ELECTRICAL NATURE OF RESISTANCE WELDING

1. ELECTRICAL NATURE OF RESISTANCE WELDING

by Sam Belkin, MSEE

Resistance welding is based upon the fundamental principle that when an electrical current is passed through a metal, friction caused by resistance to this electrical flow heats the metal. The majority of the heat is developed where the two pieces of metal to be welded are in contact. When the applied current is high enough the work pieces are melting and welding occurs.

It is well known that thermal power of the electric current is directly proportional to the square of the current multiplied by the resistance:

P=I² x R

where I = current in amperes,

R = resistance of the work in ohms,

P = thermal power in watts.

The energy or quantity of heat is the product of power and time: H = P x T. Combining these two equations gives:

H = I² x R x T

where T = time of current flow in seconds,

H = heat energy in watts per seconds (note that this is the heat generated by the electrical current.

If it is necessary to know the heat that melts the metal, the heat losses through conduction and radiation must be added).

Welding heat is proportional to the square of the welding current, the resistance of the work, and the total time of current flow. If the current is doubled, the heat generated is quadrupled and the time can be reduced accordingly, which is recommended. Resistance of the work pieces is related to the material being welded, the contact area, the pressure applied, and coating on the metal. Also, there must be a sufficient time to melt the coating. Resistance is also affected by other materials on the surface such as oil, paint, or dirt. From all of mentioned above it easy to see that there are four variables in the resistance welding process: current, time, electrode force, and electrode contact area.

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