SOLUTION

A rectifier for welding has voltage/current has Drooping characteristics.

It consists of a transformer (single-or three-phase) and a rectifier unit as shown in Fig. Such a unit has no moving parts, hence it has a long life. The only moving part is the fan for cooling the transformer. But this fan is not the basic part of the electrical system. Fig. shows a single-phase full-wave rectifier circuit of the welder. Silicon diodes are used for converting a.c. into d.c. These diodes are hermetically sealed and are almost ageless because they maintain rectifying characteristics indefinitely.

Such transformer-rectifier welding is most adaptable for shied arc welding because it provides both d.c. and a.c. polarities. It is very efficient and quiet in operation.

These welders are particularly suitable for the welding of 

• Pipes in all positions
• Non-ferrous metals
• Low-alloy and corrosion-heat and creep-resisting steel
• Mild steel in thin gauges.

The open-circuit voltage of a constant-current, rectifier type power source ranges from about 50 to 100 V hence it provides the highest potential voltage when the welding current circuit is open and no current is flowing. At the start of the weld, with striking the arc, the sharp drop in voltage takes place from the OCV. A conducting column of ionized gases is formed along with the heating up of the electrode. As there is a drop in voltage, the simultaneous increase in welding current takes place. After a certain point, the voltage/current variation becomes linear following Ohm’s law. The static volt-ampere characteristics of a welding power source are shown in Fig.