Thyristor Based Arc Welding Inverter
What is a thyristor based arc welding inverter?
The high-power, high-voltage switch tube that uses fast thyristors (SCRs) as the main inverter circuit and is controlled by its firing angle for arc welding is commonly known as thyristor-based arc welding inverter. It is controlled by firing angle and can also be called firing angle controlled arc welding inverter.
Research into thyristor-based arc welding inverters had already produced results and were reported in the late 1970s. The early to mid 1980s saw significant development in terms of capacity, from medium to large capacity. ; from electrode arc welding to CO2/MAG welding, submerged arc welding and resistance welding; from DC welding to AC square wave welding; from electronic control to microcomputer and digital control, and its application fields continued to expand.
The frequency of the inverter varies from several kilohertz to tens of kilohertz (with resonance).
However, in the late 1980s it was gradually replaced by newer technologies such as field-effect transistors and IGBT-based arc welding inverters due to its low frequency, poor control performance and noise interference.
Its application proportion has gradually decreased, but it still occupies a certain position in the world.
It should be noted that the emergence of new types of thyristors such as static induction thyristor (SITH) and gate turn-off thyristor (GTO) will change their position and contribute to their continued development and promotion.
Main Components and Basic Principles
The main components and basic principle diagram of the thyristor-based arc welding inverter are shown in Figure 1.
The main difference is that IGBT transistors are used instead of MOSFET or traditional transistors, and the frequency of the inverter is around 20-25kHz (whereas MOSFET-based inverters can reach 50kHz or higher). IGBT transistors use voltage control and a single transistor has sufficient capacity, so there is no need for parallel operation of multiple transistors.
The external characteristics of the IGBT inverter, regulation characteristics (standard parameter regulation) and acquisition and control of the output waveform are also achieved through changes (conversion, modulation) in the pulse width, including low-frequency modulation of the output waveform. output pulse wave.
In terms of the basic types of input rectifier filter circuit, inverter main circuit (various types), output filter circuit, negative feedback closed-loop control circuit and their principles, they are essentially the same of MOSFET-based welding inverters. .
Classification and applications of Igbt arc welding inverters
IGBT-based arc welding inverters can be classified according to their external characteristics or according to their output types such as DC, pulse and AC square wave.
Both types of welding inverters have universal significance and can be used not only for large-scale and comprehensive welding processes such as electric arc welding, tungsten and argon arc welding, melting electrode welding with protection against gas, plasma arc welding and cutting, but also for high power single/double wire automatic submerged arc welding processes ranging from 1250A to 2000A, air arc gouging and robotic arc welding, as well as MIG welding /MAG/double-wire pulse and three-wire submerged arc welding, among others.
Arc Welding Inverter with Soft Switching
What is a soft switching arc welding inverter?
Arc welding power supply power devices work and are controlled in analog or switch mode. There are two types of arc welding power supply with switching mode, hard switching and soft switching. The former mainly uses pulse width modulation (PWM) control technology, and the power devices operate in a forced off (current is not zero) or forced on (voltage is not zero) state.
Due to the existence of parasitic capacitance and inductance in the circuit, switching devices switch on and off at working current and voltage values that are not zero or even higher, leading to high switching losses. This loss increases proportionally with frequency, significantly reducing circuit efficiency and even causing circuit malfunction.
Traditionally designed arc welding inverters have difficulty fundamentally solving these problems. However, soft-switching arc welding inverters use resonant current conversion technology, where power devices turn on or off naturally under zero voltage or current conditions.
This technology essentially overcomes the disadvantages of hard switching arc welding power supplies, greatly reduces switching losses, and minimizes electromagnetic interference (EMI) and radio frequency interference (RFI).
It also reduces the weight of the inverter, increases the frequency, reduces the volume of transformers, inductors and capacitors in the circuit, reduces output ripple and improves the power density and dynamic performance of the system.
Therefore, the application of soft switching technology, especially in arc welding inverters, is increasingly widespread, taking arc welding power supply to a new level. Soft-switching arc welding inverters are a promising development in this field, and this section will focus on their discussion.
Basic form and working principle of soft switching inverter main circuit
The main components and basic principles of soft switching arc welding inverter are similar to those of hard switching arc welding inverter. The main difference is in the details of the inverter main circuit structure and the adjustment method of the control and drive circuit.
Resonant current conversion technology with soft switching operation mode can be controlled by two methods: variable frequency control and constant frequency control. The analysis and design of the variable frequency control circuit is complex and susceptible to interference, and the output range is small with low use of magnetic components.
Constant frequency control is based on the commonly used PWM, in which a resonant inductor and a capacitor are connected in series in the main circuit of the inverter. The control system uses phase-shifted square wave to drive the switching devices, relying on the free-wheeling diode to achieve smooth switching control of the power devices.
In the main circuit of the full-bridge inverter, the switching devices on the diagonal lines are not turned on and off at the same time, but are staggered with a time interval to achieve zero current turning off or zero voltage turning on. Output voltage or current control is achieved by adjusting the duty cycle of the bridge.
This control method is relatively easy to design, has a larger output range and a relatively simple circuit structure, making it more suitable for arc welding inverter applications.
Basic form of soft switching inverter main circuit
At present, there are several common basic forms of soft-switching inverter main circuit, including:
- Zero current switching (ZCS) resonant inverter main circuit.
- Main circuit of zero voltage switching (ZVS) resonant inverter.
- Multiresonant converter (MRC) inverter main circuit.
- Series resonant inverter main circuit.
- Parallel resonant inverter main circuit.
- DC bus resonant inverter main circuit.
- Main circuit of phase shift control resonant inverter.
Working principle of soft switching inverter main circuit
To choose the right soft-switching inverter main circuit for arc welding power supply, it is necessary to introduce in detail the following four basic soft-switching inverter main circuits:
a) Main circuit controlled by phase shift b) Drive signal timing diagram.
