A plot of output-voltage values versus output-current values usually used to describe the static characteristic of a welding power source.

Static volt-ampere characteristics are generally published by the power supply manufacturer. There is no universally recognized method by which dynamic characteristics are specified. The user should obtain assurance from the manufacturer that both the static and dynamic characteristics of the power supply are acceptable for the intended application.

Constant Current

Typical volt-ampere (V-A) output curves for a conventional constant-current power source are shown in Figure V- 1. It is sometimes called a drooper because of the substantial downward (negative) slope of the curves. The power source might have open circuit voltage adjustment in addition to output current control. A change in either control will change the slope of the volt-ampere curve.

The effect of the slope of the V-A curve on power output is shown in Figure V- 1.With curve A, which has an 80 V open circuit, a steady increase in arc voltage from 20 to 25 V (25%) would result in a decrease in current from 123 to 115 A (6.5%). The change in current is relatively small. Therefore, with a consumable electrode welding process, electrode melting rate would remain fairly constant with a slight change in arc length.

Setting the power source for 50 V open circuit and more shallow slope intercepting the same 20 V, 123 A position will give volt-ampere curve B. In this case, the same increase in arc voltage from 20 to 25 V would decrease the current from 123 to 100 A (19%), a significantly greater change. In shielded metal arc welding, the flatter V-A curve would give a skilled welder the opportunity to vary the current substantially by changing the arc length. This could be useful for out-of-position welding because it would enable the welder to control the electrode melting rate and molten pool size. Generally, however, less skilled welders would prefer the current to stay constant if the arc length should change.

Constant Voltage

A typical volt-ampere curve for a constant-voltage power source is shown in Figure V-2. This power source does not have true constant-voltage output. It has a slightly downward (negative) slope because internal electrical impedance in the welding circuit causes a minor voltage drop in the output. Changing that impedance will alter the slope of the volt-ampere curve.

Starting at point B in Figure V-2, the diagram shows that an increase or decrease in voltage to A or C (5 V or 25%) produces a large change in amperage (100 A or 50%).This V-A characteristic is suitable for constant-feed electrode processes, such as gas metal arc, submerged arc, and flux cored arc welding, in order to maintain a constant arc length. A slight change in arc length (voltage) will cause a fairly large change in welding current. This will automatically increase or decrease the electrode melting rate to regain the desired arc length (voltage). This effect has been called self regulation. Adjustments are sometimes provided with constant-voltage power sources to change or modify the slope or shape of the V-A curve. If done with inductive devices, the dynamic characteristics will also change.

Combined Constant-Current and Constant-Voltage

Electronic controls can be designed to provide either a constant-voltage or constant-current output from a single power source so that it can be used for a variety of welding and cutting purposes.

Electronically controlled outputs can also provide output curves that are a combination of constant current and constant voltage, as shown in Figure V-3. The

top part of the curve is essentially constant current; below a certain trigger voltage, however, the curve switches to constant voltage. This type of curve is beneficial for shielded metal arc welding (SMAW) to assist starting and to avoid electrode stubbing (sticking in the puddle) if a welder uses too short an arc length.

 

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