The demands on joining technology are increasing in a large number of areas, especially with regard to the quality and reproducibility of the results. Of course, this also applies to micro-arc welding.
The latest developments in the field of power sources with the associated peripherals have opened up new application possibilities.
Types of Welding Power Sources
The current strength required for arc welding at relatively low voltage values can be generated with different types of power sources.
A characteristic of all designs is the welding transformer, which is used on the one hand to adjust the current and voltage and on the other hand as a galvanic (isolating) separation between the mains supply and the welding circuit.
Related Reading: Types of Welding Machines.
However, the arrangement of the transformer in the energy path is decisive for the size and volume of the power source.
Analog current sources
The analog current source consists of a 50Hz transformer, a rectifier and a transistor cascade (many parallel-connected individual transistors), which serves as an infinitely variable series resistor.
The part of the voltage that is not required for the welding process drops at the transistor cascade. The resulting power loss heats up the semiconductors. As a rule, they are therefore cooled with an additional water-cooling circuit.
The advantage of this arrangement can be seen in its high reaction speed. The disadvantage is the enormous power loss that occurs at the line transistors.
The result is very poor electrical efficiency, which is why this device concept has practically disappeared from the market.
Secondary switched current source
The secondary clocked power source consists of a bulky 50Hz transformer, a rectifier and a transistor stage, which serves as a switch.
The transistor stage is switched on and off periodically at the clock frequency (e.g. 20,000 times per second = 20 kHz). The periodic switching on and off is referred to as “clocking”.
The transistor in this configuration can be thought of as a mechanical light switch that turns on and off.
In an ideal switch, there is no power loss either in the open or in the closed state. This allows a high electrical efficiency to be expected. Of course, semiconductor switches are not ideal switching elements, ie they are subject to power loss. However, the power loss is very low.
Another advantage of the transistors can be seen in the enormous switching speed. Modern semiconductor switches can be clocked up to 200 kHz (kilohertz) in the power range typical for welding.
The higher the clock frequency of the transistor, the lower the output current ripple and the higher the reaction speed and the possibility of better influencing the welding process.
The magnitude of the current ripple affects the stability of the plasma column of the arc. Therefore, attention should be paid to low ripple – especially in the lower power range (e.g. with 5A DCWIG).
Basically, the ripple of the welding current depends on the clock frequency. The higher the clock, the lower it is.
So that the welding power can be freely adjusted over a wide range with clocked power sources, the ratio of the switch-on to the switch-off time must be changed. This method is called pulse width modulation.
If the ratio of switch-on to switch-off time is large, a high output power (mean value) is obtained; if the ratio of switch-on to switch-off time is small, a low output power is obtained.
Primary clocked power source (inverter)
A characteristic of inverter power sources is that the welding transformer is located after the switching transistor in the energy path.
The reason for this is that, following an electrotechnical law, the weight and volume of transformers depend on the frequency at which they are operated. The higher the frequency, the lower the volume.
It is precisely this connection between volume and frequency that inverter power sources use.
Therefore, inverter power sources are light in weight and small in size without sacrificing performance.
Consequently, they are easier to transport. This is particularly important for use on construction sites.
Furthermore, due to the small volume, inverters require less floor space in the often-cramped conditions of workshops and production lines.
Another advantage can be seen in the high electrical efficiency (up to 90%).
So that the high clock frequency can be used, the mains AC voltage must first be rectified.
The term inverter power source (invert = convert) is derived from this. The DC voltage present after the primary rectifier is converted into a high frequency with a transistor switch and switched to the primary side of the transformer.
The output voltage of the transformer will be Inverter power sources can cover welding current ranges from 2 to 500A with continuous adjustment.
The lower current range is particularly important for the micro-arc welding interesting. A low current ripple is a prerequisite for a stable arc column.
Closed Loop Systems
With the use of electronics in power source technology, so-called regulated systems were developed, which keep the welding current or welding voltage constant regardless of mains voltage changes and mains cable lengths.
The central element is the closed control loop with sensors for welding current and welding voltage.
The actual values from the welding process are constantly compared by the microprocessor with the preselected target values (welding parameters) and deviations are corrected immediately via the actuator.
This is the basic requirement for the reproducibility of welding results.
Another advantage is that with transistor current sources, the welding properties are not dependent on the design of the transformer and the output choke.
This opens up unimagined opportunities to influence the quality of the weld seam and the welding process with the help of electronics.