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What is Electric Arc and what are its electrical characteristics?
Electric Arc
An electric arc is an electric discharge of gases which is produced by passing an electric current between two points called electrodes. The electric arc was first detected by British chemist Humphry Davy in 1808. The electric arc is the visible plasma between two electrodes due to the ionization of gases by the electric current. The electric arc is commonly used in welding, lighting and plasma cutting.
Electrical Properties of Electric Arc
The electrical properties or characteristics of an electric arc are given below.
Stability of Electric Arc
The electric arc has negative resistance characteristic, i.e. with the increase in the arc current, the resistance of the arc decreases. By Ohm’s law, it is can be seen that
$$\mathrm{\mathit{R}\:=\:\frac{\mathit{V}}{\mathit{I}}}$$
The IV characteristic of an electric arc is shown in the figure.
It is very difficult in manual arc welding for the welder to hold a constant arc length. If the arc length decreases, then it will result in decrease in the arc resistance and hence increase in the arc current. This will further reduce the arc resistance. In this way, the cause and effect will go on helping each other till the arc will be out of control.
Similarly, the increase in the arc length will increase in the arc resistance which will reduce the arc current and diameter of the arc resulting in further increase in the resistance of the arc. Here, again a condition approaching extinction of the arc will be reached. Therefore, it can be seen that it is very difficult task for the welder to maintain a stable arc.
In case of AC arc welding, the arc stability is achieved by putting a chock in the supply line which gives the dropping characteristics.
Arc Blow
An electric arc can be considered as a very flexible conductor that can be deflected by various factors. The space around the arc is always affected by magnetic fields which tend to deflect the arc. This phenomenon of deflection of the arc is called the arc blow.
Arc blow is more frequent in case of direct current (DC) arc welding. The arc blow causes a great interruption, especially when large currents are used since the intensity of the magnetic field is directly proportional to the square of the current. The magnetic fields deflect the arc when they are not uniformly distributed with respect to the arc. The distribution of the magnetic fields depends upon the air-gap, position and shape work-piece, etc. The effect of the arc blow is the improper penetration of the heat where it is required. This results in the poor weld.
In order to avoid the arc blow and get a good quality of weld, an AC arc welding machine is used. As in case of AC arc welding machine, the flow of electric current changes its direction rapidly and hence the magnetic field also changes the direction rapidly and this change in direction of the magnetic field will cancel the effect of arc blow and stabilize the electric arc.
With DC arc welding machine, the arc blow can be avoided by welding away from the ground or by changing the position of the work-piece.
Polarity
When the electrons leave the cathode, they strike the anode after attaining a certain velocity where their momentum is destroyed. This is the way of production of more heat at the anode than at the cathode. As we have to produce more heat at the work-piece which is to be welded, therefore it is always connected to the positive terminal of the DC source and the welding electrode to the negative terminal of the source.
Voltage
A sufficient voltage is required for striking an arc and then for maintaining it. It is because some voltage is needed for ionizing the air-gap.
Potential Gradient
An electric arc includes a relatively low potential gradient between the electrodes and a high current density.
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