Why Is SMA Welding Current Referred to as Constant Current?

Arc welding is a fusion welding process that joins metals using heat produced from an electric arc. Its applications cut across various welding methods, including Shielded Metal Arc (SMA). Of the two types of currents used in arc welding, why is SMA welding current referred to as constant current?

Although not strictly a misnomer, SMA welding current is referred to as constant current because it maintains the current within a minimal range compared to the voltage.

This article will review how current affects welding and why SMA welding current is referred to as constant current. I will also highlight the welding processes that use constant current. In conclusion, I will compare a constant voltage power source and a constant current source

How Does Current Affect Welding?

Arc welding uses an electrode to conduct the fusing process. The electrode passes current through a workpiece, and it could be consumable or non-consumable, depending on the welding process.

Gas Tungsten Arc Welding (GTAW) uses non-consumable electrodes, while SMA welding (SMAW) and Gas Metal Arc Welding (GMAW) use consumable electrodes.

Current affects the consumption or melt-off rate of the electrode, whether a stick or wire electrode. This melt-off rate is measured in kilograms per hour(kg/hr) or pounds per hour (lbs/hr). A lower current equals a lower electrode melt rate, and the higher the current, the faster the electrode melts off.

In contrast, the length of the welding arc and the resulting volume and width of the arc cone depend on the voltage.

Why Is SMA Welding Current Referred to as Constant Current?

In arc welding, an electric arc produces an intense heat of about 6500°F. The total welding current is constant, and this type of power is referred to as Dropping Arc Voltage. SMAW current is referred to as constant current, but the term constant is relative.

A welding arc is dynamic, and the power source constantly monitors it to make millisecond changes in order to keep the arc stable. Regardless of pretty significant changes in voltage, a constant current power source will maintain the current at a relatively constant level. 

Hence, the current used in SMA welding and other welding methods that require this current are said to use constant current.

What Welding Processes Use Constant Current?

Welding power supplies are devices that provide or modulate the electric current to carry out arc welding. They enable the welder to exercise control over the type of current—direct current (DC) or alternating current (AC)—and the voltage and amperage.

Most welding power supplies don’t generate power. Instead, they act as controllable transformers that permit the operator to adjust electrical properties as required. The most common power supplies in use are grouped into:

  1. Alternator or generator

They convert mechanical energy into electrical energy: modern designs usually have an internal combustion engine, while an electric motor may drive older models. In this setting, the utility power is first converted into mechanical energy before transforming into electrical energy.

This way, it can mimic the step-down system similar to what transformers produce. The generator can produce either a direct current or a higher-frequency alternating current, and it needs no rectifiers to make direct current from alternating current.

  1. Transformer

This style of welding power converts the moderate voltage and current electricity from the utility mains into a low voltage and high current supply. The transformer needs a rectifier to convert the alternating current into the direct current on more expensive machines.

Transformer machines are the least costly, and a pure design is often massive because it operates at 50 or 60 Hz.

  1. Inverter

Inverters first rectify the utility alternating current power to direct current, then they switch/invert the direct current power into a stepdown transformer. The transformer produces the welding current or voltage desired.

Other types of power sources include laser welders and spot welders.

A constant current power source is an arc welding source that maintains current at a relatively stable level, despite changes in voltage due to changes in arc length. It is also called a drooper because a steep volt-ampere curve illustrates the relationship between the output current and output voltage.

The type of welding process you use and its level of automation determines the kind of welding output that best serves it. Manual processes prefer to use a constant current type of power source, and they include:

  • SMAW
  • GTAW
  • Plasma Arc Welding (PAW)
  • Some Submerged Arc Welding (SAW)

Manual welding processes like these listed above put the control of all welding variables in your hands. You hold the TIG torch or electrode holder in your hand and control the work angle, travel angle, arch length, the rate at which the electrode is fed into the joint, and the travel speed.

Is a Constant Voltage Power Source Better than a Constant Current Source?

Constant voltage power sources/supplies keep the voltage constant regardless of the load, and they are used in automated welding processes. Examples of these automated or semi-automated welding processes are:

In these welding processes, the wire feed speed (WFS) or the rate at which the electrode is fed into the joint is controlled automatically using a constant speed wire feeder. However, you still hold the welding gun in your hand and manage the travel speed, travel angle, contact tip to work distance, and work angle.

A constant voltage power source system makes the arcs stable because of the constant position of the torch. Large percentage changes in voltage will result in small percentage current changes.

SAW prefers CV or CC output, and the determining factors for the type of output chosen are the size of the weld puddle, travel speed, and electrode diameter. 

Although these welding processes are semi-automated, it is also common to automate them completely. In a full automation mode, a machine controls all the variables and holds them at a constant rate, distance, or angle. Thus, the arc conditions experience minor change.

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