A welding rectifier is a type of power source specifically designed for welding applications. Its primary function is to convert alternating current (AC) from the power grid into direct current (DC) suitable for arc welding processes.

Understanding Voltage-Current (V-I) Characteristics

The Voltage-Current (V-I) characteristic of a power source describes the relationship between its output voltage and the current it delivers under varying load conditions. When plotting this relationship on a graph with voltage on the y-axis and current on the x-axis, we get the V-I curve. Different types of power sources have distinct V-I characteristics that make them suitable for specific welding processes.

V-I Characteristics for Welding Rectifiers

For many common welding processes, particularly Shielded Metal Arc Welding (SMAW) or stick welding, maintaining a stable arc is crucial. The arc voltage changes with the arc length. If the power source output voltage is relatively constant (flat characteristic), small changes in arc length can lead to large fluctuations in welding current, making the arc unstable.

A welding rectifier designed for these processes requires a characteristic that helps stabilize the arc against variations in arc length. This is where the 'drooping' characteristic becomes essential.

The Drooping Characteristic Explained

A power source with a drooping characteristic exhibits a decrease in output voltage as the output current increases. This type of characteristic is also often referred to as a constant current (CC) characteristic, although it is not perfectly constant.

• Imagine the V-I curve slopes downwards from left to right.
• At low current, the voltage is high.
• As the current drawn by the arc increases, the output voltage of the rectifier decreases significantly.
• This relationship helps in arc stability. If the arc length accidentally increases, the arc voltage required also increases. On a drooping curve, this higher voltage requirement leads to a significant drop in the welding current. This drop in current tends to shorten the arc length back to the desired value, thereby self-regulating the arc.
• Conversely, if the arc length decreases (e.g., electrode getting too close to the workpiece), the required arc voltage drops. On the drooping curve, this lower voltage allows the rectifier to supply a higher current. This increased current can help melt the electrode tip faster or push the arc away, thus increasing the arc length back to the desired value.
• This inherent self-regulation makes the drooping characteristic highly desirable for manual welding processes where maintaining a consistent arc length can be challenging.

Comparison with Other Characteristics

• Rising Characteristic: Voltage increases with current. This would be unstable for welding, leading to runaway current increase if the arc resistance decreases slightly.
• Flat Characteristic: Voltage stays relatively constant regardless of current. This is typical of constant voltage (CV) power sources used for processes like Gas Metal Arc Welding (GMAW/MIG/MAG) where wire feed speed controls the current, and voltage is set to control arc length. It is not the standard for rectifiers used in SMAW.
• Straight line: While a drooping characteristic might look like a straight line sloping downwards, the term 'straight line' alone doesn't specify the slope and could be confused with other characteristics. The crucial aspect for welding rectifiers (for SMAW) is the negative slope, i.e., 'drooping'.

Therefore, a welding rectifier primarily used for processes like SMAW is designed to have a drooping voltage-current characteristic to ensure arc stability and control.

Revision Table: Welding Power Source Characteristics

Additional Information on Welding Power Sources

Modern welding rectifiers can be more sophisticated, often using inverter technology. While basic rectifiers might have a fixed drooping characteristic, inverter-based power sources can offer adjustable characteristics, allowing them to perform both constant current (CC) and constant voltage (CV) welding, or even pulse welding.

• Constant Current (CC) sources: Primarily used for welding processes sensitive to current changes, such as SMAW and GTAW. They provide a relatively stable current even if the arc voltage changes. The V-I curve is steep or "drooping".
• Constant Voltage (CV) sources: Primarily used for welding processes where wire feed speed determines the current, such as GMAW (MIG/MAG) and FCAW. They provide a relatively stable voltage even if the current drawn changes. The V-I curve is flat.

The question specifically asks about a rectifier used for welding, implying a basic power source often associated with SMAW. For this application, the drooping characteristic is standard.