Before the switch is closed (tap opened) to allow the current flow, the "head of electricity", or available electrical pressure is nominated in open circuit volts. (You will note this on the machine nameplate for various welding current ranges). Our "240 volt" and "440 volt" power supplies indicate the open circuit voltage or available electrical pressure of the mains.

Once the circuit is closed then electric current will flow. This rate of flow is measured in amperes or, as it is more commonly termed, amps. The size of the current is determined both by the voltage (pressure) and the size of the conductor or its resistance to flow, which is measured in ohms. Some materials are good conductors and offer little resistance to current flow (such as copper and aluminium). Others are mediocre conductors of electricity and offer greater resistance to current flow (steel). Still others are poor conductors and are termed resistors. Very bad or "non conductors" are called insulators (bakelite, fiberglass etc.).

The resistance to flow of electric current shows its presence in the form of heat. We use this heat generated by resistance principle for our electric heating elements in stoves and radiators etc., while the remainder of the high conductivity copper circuit operates within safe heat limits. If the current flow increases to a stage where wiring could overheat to a dangerous level, the fuse, a purposely smaller section of wire conductor designed to melt when it receives an overload current, will melt or fuse and break the circuit.

From our point of view the voltage is only really important in as much as sufficient "pressure" is required to make the current flow through a circuit. In any circuit of a given resistance, it is the current which primarily determines the amount of heat generated. Thus it is very important that, while we can use small cables on the high voltage low amperage (primary) side of our AC arc welder, we must have lower resistance heavy conductors for the high amperage low voltage (secondary) welding circuit or the leads will overheat. Similarly a secondary lead which is too long or too small will cause such a drop in voltage that it can no longer maintain a stable current across the arc between the electrode and the work.

What are AC and DC?

"DC" stands for direct current in which the current flows in the one direction constantly throughout the circuit. One side of the power source is nominated as the positive (+) pole and the other as the negative (‑) pole. An automotive battery or dry cell gives DC.

In "AC" or alternating current the current periodically reverses its direction along the conductor, i.e. one fraction of a second the right‑hand terminal is "negative", the next fraction of a second it is "positive". In 50 Hertz AC current, such as is commonly used in Australia, this change from + to ‑ to + occurs as a cyclic variation 50 times a second, the current thus changing direction of flow 100 times a second. See Fig. 5.

What is an arc?

Dry air is a good insulator and it requires a very high voltage (such as used with a spark plug) to jump an air gap. For both practical and safety reasons, we must use a relatively low voltage in our welding circuit. It we touch an electrode on to the work and then withdraw it slightly, the initial heat of the high current flow on touching will vaporise some of the core wire and coating materials so that a gaseous high resistance conductor is established between the electrode tip

and the work. This short length of high resistance high current flow generates extreme heat sufficient to melt both the base material beneath and the electrode above. This is the modern electric arc used in welding.

As will be seen in Fig. 5 the current flow actually momentarily drops to zero 100 times a second, requiring a highly conductive arc atmosphere for the re‑establishment of the arc on AC.

Some electrodes, because of their type of coating, will not generate sufficient current carrying ions to maintain a stable arc at say 45 volts and require a higher pressure circuit of say80volts. Others fail to remain stable on AC at a safe voltage (80 open circuit volts) and require DC current. Thus, both AC and DC welding machines are used for the manual metal‑arc welding process (MMAW).

MMAW MACHINES AND ELECTRODES

Arc Welding Machines

The arc welder is an electrical machine capable of supplying current of sufficient magnitude to provide satisfactory welding heat at a safe voltage capable of sustaining the arc.

AC manual metal‑arc welders are invariably based on a transformer which is a static electrical machine (which will only work on AC) which can convert AC power from high voltage low amperage (as in the mains) to low voltage high amperage power (as used in welding) or vice versa. In a mechanical analogy it plays the role of a gear box that converts high RPM engine power into usable speeds. Some AC welders use two voltage ranges to cover a wider range of work, just as two‑speed gear boxes are sometimes employed on electrical drills.

DC arc welders when used in industry are either DC rotary generators driven by an AC electric motor or alternatively use an AC transformer with a rectifier attachment which is an electrical "one‑way flow valve" permitting the AC welding current to only flow in one direction, thus achieving a DC effect. In the field, rotary DC generators are driven by suitable petrol or diesel engines.

In addition, incorporated in the transformer design or as a separate device attached to the arc welder is a means of current control so that the correct amount of current can be employed on the job. In continuing the mechanical analogy, this can be likened to the throttle on the motor car.

General industry employs an AC transformer type machine providing AC welding current for general purpose arc welding.

Smaller versions of the same type of welding machine are available for use in the restricted capacity rural areas.

Where AC power is not available from a central supply authority, such as in remote rural areas or on cross‑country site welding operations, DC is usually the welding power employed. The welding generator may be driven by a suitable petrol or diesel prime mover or from the power take off pulley or shaft of a farm tractor.

Arc Welding Electrodes

The manual metal‑arc electrode is a current carrying conductor which progressively melts away due to the heat of an electric arc held between it and the work, the molten metal solidifying to form part of the weld with the base metal. The original electrodes were bare wire and the resultant weld metal was of poor quality with the arc difficult to maintain even on DC.