- Typical circuit and principle of operation of the inverter
- Upgradeable units
- Improving heat dissipation
- Welding current display
- Increasing the ON time
- Output choke winding
The characteristics of most budget inverters cannot be called outstanding, at the same time, few people will give up the pleasure of using equipment with a significant margin of reliability. Meanwhile, there are many ways to improve an inexpensive welding inverter..
Typical circuit and principle of operation of the inverter
The more expensive a welding inverter is, the more auxiliary units in its circuit are involved in the implementation of special functions. But the power converter circuit itself remains practically unchanged even with expensive equipment. The stages of the transformation of the mains electric current into welding current are quite easy to trace – a certain part of the overall process takes place at each of the main nodes of the circuit.
From the mains cable through the protective switch, the voltage is supplied to the rectifier diode bridge, coupled with high-capacity filters. On the diagram, this section is easy to see, there are impressive “banks” of electrolytic capacitors. The rectifier has only one task – to “unfold” the negative part of the sinusoid symmetrically upwards, while capacitors smooth out the ripple, bringing the direction of the current to almost a pure “constant”.
The scheme of the welding inverter
Further, according to the diagram, there is the inverter itself. This part is also easily identifiable and houses the largest aluminum radiator. The inverter is built on several high frequency field effect transistors or IGBT transistors. Quite often, several power elements are combined in a common body. The inverter again converts direct current into alternating current, but at the same time its frequency is significantly higher – about 50 kHz. Such a conversion chain allows the use of a high-frequency transformer, which is several times smaller and lighter than a conventional one..
The output rectifier removes the voltage from the step-down transformer, because we want welding on direct current. Thanks to the output filter, the nature of the current changes from a high-frequency ripple to an almost straight line. Naturally, in the considered transformation chain there are many intermediate links: sensors, control and control circuits, but their consideration goes far beyond the scope of amateur radio electronics.
Welding inverter design: 1 – filter capacitors; 2 – rectifier (diode assembly); 3 – IGBT transistors; 4 – fan; 5 – step-down transformer; 6 – control board; 7 – radiators; 8 – choke
The most important parameter of any welding machine is the volt-ampere characteristic (VAC), due to which stable burning of the arc is ensured with its different lengths. The correct I – V characteristic is created by microprocessor control: the small “brain” of the inverter changes the operating mode of the power switches on the fly and instantly adjusts the parameters of the welding current. Unfortunately, it is impossible to reprogram the budget inverter in any way – the control microcircuits in it are analog, and replacing with digital electronics requires extraordinary knowledge of circuitry.
However, the “skills” of the control circuit are quite enough to level the “curvature” of a novice welder, who has not yet learned to hold the arc stably. It is much more correct to focus on eliminating some “childhood” diseases, the first of which is a strong overheating of electronic components, leading to degradation and destruction of power keys.
The second problem is the use of radioelements of questionable reliability. Elimination of this drawback greatly reduces the likelihood of breakdowns after 2–3 years of device operation. Finally, even a novice radio technician will be quite capable of realizing the indication of the actual welding current to be able to work with special brands of electrodes, as well as carry out a number of other minor improvements.
Improving heat dissipation
The first drawback that the overwhelming majority of inexpensive inverter devices suffer from is a poor heat removal circuit from power switches and rectifier diodes. It is better to start refinement in this direction by increasing the intensity of forced airflow. As a rule, case fans are installed in welding machines powered by service circuits with a voltage of 12 V. In “compact” models, forced air cooling may be completely absent, which is certainly nonsense for electrical engineering of this class.
It is enough to simply increase the air flow by installing several such fans in series. The problem is that the “native” cooler will most likely have to be removed. To work effectively in a series assembly, fans must have an identical shape and number of blades, as well as rotational speed. It is extremely easy to assemble identical coolers into a “stack”; it is enough to pull them together with a pair of long bolts along diametrically opposite corner holes. Also, do not worry about the capacity of the service power supply, as a rule it is enough to install 3-4 fans..
If there is not enough space inside the inverter to install the fans, one high-performance duct can be fitted outside. Its installation is easier for the reason that no connection to internal circuits is required, power is removed from the terminals of the power button. The fan, of course, must be installed opposite the ventilation louvers, some of which can be cut out to reduce the aerodynamic drag. Optimal air flow direction – towards the exhaust from the housing.
The second way to improve heat dissipation is to replace the standard aluminum radiators with more efficient ones. A new radiator should be chosen with the largest number of as thin as possible fins, that is, with the largest contact area with air. It is optimal to use radiators for cooling computer CPUs for these purposes. The process of replacing radiators is quite simple, just follow a few simple rules:
- If the standard radiator is insulated from the flanges of radio elements with mica or rubber gaskets, they must be retained when replacing.
- To improve thermal contact, you need to use silicone thermal grease.
- If the radiator needs to be cut to fit into the case, the cut edges must be carefully processed with a file to remove all burrs, otherwise dust will settle on them abundantly.
- The radiator must be tightly pressed against the microcircuits, therefore, you must first mark and drill the mounting holes on it, you may need to cut a thread in the body of the aluminum sole.
Additionally, we note that it makes no sense to change the piece heatsinks of free-standing keys, only heat sinks of integrated circuits or several high-power transistors installed in a row are replaced.
Welding current display
Even if a digital current setting indicator is installed on the inverter, it does not show its real value, but a certain service value scaled for visual display. The deviation from the actual current value can be up to 10%, which is unacceptable when using special brands of electrodes and working with thin parts. You can get the real value of the welding current by installing an ammeter.
A digital ammeter of the SM3D type will cost within 1 thousand rubles, it can even be neatly built into the inverter case. The main problem is that a shunt connection is required to measure such high currents. Its cost is in the range of 500-700 rubles for currents of 200-300 A. Please note that the type of shunt must comply with the recommendations of the ammeter manufacturer, as a rule, these are 75 mV inserts with an intrinsic resistance of about 250 μOhm for a measurement limit of 300 A.
You can install the shunt either on the positive or negative terminal from the inside of the case. Usually, the size of the connecting bus is sufficient to connect an insert with a length of about 12-14 cm. It is impossible to bend the shunt, therefore if the length of the connecting bus is not enough, it must be replaced with a copper plate, a pigtail made of a cleaned single-wire cable or a piece of welding core.
The ammeter is connected with measuring outputs to the opposite terminals of the shunt. Also, for the digital device to operate, it is required to supply a supply voltage in the range of 5–20 V. It can be removed from the fan connection wires or found on the board points with potential for supplying control microcircuits. The ammeter’s own consumption is negligible.
Increasing the ON time
The duty cycle in the context of welding inverters is more reasonably called the load duration. This is the part of the ten minute interval in which the inverter is directly doing work, the remaining time it must idle and cool..
For most inexpensive inverters, the actual duty cycle is 40–45% at 20 ° C. Replacing radiators and a device for intensive airflow can increase this figure to 50-60%, but this is far from a ceiling. It is possible to achieve a PN of the order of 70-75% by replacing some radioelements:
- The capacitors of the inverter key binding must be replaced with elements of the same capacity and type, but designed for a higher voltage (600-700 V);
- Diodes and resistors from the key binding should be replaced with elements with a higher power dissipation.
- Rectifier diodes (valves), as well as MOSFETs or IGBT transistors can be replaced with similar, but more reliable.
The replacement of the power keys themselves should be discussed separately. First, you need to rewrite the marking on the element body and find a detailed datasheet for a specific element. According to the passport data, it is quite simple to choose an element for replacement, the key parameters are the frequency range limits, operating voltage, the presence of a built-in diode, the type of housing and the limiting current at 100 ° C. It is better to calculate the latter with your own hands (for the high-voltage side, taking into account losses on the transformer) and purchase radioelements with a limit current margin of about 20%. International Rectifier (IR) or STMicroelectronics are considered the most reliable manufacturers of this kind of electronics. Despite the rather high price, it is highly recommended to purchase parts of these particular brands..
Output choke winding
One of the simplest and at the same time most useful additions for a welding inverter will be winding an inductive coil that smooths out the DC ripple that inevitably remains during the operation of a pulse transformer. The main specificity of such an undertaking is that the choke is made individually for each individual device, and can also be adjusted over time as the electronic components degrade or when the power threshold changes..
To make a choke, you will need nothing at all: an insulated copper conductor with a cross section of up to 20 mm2 and a core, preferably ferrite. As a magnetic circuit, either a ferrite ring or an armored transformer core is optimal. If the magnetic core is made of sheet steel, it must be drilled in two places with an indent of about 20-25 mm and pulled together with rivets in order to be able to cut the gap without any problems.
The choke starts to work, starting from one full turn, but the real result is visible starting from 4-5 turns. When testing, turns should be added until the arc begins to stretch perceptibly, preventing separation. When it becomes difficult to cook with a separation, you need to throw off one turn from the coil and connect a 24 V incandescent lamp parallel to the choke.
Fine tuning of the throttle is carried out using a plumbing screw clamp, which can reduce the gap in the core, or a wooden wedge, which can increase this gap. It is necessary to ensure that the burning of the lamp when igniting the arc is as bright as possible. It is recommended to make several chokes for operation in ranges up to 100 A, from 100 to 200 A and more than 200 A.
All “attachments”, such as a choke or an ammeter, are best mounted with a separate attachment, which is connected to the gap of any of the welding cores by means of a bayonet plug. Thus, there will be enough space for ventilation inside the inverter housing, and additional devices can be easily disconnected as unnecessary..
It should be remembered that it will not be possible to carry out a radical, deep modernization, in other words, “RESANTU” cannot be turned into KEMPPI by reasonable forces and means. However, the manufacture of fixtures and minor modifications to the equipment is a great way to better learn the technology of arc welding and get into the professional subtleties..