Well.... I thought I answered this yesterday, but MH seems to have lost more than one of my answers. Maybe the tubes were clogged.

In any event, Arc welding describes any number of methods of welding that pass an electrical current between an electrode and the base metal to be welded, creating heat and melting the metal. It what is classically referred to as arc welding, that electrode is consumable and melts to fill in the space between the pieces being welded. The base metal and the filler melt together and form a single piece.

Spot welding also (typically) uses electricity, however no filler metal or rod is used. Two overlapping pieces are simply melted together by passing a high current through a "spot". It is typically only useful for sheet metal applications.

Now to three phase power. There are a lot of places to research this but is summary, three phase power allows more power to be carried through a smaller conductor than single phase. This is because there are three conductors, each 1/3 wave out of phase, carrying AC current. When combined they form a continuous power delivery at lower rates of current in each conductor. The bottom line is that this saves copper. In addition three phase sets up a rotating magnetic field that is especially suited to motor applications.

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