5.1 Arc Welding Power Source Classifications
Arc welding power sources, also called arc welding machines, are either ac, dc, or combination ac/dc machines. They are constructed to produce either a constant current measured in amperes, or a constant voltage (potential) measured in volts.
Alternating current (ac) power sources (Welding machines that produce alternating current) include the following types:
- Transformer.
- Motor- or engine-driven generator.
- Inverter.
- Transformer with a dc rectifier.
- Motor- or engine-driven generator with a dc rectifier.
- Inverter.
- Transformer with a dc rectifier.
- Motor- or engine-driven generator with a dc rectifier.
- Inverter.
An inverter (A special transformer-type welding power source that changes incoming ac current to dc, then back to very-high-frequency ac current) welding power source is a special transformer-type machine that changes incoming ac current to dc, then back to very-high-frequency ac current. This high-frequency alternating current then passes through a very small and efficient transformer. After passing through the small efficient transformer, the ac current is again changed back to dc current. Inverters are much smaller, lighter in weight, and more efficient than regular transformer-type machines. They can perform like either a constant current or a constant voltage machine.
To properly describe an arc welding power source, the following minimum information should be given:
- Type of power source.
- Whether the power source produces constant current or constant voltage.
- Whether the power source produces ac, dc, or both ac and dc.
An example of such a description might be: “The welding machine is a transformer, constant current, ac machine.” This information can be found on the face or front of many power sources or on the name plate on the rear of the machine. With experience, a welder can determine this by looking at the machine and knowing what welding process the machine will be used for. Other information that helps classify welding machines will be covered later in the chapter. This information consists of:
- Duty cycle.
- Welding current rating.
- Power requirement.
To meet the variety of demands on different jobs, welding machines are designed to produce ac or dc current. Each welding machine may have a different design and output capacity. The selection of a particular welding machine will also depend on such additional factors such as cost, portability, and the personal preferences of the user.
Arc welding machines may also be classified according to a graph of the electrical output of the particular machine. Such a visual representation is called the static volt-ampere curve (A graph of the electrical output of an arc welding machine.) A typical volt-ampere curve will be studied in detail under Heading 5.2 and as shown in Figure 5-2. The volt-ampere curve plots the electrical current output, measured in amperes, against the voltage output. Remember that current relates to the welding heat and that voltage relates to the length of the arc.
The general slope of the volt-ampere curve is called the output slope (The general slope of the volt-ampere curve. of the power source) Some output slopes are very flat, while others are very steep. Nearly horizontal volt-ampere curves, or output slopes, are produced by constant voltage machines. An arc welding machine that produces a steep output slope is called a constant current machine (An arc welding machine that produces constant current)