Basic Electrical Engineering for Welding [ ] --- Welding power source ---

Transcription

1 Basic Electrical Engineering for Welding [ ] --- Welding power source --- Takayoshi OHJI Professor Emeritus, Osaka University Dr. of Engineering VIRTUAL WELD CO.,LTD t-ohji@alvec.co.jp OK 0910 Output characteristics of welding power source 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 Output characteristics of welding power source The output characteristic of CV power source makes it possible t o keep the arc length constant under the condition that a thin wire is fed at a constant-speed. A slight change in arc length /voltage/ causes a relatively large change in welding current. The change in the current causes a large change of wire melting rate and makes it possible to stabilize the arc length. This effect is called self regulation. Output characteristics of power source Arc length Output characteristics of power source Arc length Voltage Voltage Welding current Constant current Welding current b Constant voltage 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.

3 Self regulation of arc length in constant voltage type power source const. Output characteristics of power source Arc characteristics Voltage Arc length Current 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.

4 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. Fig. 1 Movable iron core type Fig.2 External characteristic of power source DC power sources transformer-rectifier type Using an arrangement of diodes, it is possible to convert AC to DC. The following figure shows typical DC welding power sources, i.e., transformer-rectifier systems. As shown in the figure, the three phase power source provides a smoother output, better for arc welding than the single phase one. Half-wave single phase (a) Single phase Full-wave single phase Half-wave 3-phase (b) 3-phase Full-wave 3-phase 1) W. Galvery and F. Marlow, Welding Essentials, Industrial Press, New York, (2000)

5 DC power source of SCR-type SCR Reactor 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. 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.

6 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 thetransformer 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. Inverter controlled power source --- high time -response and compact size --- A ACurrent Current Current Current Time (10ms/div) Time (10ms/div) Courtesy of DAIHEN Corporation.