All inverter welding machines are built, despite the abundance of grades, on the same principle. The output current of the welding inverter reaches 140 A or more with an arc voltage of about 25 V. The circuit parameters are selected so that a power of about 4-5 kW is consumed from a single-phase network. The manufacturer, as a rule, is China. For some users, devices last for years, for others for several days or weeks. In most cases, a failed device can be repaired.
There are several reasons why this technique fails:
ingress of moisture (although in many products the boards are varnished) and dust, especially metal. Experienced welders recommend using a “grinder” away from the welding machine, since its cooling fan will draw conductive dust into the housing;
poor-quality contacts in the wires connecting the mains voltage, too long wires;
failures of cooling fans with their subsequent jamming.
To effectively repair these products, you need an oscilloscope that must be powered (from a 230 V / 50 Hz network) through an isolation transformer. To do this, you can use a power transformer from an old color TV. Turning on via a transformer will exclude the possibility of electric shock to the repairman, since the entire power circuit of the welding inverter is galvanically connected to a 230 V / 50 Hz network.
The repair experience of such devices shows that most of the malfunctions are associated with failures of the soft-start relay and secondary power supply (VIP). In case of VIP failure, the device does not turn on. VIPs usually produce a voltage of 12, 15 or 24 V. Its power is limited, it almost always works in heavy mode, and when power surges, jamming the blowing fans that feed from it, it immediately fails. In this case, the windings of its transformer are often destroyed. The transformer is easily disassembled after 5 minutes of boiling in water and rewinds. It is convenient to use high-temperature adhesive tape as inter-winding insulation, and in its absence, tapes cut from a kitchen sleeve for baking.
The most severe cases are when a power IGBT or FET transistor fails. Just changing them is pointless - they will “burn out” again. As a rule, “combustion” is accompanied by a short circuit in the mains supply circuit. A “dialer” with a multimeter indicates that the plus and minus smoothing capacitors of a 300 V rectifier are shorted.
In this case, immediately solder all the power transistors, all the diodes of their strapping and check. We check the rectifier diodes of the mains voltage. Sometimes half the power transistors remain intact (the first switching on can be done on them).You can try to turn on the inverter without power transistors. If the VIP is intact, the circuit will turn on, the soft-start relay will click, but the alarm indicator will light (there is no voltage at the inverter output). If 25-30 V is applied to the output terminals from an external power source, the alarm indicator should go out. At the same time, control pulses of different frequencies are observed at the output of the control board: with an accident of 10-20 kHz, without an accident - 45-50 kHz. Frequency must be checked!
Many IGBTs with a pulse frequency of 70–80 kHz shutters fail. And the quality of the ceramic capacitors of the control board is “made in China”, on which this frequency depends, you know which one. This, by the way, is one of the reasons for the “causeless” failure of power transistors, simply when the device is turned on.
It is necessary to check the presence and shape of the control pulses directly on the contacts of the IGBT inputs by soldering a capacitor with a nominal value of 1500-2000pF and in parallel a 200 Ohm resistor instead of gates. The pulses should be the same, with an amplitude of at least 12 V with some approach to the negative voltage region. At the slightest difference, check the driver elements.
In order to avoid severe damage and burnouts in the boards, the first turn on after repair is best done through a 230 V 100 W incandescent lamp that is connected in series.
Only after receiving the same control pulses, you can solder a couple of transistors, even without radiators, and try to turn on the welding inverter in the network.
Enabled, started. The accident is off. The output of the inverter is 66-80 Volts. Do not rush to cook! Check current feedback operation. In the absence of ballast, a 2-3 ohm resistor made up of several in parallel is suitable. You can put it in water. We put the knob on the minimum welding current. Observing the pulses on the gates of the output transistors with an oscilloscope, we briefly connect this impromptu load to the output of the welding inverter and we see the response of the control loop by changing the width of the control pulses under the load.
Only now can we finally assemble the power unit and try to cook.
Often in the control loop, after damage to the welding inverter, malfunctions remain, and when you try to cook, an unlimited current develops - a “bang” occurs with a bunch of failed elements ...
The main thing in the repair is not to rush, moving in order, installing power transistors in the last turn, when everything is checked and viewed.
The range of IGBT transistors is very wide in terms of stock and prices. Choose any that are available to you. It is advisable to select devices with a laser engraving of the name according to a model photo from a trusted manufacturer. Some devices do not have built-in damping diodes - such devices are cheaper, but will not work reliably. Therefore, check for dampers in advance using a datasheet.
Author: Arkady Solun, Schuchinsk, Kazakhstan
Source: Electrician №1-2 / 2018
