Electric current consists of a flow of electrons which goes from one pole (the negative) to the other (the positive).

The QUANTITY of electricity (i.e., electrons) flowing is called the "Intensity" and is measured in Amperes (Amps, in engineering slang).
For the electrons to flow, there must be a "Difference of Potential" between the two poles (the positive and negative), like there is a difference of pressure between the two ends of a pipe when water is flowing trough it: the water will always flow from the end at a higher pressure to the end at a lower pressure. The "Difference of Potential" between the two poles is called "Tension" ("Voltage", in engineering slang), and is measured in Volts.

When the tension between the two poles is constant, i.e., always the same, the electric current is called Direct, or DC. In this case, the flow direction is also constant. When the tension changes as time passes, going from zero to a maximum positive, then again to zero, then to a maximum negative and then again to zero, the electric current is called Alternating, or AC, and will change its direction from one pole to the other at the the same time the tension changes. The quantity of times this change happens in one second is called "Frequency", and is measured in Hertz. In the USA and Brazil the frequency is 60 Hertz. In other countries, as Germany and Italy, the frequency is 50 Hertz.

Multiplying one Volt per one Ampere you will have a Watt, which is the unit of electric power. One thousand Watts are called "kiloWatt". Dividing one Volt per one Ampere you will have an "Ohm", which is the unit of electric resistance.
Giovanni S. Crisi
Sao Paulo - Brazil

Good response on basic electricity theory. I think he was looking for more info on welding current characteristics
for constant current, slope control, constant voltage, AC waveforms, advantages/disadvantages for each. I don't
have a good reference on this myself. I think AWS has a good book on welding power sources though.

Marty

OK, let's go again.

When you connect the electrode to the negative pole of a DC source and the base metal to the positive one, you'll have a DC, straight polarity weld. The electrons will flow from the electrode to the base metal, which is the hottest part. When you connect the electrode to the positive pole of a DC source and the base metal to the negative one, you'll have a DC, inverse polarity weld. The electrons will flow from the base metal to the electrode, which will be the hottest part.

When you connect both electrode and base metal to an AC source, you'll have an AC weld, in which the polarity, and thus the electrons flow, will change exactly as many times as the frequency does. In the USA, it'll happen 60 times per second. In SMAW (stick electrode welding, in slang) the last number of the electrode classification will tell you whether that electrode is for straight DC, inverse DC or AC welding.

No matter what books or manufacturers catalogs might say, AC welding is not suitable for high responsibility welds. For these, only DC applies. Some examples of high responsibility welds are the following: pressure vessels, pressure piping, storage tanks designed to API or AWWA standards, penstocks and stoplogs in hydro plants, heavy weight lifting equipment, heavy structural steel and the like. AC welding is suitable for light welding such as motor car pieces, home appliances, light structural steel, fences etc.

If you want more information, access the following sites:
www.twi.co.uk
the sites of welding material manufacturers, such as Lincoln, ESAB and others, whose icones you'll find on the homepage of this very AWS website.
Regards
Giovanni S. Crisi