Zhuojin Pipe Fitting Co., LtdDesign of Inverter AC TIG Welding Control Circuit
Literature Overview
This paper by Chang Yunlong, Sun Jinggang, Wang Dianlong, and Song Wenqing from the School of Materials Science and Engineering at Shenyang University of Technology, published in the Journal of Shenyang University of Technology (2004, Vol. 26, No. 4, pp. 385-388), presents the design and analysis of a novel dual-inverter IGBT-based AC TIG welding control circuit. The study addresses the need for advanced power electronics-based welding power sources capable of providing independent control of AC waveform parameters, which is essential for welding aluminum and aluminum alloys as well as titanium alloys.
Technical Background and Design Objectives
AC TIG welding is the standard process for welding aluminum and its alloys, as well as for certain titanium alloy applications. The AC waveform provides alternating polarity, which serves two critical functions: the negative half-cycle (electrode negative) provides arc cleaning of the oxide layer on aluminum surfaces, while the positive half-cycle (electrode positive) provides deeper penetration into the base metal. The balance between these two functions is controlled by the balance ratio (the ratio of negative to positive half-cycle time).
Traditional AC TIG power sources based on transformer rectifiers or simple thyristor inverters offer limited control over waveform parameters. The design objective of this study is to develop a control circuit that provides:
- Independent control of welding current magnitude
- Adjustable AC frequency
- Independent control of positive and negative half-cycle durations (balance ratio)
- Flexible waveform shaping for different welding positions and materials
Circuit Architecture and Operating Principle
The proposed control circuit employs a dual-inverter topology consisting of a primary inverter and a secondary inverter, both utilizing IGBT (Insulated Gate Bipolar Transistor) switching devices.
Primary Inverter Stage
The primary inverter converts DC input voltage to high-frequency AC and provides the main power conversion. It is configured as a half-bridge topology driven by PWM signals. The primary inverter stage establishes the fundamental frequency of the AC welding current and provides the initial power conversion from the DC bus.
Secondary Inverter Stage
The secondary inverter performs the final waveform shaping and provides independent control of the positive and negative half-cycles. By adjusting the duty cycle and frequency of the secondary inverter switching signals, the balance ratio and current magnitude can be independently controlled.
External Characteristic Control
The external characteristic control circuit implements current closed-loop control, ensuring stable welding current despite variations in arc length, voltage, or other process disturbances. This is achieved through a feedback loop that compares the actual welding current with the reference current and adjusts the inverter modulation accordingly.
| Control Parameter | Control Method | Range |
|---|---|---|
| Welding current | External characteristic closed-loop | 20-300 A |
| AC frequency | Secondary inverter frequency control | 50-200 Hz |
| Balance ratio | Secondary inverter duty cycle | 40:60 to 70:30 |
| Current waveform | Combined modulation | Square wave, modified square |
Control Circuit Implementation Details
The control circuit is designed with the following key features:
- Simple structure: The dual-inverter topology is relatively straightforward compared to more complex multi-level inverter approaches, facilitating manufacturing and maintenance.
- Convenient parameter adjustment: The control parameters are accessible through standard potentiometers or digital interfaces, allowing operators to quickly adapt the process for different welding conditions.
- Multiple control functions: The circuit supports various welding modes including constant current (CC), constant voltage (CV), and pulse welding, providing flexibility for different applications.
The IGBT switching devices are selected for their combination of high switching frequency capability (enabling compact transformer designs) and high current handling capacity (suitable for welding applications). The gate drive circuitry includes dead-time control to prevent shoot-through in the half-bridge configurations.
Experimental Validation and Performance
Experimental testing demonstrated that the control circuit achieves the designed performance characteristics:
- Stable welding current regulation with less than 5% ripple
- Smooth transitions between positive and negative half-cycles with minimal current zero-crossing distortion
- Effective arc cleaning during the negative half-cycle as confirmed by visual inspection of weld beads on aluminum alloys
- Adequate penetration during the positive half-cycle for medium-thickness aluminum joints
The experimental results confirm that the proposed control circuit is technically viable and offers significant advantages over conventional AC TIG power sources in terms of waveform flexibility and parameter control.
Study Insights and Reflections
This paper represents an important contribution to welding power source technology, particularly in the context of the transition from thyristor-based to IGBT-based inverters. The dual-inverter approach offers a practical compromise between control flexibility and circuit complexity, making it suitable for industrial welding applications where reliability and ease of maintenance are as important as performance.
From a materials science perspective, the ability to independently control the balance ratio is particularly significant for aluminum alloy welding. Different aluminum alloy families (e.g., 5xxx series vs. 6xxx series) and different welding positions require different balance ratios to achieve optimal weld quality. The control circuit described in this paper provides the flexibility to optimize the balance ratio for each specific application.
One area for future development would be the integration of adaptive control algorithms that can automatically adjust the balance ratio and frequency based on real-time monitoring of arc characteristics. Such intelligent control would further enhance the versatility and quality consistency of the power source.
In conclusion, this study demonstrates the practical value of modern power electronics in welding power source design, providing a control architecture that balances technical sophistication with manufacturing practicality, and offering significant advantages for AC TIG welding of aluminum and titanium alloys.