First of all, electrode touches to the metal to complete the circuit. But after getting spark we retract back the electrode and maintain the distance of 2-3 mm. Now my question is if for ionisation of air voltage required is 3000 V/mm and the voltage that we are supposed to apply is only 300 V maximum. Then how the arc is still sustained and electron transfer from electrode to metal workpieces.
I sure hope you are not arc welding in air! I don't understand your second question.
An arc is generated between two conductors of electricity, cathode and anode (considering direct current, DC), when they are touched to establish the flow of current and then separated by a small distance. When it is separated the air between the electrodes is ionised by the potential difference and that reduces electrical resistance of the air column. An arc is a sustained electric discharge through this ionised gas column called plasma between the two electrodes. In order to produce the arc, the potential difference between the two electrodes (voltage) should be sufficient to allow them to move across the air gap. The larger air gap requires higher potential differences. If the air gap becomes too large for the voltage, the arc may be extinguished.
Book Manufacturing Technology, Vol. 1 Foundry, Forming and Welding
Arc can occur in vacuum but for that purpose the electrode need to be heated to a high temperature and then the electrons will flow and it will not be an arc.
Allow me to complete what has been said above by all researchers.
An electric arc expands as well in vacuum and in a gas; for vacuum arcs, the conductive medium comes from the electrodes, especially the cathode. After the initiation of the discharge, the emitted metal vapor fills the space between the electrodes. In fact, there is no longer an arc in a vacuum, but in the metal vapor.
Once the discharge starts, we observe tiny, mobile spots on the cathode surface. These small bright spots are called "cathode spots" and play a key role in switchgear apparatus (contactors, circuit breakers) , and for some sources of particles (atoms, ions, electrons).
However, there are also arcs produced from heated electrodes, where the spots are not necessary for the vaporization, nor to ensure electron emission. In this case, we obtain a stationary discharge without spots.
For more details about vacuum arcs, you can take a look to the following reference:
André Anders, "Cathodic Arcs, From Fractal Spots to Energetic Condensation" Springer, NY, USA, 2008.
Hi Mr. Akbi, i also found some mobile spot in electrode surface like sparks. can you explain more why it appears? what is the reason?
Hi Miss Ully,
Generally, the arc discharges do not rise on the entire surface of the electrode but they contract to a circular footpoint at the cathode, namely arc cathode spot. This contraction is due to processes occuring in the gas plasma and in the plasma charge sheath between cathode surface and plasma. There are many kind of spots. Some of them concern hot cathodes in high pressure discharges. The others occur in cold cathodes, and hot cathodes too, in vacuum. The physics of such phenomena is not yet clear and not well understood. However, you can take a look to the following references:
1. B. Jüttner, "Properties of Arc Cathode Spots", J. PHYS IV FRANCE 7 (1 997) , pp. C4-31 - C4-45.
2. B. Jüttner, "Cathode spots of electric arcs", Journal of Physics D: Applied Physics, Volume 34, Number 17 (2001),
https://www.researchgate.net/publication/231061284_Cathode_spots_of_electric_arcs
Article Cathode spots of electric arcs
Prof. Akbi, can we have sustained arc/plasma in continuous high pressure inert gas flow between the electrodes?
Hi Dr. Murugan,
If i have well understood your question, the answer is yes. Moreover, the position of an electric arc is stable: once it has found the shortest path, it remains there. Indeed, the energy of the arc is in this path, the weakest than along any other path since in the arc, unlike the surrounding environment, the gas is already ionized.
For more details, you can take a look to the following link:
https://en.wikipedia.org/wiki/Electric_arc
Very good information. I need some reading materials , downloadable freely,to facilitate my students to understand the basics of welding.
Kindly suggest
Great question...one can make short arcs even with 12 V car batteries when producing a short.
I would guess that when touching the electrodes first we make a short circuit, where a very high current (100A in a car battery...depending on the internal impedance of the battery...) flows through a circuit. The circuit itself is a loop, generating a magnetic field and hence flux inside. As soon as you pull the electrodes apart, current will drop to zero...dI/dt is very high, the magnetic flux breaks down FAST and as a result you induce, a very high voltage (kVs) that starts the discharge.
Then the low voltage high current source quickly drives the discharge into the arc mode and welding begins...
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