All semi-automatic carbon dioxide welding machines are equipped with voltage and current adjustment knobs (the voltage adjustment of tap type carbon dioxide welding machines is a change-over switch).
The current adjustment knob of the integrated welding machine (the wire feeder is installed inside the host) is installed on the host panel;
The current adjustment knob of the split type welding machine (the wire feeder is independent and connected with the host through cables) is installed on the wire feeder.
There are two ways to adjust the voltage:
For the thyristor rectifier and inverter welding machine, the potentiometer is used;
For the tap type welding machine, the voltage is adjusted through the transfer switch.
The first condition for the stability of the CO2 welding process is that the feeding speed of welding wire is equal to the melting speed.
Related reading: Manual Arc Welding vs CO2 Gas Shielded Welding
The energy for melting the welding wire is provided by the host.
The greater the output power of the host, the faster the welding wire melts.
For the thyristor rectifier welding machine, the output power is to adjust the conduction angle of the thyristor;
For inverter welding machine, the output power is to adjust the pulse width;
For the tap welder, the output voltage is adjusted.
According to common sense, power is the product of voltage and current.
Adjusting the output power of the welder is equivalent to adjusting the welding current.
Why is it that the welding current of carbon dioxide welding is realized by adjusting the wire feeding speed?
This problem can be explained from two aspects:
1. The current is generated in the loop (path);
2. The current is a degree with time as reference.
In the case of an open circuit (open circuit), no matter how high the voltage is, the current is always equal to zero.
In this case, the terminal voltage of the circuit is the electromotive force E of the power supply, which can be measured with a voltmeter at A and B.
We can think of this as the no-load voltage of the welding machine.
Since the circuit cannot form a loop, there is no current in the circuit and no voltage will be generated at both ends of the resistance R.
(The resistance R represents the sum of the internal resistance of the power supply and the loss voltage drop of the transmission cable in the welding arc source system.
The internal resistance of the power supply is generated by the leakage reactance of the transformer and the adjustment of the conduction angle of the rectifier components and the pulse width of the switching devices.).
If two points A and B are short circuited, or a resistance RH is indirectly connected at these two points, current will be generated in the circuit.
In the circuit, RH is the voltage drop generated at the moment when the welding current is short circuited with the workpiece through the arc and droplet, also known as the load resistance.
It can be seen from the above analysis that the smaller the values of R and RH, the greater the current in the circuit, and vice versa;
The electromotive force E of the power supply has the opposite effect.
As mentioned earlier, R is the inherent resistance in the welding circuit.
For the tap welder, the primary and secondary systems of the main transformer are made into a closely coupled structure to obtain small leakage reactance to meet the requirements of carbon dioxide welding flat characteristics.
In this kind of welding machine, we can consider that R is unchanged, but the no-load voltage E of the power supply is changed by changing the tap through the change-over switch.
In the thyristor controlled welding machine and the inverter welding machine with IGBT as the switch, the transformer has no adjustable tap, so E in the circuit can be considered as a constant.
The R in the circuit can be adjusted by adjusting the conduction angle of the thyristor and the turn-on judgment ratio of the IGBT.
The effect of R and E on the current in the circuit is usually easy to understand and pay attention to.
However, the role of RH is often not given enough attention.
This is the second problem we want to talk about – current is a degree with time as reference.
The output power of the welding machine can not only be achieved by adjusting the power supply voltage, but also depends on the load condition.
In the process of carbon dioxide welding, the welding wire deposits the workpiece (weld) in the arc in two forms:
1. Short circuit transition;
2. Fine drop transition.
The short circuit transition frequency is generally about 100 times/second, and the fine drop transition frequency is higher.
The welding wire is used as an electrode (set as point A), and the workpiece is used as the other electrode (point B).
When the arc is ignited, the welding arc is part of RH, and the other part of RH is the droplet transfer of the welding wire.
In terms of short circuit transition, the faster the wire feeding speed is, the higher the frequency of short circuit transition is, that is, the more opportunities to provide a path for this circuit in a unit time, which makes the equivalent resistance RH smaller, and then we can see that the current is also higher.
In addition, carbon dioxide welding uses a thin welding wire with high current density, and is matched with a flat characteristic power supply.
The arc self regulation plays an absolutely dominant role in the welding process.
During the welding wire feeding process, the flat characteristic power supply increases the melting speed of the welding wire, so that we have the concept of adjusting the welding current by adjusting the wire feeding speed locally.
To sum up, the welding current of carbon dioxide welding is the result of the joint action of E, R, and RH.
However, in this system, E and R have a relatively wide adaptation range, while the change of RH is more sensitive in the system.
In order to maintain the stability of the welding process and reduce the spatter, when the welding wire melting speed matches the wire feeding speed, we should often adjust the wire feeding speed.
Since this process causes changes in the welding current, we habitually call adjusting the wire feeding speed adjusting the welding current.
If we think that the welding current can only be adjusted by wire feeding speed, in order to increase the welding current, we will blindly increase the wire feeding speed, which will lead to the phenomenon of “wire jacking”.
During the welding process, it will be felt that the welding gun is pushed back, and the welding process is discontinuous.
The dry stretching injection will burn red – burst – burn red – burst, and make a snapping explosion sound.
Conversely, in order to reduce the current, only the wire feeding speed is reduced, and the welding process is discontinuous, accompanied by a large splash.
At this time, the welding gun is weak, and the weld seam is stacked high without penetration.
In order to achieve a good welding effect, experienced welders coordinate the adjustment of voltage and current (wire feeding speed), and observe the state of the weld while listening to the sound of wire transition.
Generally, beginners can refer to the CO2 welding arc characteristic curve formula to adjust, namely: UH=15+0.04I (where UH represents the arc voltage; I represents the welding current).
For example, when the welding current is 200A, the arc voltage should be about 23V.
These two data can be read through the voltmeter and ammeter on the power supply.
It should be noted here that due to the voltage drop of the welding cable and the contact resistance of each connection point in the welding circuit, the reading of the voltmeter is too large.
When a certain diameter of welding wire is used, there is not only one stable working point in the welding process.
For example, when φ1.2mm welding wire is in short-circuit transition state, the current can be adjusted from 90A to 150A, and the voltage range is between 19V and 23V;
In the particle transition state, the current can reach 160A~400A, and the voltage can work at 25V~38V.