switches, inverts it into high-frequency square wave AC. In another variation used for welding, the inverter produces sine waves in a resonant technology, with frequency modulation control. The switching of the semiconductors takes place between 1 kHz and 100 kHz depending on the component used and method of control.

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This high-frequency voltage allows the use of a smaller step down transformer. After being transformed, the AC is rectified to DC for welding. Solid-state controls enable the operator to select either constant-current or constant-voltage output and, with appropriate options, these sources can provide pulsed outputs. The capabilities of the semiconductors and particular circuit topology determine response time and switching frequency. Faster response times are generally associated with the higher switching and control frequencies, resulting in more stable arcs and superior arc performance. However, other variables, such as length of weld cables, must be considered since they may affect the power supply performance. . ADVANTAGES However, the inverter produces a unique set of advantages not attainable from conventional units. Fig. 30 shows a simplified diagram of the major components of both a conventional and inverter power source and the change in the electrical power by each component. In the conventional closed-loop-welding power source, primary power goes directly to a transformer which lowers the line voltage without changing the phase (Ph) or frequency (Hz). The next step is a rectifier assembly that changes the AC (alternating current) to DC (direct current). This goes through a filter and becomes the DC output. Sensing circuits monitor the output and compare the output to input control signals (power source setting). These circuits control the output by adjusting the output of the rectifier assembly.

Fig - 30 – Comparison of components conventional and inverter power sources

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Size and weight In conventional power sources, the transformer operating at 50Hz frequency is the largest, heaviest, most inefficient component. Transformer size is inversely proportional to operating frequency – the higher the operating frequency, the smaller the transformer. In the inverter, the operating frequency has been changed to 20,000 Hz (from 50 Hz line) or over 400 times higher than the 50 Hz operating frequency of the conventional unit. This allows the inverter transformer (and consequently the total inverter power source) to be much smaller and lighter than the conventional unit. Figure 31, shows an inverter unit for a 350 Amp 60% duty cycle and a transformer Rectifier for a 450 Amp 100% duty cycle conventional unit and. The smaller size and lower weight of inverters gives them greater portability – and they use less floor.

Fig 31 - 350 Amp 60% duty cycle Inverter Unit & a 450 Amp 100% duty cycle conventional Rectifier unit Energy Efficiency Inverters are more energy efficient than conventional power sources. Figure 32 shows the energy loss in inverter vis-à-vis conventional rectifier. Much of the reason is related to the difference in transformers. Table-1, shows the input KVA, and efficiency for conventional power sources and an inverter at the same welding output. From 200 A to 500 A inverters are much more efficient.

Fig – 32

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Input KVA determines the amount of primary line current required. The higher the primary line current, the larger size of the line or, for a given line size, the smaller the number of units that can be attached to the line. Here the inverter has a significant advantage. At 200 Amps output the inverter uses approximately one-half the input KVA. This will allow for more units to be placed on an already installed primary line. Table-1 Electrical Efficiency – Conventional V/s Inverter Power Sources S.No. Output

welding current (Amps)

Output required (KW)

Input KVA Required Efficiency (%) No load loss (watts)

Conv. Inverter Conv. Inverter Conv. Inverter

A 200A 5.6 KW 15.59 < 8.40 75.7% 85% >200W <10W

B 300A 9.6 KW 20.98 < 9.90 80.0% 88% >200W <10W

C 400A 14.4 KW 26.00 <14.60 81.4% 88.9% >220W <10W

D 500A 20.0 KW 30.80 <20.20 82.3% 89.3% >250W <10W

Table -2 Comparison of Electrical Power Requirement Between IGBT Inverter & Motor Generators S.No. Electrode Dia in mm Power consumption Units/Hrs. Power

consumption [ % Savings ]

Motor Generator Set IGBT Inverter

1 2.50mm 6.5 2.5 62%

2 3.15mm 8.5 3.0 65%

3 4.00mm 10.5 4.0 62%

4 At No-Load 4.0 Not measurable but approx. 10 Watts

Table-3 Future Trend of Inverter Technology Item / Subject Stage Conventional

Technology Future Trend

High Power Factor

Input Condenser input smoothing

Switching mode rectifier

Low EMI

Inverter Hard switching PWM converter

Soft switching PWM converter Resonant converter

Rectifier Fast Recovery Diode Soft Recovery Diode

High Speed Control Smaller Size

Inverter Thyristor BJT MOSFET

MOS Gate Thyristor IGBT

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Use of inverters will decrease the welding energy cost. Table-2 illustrates the electrical power requirement between inverter and motor generator. Performance The major advantage of inverters is a significant increase in arc performance, stability, and control. Note in Figure-26, that the sensing and control feed back is to the rectifier assembly operating at 50 cycles per second on 3 Ph power for the conventional unit, and to the power switches operating at 20,000 cycles per second for the inverter unit. The control circuitry switches are operating at 20,000 cycles per second for the inverter unit. The control circuitry can change the conventional unit output approximately 360 times per second and the inverter unit 20,000 times per second. The larger transformer of the conventional unit has more built-in inductance (resistance to change) than the smaller inverter transformer. These two conditions allow the inverter power source to give enhanced performance.

Fig 33: Comparison in spatter generation rate of various types of welding power

sources in GMA welding.

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Multi-process Capabilities Power sources of conventional design that could be used for several processes are not usually available. Generally, they were acceptable for each process in a given range but they could not give full range performance for each process. Something had to be compromised for one process to obtain acceptable results for another process. Multiple output connections and special switches settings were required with higher operator understanding. With inverter design, multiple processes are built into a single unit with maximum performance characteristics and full range for each process without compromising any process. A simple switch allows process selection and the same output control sets the output for MMAW, GTAW and GMAWP.

Fig 34 - An inverter Power source with Built-in wire-feeder for Programmable Synergic Pulse Mig with U/I & I/I Controls with MMA & GTA welding facilities. The modular concept enables the users to update the simple inverter, suitable for Pulse Tig for Pulse Mig and Synergic Pulse Mig Welding Process including air arc gauging as and when required, by addition of function panels for each module.

Utmost care should be taken to introduce the Inverter Power sources in shop floor.

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Following steps should be carefully followed to ripe the technological advantages of Inverters over and above the conventional rectifiers:- Step 1, Coordination with shop floor management:- Every new product put on the shop floor must solve a problem that cannot be solved with present product, or reduce costs through greater productivity and/or improved efficiency. The benefits to be achieved upon by shop management. Step -2, Welding testing and procedure qualification:- Generally the machine settings to obtain the desired goals with an inverter will not be the same as with a conventional power source. New settings and new travel speeds must be tested to comply with requirements for weld quality. Determine a proposed range of settings that give quality welds, meet code requirements, and accomplish the desired goals. Do not just give the machine to the operator and expect him to find the correct settings. Step-3, Operator training:- First, communicate to the operator why an inverter is being tested. Describe the desired goals. Make the operator a partner in the project. An inverter presents four very new different things to the operator that should be explained before actual welding. The small size can pose a question. “How can this smaller unit give as must power as this big one?” The arc and the machine will have a different sound. The “arc sound” is used as gauge of acceptability. The GMAW-P (Pulse) process has a very different sound compared to short circuit. Each control knob should be explained as to function and if it applies to the application. If an operator is not instructed about each control, he will attempt to find out for himself. Since most inverters are multi-process, operators should know which are to be used, which are not to be used, and why.

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Step-4, Training of maintenance personnel :- Inverters should not be described as “a box of circuit boards”. Many are designed to protect themselves from primary line spikes, dead shorting, and overheating conditions by turning the unit of before damage can occur. Inverters usually “work” or “don’t work”. There is nothing in-between. The tendency is to blame the inverter for everything that is not right. Usually, common problems such as poor wire feeding, contaminated tungsten, poor feed rolls, poor electrical connections, etc. will be the real problems. A good house-keeping can short out all these minor problems. Inverters generally conform to IP-23 specifications and CE & S marked. Hence these equipment are suitable for outdoor as well as shop floor usage, because of its compact size and light weight.

Conclusion Digitally controlled Inverter based welding power source has come to stay in the welding industry throughout the world, due to its higher efficiency, negligible no-load losses and lower power consumption and higher Arc stability. By using inverter power source, the cost of welding can be easily brought down with significant improvement in weld quality. It is a matter of pride that Indian fabrication industries had already started using inverter power sources to uplift the ultimate welding quality with reduced input cost.

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Bibliography / Reference:-

1. Jackson CE - “ The Science of Arc Welding” - 1960, Adam”s Lecture, Welding Journal , USA

2. Needham JC - “Review of New Design of Power sources for Arc welding Process” ,

- Docket XII-F-217-80, International Institute of Welding, 1980.

3. Amin M - “ Micro computer control of Synergic Pulse Mig Welding” - Docket 166/81, The Welding Institute, December 1981.

4. Lucas W - “ A review on Recent Advances in Arc Welding Power sources in Japan”

- Docket 199/82, The Welding Institute, November 1982. 5. C Shira - “Converter Power Supplies – more options for Arc welding”

- Welding Journal, USA, June 1985.

6. Rankin T - “New Power Source design breaks the traditional welding”, - Welding Journal, USA, May 1990.

7. Masao Ushio, Hideyuki Yamamoto, etc - “Recent Advances in Welding Power system

for automated welding” – International Institute of Welding, January 1995. 8. Mukhopadhyay GL - “Development of Inverter Welding Power Sources”,

- Indian Welding Journal, July 1998.

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