Welding Power Supply Current Decay Control for Arc Stability
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Solution Overview
Problem
In AC-arc welding, the inductance in welding cables causes voltage peaks during current switching, leading to unstable arcs and inferior welding results due to the decay time of current being undefined, affecting the desired frequency and rectified mean value of the current.
Innovation Solution
A welding power supply with a control unit that adjusts the current output by lowering it before switching from positive to negative polarity, allowing the current to reach a stable switching value before direction change, thus maintaining a high rectified mean value and minimizing voltage peaks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the inverter is turned off early to allow current decay before switching, then voltage peaks are reduced, but the rectified mean current value decreases leading to unstable arc
Solution Approach 1:
The control unit initiates current reduction before the switching event by controlling the inverter to operate in a current reduction mode, allowing the current to decay to a predetermined level before polarity switching occurs. This preliminary action reduces the current level proactively, thereby reducing voltage peaks during switching while maintaining arc stability through controlled current management.
Solution Approach 2:
The system dynamically adjusts the inverter operation between normal operation mode and current reduction mode based on the switching timing requirements. The control unit monitors the switching signal and dynamically transitions the inverter to current reduction mode at the appropriate moment, enabling adaptive current control that balances voltage peak reduction with arc stability.
2Object-affected harmful factors
If the current is reduced before switching, then voltage peaks are minimized, but the frequency control becomes undefined due to variable decay time
Solution Approach 1:
The control unit employs feedback control by monitoring the actual current level and comparing it with the predetermined decay level, then adjusting the inverter switching timing and duration accordingly. This feedback mechanism ensures that the current decays to the desired level while maintaining precise frequency control, compensating for variations in decay time caused by different cable inductances.
Solution Approach 2:
The system changes operational parameters by adjusting the inverter switching frequency and duty cycle during the current reduction phase. The control unit modifies these parameters dynamically to achieve the predetermined current decay level while maintaining the desired output frequency, thereby resolving the conflict between voltage peak reduction and frequency control.
3Reliability
If the inverter operates in current reduction mode, then the rectified mean current value is maintained, but the current cannot reach the set size during positive and negative periods
Solution Approach 1:
The control unit implements periodic switching between normal operation mode and current reduction mode. During normal operation, the inverter delivers full current to maintain welding power. Before polarity switching, it transitions to current reduction mode to maintain arc stability. This periodic alternation ensures both adequate current size for welding and current control for arc stability.
Solution Approach 2:
The system applies partial current reduction only during the critical switching transition period rather than continuously reducing current. The inverter operates at full capacity during most of the cycle but temporarily reduces current only when needed for switching, thereby maintaining both power delivery and arc stability without compromising welding performance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach stabilizes the welding arc, maintains a high rectified mean current value, and ensures consistent frequency, resulting in improved welding quality by adapting the current decay time to the inductance in the welding cables.
Implementation Method 1
During arc welding a high current is provided which is transmitted from a welding electrode in a welding tool to the work piece/work pieces in order to join the work pieces. The high current lead to the creation of an arc which melts parts of the work piece/work pieces and possibly also the welding electrode.
Implementation Method 2
The output of the rectifier is connected to an inverter in which the direct-current is transformed to a high frequency alternating-current voltage.
Implementation Method 3
In the welding cables between the output and the welding electrode and the output and the work piece, respectively, there is, however, an inductance which results in that the current cannot change direction momentarily. This high voltage may be harmful to the switching net and the power supply. The voltage depends among other things on the inductance in the welding cables
Data Source
Figure 1~2
Figure 3~4
AI summary
A welding power supply (1 ) for supply of current to a welding electrode (2), and a method and a computer program for controlling such a welding power supply is described. The welding power supply (1) comprises an output (3) for output of current from the welding power supply (1), and a control unit (9) for controlling the welding power supply (1). The control unit (9) is arranged to control the welding power supply (1) to supply current to the output (3) in the form of a series of alternating positive current pulses (11), having a positive top value (lmax+), and negative current pulses (12), having a negative top value (Imax), and to lower the current in the current pulses from the negative top value (Imax) and the positive top value (lmax+) to a negative switch value (Ix-) and a positive switch value (Ix+), respectively, before switch between a positive current pulse (11 ) and a negative current pulse (12). The control unit (9) is arranged to control, in dependence of the current from the output and a first set point, the welding power supply (1) at least with regard to one of the point of time for lowering of the current in the current pulses and the level for at least one of the negative switch value (Ix-) and the positive switch value (Ix+).