AC Welding Power Sources
AC Welding Power Sources
Requirements of a Welding Transformer
A welding transformer should satisfy the following requirements.
- It should have a drooping static volt-ampere characteristic.
- To avoid spatter, the surge of the welding current during a short circuit should be limited to the least possible above the normal arc current.
- The open circuit voltage should not normally exceed 80 volts and in no case 100 volts.
- The output current should be controllable continuously over the full available range.
- The open circuit voltage should be just sufficiently high for ready initiation of arc and not too high to impair the economics of welding.
Basic Types of Welding Transformer
- The high reactance type
- The external reactor type
- The integral reactor type
- The saturable reactor type
The High Reactance Type Welding Transformer
When a transformer supplies current, magnetic fluxes are produced around its windings. The lines of the resultant magnetic flux traverse the magnetic circuit and cut the primary and secondary windings. Some of magnetic flux due to primary current do not cut the secondary turns and vice-versa, since both have their paths in the air. In the other words, they are responsible for the reactance of the coils and the respective reactive voltage drops across them. As the current increases, the leakage fluxes also increase and so does the e.m.f. o self-induction. This is why an increase in the primary or secondary current results in increase in the reactive voltage drop across the respective windings.
External Reactor Type Welding Transformer
This type of welding transformer consists of a normal reactance, single phase, step down transformer and a separate reactor or choke.
The inductive reactances and resistances of the windings in such a welding transformer are low, so that its secondary voltage varies but a little with the welding current. The required drooping or negative volt-ampere characteristic is ensured by the reactor placed in the secondary of the welding circuit.
Integral Reactor Type Welding Transformer
The welding transformer of the integral reactor type has a primary winding I, a secondary winding II, and a reactor winding III. Apart from the main limbs, the core has additional limbs carrying the reactor winding. The current is adjusted by means of moving core C placed between the additional limbs.
Saturable Reactor Type Welding Transformer
In this welding transformer an isolated low voltage, low amperage dc circuit is employed to change the effective magnetic characteristics of the magnetic core. Thus, a large amount of ac is controlled by using a relatively small amount of dc, hence making it possible to adjust the output volt-ampere characteristics curve from minimum to maximum. For example, when there is no dc flowing in the reactor coil, it has its minimum impedance and thus maximum output of the welding transformer .
Parallel Operation of Welding Transformer
In welding operation sometimes there is a need for current exceeding the maximum welding current obtainable from one transformer. In such a case the desired welding current can be obtained by parallel operation of two or more welding transformers. The precaution needed for such a parallel operation is that the no-load or open circuit voltages of the transformer should be the same.
Multi-Operator Welding Transformers
A multi-arc or multi-operator welding transformer system utilises a high current constant voltage power source for providing a number of welding circuits at the same time. Such a system is used when there is a large concentration of welding points in a relatively small operating area, for example, in ship-building, construction sites for power stations, refineries, and chemical plants.