The power supply used for electric welding can be either direct current (dc) or alternating current (ac). In this article let us see the difference between ac and dc welding machines.
AC Welding :
An ac welding set or machine includes a transformer that transforms the incoming main supply to the required voltage/current value suitable for welding. The transformer can be of single-phase or three-phase. It is housed in a steel tank that is filled with transformer oil or employed with electric fans to prevent its overheating when working under heavy load conditions.
We know that in an alternating power supply there is a positive and negative cycle i.e., there will be a reversal of voltage and current for every 180°. Due to this when the welding current is zero, the arc in arc welding would extinguish and make continuous welding difficult.
This can be prevented by using special electrodes with a coating that produces more ionization in the arc stream and keeps the arc igniting as the current passes through zero.
Advantages of AC Welding :
- It is the least expensive, lightest, and small welding machine.
- There is freedom from magnetic arc blow that often occurs when welding with a dc machine.
- Operating and maintenance costs are low.
- Since there are no moving parts in the transformer, the operation is noisy less and there is hardly any wear.
- Overall electrical efficiency is high.
Disadvantages of AC Welding :
- Polarity cannot be changed.
- Because of the alternating nature of the current flow, starting of arc is more difficult than with dc.
- Low power factor.
DC Welding :
The dc power supply for dc welding can be obtained from a dc generator powered either by an electric motor or a diesel engine. The generator used can produce direct current in either normal or reversed polarity and is designed such that it will compensate for any change in voltage thus ensuring a stabilized arc. In remote places where power is not available diesel operated generators are well suited.
In case there is an availability of ac supply, a rectifier set consisting of a transformer is used. The rectifiers convert ac supply to dc supply with the help of silicon diodes that allows current to flow in one direction only.
Advantages of DC Welding :
- The spatter produced is less in dc welding compared to ac welding.
- The arc produced with dc supply is more stable.
- Nearly all ferrous and non-ferrous metals can be welded.
Disadvantages of DC Welding :
- High initial cost.
- Higher maintenance cost.
- Noisy machine operation.
- The dc welding causes arc blow.
Difference Between AC Welding and DC Welding :
| AC Welding | DC Welding |
|---|---|
| Electrodes used in ac welding must be flux coated. | In dc welding any type of electrode can be used. |
| The arc produced in ac welding is not stable. | The arc produced in dc welding is highly stable. |
| The heat developed is not uniform. | The heat developed is uniform. |
| In ac welding when only an ac supply is present, a transformer is needed. | In dc welding when only an ac supply is available then a rectifier or a motor-generator set is needed. |
| The cost of the equipment is very low when compared to dc welding. | The cost of the equipment is very high, approximately two or three times that of a transformer. |
| The operating efficiency of the ac welding machine is very high (about 80 to 85%). | The operating efficiency of the dc welding machine is low (about 30 to 40%). |
| In ac welding, the arc produced is automatically controlled. | In dc welding, the arc is very severe and also difficult to control. |
| The power factor is very low (about 0.3 to 0.4) and requires a capacitor for correction. | The power factor is high (0.6 to 0.7) when compared to ac welding. |
| The single-phase transformer used in ac welding causes unbalance in the 3-phase supply system. | In dc welding, the motor-generator set uses a 3-phase induction motor which is a balanced load. |
| Due to high operating voltage, the risk of shock is more. | Due to low operating voltage, the risk of shock is less. |
| Less amount of energy is consumed. | A huge amount of energy is consumed. |
| It is difficult to weld non-ferrous metals and alloys due to changes in polarity. | Any type of metal can be welded using dc welding. |
| AC welding equipment is small in size and lighter in weight. | DC welding equipment is larger in size and heavier in weight. |
| The maintenance and operating costs of ac welding are low. | The maintenance and operating costs of dc welding are high due to the presents of moving parts. |
| There are serve problems encountered in dc arc welding due to arc blow. | The problem of arc blow is not present in ac arc welding. |
