Inverter arc welding power source, also known as arc welding inverter, is a new type of welding power source. This type of power supply generally takes AC mains voltage of three-phase mains frequency (50 Hz), rectifies and filters it through an input rectifier, converting it to DC.
It then uses high-power switching electronics (such as SCR thyristors, GTO transistors, MOSFETs, or IGBTs) to switch the switch state by inverting it at medium-frequency AC voltage ranging from several kHz to tens of kHz, which it is then reduced by a transformer to a voltage suitable for welding.
After rectification and filtering through an inductance, it produces a stable DC welding current.
1. Inverter and inverter arc welding power supply
A device that converts DC to AC is called an inverter.
The conversion sequence can be simply represented as: AC mains frequency (after rectification and filtering) → DC (after inversion) → Medium frequency AC (after voltage reduction, rectification and filtering) → DC. If expressed in symbols, it is:
AC → DC → AC → DC
This system is generally used because if the reversed and reduced AC current is used directly for welding, the high frequency will result in a large reactive power in the welding circuit, which will greatly reduce the active power. Therefore, rectification is necessary again.
2. Characteristics of inverter power supply
The basic characteristic of inverter arc welding is that it operates at high frequency, which brings many advantages.
This is because the potential E of the transformer, whether it is the primary or secondary winding, has the following relationship with the frequency f of the current, magnetic flux density B, iron core section area S and number of turns W of the winding:
E = 4.44fBSW
And the terminal voltage of the winding U is approximately equal to E, that is:
u ≈ E = 4.44fBSW
When U and B are determined, if the frequency f is increased, S will decrease and W will decrease. Therefore, the weight and volume of the transformer can be greatly reduced. This makes the weight and volume of the entire machine significantly smaller.
Furthermore, due to the increase in frequency and other factors, it brings many advantages compared with traditional arc welding power sources. The main features are as follows:
(1) Small size, light weight, material saving and easy transportation and movement.
(2) High efficiency and energy saving, with efficiency up to 80% to 90%, saving more than a third of electricity compared with traditional welding machines.
(3) Good dynamic characteristics, easy arc starting, stable arc, beautiful weld formation and less spatter.
(4) Suitable for combination with robots to form an automatic welding production system.
(5) It can be used for various purposes, completing various welding and cutting processes.
Due to the series of advantages of inverter power sources mentioned above, it has developed rapidly since its emergence in the late 1970s. In industrialized countries such as the United States and Japan, its scope of application is quite extensive.
Switching elements used in inverter power supplies now include SCR (thyristor), GTR (transistor), MOSFET (field effect transistor), and IGBT (a type of electronic element that combines the advantages of GTR and MOSFET) .
IGBT has the potential to replace other switching elements. IGBT inverter welding machine is a significant progress in welding technology and a new development trend.
The welding machine head converts the power output from the welding power source into welding heat and continuously feeds it into the welding material while the machine head moves forward to perform welding.
Electric welding tongs used in manual arc welding need to be manually pushed down and moved forward to form a weld bead as the welding rod melts. Automatic welding machines have automatic wire feeding mechanisms and machine head moving mechanisms to move the machine head forward.
There are two commonly used types: transport and suspension types.
Welding heads for spot welding and projection welding are electrodes and their pressure mechanisms, which are used to apply pressure and electricity to the workpiece.
For seam welding, there is a transmission mechanism to move the workpiece. For butt welding, static and dynamic fixtures and fixture clamping mechanisms are required, as well as movable fixtures and disturbance mechanisms.
3. Development direction of inverter power sources
The general development trend of inverter power sources is large capacity, light weight, high efficiency, modularization and intelligence, with improvement in reliability, performance and application expansion as its core. It is increasingly used in various methods of arc welding, resistance welding, cutting processes, etc.
Efficiency and high power density (miniaturization) are the main objectives pursued by international arc welding inverters. High frequency and reduced power consumption of key components are the main technical approaches to achieve this goal.
At present, in countries such as Japan, Europe and other regions, the inverter arc welding technology around 20kHz has matured, the quality of products is high, and the products have been serialized.
Analyzing harmonic suppression of inverter welding machine power supply
1. Harmonic analysis of arc welding inverter power supply
1.1 Reasons for Harmonic Generation
Since the first 300A thyristor arc welding inverter power supply, arc welding inverter power supply has undergone significant development, experiencing thyristor inversion, high power transistor inversion, field effect inversion and IGBT inversion. Its capacity and performance have been greatly improved.
At present, arc welding inverter power supply has become the main product of welding equipment in industrialized countries.
As a typical power electronic device, although the arc welding inverter power supply has the advantages of small size, light weight and good control performance, its circuit contains rectification and inversion links, which cause waveform distortion of current and generate a large number of high-order harmonics.
There is a serious phase shift between high-order voltage and current harmonics, which results in a very low power factor of the welder. The main reasons for the generation of harmonics are the following:
(1) Inverter power supply internal interference sources
The inverter power supply is a system that combines strong and weak currents. During the welding process, the welding current can reach several hundred or even thousands of amps. Because the current generates a large electromagnetic field, especially in welding power supply systems with high inversion frequency, rectifier tubes, high-frequency transformers, control system oscillations, high-frequency arc ignition and power tube switches will produce strong harmonic interference.
In addition, when the argon tungsten arc welding machine uses high-frequency arc ignition, it uses a frequency of up to several hundred thousand Hertz and a high voltage of several kilovolts to break the air gap and form an arc, so the high frequency arc ignition is also a strong source of harmonic interference.
For computer-controlled intelligent arc welding inverter power supplies, as the operating speed of the computer control system used increases, the control board itself has become a source of harmonic interference, and higher requirements have been placed on the control board wiring.
(2) External interference sources of the inverter power supply
Pollution in the electrical network is a severe interference for the energy supply system because the loads applied to the electrical network constantly vary, causing more or less harmonic interference to the electrical network.
Large-power equipment can cause distortion of the mains voltage waveform, accidental factors can cause momentary power outages, and high-frequency equipment can generate high-frequency pulses and peak pulse components in the mains voltage waveform electrical.
In addition, in the welding workshop, due to the possibility of interconnection between grounding wires of different welding power sources during use, if corresponding measures are not taken, harmonic signals with high-frequency components can easily enter the control system , causing the power supply to malfunction or even damage it.
1.2 Characteristics and Risks of Harmonics
Arc welding inverter power supply is known for its high-efficiency power conversion. With the development of power control devices in practical and large capacity directions, arc welding inverter power supplies will also enter an era of high frequency and large capacity.
For the mains, the power supply of the arc welding inverter is essentially a large rectifier power supply. Due to the sharp rise and fall of pulses generated by power electronic components during switching, serious harmonic interference is caused.
The input current of the inverter power supply is a kind of peak waveform, which contains a large number of high-order harmonics in the power grid.
There is a serious phase shift between high-order voltage and current harmonics, resulting in a very low power factor of the welder. Low-frequency distortion is currently a common problem in power electronic equipment, attracting significant attention in the communications and home appliance industries.
In addition, at present, inverter welders mainly use rigid switching methods, inevitably causing harmonic interference in space during the power component switching process.
These interferences form interference conducted through near-field and far-field coupling, seriously polluting the surrounding electromagnetic environment and power supply environment, not only reducing the reliability of the inverter circuit itself, but also seriously affecting the operational quality of the power grid and adjacent equipment. .
2. Harmonic suppression measures commonly used in arc welding inverter power supplies
2.1 Passive Filters (PF)
The traditional method for harmonic suppression and reactive power compensation is electrical passive filter technology, also known as indirect filtering method. This method involves using electrical capacitors or other passive devices to construct a passive filter with nonlinear loads that require compensation connected in parallel, providing a low impedance path for harmonics while providing the reactive power needed by the load.
Specifically, the 50 Hz distorted sine wave is decomposed into the fundamental wave and several related principal harmonic components, and then, using the series resonant principle, each filtering branch consisting of L, C (or R) is tuned (or biased tuning) for several main harmonics. frequencies to form a low impedance path and filter them (2-3). It passively defends and reduces harmonic damage to electrical equipment that has already been generated.
Passive filtering schemes are mature, low-cost technologies, but they also have the following disadvantages:
(1) the filtering effect is affected by the system impedance;
(2) due to the fixed resonant frequency, it has low effectiveness in cases of frequency deviation;
(3) may cause overload due to resonance in series or parallel with the system impedance. In small and medium power situations, passive filters are gradually being replaced by active filters.
2.2 Active Filters (AF)
As early as the early 1970s, scholars proposed the basic principle of active power filters. However, due to the lack of high-power switching devices and corresponding control technologies at that time, only compensation currents generated by linear amplifiers and other methods could be used, which had fatal weaknesses in low efficiency, high cost, and difficulty in large scale. capacity.
With the improvement of the performance of power semiconductor switching device and the development of corresponding PWM technology, it has become possible to develop a large-capacity, low-loss harmonic current generator, making active filtering technology practical.
When a harmonic source appears in the system, a compensation current equal in magnitude and opposite in phase to the harmonic current is generated by some method and connected in parallel with the circuit becoming the harmonic source to cancel out the harmonic component of the harmonic source, allowing that the current on the DC side contains only the fundamental component, without harmonic components.
When the harmonic current generated by the harmonic source cannot predict what the higher order harmonic current is or changes at any time, the harmonic current signal ih is detected from the load current il and then modulated by the modulator and converted into a mode switching control current according to a specified method to operate the current inverter to generate compensation current ifm and inject it into the circuit to cancel the harmonic current ih.
The main circuit of the inverter generally uses a DC/AC full-bridge inverter circuit, where the switching devices can be GTO, GTR, SIT or IGBT and other high-power controllable power semiconductor devices to control the waveform of the inverter. output current by the switching device on off state, generating the required compensation current.
Active electrical filters are the most promising power devices for suppressing grid harmonics and compensating reactive power, improving the quality of power supply.
Compared to electrical passive filters, they have the following advantages:
(1) dynamic compensation is achieved, and changes in the frequency and magnitude of harmonic and reactive power can be compensated, with a very fast response to changes in the compensation object;
(2) simultaneous compensation of harmonics and reactive power is possible, with the size of the compensated reactive power continuously adjustable;
(3) no energy storage device is needed when compensating reactive power, and the required capacity of the energy storage device when compensating harmonics is not large;
(4) even if the compensated current is very large, the active electrical filter will not overload and can work normally for compensation;
(5) is not easily affected by mains impedance and does not easily resonate with mains impedance;
(6) can track changes in power grid frequency, and the compensation performance is not affected by changes in frequency;
(7) can compensate single harmonic and reactive power or focus on compensating multiple harmonics and reactive power.
3. Smooth switching technology
As power electronics technology develops toward high frequency and high power density, switching loss and harmonic interference from physical switching become increasingly prominent.
Soft switching technology is beneficial for any switching power converter in terms of improving conversion efficiency, device utilization, enhancing electromagnetic compatibility and device reliability.
It is particularly necessary in some special cases (such as power density requirements or limited heat dissipation conditions). Between the two types of soft switching technology, passive soft switching without additional switching devices, detection methods and control strategies has many advantages such as low additional cost, high reliability, high conversion efficiency and high performance-price ratio.
In the field of single-ended converter manufacturing, it has basically established a dominant position.
As for topology, the series inductance and parallel capacitance method is the only means of passive soft switching, and the so-called passive soft switching technology derived from it is actually a lossless absorption technology.
Regarding bridge inverter circuits, from the initial power absorption type to the later proposed partial feedforward type and lossless solutions, they all have problems such as strong load dependence, narrow working frequency range, high additional stress, network excessively complex, etc., making its practicality is relatively poor.
At the same time, under the modularization trend of switching power devices, the space available for placing absorbing elements is becoming smaller and smaller, and lossless absorption technology suitable for inverter modules is rarely seen in the literature.
Overall, passive absorption technology suitable for inverter module applications is still undergoing further research and development due to its special structure and difficulty.
4. Conclusion
Arc welding inverter power supplies generate a large amount of harmonics, which can cause serious damage.
To suppress harmonics and improve the power factor, corresponding suppression measures must be taken. The traditional passive filter method has obvious limitations, which restricts its application, while the active filter method can compensate for the shortcomings of passive filters, effectively suppressing harmonics in arc welding inverter power supplies and has been widely used. Soft switching technology can also achieve good filtering effects to a certain extent.
