The current and voltage requirements during arc welding process vary considerably. In arc welding, short circuits keep on taking place frequently. For example, whenever the operator touches the electrode to the work in order to strike an arc, short circuit is created.
Similarly when the molten globules of weld metal cross the arc, electric short circuits (around 20 nos. per sec.) are caused. Under short circuit condition, current drawn is excessively high because resistance falls to very low value.
Excessive heat caused by high current surges results in excessive spatter and sticking of the electrode. A good arc-welding generator is designed to limit these instantaneous surges of current.
When short-circuit is there, voltage falls almost to zero, but when it is cleared, considerable voltage is required to keep the are alive. Similarly if arc length is quickly changed, adequate recovery of voltage instantaneously is called for.
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Thus electric supply source of arc welding should provide sufficient current depending on size of electrode, a suitable voltage for striking and maintaining the arc, and of suitable characteristics to provide stability for the arc. The voltage across the arc may vary from 20 to 45 volts and current from 75 to 600 amperes.
Various sources of electric supply for arc welding process are:
(i) D.C. generator with variable voltage characteristics.
(ii) D.C. generator with constant voltage characteristics.
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(iii) D.C. power line in conjunction with resistors to reduce the voltage and current.
(iv) Special a.c. generator or transformer.
For most steel welding applications, a.c. source is normally used because of use of cheap a.c. transformers. D.C. supply is preferred for welding sheet metal, non-ferrous metals and stainless steel through it requires transformer-rectifier, (costly equipment).
The welding circuit consists of a welding machine, two leads, an electrode holder, an electrode and the work itself.
Fig. 9.31 (a) shows how the voltage-current relationship exists for arc welding process. It would be noted that for a given spacing and the electrode material, voltage reduces upto current of 50 A and starts increasing gradually and very slowly with further increase in current. This relationship determines the characteristics of the power source.
The electric power supply usually has a drooping characteristic, i.e. with increase in current, voltage drops. The drop may be either sharply drooping (suited for manual arc welding) or nearly flat (suited for semi-automatic arc welding) depending on type of power supply source.
The operating point is determined where two curves intersect. The stable operating point is obtained on the right hand side because any disturbance is automatically opposed and operating point returns to the original value.
Fig. 9.31 (b) shows how potential difference across are is distributed. Anode spot is the area on the anode surface where the electrons are absorbed. Anode space is the gaseous region (around 10 µm thick) adjacent to the anode surface where a sharp drop in voltage takes place. Arc column is the visible portion of the are consisting of plasma where the voltage drop is not sharp.
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Cathode space is the gaseous region adjacent to the cathode (around 10 µm thick). Sharp voltage drop in this region is necessary as the electrons have to be pulled up from this region. Relative voltage drops in various zones re dependent on the spacing, current, and electrode materials.