A rectifier welding machine is used for welding processes or electrodes that require direct current rather than alternating current. It is a machine in which a-c input power is changed to d-c welding power. Alternating current is supplied to the rectifier from the power line through a transformer. The welding current control may be incorporated in the transformer, or may be a separate reactor between the transformer and the rectifier.

Rectifier welding machines may have either single-phase or three-phase input. While some machines may supply either a-c or d-c output, the most efficient are those designed for d-c welding only. The three-phase welding machine will show the lowest ripple percentage; that is, it will exhibit very smooth arc characteristics. Rectifier welding machines may be divided broadly into two general types, according to volt-amperage curves and application.

Constant-Current Welding Machines- A constant-current welding machine has characteristically drooping volt amperage curves, producing relatively constant current within a limited change in load voltage. This type of welder is conventionally used with shielded metal arc welding, gas tungsten arc welding, plasma arc welding or air carbon arc cutting. Constant current welding units, when adjusted for full-rated output, should maintain the current within 5% of its rated value, with a variation of 1% above or below normal arc voltage.

A constant current welding machine is best suited for most manual operations where variations in the arc length are most apt to occur because of the individual technique of the operator. It may also be used, however, in automatic and semi-automatic operations with a variable electrode feed mechanism, and in operations in which an effort is made to maintain a constant arc length by automatic changes in the wire feed speed.

Constant Potential- Constant-potential power supplies are designed specifically to power the various automatic welding processes which use a continuous wire electrode that is fed at a constant speed. In this type of welding machine, the arc voltage curve approaches a horizontal line and maintains its voltage within 5% of the rated full-load setting, over the range from open circuit to full load.

The methods of current control on rectifier type welders vary between different equipment manufacturers. Among commercial designs, the means of current control are movable coil transformers, movable core reactors, saturable reactors, magnetic linkage controls, and various solid-state devices.

The advantages of mechanical current controls are stability and the capacity of duplicating current settings. The principal advantages of electrical controls are convenience of operation and adaptability to automatic welding process control.

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