How to repair welding inverters with their own hands

Welding in the inverter way has found wide application today. With all the reliability of inverters, there are often cases of their failure for various reasons. In such moments, the question of how to repair the welding inverter with your own hands is acute.

Welding inverter device

The device welding inverter.

Repair of welding inverters itself is not a big problem. For its implementation, it is necessary to know the design of the apparatus and the basic principles of repair of equipment of this type. When repairing you will need basic knowledge of the basics of electrical engineering and radio engineering in the installation of simple circuits.

General information about inverters

Welding inverter functionality

The functionality of the welding inverter.

The inverter is a source of direct current for ignition and maintenance of the electric arc during welding of metals. The principle of operation of welding inverters is based on the fact that a welding current of considerable power is obtained by high-frequency transformation, which allows to significantly reduce the size of the transformer, as well as increase the stability and controllability of the output current.

The whole process of obtaining the desired current includes the following steps: primary rectification of the current received from the mains; transformation of primary direct current into high frequency current; an increase in current with a corresponding decrease in the voltage in a high-frequency transformer; secondary rectification current output power.

The current is rectified by means of diode bridges of corresponding power. The frequency change is carried out by powerful transistors. The required output current is provided by a high-frequency transformer.

Back to table of contents

Inverter design

Welding inverters consist of several main blocks. The power supply provides stabilization of the input signal. The block circuit is based on a multiwinding choke with control using transistors and energy storage in a capacitor. In addition, diodes are used in the throttle control system. The power supply is located separately from other units and, as a rule, is separated from them by a metal partition.

Electric circuit welding inverter

Electric circuit welding inverter.

The basis of the welding inverter is the power unit, which provides all the conversion from the primary current coming from the power supply to the output welding current. The power unit consists of the following boards: a primary rectifier, an inverter converter, a high-frequency transformer, and a secondary rectifier.

The primary rectifier is a diode bridge to which an electric current of no more than 40 A (most common 25-32 A) with a voltage of 200-250 V and a frequency of 50 Hz is applied. The inverter converter is a power transistor with a capacity of at least 8 kW (at a current of 32 A) with an operating voltage of up to 400 V. The signal from the converter goes out with a frequency of up to 100 kHz (most often 50-55 kHz).

The high-frequency transformer has tape windings and increases the current to 200-250A at a voltage in the secondary winding of not more than 40 V. A secondary rectifier is assembled on the basis of high-power diodes with an operating current of at least 250 A for an operating voltage of up to 100 V. There is a mandatory cooling with radiators, fans are also installed. To stabilize the output signal on the output plate is installed choke.

Back to table of contents

Control and protection units

Sectional welding inverter

Welding inverter in section.

The control unit is assembled on the basis of the master oscillator or wide-pulse modulator. If the circuit is assembled on the basis of a generator, then a microcircuit is used in its quality. In addition to it, on the control plate are placed a resonant choke and resonant capacitors in the amount of 6 or 10 pieces. Cascade control is provided by a transformer.

Protection schemes are usually assembled on the plateau of the power unit to protect the corresponding element. To protect against overloads, a circuit based on the 561LA7 chip is used. In the system of protection of rectifiers and converter, snubbers based on capacitors K78-2 and resistors are used. Thermal protection of the elements of the power unit is provided by the installation of thermal switches.

Back to table of contents

Causes of inverter failure

Most of the breakdowns of welding inverters are caused by impaired operation of devices. Frequent cause of short circuits in electrical circuits is moisture ingress. Unpredictable effects can cause dust concentration inside the inverter.

Often the reason for the repair of the device is the attempt to produce work for which the inverter is not designed. For example, a small-sized apparatus is not able to provide cutting of a railroad rail - this will cause unplanned overloads.

Simplified circuit of the power part of the welding inverter

Simplified circuit of the power part of the welding inverter.

Of domestic reasons, it should be noted a strong decrease in voltage in the network. This can be observed everywhere, but is especially important for country work and in rural areas. Reducing the voltage to 190 V can have an extremely negative effect on the performance of the inverter.

Quite often, the failure of the inverter is caused by poor-quality fixing of the supply or outgoing cable in the terminal blocks (terminal clamps). With the weakening of the contact at the junction there is an overheating zone, and sometimes sparking.

Repair of welding inverters due to failure of the circuit elements most often occurs when using low-quality parts. In addition, damage in electrical circuits may occur due to overheating of the power elements, i.e. if their cooling is not enough.

Back to table of contents

Main types of faults

Among the many possible faults should highlight the main types. First of all, these are the cases when in the presence of an input voltage there is no current at the output of the inverter. Such a malfunction is due to blown fuses or violation of the integrity of the electrical circuit, which can occur in any area of ​​the inverter.

Scheme welding inverter with a soft ignition system

The scheme of the welding inverter with system of soft ignition.

Another type of malfunction: the welding output current does not reach the desired values ​​even at maximum settings. This inverter malfunction can be caused both by an insufficient input voltage and a loss in the terminals, as well as faults that have occurred in the power unit.

Frequent spontaneous disconnections of the inverter indicate that there is a short circuit in the electrical circuit or excessive overheating of the power unit elements. In this case, the protection system operates normally and provides emergency shutdown.

The instability of the welding arc and the absence of adjustment of the welding current indicate the presence of faults in the power unit or the control unit. Increased noise emitted by the inverter indicates the presence of overloads and may lead to subsequent failure of the system. The protection system of the inverter works with violations. The same can be said in the case when there is a heating of the apparatus itself. In the latter case, weak cable fastening in the terminal block is added to the possible reasons.

Back to table of contents

General procedure for repairing welding inverters

Any repair of the inverter for welding should begin with an external inspection. Visually determined by the presence of mechanical damage to the body and traces of a short circuit (blackening, burn-through). Then the quality of fixing the cables in the terminal blocks (at the input and output of the inverter) is checked.

Block diagram of the input rectifier

Block diagram of the input rectifier.

Regardless of the test results, tighten the clamping elements with a screwdriver or key. Check the fuse integrity with a tester and, if necessary, replace it.

If the cause of the fault has not been eliminated, the cover of the inverter is removed. After the cover is removed, a visual inspection is carried out in order to identify breaks in the electrical circuit or traces of the effects of a short circuit. Measured input voltage and input current strength, as well as their values ​​at the output of the welding inverter using a tester or multimeter.

In the absence of obvious faults, block-wise control of the integrity of the electrical circuit is carried out. The test starts with the power supply and goes gradually to other blocks.

Back to table of contents

Repair power inverter unit

Checking the power unit and the control unit is to check the main elements. The most frequent malfunction of the power unit is the failure of the power transistor, therefore, the test of this unit should begin with it.

Usually a faulty power transistor has obvious signs of damage (deformation of the body, traces of burnout). If visual inspection is not enough, then the transistor should ring a multimeter. A failed transistor must be replaced. Installing it on a plateau is carried out using thermal paste KPT-8.

The fault of the transistor, as a rule, is accompanied by the failure of one of the drivers. These control transistors (or chips) should be checked with an ohmmeter. All defective parts must be unsoldered and replaced with new ones.

Rectifier diode bridges are usually more reliable than transistors, but they should also be checked. For a more accurate determination of the fault, remove the diode bridge completely from the plateau and ring it in a state where all the diodes are interconnected. When the resistance is close to zero, you should look for a specific diode that has failed. The faulty diode is being replaced.

It should be borne in mind that when installing an analogue, it is necessary to fulfill the condition that inverters usually use high-speed diodes, and they must also be changed for high-speed diodes of the same power. When replacing special attention should be paid to the mounting radiator cooling. When they are connected to a diode, KPT-8 heat conductive paste should be used.

Repair of the control unit is associated with the verification of the parameters of parts that issue signals of complex types. This causes problems in troubleshooting with oscilloscopes. Repair of the unit should be entrusted to specialists.

If at overheating of the elements in the power unit the device does not shut down, then the fault should be sought in the thermal switches.

To do this, you first need to check the reliability of their attachment to the part on which the temperature is controlled. If it turns out that the thermal switch does not work, it must be replaced.

Back to table of contents

Necessary tool

During the inspection and repair of inverters there is a need for the following tools and measuring instruments:

  • pliers;
  • 40 W and not less than 100 W soldering irons;
  • screwdriver;
  • wrenches and end wrenches;
  • knife;
  • clippers;
  • tester;
  • ammeters 50 A and 250 A;
  • voltmeters at 50 V and 250 V;
  • frequency counter;
  • oscilloscope;
  • ohmmeter;
  • calipers;
  • micrometer.

Failure of a welding inverter is not always a big problem. If you properly analyze the causes of its failure and find a fault, then the repair can be done on its own.

Add a comment