A simple control circuit for a three-phase voltage inverter. Three-phase pulse generator Circuits of three-phase pulse sequence generators

To power various household and industrial devices, a three-phase alternating current network with a frequency of 200 or 400 Hz is required. To obtain such voltage, in most cases, an appropriate electromechanical three-phase generator is used, the rotor of which is driven by a single-phase electric motor powered from a 220V network.

The proposed electronic generator allows us to solve this problem with best odds useful action.

If you examine the three-phase voltage diagram, you can see three sinusoidal signals shifted in series by 1/3 of the cycle. If a frequency of 200 Hz is assumed, then the period is 5 mS. Therefore, 1/3 of the period is equal to 1.666... ​​mS. Thus, it turns out that if we have an initial single-phase voltage of 200 Hz, passing it through two delay lines connected in series, each of which introduces a delay of 1.666.. mS, we will obtain a three-phase voltage, one phase is the original voltage, and two phases of voltage with outputs of the corresponding delay lines.

Schematic diagram A device operating on this principle is shown in the figure. All source signals are rectangular, their conversion to sinusoidal occurs in the inductances of the output transformers T1-T3.

The multivibrator on chip D1 produces rectangular pulses with a frequency of 200 Hz. These pulses are supplied to the input of an electronic high-voltage switch on transistors VT1 and VT4, at the output of which the primary winding of transformer T1 is switched on. As a result, a pulse voltage of 300V is supplied to the winding. The self-induction EMF smooths out these pulses to a shape close to sinusoidal and is formed on the secondary winding T1 AC voltage frequency 200 Hz. Thus, phase “A” is formed.

To form phase “B”, pulses with a frequency of 200 Hz from output D1 are supplied to a delay circuit having a time constant equal to 1.666 mS. From output D1.2, a pulse voltage shifted by 1/3 phase compared to the voltage at output D1.3 is supplied to the second switch on transistors VT2 and VT5, which operates similarly to the previous one. On the secondary winding T1 there is phase "B".

Then, from the output of element D2.2, the pulse voltage, already shifted by 1/3 phase, is supplied to the second delay line on elements D2.3 and D2.4, in which another shift by 1/3 phase occurs. Pulses from the output of element D2.4 are supplied to the third switch on transistors VT3 and VT6, in the collector circuit of which the primary winding of transformer T3 is switched on, and an alternating voltage of the third phase is released on its secondary winding.

Microcircuits: D1 - K561LE5, D2 -K561LP2. The microcircuits may be from the K176 series, but in this case the supply voltage must be lowered to 9V (instead of 12V). KT604 transistors can be replaced with KT940, KT848 transistors with KT841. Transformers T1-T3 are identical transformers, designed to obtain the required voltage when a voltage of 220V is applied to their primary winding. For example, if you need to obtain a three-phase voltage of 36V, you need to take 220V/36V transformers for the required power. Used to power microcircuits

constant stabilized voltage source 12V. The +300V voltage is obtained by rectifying the 220V mains voltage using a diode bridge, for example on D242 diodes or other powerful diodes with a voltage of at least 300V. Ripple smoothing is carried out by a 100 µF/360V capacitor (as in the power supply of an USCT TV). This constant voltage is applied to the “+300V” point. You can also apply a lower voltage, and the output voltages will change accordingly.

During the setup process, you need to select resistance R1, use a frequency meter to set the frequency at pin 10 D1 equal to 200 Hz, and then select R2 and R3, use a phase meter to set the phase shift to 120°.

If a three-phase voltage with a frequency of 400 Hz is required, the values ​​of the elements change to the following: R1 = 178 kohms, R2 = 60 kohms, R3 = 60 kohms. All parts, except output transistors and transformers, are mounted on one printed circuit board made of single-sided fiberglass. The output transistors must be installed on heat sinks with a surface area of ​​at least 100 cm2.

View printed circuit board three-phase voltage source

The generator, the diagram of which is shown in Fig. 1, can find application in various converters single-phase voltage to three-phase. It is simpler than those described in.

Rice. 1 Three-phase pulse generator circuit

The device consists of generator clock pulses DD1.1...DD1.3, driver DD2 and inverters DD1.4...DD1.6. Clock frequency generator choose 6 times higher frequency than required three-phase voltage and calculated using the approximate formula

The shaper is made on a shift register connected according to the counter-frequency divider circuit by 6. At outputs 1, 3 and 5 (pins 5, 6, 13)

Rice. 2 Output signals of three-phase pulse generator

DD2 produces rectangular pulses shifted by 1/3 of a period with a duty cycle of 2. Inverters DD1.4...DD1.6 are connected to the outputs of DD2 for decoupling. The output signals of the generator are shown in Fig. 2.

A. ROMANCHUK

Literature

1. Shilo V.L. Popular digital microcircuits. - Radio and communications, 1989, p.60.

2. Ilyin A. Connecting three-phase consumers to a single-phase circuit. - Radio Amateur, 1998, N10, P.26.

3. Kroer Yu. Three-phase 200 Hz from 50 Hz. - Radio Amateur, 1999, N10, P.21.

4. Pyshkin V. Three-phase inverter. - Radio, 2000, N2, P.35.

Three-phase asynchronous motors are widely used in industry and in everyday life due to their simplicity and reliability. The absence of sparking and heating commutator-brush assembly, as well as simple design rotors ensure a long service life and simplify prevention and maintenance. However, if it is necessary to regulate the shaft speed of such an engine, difficulties arise. For this purpose, special converters are usually used, called frequency regulators, which change the frequency of the voltage supplying the motor. Such regulators often allow a three-phase motor to be powered from a single-phase network, which is especially important when using them in everyday life.

Quite a lot of articles are devoted to frequency regulators, for example,. Unfortunately, most of the designs described are not very suitable for replication because they are either too complex or (like the regulator described in) built from expensive parts that cost half the price of a commercially manufactured regulator. Additional functions regulators are not always necessary. Therefore, for many simple applications such a regulator is unprofitable. The device described in is simple in design, but it is difficult to organize smooth control of the rotation speed with its help.

The device described in can be considered optimal for repetition, if it is slightly simplified. It is built on cheap, widely available chips, so there is no need to buy expensive microcontrollers or specialized modules. In the device described in this article, only the control pulse shaper is left. The rest has been changed for simplicity.

As is known, when the frequency of the voltage supplying the motor decreases, its amplitude must be proportionally reduced. The easiest way to do this is using pulse-width modulation of the generated voltage. A separate generator and five microcircuits are used for this. This is not very convenient, since it requires using a dual variable resistor to control the engine and setting up two generators, and the number of microcircuits can be reduced.

I used a different method of implementing pulse-width modulation, which simplifies the device and its setup. Now it consists of a frequency-controlled generator of pulses of constant duration, a counter-divider of the pulse repetition rate of the generator into three, a control pulse shaper and optocouplers that control the power switches of the DC-to-three-phase AC inverter.

The control pulse shaper divides the frequency of the pulses received by it by six. The emitting diodes of the optocouplers are connected in such a way that current flows through them only during periods of time when the logical voltage level is set at the output of the generator and the logic voltage level at the corresponding output of the control pulse shaper is set to low. Therefore, each half-cycle of the voltage applied to the motor winding consists of nine pulses of constant duration, but with adjustable pauses between them. In this case, a decrease in the effective value of the voltage supplied to the windings occurs automatically according to the desired law due to an increase in the duty cycle as its frequency decreases.

The schematic diagram of the master oscillator of a frequency regulator using this principle is shown in Fig. 1. It is designed for axial fan power supply system with 0.37KW three-phase motor. A pulse generator is built on a Schmitt trigger DD3.4 and transistor VT1. Let's consider its operation from the moment when capacitor C9 is discharged and the output of trigger DD3.4 is set to a high logical level, and the outputs of parallel-connected triggers DD3.5 and DD3.6 are set to low.

Rice. 1. Schematic diagram of the frequency regulator master oscillator

Capacitor C9 begins to charge through resistor R12 and the drain-source resistance of transistor VT1, which depends on the voltage at its gate. At some point in time, the voltage on the capacitor will exceed the upper switching threshold of the trigger, the output level of which will become low. Next, capacitor C9 will begin to discharge. After the voltage on the capacitor reaches the lower switching threshold of the trigger, everything will repeat from the beginning.

The duration of the low level pulse at the output of the trigger DD3.4 and the high level at the outputs of the triggers DD3.5 and DD3.6 is unchanged and is determined by the time constant of the C9R13 circuit. And the duration of pauses between pulses depends on the voltage at the gate of field-effect transistor VT1, which is set by variable resistor R3. The higher it is, the lower the drain-source resistance of the transistor, therefore, the shorter the pause between pulses and the higher their repetition frequency. At maximum frequency, pauses between pulses are minimal, so the voltage supplied to the motor windings is close to the voltage of the power switches.

As the frequency decreases, the duration of the pauses increases, which leads to a decrease in the average voltage on the motor winding.

The variable resistor R3 is used to regulate the engine speed, and the trimming resistor R4 is used to set its minimum value. Resistor R12 determines the minimum duration of pauses between pulses.

This generator is more complex than in , but is used for several reasons. Firstly, it allows you to obtain a wide frequency control interval with a small resistance of the variable resistor R3. With most variable resistors, when the moving contact moves from a metal contact to a resistive coating (or vice versa), a sharp change in resistance occurs. Moreover, the greater the nominal resistance of the resistor, the more clearly this property manifests itself. And in a conventional generator, in order to obtain a wide control interval, high-resistance variable resistors are required. In practice, this effect manifests itself as a sharp jerk of the motor shaft and a surge in the current it consumes when the variable resistor motor approaches the extreme position.

Secondly, it became possible to implement a smooth engine start without significantly complicating the device. This is relevant for fans, especially centrifugal ones, since the moment of inertia of the impeller is, as a rule, quite large, which contributes to long-term operation of the engine in starting mode with a significant excess of the rated current consumption.

Thirdly, due to the fact that the generator frequency is controlled by changing the DC voltage, if necessary, it is easy to organize remote control of the engine shaft speed.

To implement a soft start, elements C2, R1, R2, VD1, as well as relay K2, are used. At the moment the power is turned on, the relay winding circuit K2 is broken, the emitting diodes of the optocouplers U1-U6 are disconnected from the pulse generator, and capacitor C2 is discharged. In this state, trimming resistor R2 sets the minimum pulse repetition rate of the generator, from which the engine will start. It should be noted that the minimum frequency depends to some extent on the position of the variable resistor R3.

When you press the SB1 “Start” button, relay K2 with its contacts K2.2 will connect the optocouplers to the generator. Capacitor C2 will begin to charge mainly through resistor R2. The voltage at the transistor gate, and therefore the generator frequency, gradually increases. By selecting the capacitance of capacitor C2, you can change the acceleration speed of the engine. When the generator frequency reaches the value set by variable resistor R3, diode VD1 will close. Capacitor C2, charging to the supply voltage through resistor R2, does not affect the further operation of the generator.

When you press the SB2 "Stop" button, relay K2 turns off the optocouplers, and contacts K2.1 discharges capacitor C2. Relay K1 controls the node current protection frequency regulator. When overloaded, it opens the power supply circuit to relay coil K2. For additional protection, the frequency regulator is connected to the network through a circuit breaker with a shutdown current of 3 A.

If soft start and control of the frequency regulator using buttons are not required, all the elements located on the diagram inside the dash-dotted frame do not need to be installed. Instead of the drain-source section of transistor VT1, a variable resistor with a resistance of 100 kOhm should be connected according to the rheostat circuit. It is better to increase the capacitance of capacitor C9 to 470 nF, and select the resistance of resistors R12 and R13 accordingly
200 Ohm and 1.6 kOhm. The anodes of the emitting diodes of optocouplers U1-U6 should be connected to the outputs of triggers DD3.5 and DD3.6 directly.

From the output of trigger DD3.4, pulses are supplied to the input of counter DD4, the division coefficient of which is set to three. The control pulse generator is built on a counter DD1, 3OR-NOT elements of the DD2 microcircuit and Schmitt triggers DD3.1-DD3.3. His work is described in sufficient detail in and.

The operation of the control unit is illustrated by timing diagrams of signals at some of its points, shown in Fig. 2. As output signals of phase A, the currents flowing through the emitting diodes of optocouplers U1 and U4 are shown. Since, unlike in the device under consideration, all processes are synchronized with the frequency of the generator, the so-called dead time At between the open states of different power switches, equal in duration to the pause between generator pulses, is provided automatically. With the values ​​of resistor R12 and capacitor C9 indicated in the diagram and the maximum pulse frequency, its duration is at least 30 μs.

Rice. 2. Signal timing diagrams

Field-effect transistor KP501A can be replaced with BSN304 or KP505 series. Instead of the 74НСТ14 microcircuit, it is better to install one of its functional analogs KR1554TL2, 74AS14, which are characterized by increased load capacity. Microcircuits of the K561 series, much less K176, should not be used here.

Literature

1. Naryzhny V. Power supply for a three-phase electric motor from a single-phase network with speed control. - Radio, 2003, No. 12, p. 35-37.

2. Galichanin A. Frequency control system for an asynchronous motor. - Radio, 2016, No. 6, p. 35-41.

3. Khitsenko V. Three phases from one. - Radio, 2015, No. 9, p. 42, 43.

Publication date: 17.05.2017

Readers' opinions

• Peter / 09.10.2018 - 17:16
  Pin numbers kr1561le10 do not correspond to the reference book
• Alexander / 05.24.2017 - 19:40
  The output signals of phase A show the currents flowing through the emitting diodes of the optocouplers U1 and U4 Through U1 and U2 Why invert the signal for drivers - (A, B, C)

The invention relates to converter technology devices and can be used to power at a frequency of 400 Hz on-board systems of aircraft, as well as to power a high-frequency instrument with a frequency of 400 Hz or 200 Hz. The technical result consists in simplifying the design, reducing the weight and size of the device, increasing the reliability and quality of the output voltage by monitoring and controlling the pause generator. For this purpose, the claimed device, which is made according to a bridge circuit, containing fully controllable switches with back-to-back diodes, phase loads connected according to a star circuit, and a control unit, includes a new, according to the technical solution, control unit, consisting of a master oscillator, a generator pauses for switching on the control keys, the three-phase pulse sequence generator and the parameter setter for the output voltage period T and the load power factor cos φ n, the input of which is connected to the load circuit. Another object is a method for controlling a three-phase inverter with a link direct current is equipped with a control unit that forms a pause between switching on the controlled keys, and the duration of the pause between switching on the controlled keys of the inverter's antiphase arms at values ​​of cos φ n =1.0÷0.8 is 0.05T÷0.044T. 2 n.p. f-ly, 2 ill.

The invention relates to converter technology devices; it can be used to power at a frequency of 400 Hz on-board systems of aircraft, as well as to power a high-frequency instrument with a frequency of 400 Hz or 200 Hz.

Three-phase inverters are known with a DC link, the load is connected according to a star circuit, with the duration (λ) of the open state of the controlled switches of half the period (λ = 180° el.), in which the phase voltage at the load has a two-stage form [Handbook of converter technology. Ed. I.M. Chizhenko. Kyiv. Publishing house: Tekhnika, 1978, pp. 131, 132, Fig. 3.38 and 3.39b,c].

The disadvantages of such inverters are relatively low reliability due to the possibility of through currents flowing through antiphase controlled valves of all phases during switching, as well as a high coefficient nonlinear distortion, i.e. significant difference in output voltage from sinusoidal.

There are schemes for generating three-phase sequences of control pulses for valves of each phase, but they do not allow forming the interval between switching on antiphase valves [V.L. Shilo. Popular digital microcircuits: Directory. - M.: Metallurgy, 1988, p.59, Fig. 1.38a, b].

The closest technical solution This invention is a three-phase inverter with a DC link, which is made according to a bridge circuit, containing fully controllable switches with back-to-back diodes connected in parallel, phase loads connected in a star configuration, a control unit and auxiliary switches connected to the corresponding load phases and an additional capacitor , and the main switches are in a conducting state of 5/12T, and auxiliary ones are 1/12T, where T is the period of the output voltage [Patent (RF) No. 2125761, N02M 7/5387,1999].

Disadvantages of this device are big number additional elements, complexity, and relatively low reliability.

The problem to be solved by the claimed invention is to simplify the design, reduce the weight and size of the device, increase the reliability and quality of the output voltage by monitoring and controlling the pause generator.

The problem is solved by the fact that in a three-phase inverter with a DC link, made according to a bridge circuit, containing fully controllable switches with back-to-back diodes, phase loads connected according to a star circuit, a control unit, according to the invention, the control unit contains a master oscillator, a three-phase driver sequences of pulses and a parameter setter for the period of the output voltage T and the load power factor cos φ n, the input of which is connected to the load circuit, the pause generator for turning on the controlled keys and the first, second, third decoder of control pulses of the keys of antiphase arms of the corresponding phases of the inverter, the inputs of which are connected to the output the pause generator for turning on the controlled keys and the corresponding outputs of the three-phase pulse sequence generator, the output of the master oscillator is connected to the first input of the pause generator for turning on the controlled keys and the second input of the parameter setter for the period of the output voltage T and the load power factor cos φ n.

The problem is also solved by a method for controlling a three-phase inverter with a DC link, according to which, according to the invention, the duration of the pause between turning on the controlled switches of the inverter's antiphase arms at cos φ n =1.0÷0.8 is set to 0.05T÷0.044T.

The essence of the invention is illustrated by drawings. Figure 1 shows a diagram of a three-phase inverter, Figure 2 shows voltage timing diagrams.

The inverter consists of power modules 1-6, consisting of switches and diodes connected counter-parallel to the keys, which are connected via a bridge circuit by one terminal to the negative terminal of the power source 7, and the other to the corresponding load phase 8. The control unit 9 consists of a master generator 10, three-phase pulse sequence generator 11, first control pulse decoder 12, second control pulse decoder 13, third control pulse decoder 14 of each phase A, B, C, pause generator 15 and parameter setter for the output voltage period T, load power factor cos φ n 16 (Fig. 1).

From the master oscillator 10, pulses (U10) (Fig. 2) are supplied to the three-phase pulse sequence generator 11, which issues control pulses (U11) to the upper and lower power modules 1-6 of each arm of the bridge during a half-cycle of the output voltage. The duration of the pause between switching on the antiphase arms of the inverter (tp) is set by the pause generator 15, to the input of which pulses are supplied from the master oscillator 10. The pause generator 15 simultaneously introduces a pause into the first, second, and third decoders of control pulses 12, 13, 14. The pulses arrive from control unit 9 to the upper (U1) and lower (U2) power modules 1-6 of each bridge arm with a pause between switching on the antiphase arms of the inverter. The parameter setter for the period of the output voltage T and the load power factor cos φ n 16, the input of which receives pulses from the master oscillator 10, monitors and controls the pause generator 15 based on the obtained values ​​of the period of the output voltage T, the load power factor cos φ n from the load phases 8 .

As can be seen from the timing diagrams, the load voltage (U8) has a three-stage shape with a pause between switching on the controlled switches of the inverter's antiphase arms, which brings the phase voltage shape closer to a sinusoidal one. This leads to a reduction in the odd harmonic content, therefore improving the quality of the device's output voltage.

An example of a specific implementation of the method.

From the master oscillator 10, pulses are supplied to the three-phase pulse sequence generator 11, which issues control pulses to the upper and lower power modules 1-6. The duration of the pause between switching on the antiphase arms of the inverter for the value of cos φ n =1.0 is set by the pause generator 15, equal to the value of 0.05T. The pause generator 15 simultaneously introduces the value 0.05T into the first, second, and third control pulse decoders 12,13,14. Pulses arrive from the control unit 9 to the upper and lower power modules 1-6 of each bridge arm with a pause equal to a value of 0.05 T between switching on the antiphase arms of the inverter, forming a three-stage output voltage.

The use of this three-phase inverter makes it possible to simplify the circuit, reduce dimensions and weight, and increase the reliability of the device. The method of controlling a three-phase inverter with a DC link brings the shape of the output voltage closer to sinusoidal, which improves the quality of the output voltage at values ​​of cos φ n = 1.0÷0.8.

1. A three-phase inverter with a DC link, made according to a bridge circuit, containing fully controllable switches with back-to-back diodes connected, phase loads connected in a star circuit, a control unit, characterized in that the control unit contains a master oscillator, a three-phase pulse sequence generator and a parameter setter for the period of the output voltage T and the load power factor cos φ n, the input of which is connected to the load circuit, the pause generator for turning on the controlled keys and the first, second, third decoders of the control pulses of the keys of the antiphase arms of the corresponding phases of the inverter, the inputs of which are connected to the output of the pause generator switching on the controlled switches and the corresponding outputs of the three-phase pulse sequence generator, the output of the master oscillator is connected to the first input of the pause generator for switching on the controlled keys and the second input of the parameter setter for the period of the output voltage T and the load power factor cos φ n.

2. A method of controlling a three-phase inverter with a DC link, characterized in that the duration of the pause between turning on the controlled switches of the inverter's antiphase arms at cos φ n =1.0÷0.8 is set to 0.05÷0.044T.

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The invention relates to the field of converter technology and can be used, for example, in variable AC electric drive systems and in secondary power supply systems. The technical result consists in the development of an autonomous voltage inverter, which allows reducing power losses by ensuring minimal resistance of the circuit through which the current of each phase flows, while maintaining a low level of higher voltage harmonics on the motor phases. For this purpose, the claimed device contains a first electrical bridge consisting of three parallel-connected half-bridges made of several series-connected transistors, shunted by reverse diodes, a second six-arm electrical bridge consisting of three parallel-connected half-bridges made of two series-connected pairs of transistors, each of which consists of two transistors connected by opposite power terminals, and a voltage divider made of three series-connected capacitors. The first and fourth outputs of the voltage divider are connected to the inputs of the first electrical bridge, and its second and third outputs are connected to the inputs of the second electrical bridge. The outputs of the same-name half-bridges of the first and second bridges are interconnected and connected to the corresponding motor phase. 1 ill.

The invention relates to power converter technology and is a device that implements an energy-efficient pulsed method for regulating the power transmitted to the load. The technical result is increased energy efficiency and reliability. The device is a push-pull bridge voltage converter, which contains transistors (power controlled switches) forming a transistor bridge circuit, and a two-terminal load circuit of the transistor bridge circuit. The first and second transistor bridge transistors connected in series form a first transistor circuit that is connected between the power rails. The third and fourth transistors of the transistor bridge circuit, connected in series, form a second transistor circuit, which is connected between the power buses. The midpoints of the first and second transistor circuits are, respectively, the first and second terminals of the output circuit of the transistor bridge circuit, and the first and second terminals of the two-terminal load circuit of the transistor bridge circuit are connected to them. The first and second transistors are controlled by paraphase pulse signals of their first sequence, and the third and fourth transistors are controlled by paraphase pulse signals of their second sequence. The second sequence of paraphasic pulse signals is shifted in time relative to the first sequence. The set goals are achieved by introducing additional chokes and C-circuits containing capacitors. The first terminal of the winding of the first inductor is directly connected to the first terminal of the output circuit of the transistor bridge circuit, and the second terminal of the winding of the first inductor is connected to the power buses or to the power bus through capacitors or a capacitor of the first C-circuit. The first terminal of the winding of the second inductor is directly connected to the second terminal of the output circuit of the transistor bridge circuit, and the second terminal of the winding of the second inductor is connected to the power buses or to the power bus through capacitors or a capacitor of the second C-circuit. In the first version of the circuit of the proposed device, additional capacitors are introduced, and in the first and second transistor circuits, each of the transistors contained in them or one of them is shunted by a corresponding additional capacitor. In the second version of the circuit of the proposed device, additional diodes are introduced. The second terminal of the winding of the first inductor is connected to the first and second power buses through the first and second additional diodes, respectively. The second terminal of the winding of the second inductor is connected to the first and second power buses through the third and fourth additional diodes, respectively. 2 salary f-ly, 3 ill.

The invention relates to converter technology devices and can be used to power at a frequency of 400 Hz on-board systems of aircraft, as well as to power a high-frequency instrument with a frequency of 400 Hz or 200 Hz

This article discusses the circuit of a simple device that allows you to implement control of the power circuit of a frequency asynchronous drive. The article is aimed at radio amateurs interested in the development and manufacture of homemade speed controllers for asynchronous motors, including when they are powered from a household single-phase network.

Important note. The article does not discuss auxiliary systems, without which the construction of a complete drive circuit is impossible, namely: power supplies for all drive units, the interface circuit between the low-voltage control circuit and the inverter power circuit (power switch drivers), and the inverter power circuit itself. The development of these nodes is left to the discretion of the readers.

Frequency controlled (or variable) asynchronous drive(hereinafter simply the drive) is usually built according to the scheme "supply network - rectifier - filter - three-phase voltage inverter - driven asynchronous motor (hereinafter - IM)". The supply network can be either domestic single-phase or industrial three-phase, and accordingly the rectifier is made single- or three-phase. As a rule, L-shaped LC filters are used as a filter; in low-power systems, the use of a conventional anti-aliasing C-filter is acceptable.

The most complex component is the voltage inverter. In recent years, it has been built on the basis of fully controlled power switches - transistors ( MOSFET or IGBT), and more recently, circuits based on semi-controlled switches (thyristors) were used. The task of the inverter is to obtain from a direct voltage a three-phase voltage regulated in frequency and effective value. Frequency regulation is not particularly difficult, but to regulate the effective voltage value you have to use PWM modulation, which is far from simple.

The power switches of the inverter are controlled by a special control controller (in other words, a control circuit) according to a certain algorithm. The control algorithm implies not only the implementation of functions for regulating the frequency and effective value of the output voltage, but also the implementation of protection of power switches from overloads and short circuits. In some cases, the functions of regulating the torque on the IM shaft and other specific tasks that are irrelevant for amateur use are additionally implemented.

Developing an inverter control circuit with a full set of functions is too complex a task to recommend it to a wide range of electronics enthusiasts, but it is possible to solve it in a truncated form, but sufficient for domestic use (and even for some special industrial cases, for example, ventilation drives) - see. magazine articles Radio No. 4 for 2001 And No. 12 for 2003(can be downloaded from) . Unfortunately, these designs have several drawbacks, in particular, low stability of parameters due to the mixed semi-analog-semi-digital approach, poor protection systems, etc. An attempt to get rid of these shortcomings and at the same time expand functionality control system resulted in the creation of a voltage inverter control circuit on an inexpensive microcontroller (see. Picture 1), which is proposed for repetition.

Figure 1. Circuit diagram

Brief characteristics and features:

• generating a sequence of control pulses for power switches using an algorithm that implements a linear dependence of the effective voltage value on frequency;
• regulation of the inverter output voltage frequency from 5 to 50 Hz;
• fast-acting protection of inverter power switches from short-circuit currents;
• the possibility of using a protection circuit as a current sensor as a specialized sensor (for example, L.E.M.), and a conventional shunt;
• possibility of connecting an additional display with serial interface to indicate the current and set frequency;
• extreme simplicity of the circuit - only 4 chips, including a microcontroller.

The circuit uses an inexpensive microcontroller AT89C2051-24PI. It implements all the required functions using a specially developed program.

Connector XP3 serves to connect the supply voltage to the 5 V control circuit (pins 1 and 4), as well as to connect the inverter power switch drivers to the circuit (pins 12 - 17).

Connector XP1 serves to connect the signal from the inverter current sensor. If a current sensor from a company is used L.E.M. or similar, then a load resistor is required R0, its resistance is determined by the type of sensor. If a shunt is used as a sensor, then this resistor is not needed. The shunt must be designed so that, in the presence of a short-circuit current in the DC circuit of the inverter, the voltage across it drops from 3 to 5 V. If the voltage is significantly lower, an additional amplification stage may be required.

The protection circuit is based on a comparator DA1A and trigger DD1.1 and it works like this. Voltage from current sensor via protective circuit R1-VD1 goes to the non-inverting input of the comparator DA1.A, and the threshold voltage from the trimming resistor is supplied to its inverting input R2. When the voltage from the current sensor exceeds the threshold, the comparator will operate, and a high logic level from its output will go to the clock input of the trigger DD1.1, which will switch and use a signal from its pin 5 to put the microcontroller into the reset state. Power on trigger DD1.1 set to reset state by circuit R5-C1. To reset the protection circuit to the operating position and thereby start the inverter, briefly press the button SB1.

When a reset signal arrives at the microcontroller DD2 stops, it will begin executing its program. First, the microcontroller is internally initialized, and then the bus buffer enable signal is sent. DD3 "GATE ". This buffer is used to quickly turn off output control signals when the protection is triggered, because when a reset signal arrives at the microcontroller, a high logical level is set at all its output ports, including the line " GATE ", which translates the outputs DD3 into the Z-state. Thanks to resistors R9-R14 at the control circuit outputs marked " VT1 " - "VT6 ", a low logical level is set, which corresponds to the locked state of all inverter power switches. LED HL1 indicates the operating mode of the control circuit: green light is “operation”, red light is “protection”.

This design of the protection circuit is due to the fact that the speed of modern inexpensive microcontrollers is clearly not enough to implement protection software. This applies not only to the microcontroller used, but also to faster AVRs and PICs.

Using a resistor R8 the desired value of the inverter output voltage frequency is set. Regardless of engine position R8, immediately after starting operation, the inverter generates output signals for a voltage frequency of 5 Hz. Then, having analyzed the position of the slider of this resistor, the microcontroller begins to gradually increase the frequency to a given level. The frequency changes discretely in steps of 1 Hz, and the rate of change is set to 2 Hz/sec. This is done to eliminate abrupt changes in the output frequency, which can lead to shock currents in the IM and mechanical overloads in the drive mechanism.

To connector XP2 You can connect a display with a serial interface, with which the set and current frequency values ​​are displayed; the presence of a display is not necessary for the operation of the circuit. In the author's version it is used on six seven-segment LED indicators and six registers with serial input and parallel output.

Figure 2 Drawing of PCB sides

Figure 3 Arrangement of elements on the board.

A printed circuit board has been designed for the control circuit (see Fig. Figure 2). The placement of circuit elements shows Figure 3. The connectors used are pin plugs of the type PLS. Microcontroller DD2 installed in the panel to allow reprogramming. Two-color LED - any, red crystal is connected to a resistor R16. Button SB1- any clock, trimming resistor R3 type SP5-16, variable R8- any. The type of resistors and capacitors is not of fundamental importance; it is only important that the voltage of electrolytic capacitors is at least 10 V. Non-electrolytic capacitors are ceramic disk capacitors.

The operation algorithm of the control circuit is explained by the diagrams of the output signals and the corresponding diagrams of the inverter output voltages (with an active load) - see. Figure 4 And Figure 5. The duration of the pulses is 1.11 milliseconds, and the duration of the pause between them (inside the burst) depends on the frequency, and at an inverter output voltage frequency of 50 Hz it is about 20 microseconds (a protective interval that completely eliminates the possibility of through currents occurring in the inverter).

Figure 4 Control Circuit Output Diagram

Figure 5 Shape of inverter output voltages with active load

The control circuit has been tested using a high-power inverter at IGBT transistors MBN1200C33(HITACHI), to which was connected an IM with a power of 55 kW with a rated rotation speed of 1500 min-1, loaded on a centrifugal fan. There were no malfunctions in the operation of the control circuit. The actual shape of the voltage at the output of the inverter with the above-mentioned blood pressure is demonstrated by oscillograms - see. Figure 6 And Figure 7.

Figure 6 Phase voltages on the motor

Figure 7 Phase voltages on the motor

High-quality images of the circuit, the pattern of the printed circuit board conductors, binary file firmware can be downloaded from, and some additional information details of the construction of the remaining drive and inverter components not discussed in this article can be obtained from the additional article-appendix located there.