The traditional welding machine, which necessarily includes a bulky transformer, has recently been actively replaced by inverters. In order to understand how a welding inverter works, it is necessary to understand its design, principle of operation, and operational features, which determine the advantages and reveal the disadvantages of this device.
The invertor welding machine is used for welding of various details from metal.
General principles of the inverter
Unlike the more familiar welding transformers, in this device the conversion of electrical voltage into a welded current occurs in several stages: by means of a low-power transformer, in dimensions almost commensurate with a pack of cigarettes, and an electronic circuit. Also, the inverter unit has a control system (unit), which greatly facilitates the welding process and allows you to create a high-quality weld. How does an inverter welding machine work?
Device of the invertor welding machine.
First, the input current of 220 V with a frequency of 50 A passes through the rectifier of the welding machine, is converted into a constant one and is simultaneously smoothed by filters (usually in the form of electrolytic capacitors). The resulting DC voltage by means of a modulator assembled on semiconductors is converted again into an alternating one, but at a higher frequency (up to 100 kHz). Next is straightening and lowering the voltage to the value required for welding the metal.
The use of a high-frequency converter allowed the use of a transformer of relatively small size, as a result of which the dimensions and weight of the inverter apparatus were significantly reduced. For example, to get a welding current of 160 amperes in the inverter, you will need a transformer weighing approximately 0.25 kg: to achieve a similar result on a traditional welding unit, you will have to use a transformer weighing at least 18 kg. When an inverter welding machine is in operation, electronics play an important role: it provides feedback to the electric arc, which makes it possible to strictly control and maintain its parameters at the required level. Their slightest deviation is immediately "suppressed" by microprocessors. All these "additions" guarantee a stable arc, which guarantees high quality of work when using an inverter type welding machine.Back to table of contents
How does the main electronic circuit work?
Internal device welding inverter.
In a network rectifier, the electric current (220 V) is rectified by means of a strong diode bridge (usually a diode assembly), the smoothing of alternating current pulsations is produced by electrolytic capacitors. Since Since the diode bridge is extremely hot during operation, it is installed on cooling radiators. Plus, there is a thermal fuse that triggers when the diodes are heated over + 90 ° C and protects an expensive diode assembly. Next to the rectifier bridge, electrolytic capacitors (round “barrels”) stand out with their dimensions, the capacitance of which varies between 140-800 microfarad. Additionally, a filter is installed in the welding machine that prevents radio interference.
The circuit of the inverter itself includes 2 powerful transistors (usually MOSFET or IGBT), also installed on radiators. These semiconductors switch the current passing through a pulse transformer: at the same time, the switching frequency reaches tens of kHz. As a result, an alternating current of high frequency is formed. To protect expensive transistors from voltage surges, protective circuits are used, including resistors and small capacitors. After the transistors have “worked out”, a lower voltage is removed from the secondary winding of the step-down transformer (up to 70 V), but the current can be 130-140 or more amperes.
The electronic circuit of the inverter welding machine.
In order to obtain a constant voltage at the output, a reliable output rectifier is used. Usually this device is assembled on the basis of dual diodes having a common cathode. These devices are characterized by maximum speed, i.e. quickly open and close, while the recovery time does not exceed 50 nanoseconds. The last quality is very important, because These diodes rectify the current of a very high frequency: ordinary semiconductors could not cope with this task, they would not have time to switch. Therefore, when repairing, it is important to replace these diodes with the same high-frequency ones (the most common devices of type VS 60CPH03, STTH6003CW, FFH30US30DN), which should be designed for a reverse voltage of 300 V and a current of 30 A.
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Work Management Board
A voltage regulator designed for 15 V and mounted on a heat sink radiator is used to power the board elements. The supply voltage comes from the main rectifier. One of the functions of the power conditioner is to supply voltage to the relay, which ensures a “smooth start” of the device. When a voltage is applied, capacitors begin to charge: the voltage increases and, in order to protect the diode assembly, a limiting circuit is applied, which includes a powerful (8 W) resistor. As soon as the capacitors are charged, the inverter will work, the relay will close its contacts, and the resistor will not participate in further work.
Welding machine control.
In addition to the voltage regulator, in the electronic circuit of the inverter there are many other systems that provide high performance device. The main of these electronic components are:
- Control system and drivers: here the main element is the PWM controller chip, which “deals” with controlling the operation of powerful transistors;
- Adjusting and control circuits: the main element is a current transformer, whose task is to control the current output of the transformer;
- The system for monitoring the supply voltage and output current: consists of an op-amp (operational amplifier) assembled on a microchip (for example, LM324). The purpose of the system is, if necessary, to include emergency protection, to monitor the operation and operability of the main elements of the electronic unit.
Special features of inverters
In addition to the advantage of low weight, inverter-type welding machines allow the use of electrodes for both alternating and direct current. This is especially important when welding elements from cast iron, non-ferrous metals. Most models have options that make the welding process more convenient, especially these additions are suitable for those who are just learning to master welding:
- hot start (or Hot start): sets the optimal parameters for ignition of the arc;
- anti-sticking (or AntiSticking): in case of a short circuit, the welding current is automatically reduced to the minimum, as a result of which the electrode does not stick to the part:
- ArcForce: this option delivers the optimal current at the time of metal separation from the electrode, which also prevents sticking.
Good arc ignition in the welding inverter is realized due to the independence of the output voltage from the input that is present in traditional welding devices. In conventional welding, too little current causes the electrode to stick, and too much is fraught with over-burning the metal part. Those. when working with an inverter, it is impossible to “underbore” or “burn” the part, which guarantees the strength of the seam (there are no shells or cracks in it).
Another feature of the inverter - the lack of compliance with the length of the arc.
In a conventional apparatus, it is necessary to maintain a distance of about 2 diameters of the electrode to the junction of the parts to be joined, otherwise the current value will vary. Inverters keep the current in a strictly defined framework, moreover, it is constant, not alternating. This allows not so critical to look at the length of the arc, which facilitates the work, especially if the welder is a beginner. The quality of the seam does not depend on the length of the arc.