Discussion on the development direction of electric resistance welding power source
Text / Wang Qing
ã€Abstract】The electric resistance welding process is one of the most widely used welding methods in the world. This paper discusses the future development trend of electric resistance welding power supply. The main development direction of electric resistance welding power supply in the new century is put forward, that is, the development of high-efficiency and energy-saving three-phase equalization electric resistance welding power supply will remain the theme of the 21st century; various inverter resistance welding machines featuring soft switching will be obtained. Long-term development; and the new technical principles and market prospects of resistance welding power supply are introduced in detail.
Key words: resistance welding power supply
After more than 100 years of development, the electric resistance welding power supply has become one of the indispensable welding equipments in industrial production today. Its development has also experienced a development process from simple to complex, from primitive to perfect, and the research on electric resistance welding power source has made great achievements. Despite this, with the continuous advancement and development of the society, both in terms of energy saving and material saving, and in further improving the welding quality and expanding the application field of resistance welding, higher requirements are placed on the electric resistance welding power supply, and resistance is also required. The welding power supply is continuously improved and improved. In the future, where the electric resistance welding power source will develop is a question that deserves serious consideration and discussion.
1. Continue to develop energy-efficient three-phase balanced resistance welding power supply will remain the theme of the future
At present, the most widely used electric resistance welding power source at home and abroad is still a single-phase power frequency resistance welding machine, but from the energy point of view, the single-phase power frequency resistance welding machine is a very unsatisfactory power source. First, the power factor of the single-phase power frequency resistance welder is very low. For example, the rated power factor of the FN1 series seam welder is 0.35; the rated power of the DN2 series spot welder is 0.65, but if the extension length of the welder arm is greater than When 1 × 103 mm, its power factor drops below 0.30. Secondly, because it is a single-phase power supply, when the welding machine with higher power is welded, it will have a great impact on the power supply grid. For example, a 100kVA single-phase power frequency resistance spot welder operating at rated power, at 380V In the case of power supply, the initial current of the welding can reach several hundred amperes. Usually, the welding time of the spot welding machine is very short. About a few or dozens of cycles can be used to form a solder joint, so it will definitely be welded. It has caused a great impact on the power grid and caused serious pollution to the power supply grid. Furthermore, the single-phase power frequency resistance welding machine has a problem that the welding current has 100 zero-crossing points per second during welding, which will cause discontinuity in welding heating. Therefore, it is not conducive to the use of such a welder for the welding of light metals such as aluminum and titanium.
In view of the problems existing in single-phase power frequency resistance welding machines, people have been seeking an ideal electric resistance welding power source and have made great achievements. Since the 1940s, people have done a lot of work in the development of welding power supply using three-phase power supply, and have developed three-phase low-frequency resistance welding machine, secondary rectification resistance welding machine, capacitor energy storage resistance welding machine and Inverter resistance welding machines, although these welders have successfully solved the problems of single-phase power frequency resistance welding machines, they still have some improvements. Therefore, the continued development of energy-efficient three-phase balanced resistance welding power supply will remain the theme of the 21st century.
2. Various inverter resistance welders featuring soft switches will be greatly developed.
Inverter resistance welding machine is one of the best resistance welding machines with various technical indicators. However, in the inverter electrical appliances of most of the inverter resistance welding machines currently used, the method of keeping the switching frequency of the power switching tube fixed and changing the length of the power switching tube (ie, the width of the pulse) is generally adopted. To adjust the welder input energy, this control method is called pulse width modulation (PWM). In this case, since the power switch tube is turned on and off according to the applied control pulse, and the control pulse is emitted independently of the current flowing through the power switch tube and the voltage applied across the two ends, such a power switch tube is Called "hard switch."
Inverter circuit controlled by hard switch, when the operating frequency is high, due to the overlap of current and voltage on the power switch tube, high switching loss current and voltage stress are generated. The relevant data shows that the operating frequency in the hard-switching state is 20 kHz, and the arc-welding inverter power supply using the IGBT power switch tube has a switching loss of 60% to 70% of the total loss of the power switch tube, or even larger. At the same time, the parasitic inductance of the circuit and the parasitic capacitance of the power device will generate severe voltage spikes and current spikes when operating at high frequencies. To eliminate the effect, a buffer circuit is usually placed on both sides of the switch. However, the snubber circuit consumes energy. The higher the operating frequency of the inverter, the greater the energy consumption, which will lower the overall efficiency of the entire inverter.
In view of some problems with the hard-switching inverter power supply, after the 1980s, people began the development of soft-switching inverter power supplies. The so-called soft switch refers to the control method to make the power switch turn on when the voltage across it is zero, or turn off when the current flowing through the power switch is zero. This switch is called a soft switch. The ideal value of the turn-on and turn-off loss of the soft switch is zero. In the development of soft-switching inverter circuits, people have also undergone a process of continuous improvement.
In order to satisfy the condition that the voltage or current on the power switch is zero, a resonance method can be employed. In fact, because the forward and reverse LC loop values ​​on the main circuit of the welding are different, the oscillation frequency of the forward and reverse will be different, and the amplitude of the oscillating current will be different, forming a oscillating asymmetry. This oscillation is called quasi-oscillation. A quasi-oscillation method to create a zero-voltage turn-on or zero-current turn-off condition is called a quasi-resonant inverter. In practical circuits, either a series LC or a parallel LC produces quasi-resonance.
Both the resonant inverter and the quasi-resonant inverter use the adjustment of the switching frequency to regulate the output voltage and the output current, so it is called a frequency modulation system. Over the years, a lot of work has been done on the development of inverter welding power supplies using quasi-resonant inverters, and some progress has been made. However, there are many shortcomings in quasi-resonant inverters. First, the control is complicated compared with PWM. Secondly, the peak value of the resonant voltage (or resonant current) generated in the resonant circuit is high, and the pressure on the power switching element is large. The output adjustment range is small. In order to obtain a large adjustment range, the switching frequency is required to vary within a wide range, which makes the design of the welding transformer and the filter very difficult and reduces the utilization of the magnetic component.
In the 1990s, research hotspots have been moved to zero-switch-PWM transformers and zero-conversion-PWM converters.
Zero-conversion-PWM converter refers to adding an auxiliary switch control circuit to the quasi-resonant converter to make the converter zero-current switch (ZCS, Zero-Current-Switching) or zero-voltage switch (ZVS) in one cycle. , Zero-Voltage-Switching) The quasi-resonant converter works, and the other part works according to the PWM converter. The former is called a ZCS-PWM converter, and the latter is called a ZVS-PWM converter. In this way, the converter has the characteristics of soft switching controlled by voltage zero crossing (or current zero crossing), and has the characteristics of constant frequency widening. The inductance in the resonant network is then in series with the main switch. Many achievements have been made in the development of inverter welding power supply using zero-switch-PWM technology, which has been reported in both domestic and foreign literatures. However, since the resonant inductance of such a soft switch is connected in series in the main circuit, the condition of the zero switch is related to the range of variation of the power supply voltage and the load current, and it is possible to lose the condition of the zero switch under light load.
The zero-conversion-PWM converter is essentially indistinguishable from the zero-switch-PWM converter. It is also a combination of a soft-switch and a PWMR, except that its resonant tank is connected in parallel with the main electronic switch, thus improving the condition of the zero-switch. Analysis and experiments show that the zero-transform-PWM converter has the lowest conduction loss and switching loss, can achieve zero switching characteristics without increasing the current and voltage stress of the switch, and is suitable for higher voltage and high power converters. In recent years, people have carried out theoretical and experimental research on the application of zero-conversion-PWM converter technology in welding power supply, and have obtained many research results. On the other hand, the circuit also has a problem of duty cycle loss and the ZVT range cannot be too wide.
In view of the maturity of the soft-switching inverter circuit and its own advantages, it is believed that the inverter-type resistance welding machine based on zero-switch-PWM converter or zero-switch-PWM converter will become a hot spot in the 21st century. And the mainstream.
3, reduce or eliminate the various drawbacks caused by secondary rectification
The secondary rectification resistance welder was developed after the 1970s and has become one of the main development directions of resistance welding power supply. The secondary rectification resistance welding machine can be divided into two types: single-phase secondary rectification and three-phase secondary rectification. Although the single-phase secondary rectification resistance welder has made great progress in energy saving and power factor improvement compared with the single-phase power frequency resistance welder, the welder cannot solve the single-phase power supply and the power supply grid. Impact and pollution. The three-phase secondary rectification resistance welding machine can provide three-phase power supply, and can greatly reduce the line current of each phase of the power supply. However, due to the use of high-power secondary rectifier diodes, new energy losses will be brought. . In general, the secondary tube used for secondary rectification has a tube pressure drop of about 1.5V. Generally, the secondary voltage of the resistance welding machine is about 5V, that is, it is used to overcome the voltage drop generated by the rectifying diode at about 1.5V in the secondary, which accounts for about 32% of the total output energy. In addition, in order to prevent the damage of the rectifier diode, it is necessary to increase the corresponding overload protection, which not only increases the cost of the whole welder, but also increases the maintenance difficulty of the welder.
Inverter resistance welding machine is a new type of electric resistance welding power source developed after the 1980s, but it also faces the same secondary rectification problem. It is well known that under the same conditions, the core cross section of the welding transformer and its The frequency of the input voltage is inversely proportional. In order to maximize the advantages of the inverter resistance welder, it is always desirable to have the inverter frequency as high as possible during operation. But on the other hand, the magnitude of the inductive reactance in the circuit is proportional to the frequency of its input voltage. When the inverter resistance welding machine is working, its secondary circuit together with the workpiece to be welded constitutes a large coil with a number of turns. When the inverter's inverter frequency is increased, it is bound to take the inductive reactance in the secondary circuit. Doubled. In order to eliminate the influence of the secondary inductive reactance, it is usually necessary to connect the rectifying element in the secondary circuit. Only when the area enclosed in the secondary circuit is small, and the operating frequency of the welding machine is less than 400 Hz, the secondary device can be used. Rectification is used directly, which greatly limits the working range of the welder. After adding the secondary rectification circuit, it not only brings the energy consumption problem of the secondary rectifying component, but also because the working current of the electric resistance welding machine is generally thousands and tens of amps, which is far greater than the current of the arc welding power source, and is subject to secondary rectification. The limitation of the component di/dt, the resistance welding inverter power supply is not likely to be as high as the inverter frequency of the arc welding inverter power supply.
In view of the shortcomings of secondary rectification, it is foreseeable that in the 21st century, people will intensively study how to overcome their shortcomings and make the resistance welding power supply more perfect. There are two ways to solve the above drawbacks caused by the secondary access to the rectifying element: one is to start from the rectifying element itself, and reduce its tube pressure drop as much as possible, but this method does not solve the problem fundamentally; The second is to design a new type of resistance welding machine to achieve secondary rectification of the resistance welding machine while maintaining the main advantages of the secondary rectification welder.
4, a variety of special resistance welding machines will be widely used
A dedicated resistance welder is a general term for a resistance welder specially developed to meet a specific production purpose. Because the special resistance welding machine can combine the high-quality welding process with the machine that is easy to mechanize and automate the resistance welding itself, and give full play to their respective strengths to achieve the effect of improving welding quality and labor productivity. In developed industrial countries such as the United States and Japan, especially in Germany, due to the high labor costs in the country, and at the same time to improve labor productivity and welding quality, various electric resistance welding machines have reached a high proportion.
In contrast, in China's resistance welding machine market, resistance welding machines account for a small share of the entire welding machine market, about 10%, and the proportion of resistance welding machines is smaller. Therefore, we can be sure that all kinds of resistance welding machines in the 21st century will have broad prospects in China.
5. The higher harmonics generated by the resistance welder will be suppressed.
The periodic functions encountered in engineering can generally be expanded into a sum of a series of sinusoids with a frequency of f(t) and an integer multiple of the frequency. In the circuit analysis, the constant term in the above equation is called the DC component; the sine of the angular frequency ω is the fundamental or first harmonic, and its frequency is the same as the frequency of f(t); the angular frequency is 2ω, The positive arcs of 3ω, 4ω, etc. are second harmonic, third harmonic, fourth harmonic, etc., and the harmonics of the second and above are collectively referred to as higher harmonics.
In various resistance welding machines, either by adjusting the conduction thyristor conduction angle of the main circuit of the welding, or by adjusting the pulse width or frequency of the input square wave of the welding transformer to adjust the welding input power, that is, the welding voltage used. And the current waveform is not a standard sinusoidal waveform, so it will be accompanied by a large number of higher harmonic components. In addition, the resistance welding machine has high input power and short welding time, and the problem of high-order harmonic pollution generated during the welding process will be more serious.
Higher harmonics will cause serious pollution to the power system, as shown in:
(1) Seriously pollute the public power grid and interfere with the normal operation and safety of other equipment;
(2) The increase of the harmonic content will reduce the power factor of the device and increase the power capacity of the pre-stage device, resulting in waste of power capacity;
(3) The error in the calculation of electric energy, thereby damaging the interests of the electricity users.
At present, harmonics have been called a kind of power pollution. In order to eliminate the impact, many countries have already specified the standards for harmonic limits of power system harmonics and electrical equipment. The International Electrotechnical Commission (IEC) and the International Committee of Electrical and Electronics Engineers (IEEE) have established special working groups to develop harmonic limit standards (IEC61000-3-2 and IEEEE519-92) and have been in the European Union since 1996. Implementation began within the country. Since 1998, China has begun to establish restrictions on the harmonics of electrical equipment, and first implemented it in the home appliance and postal and telecommunications industries. If the harmonic limit standard is enforced in the future, most of the types of welders currently in use and produced are unqualified products. Therefore, it is imperative to develop a new type of low-harmonic resistance welding power source.
6. Power electronic simulation technology will be widely applied to the development of various resistance welding machines.
The model is built according to the actual circuit (or system), and the purpose of developing and developing the actual circuit (or system) is achieved through computer analysis, research and experiment on the model. This process is called computer simulation. Due to the high efficiency, high precision, high economy and high reliability of computer simulation, it has received much attention. It has been widely used in the analysis of power electronic circuits (or systems) in the past 20 years and has become a must for research in this area. An important tool that is indispensable.
In the design of power electronic circuits, computer simulation is mainly used to verify the design, predict the performance of the system, discover potential problems of new products, and evaluate methods to solve problems.
The computer simulation technology of the circuit mainly needs to solve how to establish the circuit equation (that is, establish the simulation mathematical model) and how to solve the circuit equation.
Resistance welding power supplies are nonlinear time-varying systems, and it is often difficult to accurately analyze their steady-state and dynamic performance. Establishing accurate mathematical models has always been a problem in the field of power electronics. Usually only a certain condition is assumed, and some minor factors are neglected to obtain a mathematical model that can be applied within a certain range, which helps to analyze and design the circuit.
Since the 1970s, the analytical modeling methods used in circuit simulation mainly include state variable method, node analysis method, improved node analysis method and state space averaging method. Each of these modeling methods has its own limitations and has its own scope of use. In the specific use, it is necessary to establish a specific simulation model according to the specific purpose.
The difficulty and key point of power electronic simulation lies in the establishment of power electronic device model. The accuracy of the model is the guarantee of computer simulation reliability. The simulation of electronic power circuit is divided into three levels.
(1) Study the device-level simulation of the microscopic characteristics of the circuit and the operating state of the device;
(2) Study circuit topology and circuit-level simulation of its transient process.
(3) System level simulation of low frequency of circuit system. The level of simulation is different. It can also be said that the focus of the research problem is different. Considering the speed and convergence of the simulation, models with different precisions should be adopted. In recent years, domestic and foreign scholars have done a lot of research work on the detailed models of power switching components and magnetic devices commonly used in inverter circuits, and published many research results.
With the wide application of computer simulation technology, various simulation softwares have appeared, such as the system-oriented switching converter simulation software Pspice developed by the Virginia Power Electronics Center, the Analgy Saber software and the power system transient analysis software EMTP. Simulation software ATP for power converter and electric drive, and simulation software PECAN for power electronic closed-loop system analysis. In 1980, the MATLAB language developed by Dr. Cleve Moler of the United States played a huge role in promoting the later control system theory and computer-aided design technology. In recent years, with the continuous enrichment and improvement of the MATLAB language, it has produced a huge impact on the international control community. Impact. Currently, the language has become the most important and popular language in the international control community. The MATLAB language is very convenient to use and powerful. In addition to traditional interactive programming, it also provides convenient and reliable tools for matrix operations, graphics rendering, data processing, image processing and convenient Windows programming.
It can be firmly believed that with the application of electronic simulation technology, the development process of the new generation of resistance welding machine will be greatly accelerated.
7, the conclusion
It is foreseeable that the development of the above new technologies will have great development potential and market in the future. At the same time, some traditional resistance welding research techniques, such as non-destructive testing of welding quality, will be further developed. In short, resistance welding technology is about to face new development opportunities and challenges. Every scientific and technical worker engaged in resistance welding technology research should have a clear understanding of this.
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