For striking a welding arc it is essential to have the desired current in the welding circuit. Depending upon the type of current required (ac or dc) it may be obtained from the following sources.
- A welding machine or frequency changer (for ac welding machine).
- A dc generator of a welding rectifier (for dc welding).
- The no load or open circuit voltage (OCV) should be high enough to strike an arc, but not so high as to endanger the welding operator, for example, for a 2000A transformer the OCV should not be over 90 volts through for lower currents of say upto 600A OCV may be upto 100 volts.
- With direct current, the OCV must be at least 30 to 35 volts while with alternating current it should not be lower than 50 to 55 volts. An open welding arc can be sustained at 18 to 40 volts.
- The minimum voltage for arc ignition is 25 volts which must be attained within 0.05 second for the maintenance of a stable arc.
- The power of the welding source should be sufficient to provide the required welding current.
- It should be possible to tap the desired current within the available limits either in infinite steps 9stepless regulation) or in a large number of discreet steps (stepped regulation).
- The voltage of the power source should adjust itself rapidly with the change in arc length.
- The short-circuit current must be within limits of safety for power source windings.
- It should have output characteristics which match the arc characteristics.
- It should incorporate a low voltage supply for the operation of auxiliary units.
- It should have a desired duty cycle and provide the required volt-ampere characteristics.
- It should be light in weight, small in size, low in cost, convenient in service and easy to maintain.
Volt-Ampere Characteristics of a Welding Power Source
All welding power sources have two kinds of operating characteristics viz., static characteristic and dynamic characteristic. The static output characteristic can be easily established by measuring the steady-state output voltage and current by conventional method of loading by variable resistors. Thus, a curve showing the output current versus output voltage for a given power source constitutes its static characteristic.
The dynamic characteristic of an arc welding power source is determined by recording the transient variations occurring over a short intervals in the welding current and the arc voltage. Thus, it describes instantaneous variations occurring over a short interval of time say a millisecond. Arc stability is determined by the combined interaction of the static and dynamic colt ampere (V-I) characteristics of the welding machine power source.
The intrinsic transient nature of a welding arc is the main reason for great importance of the dynamic characteristics of an arc welding power source. Most welding arcs have continuously changing conditions which are mainly associated with striking of the arc, metal transfer from the electrode to the weld pool, and arc extinction and reignition during each half cycle of ac welding.
The transient nature of the welding arc is also due to variation in arc length, arc temperature and electron emission characteristic of the cathode.
The rate of change of voltage and current in arc welding machine is so fast that the static volt-ampere characteristic of a power source can hardly be of any significance in predicting the dynamic characteristic of a welding arc. However, it is only the static volt-ampere characteristics of a welding power source which are supplied by the manufacturer. Through they cannot give the nature of behaviour of the power source regarding its dynamic response but they are of considerable importance in determining the general overall response in controlling the process parameters.