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 - a method for controlling a three-phase inverter with a DC link is equipped with a control unit that forms a pause between switching on of the controlled keys, and the duration of the pause between switching on of the controlled keys of the inverter's antiphase arms at values of cos φ n = 1.0÷0.8 is 0.05 T÷ 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 of 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 to 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, phase loads connected in a star configuration, a control unit and auxiliary switches connected to the corresponding phases load and an additional capacitor, with the main switches being in a conducting state of 5/12T, and the auxiliary ones 1/12T, where T is the period of the output voltage [Patent (RF) No. 2125761, N02M 7/5387,1999].
The disadvantages of this device are the large number of 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 decoder of 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 of 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 switching on of 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 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
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Master generator for a three-phase inverter.
The topic of powering a three-phase electric motor from a single-phase network is not new, but still remains relevant. Today we bring to our readers another technical solution to the problem. To simplify the master generator - the basis of a three-phase inverter that provides power to such a motor - the author of the article suggests using the PIC12F629 (PIC12F675) or PIC16F628 (PIC16F628A, PIC16F648A) microcontroller. The frequency of the generated oscillations can be changed from the nominal (50 Hz) both downwards (33 and 25 Hz) and up (67 Hz). A description of the program is given that allows you to change the frequency of the generated pulses and their duty cycle. In addition, this program, when loaded into the memory of the PIC12F629 (PIC12F675) microcontroller, is capable of controlling the operation of a six-LED display that simulates the rotation of the rotor of a three-phase electric motor. The microcontroller program files and the “Setting up a three-phase generator” program will be placed on our FTP server at
One of the first converter circuits for powering a three-phase motor was published in Radio magazine No. 11, 1999. The developer of the scheme, M. Mukhin, was a 10th grade student at that time and was involved in a radio club.
The converter was intended to power a miniature three-phase motor DID-5TA, which was used in a machine for drilling printed circuit boards. It should be noted that the operating frequency of this motor is 400Hz, and the supply voltage is 27V. In addition, the middle point of the motor (when connecting the windings in a star) is brought out, which made it possible to simplify the circuit extremely: only three output signals were needed, and only one output switch was required for each phase. The generator circuit is shown in Figure 1.
As can be seen from the diagram, the converter consists of three parts: a three-phase sequence pulse generator on DD1...DD3 microcircuits, three switches on composite transistors (VT1...VT6) and the electric motor M1 itself.
Figure 2 shows the timing diagrams of the pulses generated by the generator-shaper. The master oscillator is made on the DD1 chip. Using resistor R2, you can set the required engine speed, and also change it within certain limits. More detailed information about the scheme can be found in the above magazine. It should be noted that according to modern terminology, such generator-shapers are called controllers.
Picture 1.
Figure 2. Generator pulse timing diagrams.
Based on the considered controller by A. Dubrovsky from Novopolotsk, Vitebsk region. The design of a variable frequency drive for a motor powered by a 220V AC network was developed. The device diagram was published in Radio magazine in 2001. No. 4.
In this circuit, practically without changes, the controller just discussed according to M. Mukhin’s circuit is used. The output signals from elements DD3.2, DD3.3 and DD3.4 are used to control the output switches A1, A2, and A3, to which the electric motor is connected. The diagram shows key A1 in full, the rest are identical. The complete diagram of the device is shown in Figure 3.
Figure 3.
To familiarize yourself with connecting the motor to the output switches, it is worth considering the simplified diagram shown in Figure 4.
Figure 4.
The figure shows an electric motor M controlled by keys V1...V6. To simplify the circuit, semiconductor elements are shown as mechanical contacts. The electric motor is powered by a constant voltage Ud received from the rectifier (not shown in the figure). In this case, the keys V1, V3, V5 are called upper, and the keys V2, V4, V6 are called lower.
It is quite obvious that opening the upper and lower keys at the same time, namely in pairs V1&V6, V3&V6, V5&V2 is completely unacceptable: a short circuit will occur. Therefore, for the normal operation of such a key circuit, it is necessary that by the time the lower key is opened, the upper key has already been closed. For this purpose, control controllers create a pause, often called a “dead zone”.
The length of this pause is such as to ensure guaranteed closure of the power transistors. If this pause is not sufficient, then it is possible to briefly open the upper and lower keys simultaneously. This causes the output transistors to heat up, often leading to their failure. This situation is called through currents.
Let's return to the circuit shown in Figure 3. In this case, the upper keys are 1VT3 transistors, and the lower ones are 1VT6. It is easy to see that the lower keys are galvanically connected to the control device and to each other. Therefore, the control signal from output 3 of element DD3.2 through resistors 1R1 and 1R3 is supplied directly to the base of the composite transistor 1VT4…1VT5. This composite transistor is nothing more than a lower switch driver. In exactly the same way, elements DD3, DD4 control the composite transistors of the lower key drivers of channels A2 and A3. All three channels are powered by the same rectifier VD2.
The upper switches do not have a galvanic connection with the common wire and the control device, so to control them, in addition to the driver on the composite transistor 1VT1...1VT2, it was necessary to install an additional 1U1 optocoupler in each channel. The output transistor of the optocoupler in this circuit also performs the function of an additional inverter: when the output of element 3 of DD3.2 is high, the transistor of the upper switch 1VT3 is open.
To power each upper switch driver, a separate rectifier 1VD1, 1C1 is used. Each rectifier is powered by an individual winding of the transformer, which can be considered as a disadvantage of the circuit.
Capacitor 1C2 provides a switching delay of about 100 microseconds, the same amount is provided by optocoupler 1U1, thereby forming the above-mentioned “dead zone”.
Is frequency regulation enough?
As the frequency of the AC supply voltage decreases, the inductive reactance of the motor windings decreases (just remember the formula for inductive reactance), which leads to an increase in the current through the windings, and, as a consequence, to overheating of the windings. The stator magnetic circuit also becomes saturated. To avoid these negative consequences, when the frequency decreases, the effective value of the voltage on the motor windings must also be reduced.
One of the ways to solve the problem in amateur frequency generators was to regulate this most effective value using an LATR, the moving contact of which had a mechanical connection with a variable resistor of the frequency regulator. This method was recommended in the article by S. Kalugin “Refinement of the speed controller of three-phase asynchronous motors.” Radio magazine 2002, no. 3, p. 31.
In amateur conditions, the mechanical unit turned out to be difficult to manufacture and, most importantly, unreliable. A simpler and more reliable method of using an autotransformer was proposed by E. Muradkhanyan from Yerevan in the magazine “Radio” No. 12 2004. The diagram of this device is shown in Figures 5 and 6.
The 220V network voltage is supplied to the autotransformer T1, and from its moving contact to the rectifier bridge VD1 with filter C1, L1, C2. The output of the filter produces a variable constant voltage Ureg, which is used to power the motor itself.
Figure 5.
The voltage Ureg through resistor R1 is also supplied to the master oscillator DA1, made on the KR1006VI1 microcircuit (imported version). This connection turns a conventional square wave generator into a VCO (voltage controlled oscillator). Therefore, as the voltage Ureg increases, the frequency of generator DA1 also increases, which leads to an increase in engine speed. As the voltage Ureg decreases, the frequency of the master generator also decreases proportionally, which avoids overheating of the windings and oversaturation of the stator magnetic circuit.
Figure 6.
Figure 7.
The generator is made on the second trigger of the DD3 chip, designated in the diagram as DD3.2. The frequency is set by capacitor C1, frequency adjustment is carried out by variable resistor R2. Along with the frequency adjustment, the pulse duration at the generator output also changes: as the frequency decreases, the duration decreases, so the voltage on the motor windings drops. This control principle is called pulse width modulation (PWM).
In the amateur circuit under consideration, the motor power is low, the motor is powered by rectangular pulses, so the PWM is quite primitive. In real high-power applications, PWM is designed to generate almost sinusoidal voltages at the output, as shown in Figure 8, and to operate with various loads: at constant torque, at constant power and at fan load.
Figure 8. Output voltage waveform of one phase of a three-phase PWM inverter.
Power part of the circuit
Modern branded frequency generators have outputs specifically designed for operation in frequency converters. In some cases, these transistors are combined into modules, which generally improves the performance of the entire design. These transistors are controlled using specialized driver chips. In some models, drivers are produced built into transistor modules.
The most common chips and transistors currently used are International Rectifier. In the described circuit, it is quite possible to use IR2130 or IR2132 drivers. One package of such a microcircuit contains six drivers at once: three for the lower switch and three for the upper, which makes it easy to assemble a three-phase bridge output stage. In addition to the main function, these drivers also contain several additional ones, such as protection against overloads and short circuits. More detailed information about these drivers can be found in the Data Sheets for the corresponding chips.
Despite all the advantages, the only drawback of these microcircuits is their high price, so the author of the design took a different, simpler, cheaper, and at the same time workable route: specialized driver microcircuits were replaced with integrated timer microcircuits KR1006VI1 (NE555).
Output switches on integral timers
If you return to Figure 6, you will notice that the circuit has output signals for each of the three phases, designated as “H” and “B”. The presence of these signals allows you to control the upper and lower keys separately. This separation allows a pause to be formed between switching the upper and lower keys using the control unit, and not the keys themselves, as was shown in the diagram in Figure 3.
The diagram of output switches using KR1006VI1 (NE555) microcircuits is shown in Figure 9. Naturally, for a three-phase converter you will need three copies of such switches.
Figure 9.
KR1006VI1 microcircuits connected according to the Schmidt trigger circuit are used as drivers for the upper (VT1) and lower (VT2) keys. With their help, it is possible to obtain a gate pulse current of at least 200 mA, which allows for fairly reliable and fast control of output transistors.
The microcircuits of the lower DA2 switches have a galvanic connection with the +12V power source and, accordingly, with the control unit, so they are powered from this source. The upper switch chips can be powered in the same way as shown in Figure 3 using additional rectifiers and separate windings on the transformer. But this scheme uses a different, so-called “booster” method of nutrition, the meaning of which is as follows. The DA1 microcircuit receives power from the electrolytic capacitor C1, the charge of which occurs through the circuit: +12V, VD1, C1, open transistor VT2 (through drain - source electrodes), “common”.
In other words, the charge of capacitor C1 occurs while the lower switch transistor is open. At this moment, the negative terminal of capacitor C1 is practically short-circuited to the common wire (the resistance of the open “drain-source” section of powerful field-effect transistors is thousandths of an Ohm!), which makes it possible to charge it.
When transistor VT2 is closed, diode VD1 will also close, the charging of capacitor C1 will stop until the next opening of transistor VT2. But the charge of capacitor C1 is sufficient to power the DA1 chip for as long as transistor VT2 is closed. Naturally, at this moment the upper switch transistor is in the closed state. This power switch circuit turned out to be so good that it is used without changes in other amateur designs.
This article discusses only the simplest circuits of amateur three-phase inverters on microcircuits with a low and medium degree of integration, from which it all began, and where you can even look at everything “from the inside” using the circuit diagram. More modern designs have been made, the diagrams of which have also been repeatedly published in Radio magazines.
Microcontroller control units are simpler in design than those based on medium-integrated microcircuits; they have such necessary functions as protection against overloads and short circuits, and some others. In these blocks, everything is implemented using control programs or, as they are commonly called, “firmware”. It is these programs that determine how well or poorly the control unit of a three-phase inverter will work.
Quite simple controller circuits for a three-phase inverter were published in the magazine “Radio” 2008 No. 12. The article is called “Master generator for a three-phase inverter.” The author of the article, A. Dolgiy, is also the author of a series of articles on microcontrollers and many other designs. The article shows two simple circuits on the PIC12F629 and PIC16F628 microcontrollers.
The rotation speed in both circuits is changed in steps using single-pole switches, which is quite sufficient in many practical cases. There is also a link where you can download ready-made “firmware”, and, moreover, a special program with which you can change the parameters of the “firmware” at your discretion. It is also possible to operate the generators in “demo” mode. In this mode, the generator frequency is reduced by 32 times, which allows you to visually observe the operation of the generators using LEDs. Recommendations for connecting the power section are also given.
But, if you don’t want to program a microcontroller, Motorola has released a specialized intelligent controller MC3PHAC, designed for 3-phase motor control systems. On its basis, it is possible to create inexpensive three-phase adjustable drive systems containing all the necessary functions for control and protection. Such microcontrollers are increasingly used in various household appliances, for example, in dishwashers or refrigerators.
Complete with the MC3PHAC controller, it is possible to use ready-made power modules, for example IRAMS10UP60A developed by International Rectifier. The modules contain six power switches and a control circuit. More details about these elements can be found in their Data Sheet documentation, which is quite easy to find on the Internet.
Three-phase asynchronous motors are widely used in industry and in everyday life due to their simplicity and reliability. The absence of a sparking and heating commutator-brush assembly, as well as the simple design of the rotor, ensures a long service life and simplifies 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 of the regulator 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 complicated 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 current protection unit of the 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 sent 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
The KP501A field-effect transistor can be replaced with a 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)
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 the functionality of the 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;
- the ability to connect an additional display with a 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 by 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 original version, it is used on six seven-segment LED indicators and six registers with serial input and parallel data 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 rpm, 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, and the binary firmware file can be downloaded from, and some additional information about the construction features of the remaining drive and inverter components not discussed in this article can be obtained from the additional article-appendix located there.
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