A transformer is a necessary element of any welding source. It reduces the network voltage to the arc voltage level, and also provides galvanic isolation of the network and the welding circuit. It is known that the dimensions of a transformer are determined by its operating frequency, as well as the quality of the magnetic core material.
Note.
As the frequency decreases, the dimensions of the transformer increase, and as the frequency increases, they decrease.
Transformers of classical sources operate at a relatively low network frequency. Therefore, the weight and dimensions of these sources were mainly determined by the mass and volume of the welding transformer.
Recently, various high-quality magnetic materials have been developed that make it possible to somewhat improve the weight and size parameters of transformers and welding sources. However, a significant improvement in these parameters can only be achieved by increasing the operating frequency of the transformers. Since the frequency of the mains voltage is standard and cannot be changed, it is possible to increase the operating frequency of the transformer using a special electronic converter.
Block diagram of an inverter welding source
A simplified block diagram of an inverter welding source (IWS) is shown in rice. 1. Let's look at the diagram. The mains voltage is rectified and smoothed, and then supplied to the electronic converter. It converts direct voltage into high frequency alternating voltage. High-frequency alternating voltage is transformed using a small-sized high-frequency transformer, then rectified and fed into the welding circuit.
Transformer types
The operation of the electronic converter is closely related to the magnetization reversal cycles of the transformer. Since the ferromagnetic material of the transformer core is nonlinear and saturated, the induction in the transformer core can only grow to a certain maximum value Vm.
After reaching this value, the core must be demagnetized to zero or remagnetized in the opposite direction to the value – Vm. Energy can be transmitted through a transformer:
- in the magnetization cycle;
- in the magnetization reversal cycle;
- in both cycles.
Definition.
Converters that provide energy transfer in one cycle of transformer magnetization reversal are called single-cycle.
Accordingly, converters that provide energy transfer in both magnetization reversal cycles of the transformer are called two-stroke.
Single-ended forward converter
Advantages of single-ended converters. Single-cycle converters are most widely used in cheap and low-power inverter welding sources designed to operate from a single-phase network. Under conditions of sharply variable load, such as the welding arc, single-cycle converters compare favorably with various push-pull converters:
- they do not require balancing;
- they are not susceptible to such a disease as through currents.
Therefore, to control this converter, more simple circuit control, compared to what would be required for a push-pull converter.
Classification of single-cycle converters. According to the method of transferring energy to the load, single-cycle converters are divided into two groups: forward and flyback ( rice. 2). In forward converters, energy is transferred to the load at the moment of the closed state, and in flyback converters - at the moment of the open state of the key transistor VT. In this case, in a flyback converter, energy is stored in the inductance of the transformer T during the closed state of the switch and the switch current has the shape of a triangle with a rising edge and a steep cutoff.
Note.
When choosing the type of ISI converter between forward and flyback, preference is given to a forward single-ended converter.
Indeed, despite its great complexity, a forward converter, unlike a flyback converter, has high power density. This is explained by the fact that in a flyback converter a triangular current flows through the key transistor, and in a forward converter, a rectangular current flows. Consequently, at the same maximum switch current, the average current value of a forward converter is twice as high.
Main advantages flyback converter is:
- lack of a choke in the rectifier;
- possibility of group stabilization of several voltages.
These advantages provide an advantage to flyback converters in various low-power applications, such as power supplies for various household television and radio equipment; as well as service power supplies for the control circuits of the welding sources themselves.
Transformer of a single-transistor forward converter (SFC), shown on rice. 2, b, has a special demagnetizing winding III. This winding serves to demagnetize the core of the transformer T, which is magnetized during the closed state of the transistor VT.
At this time, the voltage on winding III is applied to diode VD3 in blocking polarity. Due to this, the demagnetizing winding does not have any influence on the magnetization process.
After turning off the transistor VT:
- the voltage on winding III changes its polarity;
- diode VD3 is unlocked;
- the energy accumulated in the transformer T returns to the primary power source Up.
Note.
However, in practice, due to insufficient coupling between the transformer windings, part of the magnetizing energy is not returned to the primary source. This energy is usually dissipated in the VT transistor and damping circuits (on rice. 2 not shown), degrading the overall efficiency and reliability of the converter.
Oblique bridge. This disadvantage is not present in two-transistor forward converter (DFC), which is often called "oblique bridge" (rice. 3, a). In this converter (due to the introduction of an additional transistor and diode), the primary winding of the transformer is used as a demagnetizing winding. Since this winding is completely connected to itself, the problems of incomplete return of magnetization energy are completely eliminated.
Let us consider in more detail the processes occurring at the moment of magnetization reversal of the transformer core.
A common feature of all single-ended converters is that their transformers operate in conditions with one-way magnetization.
Magnetic induction B (in a transformer with one-way magnetization) can only vary within the range from maximum Bm to residual Br, describing a partial hysteresis loop.
When transistors VT1, VT2 of the converter are open, the energy of the power source Up is transferred to the load through transformer T. In this case, the transformer core is magnetized in the forward direction (section a-b on rice. 3, b).
When transistors VT1, VT2 are locked, the current in the load is maintained by the energy stored in the inductor L. In this case, the current is closed through the diode VD0. At this moment, under the influence of the EMF of winding I, the diodes VD1, VD2 open, and the demagnetization current of the transformer core flows through them in the opposite direction (section b-a on rice. 3, b).
The change in induction ∆B in the core occurs practically from Bm to Br and is significantly less than the value ∆B = 2·Bm possible for a push-pull converter. Some increase in ∆B can be obtained by introducing a non-magnetic gap into the core. If the core has a non-magnetic gap δ, then the residual induction becomes less than Br. If there is a non-magnetic gap in the core, the new value of the residual induction can be found at the intersection point of a straight line drawn from the origin at an angle Ѳ to the magnetization reversal curve (point B1 on rice. 3, b):
tgѲ= µ 0 · l c/δ,
where µ 0 – magnetic permeability;
l c – length of the average magnetic field line of the magnetic core, m;
δ – length of non-magnetic gap, m.
Definition.
Magnetic permeability – this is the ratio of induction B to tension H for a vacuum (also valid for a non-magnetic air gap) and is a physical constant, numerically equal to µ 0 = 4π·10 -7 H/m.
The value tgѲ can be considered as non-magnetic gap conductivity, reduced to the length of the core. Thus, introducing a non-magnetic gap is equivalent to introducing a negative magnetic field strength:
Н1 = -В1/ tgѲ.
Push-pull bridge converter
Advantages of push-pull converters. Push-pull converters contain more elements and require more complex control algorithms. However, these converters provide lower input current ripple and greater output power and efficiency from the same discrete key components.
Scheme of a push-pull bridge converter. On rice. 4, a shows a diagram of a push-pull bridge converter. If we compare this converter with single-ended ones, then it is closest to a two-transistor forward converter ( rice. 3) . A push-pull converter is easily converted into it if you remove a pair of transistors and a pair of diodes located diagonally (VT1, VT4, VD2, VD3 or VT2, VT3, VD1, VD4).
Thus, a push-pull bridge converter is a combination of two single-cycle converters operating alternately. In this case, energy is transferred to the load during the entire period of operation of the converter, and the induction in the transformer core can vary from -Bm to +Bm.
As in the DPP, diodes VD1-VD4 serve to return the energy accumulated in the leakage inductance Ls of the transformer T to the primary power source Up. MOSFET internal diodes can be used as these diodes.
Operating principle. Let us consider in more detail the processes occurring at the moment of magnetization reversal of the transformer core.
Note.
A common feature of push-pull converters is that their transformers operate in conditions with symmetrical magnetization reversal.
Magnetic induction B, in the core of a transformer with symmetrical magnetization reversal, can vary from negative -Bm to positive +Bm maximum induction.
In each half-cycle of the DMP operation, two keys located diagonally are open. During pause, all transistors of the converter are usually closed, although there are control modes when some transistors of the converter remain open during pause.
Let's focus on the control mode, according to which all DMP transistors are closed during a pause.
When transistors VT1, VT4 of the converter are open, the energy of the power source Up is transferred to the load through transformer T. In this case, the transformer core is magnetized in the conventional reverse direction (section b-a in Fig. 4, b).
During a pause, when transistors VT1, VT4 are closed, the current in the load is maintained by the energy stored in the inductor L. In this case, the current is closed through the diode VD7. At this moment, one of the secondary windings (IIa or IIb) of transformer T is short-circuited through an open diode VD7 and one of rectifier diodes(VD5 or VD6). As a result of this, the induction in the transformer core remains virtually unchanged.
After the pause is completed, transistors VT2, VT3 of the converter open, and the energy of the power source Up is transferred to the load through transformer T.
In this case, the transformer core is magnetized in the conventional forward direction (section a-b on rice. 4). During a pause, when transistors VT2, VT3 are closed, the current in the load is maintained by the energy stored in the inductor L. In this case, the current is closed through the diode VD7. At this moment, the induction in the transformer core remains virtually unchanged and is fixed at the achieved positive level.
Note.
Due to the fixation of inductions in pauses, the core of the transformer T is capable of reversing magnetization only when the diagonally located transistors are open.
In order to avoid one-sided saturation under these conditions, it is necessary to ensure equal open time of the transistors, as well as symmetry of the power circuit of the converter.
The power part of our homemade inverter-type semi-automatic welding machine is based on an asymmetric bridge circuit, or, as it is also called, an “oblique bridge”. This is a single-ended forward converter. The advantages of such a scheme are simplicity, reliability, minimal number of parts, and high noise immunity. Until now, many manufacturers produce their products using the “oblique bridge” design. You also cannot do without disadvantages - these are large pulse currents from the power supply, lower efficiency than in other circuits, and large currents through the power transistors.
Block diagram of a forward converter “oblique bridge”
The block diagram of such a device is shown in the figure:
Power transistors VT1 and VT2 operate in the same phase, i.e. they open and close at the same time, therefore, compared to a full bridge, the current through them is twice as large. The TT transformer provides current feedback.
You can learn more about all types of inverter converters for welding machines from the book.
Description of the inverter circuit
Semiautomatic inverter welding machine, operating in MMA (arc welding) and MAG (special wire welding in a gas environment) modes.
Control board
The following inverter components are installed on the control board: a master oscillator with a galvanic isolation transformer, current and voltage feedback units, a relay control unit, a thermal protection unit, and an “anti-stick” unit.
Master oscillator
The current control unit (for MMA mode) and the master oscillator (OG) are assembled on LM358N and UC2845 microcircuits. UC2845 was chosen as the MG, rather than the more common UC3845 due to the more stable parameters of the former.
The generation frequency depends on the elements C10 and K19, and is calculated by the formula: f = (1800/(R*C))/2, where R and C are in kilo-ohms and nanofarads, the frequency is in kilohertz. In this circuit, the frequency is 49KHz.
Another important parameter is the fill factor, calculated using the formula Kzap = t/T. It cannot be more than 50%, and in practice it is 44-48%. It depends on the ratio of denominations C10 and R19. If the capacitor is taken as small as possible, and the resistor as large as possible, then Kzap will be close to 50%.
The generated SG pulses are fed to the VT5 switch, which operates on the T1 galvanic isolation transformer (TGR), wound on an EE25 core used in electronic starting units fluorescent lamps(electronic ballasts). All windings are removed and new ones are wound according to the diagram. Instead of the IRF520 transistor, you can use any of this series - IRF530, 540, 630, etc.
Current feedback
As mentioned earlier, for arc welding What is important is a stable output current, for semi-automatic - a constant voltage. Current feedback is organized on the TT current transformer; it is a ferrite ring of size K 20 x 12 x 5, placed on the lower (according to the diagram) terminal of the primary winding of the power transformer. Depending on the primary winding current T2, the pulse width of the master oscillator decreases or increases, maintaining the output current unchanged.
Voltage feedback
Welding semi-automatic inverter type requires voltage feedback; for this, in MAG mode, switch S1.1 supplies the voltage from the output of the device to the output voltage adjustment unit, assembled on elements R55, D18, U2. Powerful resistor K50 sets the initial current. And with contacts S1.2, the key on the transistor VT1 short-circuits the regulator R2 to the maximum current, and the key VT3 turns off the “anti-stick” mode (turning off the ignition when the electrode sticks).
Thermal protection block
A homemade semi-automatic welding machine includes an overheat protection circuit: this is provided by a unit on transistors VT6, VT7. Temperature sensors at 75 degrees C (there are two of them, normally closed, connected in series) are installed on the radiator of the output diodes and on one of the radiators of the power transistors. When the temperature is exceeded, transistor VT6 shorts pin 1 of the UC2845 to ground and disrupts the generation of pulses.
Relay control unit
This block is assembled on a DD1 CD4069UB microcircuit (analogous to 561LN2) and a VT14 BC640 transistor. These elements provide the following operating mode: when you press the button, the gas valve relay is immediately turned on, after about a second, the VT17 transistor allows the generator to start and at the same time the pull-out mechanism relay turns on.
The relays that control the “pulling” and the gas valve, as well as the fans, are powered by the stabilizer on the MC7812 mounted on the control board.
Power unit based on HGTG30N60A4 transistors
From the TGR output, pulses pre-generated by drivers on transistors VT9 VT10 are supplied to power switches VT11, ME12. “Snubbers” are connected parallel to the collector-emitter terminals of these transistors - chains of elements C24, D47, R57 and C26, D44, R59, which serve to keep powerful transistors in the range of permissible values. In the immediate vicinity of the keys there is a capacitor C28, assembled from 4 capacitors 1 µm x 630v. Zener diodes Z7, Z8 are necessary to limit the voltage at the switch gates to 16 volts. Each transistor is installed on a radiator from a computer processor with a fan.
Power transformer and rectifier diodes
The main element of the semiautomatic welding circuit is the powerful output transformer T2. It is assembled on two E70 cores, N87 material from EPCOS.
Calculation of a welding transformer
The turns of the primary winding are calculated according to the formula: N = (Upit * timp)/(Badd * Ssec),
where Upit = 320V – maximum supply voltage;
timp = ((1000/f)/2)*K – pulse duration, K = (Kzap*2)/100 = (0.45*2)/100 = 0.9 timp = ((1000/49)/2 )*0.9 = 9.2;
Vdop = 0.25 – permissible induction for the core material;
Ssection = 1400 – core section.
N = (320 * 9.2)/(0.25 * 1400) = 8.4, rounded to 9 turns.
The ratio of secondary to primary turns should be approximately 1/3, i.e. we wind 3 turns of the secondary winding.
The power transformer can be wound on a different standard size; the turns are calculated using the above formula. For example, for a core 2 x E80 at f = 49Khz, turns in the primary: 16, secondary: 5.
Selecting the cross-section of the wires of the primary and secondary windings, winding the transformer
We select the wire cross-section at the rate of 1mm.kv = 10A output current. This device should produce approximately 190A under load, so we take the secondary cross-section of 19mm.kv (a bundle of 61 wires with a diameter of 0.63mm). The cross section of the primary is selected 3 times smaller, i.e. 6mm.sq. (harness of 20 wires with a diameter of 0.63 mm). The cross-section of the wire depending on its diameter is calculated as: S = D²/1.27 where D is the diameter of the wire.
Winding is carried out on a frame made of 1mm PCB, without side cheeks. The frame is mounted on a wooden frame according to the dimensions of the core. The primary winding is wound (all turns in one layer). Then 5 layers of thick transformer paper, with the secondary winding on top. The coils are compressed with plastic ties. Then the frame with windings is removed from the mandrel and impregnated with varnish in a vacuum chamber. The chamber was made from a liter jar with a tight lid and a hose attached to the suction tube of the compressor from the refrigerator (you can simply dip the trans in varnish for a day, I think it will also become saturated).
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Most often, when constructing welding inverters, three main types of high-frequency converters are used: half-bridge, asymmetric bridge (or “oblique bridge”) and full bridge. Under the guise of a half bridge and a full bridge, there are resonant converters. Depending on the control system for the output parameters, converters are available with PWM (pulse width), with PFM (frequency control), with phase control, and combinations of these three. All these types of converters have their advantages and disadvantages. Let's start with a half bridge with PWM. The block diagram of such a converter is shown in Fig. 3.
This is the simplest converter of the two-stroke family, but no less reliable. The disadvantage of this circuit is that the voltage “swing” on the primary winding of the power transformer is equal to half the supply voltage. But on the other hand, this fact is a plus; you can use a smaller core without fear of entering saturation mode.
For low-power inverters (2-ZkW), such a converter is very promising. But PWM control requires special care when installing power circuits; drivers must be installed to control power transistors. Transistors of such a half-bridge operate in hard switching mode, so increased demands are placed on control signals.
There must be a “dead time” between two antiphase pulses, the absence of a pause, or its insufficient duration, always leads to the occurrence of a through current through the power transistors.
The consequences are easily predictable - failure of transistors. A very promising type of half-bridge converter is the resonant half-bridge. The block diagram of such a half-bridge is shown in Fig. 4.
The current flowing through the power circuits has a sinusoidal shape, and this removes the load from the filter capacitors.
With this design, power switches do not require drivers! An ordinary pulse transformer is enough to switch the power transistors. The quality of control pulses is not as significant as in a PWM circuit, although there should be a pause (“dead time”).
Another plus is that this circuit allows you to do without current protection and the shape of the current-voltage characteristic (volt-ampere characteristic) has an immediately falling form and does not require parametric shaping.
The output current is limited only by the magnetizing inductance of the transformer and can reach significant values during a short circuit; this must be taken into account when choosing output diodes, but this property has a positive effect on the ignition and burning of the arc!
Typically, the output parameters are regulated by changing the frequency, but the use of phase control gives much more advantages and is the most promising for a welding inverter, since it allows you to bypass such an unpleasant phenomenon as the coincidence of resonance with the short-circuit mode, and the range of adjustment of the output parameters is much wider. Phase adjustment allows you to change the output current practically from 0 to Imax.
The next scheme is an asymmetric bridge, or “oblique bridge”. The block diagram of such a converter is shown in Fig. 5.
The asymmetric bridge is a single-cycle, forward-flow converter.
A converter of this configuration is very popular both among manufacturers of welding inverters and among radio amateurs. The first welding inverters were built exactly like an “oblique bridge”. Simplicity and reliability, ample opportunities for adjusting the output current, noise immunity - all this still attracts developers of welding inverters.
And although the disadvantages of such a converter are quite significant, these are large currents through transistors, high requirements for the shape of control pulses, which implies the use of powerful drivers to control power switches, high requirements for the installation of power circuits, large pulse currents place high demands on input filter capacitors, electrolytic Capacitors really don't like large pulse currents. To keep transistors in the ODZ (permissible value range), RCD chains (snubbers) are required.
But, despite all these shortcomings and low efficiency, the “oblique bridge” is still used in welding inverters to this day. Transistors T1 and T2 operate in phase, opening together and closing together. The energy is not stored in the transformer, but in the output inductor of the inductor. The duty cycle does not exceed 50%, which is why to obtain the same power with a bridge converter, double current through the transistors is required. The operation of such a converter will be examined in more detail using the example of a real welding inverter.
The next type of converter is a full bridge with PWM. Classic push-pull converter! The block diagram of the full bridge is shown in Fig. 6.
The bridge circuit makes it possible to obtain power 2 times more than a half-bridge, and 2 times more than an “oblique bridge”, with the same values of currents and switching losses. This is explained by the fact that the “swing” of the voltage of the primary winding of the power transformer is equal to the supply voltage.
Accordingly, to obtain the same power, for example, with a half-bridge (in which the drive voltage is 0.5U supply), the current through the transistors will be 2 times less! Full bridge transistors operate diagonally when T1 - T3 are open, T2 - T4 are closed, and vice versa. The current transformer monitors the amplitude value of the current flowing through the switched on diagonal. You can regulate the output current of such a converter in two ways:
1) change the duration of the control pulse, leaving the cutoff voltage unchanged;
2) change the level of the cutoff voltage coming from the current transformer, leaving the duration of the control pulses unchanged.
Both of these methods allow you to change the output current within a fairly wide range. The disadvantages and requirements of a full bridge with PWM are exactly the same as those of a half bridge with PWM. (See above). And finally, let's consider the most promising RF converter circuit for a welding inverter - a resonant bridge. The block diagram is shown in Fig. 7.
As it may seem at first glance, the resonant bridge circuit is not very different from a PWM bridge, and this is true. In practice, only an LC resonant circuit is additionally introduced, connected in series with the power transformer. However, the introduction of this chain completely changes the processes of power transfer. Losses are reduced, efficiency increases, the level of electromagnetic interference is reduced by orders of magnitude, and the load on the input electrolytes is reduced. As you can see, you can completely remove current protection; power transistor drivers may only be needed if MOSFET transistors with a gate capacitance greater than 5000pF are used. For IGBT transistors, one pulse transformer is sufficient.
The output current of the resonant converter can be controlled in two ways: frequency and phase. Both of them were mentioned earlier, in the description of the resonant half-bridge. And the last type of RF converter is a full bridge with a leakage choke. Its circuit is practically no different from the circuit of a resonant bridge (half-bridge), just like the LC circuit is connected in series with a transformer, only it is not resonant. C = 22 µFx63V works as a balancing capacitor, and L of the inductor acts as a reactance, the value of which linearly depends on the frequency. The control of such a converter is frequency. As the frequency increases, the resistance L increases. Current through power transformer decreases. Simple and reliable. Most industrial inverters are built on this principle of adjusting and limiting the output current.
The arc welding machine must provide a decreasing current-voltage characteristic in the load (arc). In bridge inverters, as a rule, the falling characteristic is provided by rather complex electronics with mandatory feedback by current. From the point of view of ease of control, in my opinion, the resonant bridge is the most attractive. In it, the falling characteristic of the welding current source is ensured by the parametric properties of the resonant circuit in the primary circuit of the inverter.
A feature of the inverter presented in this article is not only the use of a full resonant bridge, but also its control using a PIC16F628-20I/P microcontroller.
Let us immediately note that the maximum welding current of the inverter depends on the setting. Its value is entirely determined by the width of the non-magnetic gap in the magnetic circuit of the resonant choke. For the power elements used in the inverter, subject to their thermal conditions, the welding current can reach 200 A.
The inverter circuit diagram is divided into two parts. On Fig.1 the power section is shown, and Fig.2— diagram of the power supply with the control unit. A classic bridge welding inverter consists of a mains voltage rectifier with filter capacitors. A direct voltage of 300 V is converted using 4 switches into an alternating voltage of a higher frequency, which is lowered and then rectified using a welding transformer.
Power part
In resonant converters, a resonant inductor L1 and a resonant capacitor C1-C10 are connected in series with the primary winding of the welding transformer T1 (see Fig. Fig.1 on which the power circuits are highlighted with bold lines). The inductance of the series circuit consists of the inductance of the resonant choke L1 and the inductance of the primary winding of the transformer T1. The secondary winding T1 is loaded with a welding arc. If the capacitance C1-C10 and inductance L1 are constant values, then the inductance of the primary winding T1 depends on the load resistance in the secondary winding, i.e. from welding current. The maximum inductance of the primary winding T1 corresponds to the “no-load” mode of the inverter, and the minimum - the short circuit. The load resistance also determines the quality factor of the circuit. Thus, the resonant frequency of the circuit is minimal in the “no-load” mode (with maximum inductance of the primary winding T1) and maximum in short-circuit mode (with minimal inductance of the primary winding T1). When the inverter load is a welding arc, the resonant frequency of the circuit depends on the current in the arc.
From all that has been said above, it is obvious that the frequency of the inverter when operating at maximum power in the arc should be lower than the natural frequency of the resonant circuit of the inverter in short-circuit mode and higher than it in the “idle” mode. It is optimal for resonance to occur at the circuit’s natural frequency, at which maximum power develops in the arc (f MAX. POWER). This is precisely the main criterion correct settings inverter. If in this case the inverter frequency is increased relative to f MAX. POWER , the arc current decreases due to an increase in the inductive reactance of the resonant inductor L1. This is how frequency regulation of the current in the welding arc is carried out.
Resonance in the inverter circuit due to a short circuit and incorrect settings of the inverter is possible at a frequency higher than f MAX. POWER .
Note also that resonance is unacceptable in short-circuit mode for transistor switches of the inverter due to the occurrence of overcurrent in the primary circuit. Since short circuit mode is the normal mode for the welding machine, it is necessary to prevent the inverter from operating at frequencies above f MAX. POWER in case of a short circuit in the welding circuit.
To do this, the microcontroller in this inverter continuously monitors the fact of a short circuit in the welding wires using a special detector. When a short circuit occurs, the microcontroller automatically reduces the inverter frequency to the previously set value f MAX. POWER - at this frequency, resonance in a short circuit is impossible, which prevents excessive current from flowing in the primary circuit and, accordingly, through the switches.
In the power section (Fig.1) R13 - starting resistor. It limits the charging current of oxide capacitors C16, C17 when the device is turned on. The diode bridge VD14-VD21 is designed to rectify the mains voltage 220 V / 50 Hz, which is smoothed by capacitors C15-C17 and supplied to the output bridge of the circuit, consisting of 4 switches on IGBT transistors VT1-VT4.
Suppressors VD3, VD9 and VD22 protect keys from voltage surges. Resistors R5, R6 discharge the resonant capacitor when the inverter is turned off. Zener diodes VD1, VD2, VD4, VD5 do not allow the voltage on the gates of the switches to exceed 18 V. Resistors R1, R3, R7 and R9 limit the output current of the drivers at the moments of charge and discharge of the gate capacitances of the switches. Resistors R2, R4, R8, R10 ensure reliable closing of the keys at moments when there is no power to the drivers.
Welding transformer T1 with a transformation ratio of 6 reduces the voltage and provides galvanic isolation of the output relative to the network part of the inverter. The alternating voltage from the secondary winding of the welding transformer is rectified by diodes VD6, VD7 and is supplied through the welding wires to the electrode and the surfaces being welded. Chains R11C13 and R12C14 serve to absorb the energy of the reverse voltage emissions of the output rectifier. For stable arc burning at low currents, as well as to facilitate its ignition, a voltage doubler is provided, assembled on elements C11, C12, VD10-VD13, C19, C20 and L2. Resistor R14 serves as a load for the doubler. The VD8 suppressor protects the output rectifier diodes from reverse voltage surges.
power unit
Built according to a flyback converter circuit based on a specialized DA6 TNY264 microcircuit according to a standard circuit (Fig.2). It provides power to the drivers, relays and microcontroller control unit. The power supply of the upper switch drivers is galvanically isolated from the 24 V relay power supply channel and the power supply channel of the lower drivers. To power the microcontroller DD1 (5 V), a parametric stabilizer DA7 is used. Drivers DA1-DA4 type HCPL3120 are designed to control VT1-VT4 switches and provide steep edges of control pulses on the gates of these transistors.
The short circuit detector is assembled on elements R25, R27, R28, DA8, VD32, VD33, C38. When the voltage on the welding wires is below 9 V (short circuit), a high logical level appears at the RB4 input of the DD1 controller, and when the voltage is more than 9 V (no short circuit), a low logical level appears at the RB4 input.
Position DD1 uses the widely used microcontroller (MCU) PIC16F628-20I/P in a DIP package.
Inverter operation
As soon as the power supply starts, the microcontroller program starts running. After a delay of approximately 5 s, the buzzer will sound and the inverter will start operating. As soon as the voltage in the welding wires exceeds 9 V, the MK will open key VT5, which will turn on relay K1, and the relay contacts will be bypassed by charging resistor R13. The buzzer will also turn off. From this moment the inverter is ready for operation. The operating frequency of the inverter will be determined by the position of potentiometer R18. Moreover, the minimum frequency (aka f MAX. POWER) corresponds to the maximum welding current, and the maximum frequency corresponds to the minimum current. The frequency changes in steps (discretely). Only 17 positions are used. When rotating potentiometer R18, the frequency change is accompanied by a short sound signal buzzer Thus, by the sound of the buzzer, you can change the frequency of the welding current to the required number of positions.
If there is a short circuit in the welding leads, the inverter automatically starts operating at frequency f MAX. POWER ,- Operation of the inverter in short circuit mode is accompanied by a buzzer sound. If the short circuit lasts more than 1 s, the inverter operation is blocked and resumes after 3 s. This is how the anti-stick electrode function is implemented.
In the absence of a short circuit, a low logic level is applied to input RB4, and the inverter frequency is determined by the position of potentiometer R18.
To protect the output switches from overheating, two thermostats TS1 and TS2 are used as sensors. If at least one of the thermostats is turned off, the operation of the inverter is blocked. The buzzer emits an intermittent, rapid beep until the radiator on which the triggered thermostat is installed cools down.
Construction and details Resonant choke L1 is wound on an ETD59 magnetic core, material No. 87 from EPCOS and contains 12 turns of copper wire with a diameter of 2 mm in varnish insulation. The wire is wound with a mandatory gap between the turns. To provide clearance, you can use a thick thread. To fix the winding, you need to coat the turns with epoxy glue. The halves of the magnetic circuit are joined with a non-magnetic gap of 1...2 mm. A more accurate value of the non-magnetic gap is selected when setting the resonant frequency. During operation of the inverter, the magnetic circuit of the resonant choke can become very hot. This is due to the saturation of ferrite when operating in resonance. To ensure reliable fixation of the gap of the magnetic core, its halves must be tightened with metal pins. In this case, it is necessary to ensure a distance from the gap to the studs of at least 5 mm. Otherwise, the studs may melt near the gap. For the same reason, it is unacceptable to tighten the throttle with a solid metal casing.
Transformer T1 is wound on an E65 magnetic core, material No. 87 from EPCOS. First, the primary winding is wound in one row - 18 turns of copper wire with a diameter of 2 mm in varnish insulation. Windings II and III are wound on top of the primary winding. Each of them occupies half of the frame. Windings II and III each contain 3 turns of four copper wires with a diameter of 2 mm. The halves of the transformer magnetic core are joined without gaps and securely fixed.
Choke L2 contains 20 turns of mounting wire with a cross section of 1.5 mm 2, wound on a K28x16x9 ferrite ring.
Transformer T2 is wound on ferrite Ш5х5 with a permeability of 2000 NM. The halves of the magnetic circuit are joined with a gap of 0.1…0.2 mm. Winding I contains 180 turns of PEV-1 wire with a diameter of 0.2 mm. Winding II is wound in one row and contains 47 turns of the same wire. Windings III, IV and V each contain 33 turns of PEV-1 wire with a diameter of 0.25 mm. Between the windings you need to lay 2 layers of insulation (for example, masking tape). The phasing of the winding connections is indicated on Fig.2.
It is permissible to use only high-quality film capacitors C1-C10 for a voltage of at least 1000 V. It is preferable to use capacitors of the K78-2 type. The blocking capacitor C15 should be of the same type.
The power supply does not require configuration and, if the parts are in good condition, starts working immediately. It is only necessary to check the voltage values for powering the drivers 16...17 V. When checking the power supply, you can apply a 220 V mains voltage to its input terminals GND and +300 V. The power supply should be powered in the same way when setting the resonant frequency.
During operation of the inverter, all its power elements heat up. The time of continuous operation of the device and its durability will depend on how well these elements are blown. Radiators with large area must be provided for the input rectifier VD14-VD21, transistors VT1-VT4 and the output rectifier VD6, VD7. Forced air cooling is also required for the resonant choke L1, the welding transformer T1 and the doubler diodes VD10-VD13. Safety thermostats TS1 and TS2 type KSD250V are installed on the radiators of the upper switches and output diodes. All other elements of the inverter do not require airflow and radiators.
Setting the resonant frequency
To configure the inverter, you need an LATR and a load rheostat with a resistance of 0.15 Ohm. The rheostat must withstand a short-term current flow of up to 200 A. The gap of the magnetic circuit of the resonant choke is set to approximately 1 mm. A jumper is installed between pins 3 and 4 of the DA8 optocoupler. Install the “stitched” microcontroller into the control unit.
When setting up, the power supply should be powered separately. To do this, without turning on the device to the network, you need to apply a mains voltage of 220 V to the GND and +300 V wires of the power supply.
The power section is still de-energized. After turning on the power, the buzzer should sound after 5 seconds, then the sound should stop and the relay should turn on. Press both buttons SB1 and SB2 simultaneously. Hold the buttons until the buzzer sounds. Let's release the buttons. The continuous sound will stop and the buzzer will start beeping intermittently for approximately 2 seconds. This corresponds to the resonant frequency tuning mode.
If everything is so, then using an oscilloscope we monitor the presence of bipolar pulses between the gates of transistors VT2 and VT4 with a frequency of 30 kHz, an amplitude of at least 15 V and a “dead time” step of 2 μs. The same signal should be between gates VT1 and VT3. If everything is so, we power the power section through LATR and set the voltage to 20...30 V.
You can connect a 12 V light bulb to the welding wires. If the light is on, connect a 0.15 Ohm rheostat and a DC voltmeter to the welding wires. We set the voltage on the LATR to 30...40 V and begin the setup. Use the SB1 and SB2 buttons to decrease or increase the inverter frequency. Frequency change limits 30…42 kHz. By adjusting the frequency with the buttons, we achieve maximum voltage on the rheostat. If the voltage continues to increase when the frequency decreases to 30 kHz, then it is necessary to increase the gap in the magnetic circuit of the resonant choke and repeat the adjustment again. If, when the frequency increases to 42 kHz, the voltage on the rheostat continues to increase, it is necessary to reduce the gap in the magnetic circuit of the resonant choke and repeat the adjustment again.
It is necessary to achieve resonance, i.e. configure the circuit so that an increase or decrease in the inverter frequency would lead to a decrease in the voltage on the rheostat. With the elements indicated in the diagram, it is best to achieve such a gap in the resonant choke so that resonance with a load of 0.15 Ohms occurs at a frequency of 33...37 kHz. Resonance at a higher frequency will increase the maximum welding current, but the switches and output diodes will work at their limit.
Once the resonant frequency is set, press both buttons simultaneously. After a long sound signal, the value of the resonant frequency will be written to the non-volatile memory of the microcontroller. By rotating potentiometer R18, we check the operation of frequency regulation. The minimum frequency must be equal to the resonant frequency. When rotating the potentiometer, the change in frequency should be accompanied by a short sound signal (17 steps in total).
If everything happens this way, we assemble the entire inverter circuit. Remove the jumper between pins 3 and 4 of the DA8 optocoupler. We turn on the inverter to the network. After 5 seconds the buzzer will sound, then the relay will turn on and the sound will stop. Using potentiometer R18 we set the minimum frequency (aka f MAX. POWER), corresponding to the maximum current. We briefly load the inverter with a rheostat with a resistance of 0.15 Ohm and measure the voltage in the load. If this voltage exceeds 23 V, then the setup can be considered complete. If it is less, then you should increase the gap in the magnetic circuit of the resonant choke and repeat the adjustment from the beginning.
Schematic diagram of the factory welding inverter "Resanta" (click to enlarge)
Inverter circuit from the German manufacturer FUBAG with a number of additional functions (click to enlarge)
An example of principle electrical diagram welding inverter for self-production (click to enlarge)
The electrical circuit diagram of the inverter device consists of two main parts: the power section and the control circuit. The first element of the power section of the circuit is a diode bridge. The task of such a bridge is precisely to convert alternating current into direct current.
In the direct current converted from alternating current in the diode bridge, pulses may occur that need to be smoothed out. To do this, a filter consisting of capacitors of predominantly electrolytic type is installed after the diode bridge. It is important to know that the voltage that comes out of the diode bridge is approximately 1.4 times greater than its value at the input. When converting AC to DC, rectifier diodes become very hot, which can seriously affect their performance.
To protect them, as well as other elements of the rectifier from overheating, radiators are used in this part of the electrical circuit. In addition, a thermal fuse is installed on the diode bridge itself, the task of which is to turn off the power supply if the diode bridge has heated up to a temperature exceeding 80–90 degrees.
High-frequency interference generated during operation of the inverter device can enter the electrical network. To prevent this from happening, an electromagnetic compatibility filter is installed in front of the rectifier block of the circuit. Such a filter consists of a choke and several capacitors.
The inverter itself, which converts direct current into alternating current, but has much more high frequency, is assembled from transistors using an “oblique bridge” circuit. The switching frequency of transistors, due to which the alternating current is generated, can be tens or hundreds of kilohertz. The high-frequency alternating current thus obtained has a rectangular amplitude.
A voltage-reducing transformer installed behind the inverter unit allows you to obtain a current of sufficient strength at the output of the device so that you can effectively perform welding work with its help. In order to obtain direct current using an inverter apparatus, a powerful rectifier, also assembled on a diode bridge, is connected after the step-down transformer.
Inverter protection and control elements
Several elements in its circuit diagram allow you to avoid the influence of negative factors on the operation of the inverter.
To ensure that transistors that convert direct current into alternating current do not burn out during their operation, special damping (RC) circuits are used. All electrical circuit blocks that operate under heavy load and become very hot are not only provided with forced cooling, but are also connected to temperature sensors that turn off their power if their heating temperature exceeds a critical value.
Due to the fact that the filter capacitors, after being charged, can produce a high current, which can burn the inverter transistors, the device must be provided with a smooth start. For this purpose, stabilizers are used.
The circuit of any inverter has a PWM controller, which is responsible for controlling all elements of its electrical circuit. From the PWM controller, electrical signals are sent to a field-effect transistor, and from it to an isolation transformer, which simultaneously has two output windings. The PWM controller, through other elements of the electrical circuit, also supplies control signals to the power diodes and power transistors of the inverter unit. In order for the controller to effectively control all elements of the inverter's electrical circuit, it is also necessary to supply electrical signals to it.
To generate such signals, an operational amplifier is used, the input of which is supplied with the output current generated in the inverter. If the values of the latter diverge from the specified parameters, the operational amplifier generates a control signal to the controller. In addition, the operational amplifier receives signals from all protective circuits. This is necessary so that he can disconnect the inverter from the power supply at the moment when a critical situation arises in its electrical circuit.
Advantages and disadvantages of inverter-type welding machines
The devices that replaced the usual transformers have a number of significant advantages.
- Thanks to a completely different approach to the formation and regulation of welding current, the weight of such devices is only 5–12 kg, while welding transformers weigh 18–35 kg.
- Inverters have very high efficiency (about 90%). This is explained by the fact that they spend significantly less excess energy on heating the components. Welding transformers, unlike inverter devices, get very hot.
- Due to such high efficiency, inverters consume 2 times less electrical energy than conventional transformers for welding.
- The high versatility of inverter machines is explained by the ability to regulate the welding current over a wide range with their help. Thanks to this, the same device can be used for welding parts made of different metals, as well as for welding using different technologies.
- Most modern inverter models are equipped with options that minimize the impact of welder errors on the technological process. Such options, in particular, include “Anti-stick” and “Arc Force” (fast ignition).
- Exceptional stability of the voltage supplied to the welding arc is ensured by the automatic elements of the inverter electrical circuit. In this case, automation not only takes into account and smoothes out differences in input voltage, but also corrects even such interference as the attenuation of the welding arc due to strong wind.
- Welding using inverter equipment can be performed with any type of electrode.
- Some models of modern welding inverters have a programming function, which allows you to accurately and quickly configure their modes when performing a certain type of work.
Like any complex technical devices, welding inverters also have a number of disadvantages that you also need to know about.
- Inverters are highly expensive, 20–50% higher than the cost of conventional welding transformers.
- The most vulnerable and often failing elements of inverter devices are transistors, the cost of which can be up to 60% of the price of the entire device. Accordingly, it is quite an expensive undertaking.
- Due to the complexity of their electrical circuitry, inverters are not recommended for use in bad weather conditions and at low temperatures, which seriously limits their scope of application. In order to use such a device in field conditions, it is necessary to prepare a special closed and heated area.
When welding work performed using an inverter, long wires cannot be used, as they induce interference that negatively affects the operation of the device. For this reason, the wires for inverters are made quite short (about 2 meters), which makes welding work somewhat inconvenient.
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