Exercises in Welding Process and Equipment --- Part 3: Power-source and Equipment ---

Transcription

1 JICA_OHJI Exercises in Welding Process and Equipment --- Part 3: Power-source and Equipment --- Takayoshi OHJI Professor Emeritus, Osaka University Dr. of Engineering VIRTUAL WELD CO.,LTD OK 0912 Ex. Explain briefly the following technical terms. 1 Reactance 2 Power factor 1

2 Ex. Power source Welding processes are given in the column A in the following table. The external characteristics of the power sources and the wire (including rod) feeding systems or electrode adjusting systems are given in Group B and Group C below. Select and put a mark of the appropriate word in the column B and C respectively. Group B: Power source (a) Drooping (or constant current) characteristics (b) Constant potential characteristics Group C: Wire feeding system or electrode adjusting system (1)Manually feeding or adjusting (2)Voltage feed-back control feeding or adjusting (3)Constant speed feeding or adjusting A Welding process B Power source C Wire feeding system SMAW TIG welding (GTAW) MIG welding (GMAW) MAG welding (GMAW) SAW JWES 0808 Output characteristics of welding power source 1 The output characteristics of welding power source is classified into two types, i.e., the constant current (or the drooping characteristics) and the constant voltage types. The constant-current type of power source (CC) is generally applied to manual arc welding processes such as shielded metal arc welding, TIG welding or plasma arc welding, where the variation in arc length caused by the manual operation is inevitable. The constant-voltage type of power source (CV) is generally applied to MAG or MIG welding process where a consumable electrode-wire is used and the electrode is fed at a constant speed to make use of the self regulation effect. Output voltage Constant current characteristics Drooping characteristics Output voltage Constant voltage characteristics Output current Output current (a) Drooping characteristics (b) Constant voltage characteristics Fig.1 Constant voltage and constant current welding power sources 2

3 Ex. Power Sources for Welding _No.1 A constant voltage type of power source is applied to MAG arc welding. The power source with a constant-speed wire feeder sustains a nearly constant arc voltage (arc length) during the welding operation. Explain the mechanism of the self regulation of arc length. Self regulation of arc length in constant voltage type power source WF>MR WF=MR WF<MR WF L l MR L 0 L s WF: const. Output characteristics of power source MR f(i) Arc characteristics L l L 0 Voltage L s Arc length: L s <L 0 <L l I 0 Current 3

4 Ex. Power Sources for Welding _No.2 In AC Welding Power Sources mostly used in Japan, the drooping external characteristics are normally provided by means of the movable iron core installed in the welding transformer. The welding current adjustment is made by taking in or taking out the movable iron core. Explain this principle, showing a schematic diagram of the welding transformer and an external characteristics diagram of the welding power source. A.C. Welding Power Source In AC Welding Power Sources mostly used in Japan, the drooping characteristics are generally realized by using a movable iron core, installed in the welding transformer. The welding current, the output is adjusted by taking in or taking out the movable core. For example, as shown in Fig.1, a third iron core M3 is inserted between the cores M1 and M2 of a normal transformer, so that when the load current flows, the magnetic flux partially leaks and passes through the iron core M3, which then acts as a series reactor. Since the magnetic leakage flux, i.e. reactance, becomes larger as M3 is inserted deeper into the main transformer, the external characteristics of the power source changes, as shown in Fig.2, and the output current (arc current) becomes smaller. Primary coil Secondary coil Leakage flux: small Primary input M 1 M 2 M 3 Electrode Output voltage Base metal Leakage flux: large Movable core Output current Fig. 1 Movable iron core type Fig.2 External characteristic of power source 4

5 Ex. Duty cycle in arc welding power source There is an A.C. welding power source of rated secondary current: 300A and rated duty cycle: 40%. Check the possibility of burning (thermal damage of welding transformer) when this power source is used for welding with current of 200A and duty cycle of 60%. JWES 0808 Duty cycle Generally, welding power sources are used intermittently. The components of the welding power source are heated up during arc-on time. Duty cycle is expressed as a percentage of the maximum time that the power source can be operated at its rated output successively without overheating as shown in the figure. In Japan, a ten minute period is used. Load on off on off on off period Time t Rated duty cycle in welding power source The following equation is applied to estimate the duty cycle at other than rated output, T R I R2 = T I 2 (1) where, T R : rated duty cycle (%), T: duty cycle (%) under the output required, I R : rated current, I: required output current. 5

6 JICA_OHJI Ex. Primary current of arc welding power source The power source of three-phase is generally used as the input power of a D.C. welding machine. There is a three-phase D.C. welding machine of the rated input voltage: 200V, rated output: 300A and the rated primary input: 21 kva. Calculate the rated primary current of this machine. JWES 0808 Three-phase circuits Most electric power systems are three-phase where they involve three voltage sources having the same amplitude and frequency but displaced in time from each other by 120. v V t (1) a= m sinω v = V sin( t 120 ) (2) b m ω (3) vc= V sin( ωt 240 ) = Vm sin( ϖt+ 120 ) m Fig. 1 Three-phase power and its expression by complex vectors For a three-phase power source, the following expressions of power are given: 3 V :apparent power (VA) l I l P = 3 V l I :active or real power (W) l cosθ Q = 3 V l I :reactive power (var) l sinθ where V l :line voltage (effective value), I l :line current (effective value). [J.J.Cathey and S.A.Nasar, Basic Electrical Engineering, McGraw-Hill,1997 ] 6

7 Ex. Power Sources for Welding _No.3 Explain the principle of the inverter controlled welding machine, and give two items of the advantage of the inverter controlled machine compared with the conventional SCR-type welding machine. DC power source of SCR-type SCR Reactor A.C. D....C/smoothed Fig.1 Single phase DC power source with SCR Figure 1 shows a single phase DC power source of SCR type. The silicon-controlled rectifier (SCR), so called a thyristor, is a sort of diode with a trigger, called a gate. The SCR is non-conducting until a proper signal is applied to the gate and the conduction stops only if the anode voltage is less than the cathode voltage. The output current is controlled by adjusting the gate-on timing of SCR and is smoothened by using the reactor to reduce the fluctuation of welding current. 7

8 Inverter controlled welding machine The inverter controlled welding machine is based on the fact that transformer size is significantly reduced if its operating frequency is increased. As shown in the figure, the primary AC input is rectified first and the DC voltage is converted to a high frequency AC voltage by using a inverter. And the high frequency AC voltage is adjusted to suit the welding by using a high frequency transformer. Then the transformer output is rectified again to supply the DC power for arc welding. The welding machine of inverter type, where the frequency of operation is between 5000 and 50000Hz, has the following advantage, compared with the conventional machine of SCR type, (1) Mechanical: compact and light. (2) Electrical: high response and accurate control. Rectifier Inverter Transformer Mains power line Torch Work-piece A.C. D.C. High Frequency A.C. D.C.(smoothed smoothed) OK 0806 Inverter controlled power source --- high time-response and compact size Current A () ms 0 Time (10ms/div) 10ms 10 ()Current A Time (10ms/div) 10ms 10 kg ower source of CR-type 2 kg Inverter controlled power source Courtesy of DAIHEN Corporation. 8

9 Ex. Wire feeding system in MAG welding In MAG/MIG welding, three methods of wire feeding system have been used. Explain the methods and their features in brief. Wire feeding system in MAG welding 1 The wire feeding systems for MAG welding machines are classified into three types, the push type, pull type and push-pull type. The push type in figure (a) is equipped with a feeding motor near the wire reel. This type is suitable for steel wires which are hard to buckle and has excellent operability with a light welding torch. The pull type in (b) is quipped with a feeding motor near the torch. This type is suitable for thin wires and soft wires, but the conduit cable can not be very long. Wire reel Feed roller Flexible conduit Torch Feeding motor (a) Push type Flexible conduit Feed roller Torch Wire reel Motor (b) Pull type OK

10 Wire feeding system in MAG welding 2 The push-pull type in (c) is a combined system of the push type and the pull one, which makes it possible to feed a soft wire stably and smoothly. Wire reel Feed roller Flexible conduit Torch Motor Feed roller Motor (c) Push-pull type Push type Pull type Torch Torch Push type Robot Wire reel (a) Push type (c) Push-pull type Courtesy of DAIHEN Corporation. Ex. Explain briefly the following technical terms. 1 Duty cycle 2 Voltage reducing device 10

11 Voltage reducing device When an AC arc welding machine is no load, the voltage between the electrode and the base plate is high. The voltage is dangerous from the viewpoint of the electric shock. To avoid the electric shock, Safety and Health Administrative Regulations in Japan requires a voltage reducing device for the welding in narrow spaces or in locations more than 2m from the ground. The device reduces the voltage between the electrode and the base plate to less than 25 volts when the welding arc is off, as shown in the figures bellow. JIS C 9311 requires : time to reduce no load voltage : 1.0±0.3 sec. controller transformer for voltage reducing S 2 current transformer 200V S 1 primary secondary 80V welding transformer output voltage electrode contact no load voltage 25V arc voltage (30V~40V) 25V reduced voltage starting time delay time reduced voltage (with in 0.06s) ( 1.0s ) time OK 0912 Arc welding robot The industrial robot plays an important role in the flexible manufacturing system. As shown in the figure, the welding robot is classified into two types, articulated (jointed) and rectangular coordinates types. Up & Down Bend Back & Forth Bend Twist Up and Down Back & Forth Right & Left Twist Swing (a) Articulated type (b) Rectangular coordinates type Recently, the articulated type of robot is preferred to the rectangular coordinates type because the articulated robot is capable of flexible and quick motion, and the work envelope is wide for the installation space. 11

12 Ex. Arc sensor (through-arc sensor) In the robotic welding, the arc sensor technique is applied to the seam tracking in process. Explain the principle of the arc sensor in robotic welding. Arc sensor Arc sensor techniques utilize the change of the arc current or voltage during weaving of the welding torch in the groove. If the arc is oscillated laterally across the seam, the stand off (torch-head to work-piece distance) will change. In MAG arc welding with a constant voltage power source, the arc current will change corresponding to the torch position as shown in the figure. Accordingly, in the arc sensor system, the torch position can be estimated from the pattern of current or voltage fluctuation during the oscillation. The arc sensor seam tracking system is commonly available on welding robots and applied to the fillet and heavy section butt welding. arc current weaving position Seam tracking by arc-sensor 12

13 Wire touching sensor Wire-touching sensor utilize the change in voltage or current when the welding wire tip touches the base metal to detect the position of the groove or the base metal. In the typical sensing process as shown in the figure, the touching procedure is repeated several times before welding to detect a particular position such as the groove and the edge of the work-piece. Welding wire Welding torch Power source for detecting circuit Detecting circuit Principles of wire-touching sensor (Typical sensing process for fillet welding) 13