Frequency switch for a three-phase electric motor connection diagram. Frequency converter - types, principle of operation, connection diagrams

Asynchronous motors are the devices most commonly used in industry.

( ArticleToC: enabled=yes )

For a smooth start, frequency converters are used that can control the starting current and allow you to regulate the rotation speed. But, it is important to understand that the frequency converter for single-phase electric motor different from that required by three-phase.

Asynchronous motors compared to others electric machines more powerful and productive, but having such a disadvantage as the need to equip with additional elements responsible for the rotor rotation speed.

The same applies to the starting current, which is 5-7 times higher than the rated current, due to which shock loads lead to loss of energy and all together reduce its service life.

To combat these problems, there is a class of devices that automatically controls inrush currents. They are called frequency converters.

With their help, it is possible to reduce starting currents by 5 times, achieving a smooth start.

In addition, by adjusting frequencies with voltage, the rotor is controlled.

In addition to these advantages, the use of such devices has the following:

• at the time of start-up, up to 50% energy is saved;
• with their help, feedback is carried out between adjacent conductors. Their
• can be called three-phase voltage generators of the desired value and frequency.

They are based on a double conversion inverter.

The operating principle is as follows:

• first, the input current sinusoidal 220 or 380V is rectified, passing through the diode bridge;
• after that, it goes to the capacitor group, where it is smoothed; having passed through the capacitors, it is fed to control microcircuits and a bipolar BTI transistor, or rather bridge switches, where it is formed specified parameters pulse-width three-phase sequence;
• the received pulses, having the shape of a rectangle, under the influence of the inductance of the windings are converted at the output into a sinusoidal voltage.

Below is a diagram to help you understand how a frequency converter works:

Selection of frequency converters

For manufacturers of these devices to win the market, price is important, as with any electronic equipment. To reduce it, they create devices with a minimal set of functions, i.e. The more expensive a frequency converter is, the more versatile the device is, which is important for a consumer who wants to extend the life of the engine.

Main selection criteria

These include:

• control. According to this indicator, frequency converters are divided into scalar and vector, which are more common, but more expensive. This is explained by the fact that they are able to provide more high-precision adjustment, which the former cannot provide. Scalar ones can only maintain a given ratio of output voltage and frequency. Therefore, they are placed in devices with a low load on the motor;
• power. It is clear that the larger this parameter, the better. But, in addition to the numbers, the manufacturer is important: equipment that is “closely related” works much more efficiently. In addition, the use of single-brand converters is important for interchangeability;
• mains voltage. To protect devices from power surges, which often occur in domestic networks, it is desirable that the voltage have a large operating range;
• frequency adjustment range. This is based on the requirements of a specific device. In practice, converters with a frequency of 10-100 Hertz are used; discrete inputs. They are designed to transmit commands. They also ensure engine starting and stopping, rotation in the opposite direction and braking;
• analog inputs. Thanks to them, they monitor when the engine is running and adjust the drive;
• digital. Their purpose is to transmit high-frequency signals that are generated by rotation angle sensors. The more inputs, the better it is, but the device is more expensive;
• in addition to the entrances, discrete outputs are important, from which the signal reports any malfunctions that have occurred (overheating, accidents, deviation of the input voltage from the norm, etc.);
• analog outputs are responsible for the transfer feedback. They are selected according to the principle described above;
• at the control bus the number of inputs and outputs must match the converter circuit. But, it is better if she has a reserve that may be needed when improving the device;
• overload capacity. It is considered normal when the power frequency converter 10-15% more than the engine. Its current must be higher than the rated one.

They are produced with a power of 5-10 W. This is enough to operate centrifuges, household refrigerators, washing machines, processing machines, etc. Their technical characteristics are worse compared to three-phase ones:

The power is only 70% of three-phase, and the overload capacity is lower.

The IM stator contains windings - main and starting. The latter is used when starting a squirrel-cage rotor.

To understand why a starting winding is needed, let’s look at an example: the motor is connected only to the working winding (220V).

In it, I1 (single-phase current) creates a pulsating magnetic field. It can be decomposed into two - with the same amplitude and rotation speeds, but oppositely directed - Fa and Fv. With a stationary rotor, these fields create torques M1 and M2 that are different in sign, but equal in magnitude.

The resulting starting torque is zero (Mn= M1 – M2), i.e. the motor will not be able to rotate without applying a load to the shaft.

Therefore, a starting winding is required. The field it creates causes the motor to rotate. The direction of rotation determines the starting initial torque.

An electric motor is a machine that transforms electrical energy into a mechanical one, thanks to which mechanisms are set in motion. When converting energy back, these devices act as a generator. The rotor (rotating) and stator (stationary) are the main components of electric motors.

To create a rotating field, two windings on the stator are required, offset in space at a certain angle. The starting unit is placed on the stator in accordance with this with an offset relative to the working one of 90 degrees. To ensure a current shift, when connecting it to the network, a phase-shifting element is used - a coil, capacitor or active resistor.

When current flows through a conductor, a magnetic field is created that acts on it with a force F. If the conductor is bent into a frame and placed in a magnetic field, two sides that are at an angle of 90 degrees to the field will experience the same force, but in the opposite direction , which create torque.

A small-sized single-phase frequency converter is needed to control an asynchronous motor with capacitor start (AIRE, ABE, etc.)

Such motors are installed in electric fans, washing machines, refrigerators, etc.

On the website h ttp://xn--80aqahnfuib9b.xn--p1ai/esq_A200.html You can see all the characteristics of the device. Here you can buy it, having decided on the table with the model.

In the online store http://npf-oberon.com.ua/index.php?route=product/product&path=59_63_65&product_id=62/ its cost is 170 dollars. You can also see the characteristics there.

It is used to control motors installed in agricultural equipment, conveyors, mixers, and powerful pumps.

Huge selection of single and three-phase converters different manufacturers on the website https://chastotnik.com.ua/preobrasovateli//p5 .

To say whether a single-phase frequency converter or a three-phase one is better, you need to clearly know what it is needed for. In single-phase motors they are needed for control and regulation. Such frequency converters convert alternating voltage into pulsed voltage, whose frequency is 0-1000 oscillations/sec. The speed at which the rotor of an asynchronous motor rotates, receiving a sinusoidal voltage, changes in proportion to the frequency of such power supply.

The frequency converter for electric motor 380 differs from motors operating from a household network in the voltage supplied to the inverter. The frequency of the three-phase voltage at the output is in the range of 0-1 kHz.

The motor is subsequently powered from it, i.e. Such a converter allows the drive to be powered from a household network and simultaneously regulates its characteristics.

Today, such devices are rarely used, since they have been replaced by three-phase frequency converters, which have much wider capabilities. Three-phase frequency converter for three-phase electric motor capable of converting industrial mains voltage (three-phase).

They are connected to an asynchronous motor with a “star”, and single-phase ones are connected with a “triangle”, i.e. they regulate a larger number of parameters, which makes it possible to select the optimal mode.

They have significantly smaller dimensions and larger functionality, high levels of durability and reliability, quite reasonable cost.

Video: Frequency converter. Connection three phase motor to a single-phase 220V network.

With the ever-increasing growth of automation in the household sector, there is a need for modern systems and electric motor control devices.

Control and frequency conversion in small-power single-phase asynchronous motors, started using capacitors, allows you to save energy and activates the energy saving mode at a new, progressive level.

Operating principle of a single-phase asynchronous machine

The operation of an asynchronous motor is based on the interaction of a rotating magnetic field stator and the currents induced by it in the motor rotor. When the rotation frequency of the pulsating magnetic fields differs, a torque occurs. It is this principle that is followed when regulating the rotation speed of an asynchronous motor using.

The starting winding occupies 1/3 of the slots in the stator structure, and the main winding accounts for 23 stator slots.

The rotor of a single-phase motor, short-circuited, placed in a stationary magnetic field of the stator, begins to rotate.

Fig. No. 1 Schematic drawing of a motor, demonstrating the principle of operation of a single-phase asynchronous motor.

Main types of single-phase electric drives

Air conditioners, refrigeration compressors, electric fans, blowers, water, drainage and sewage pumps, washing machines use an asynchronous three-phase motor in their design.

All types of frequency converters convert alternating mains voltage into constant voltage. Serve to generate single-phase voltage with adjustable frequency and specified amplitude to control rotation asynchronous motors.

Rotation speed control of single-phase motors

There are several ways to regulate the rotation speed of a single-phase motor.

1. Controlling motor slip or voltage variation. The method is relevant for units with a fan load; it is recommended to use motors with increased power. The disadvantage of this method is heating of the motor windings.
2. Step control of engine rotation speed using an autotransformer.

Fig. No. 2. Adjustment circuit using an autotransformer.

The advantages of the circuit are that the output voltage has a pure sinusoid. The transformer's ability to withstand overloads has a large power reserve.

Disadvantages - the autotransformer has large overall dimensions.

Using a thyristor. Thyristor switches connected back-to-back are used.

Rice. No. 3. Scheme of thyristor control of a single-phase asynchronous electric motor.

When used to regulate the rotation speed of single-phase asynchronous motors, to avoid negative influence induction load, the circuit is modified. LRC circuits are added to protect the power switches, a capacitor is used to correct the voltage wave, the minimum engine power is limited, this ensures that the engine starts. The thyristor must have a current higher than the motor current.

Transistor voltage regulator

The circuit uses pulse-width modulation (PWM) using an output stage built using field-effect or bipolar IGBT transistors.

Rice. No. 4. Scheme of using PWM to regulate a single-phase asynchronous electric motor.

Frequency regulation of an asynchronous single-phase electric motor is considered the main method of regulating power, efficiency, speed and energy saving indicators.

Rice. No. 5. Electric motor control circuit without excluding the capacitor from the design.

Frequency converter: types, principle of operation, connection diagrams

Allows its owner to reduce energy consumption and automate processes in equipment and production management.

The main components of the frequency converter: rectifier, capacitor, IGBT transistors assembled into an output stage.

Thanks to the ability to control the parameters of the output frequency and voltage, a good energy-saving effect is achieved. Energy saving is expressed in the following:

1. The engine maintains a constant current torque of the shaft. This is due to the interaction of the output frequency of the inverter converter with the engine speed and, accordingly, the dependence of voltage and torque on the engine shaft. This means that the converter makes it possible to automatically regulate the output voltage when it detects a voltage value that exceeds the norm with a certain operating frequency necessary to maintain the required torque. All inverter converters with vector control have the function of maintaining constant torque on the shaft.
2. The frequency converter is used to regulate the operation of pumping units (). When receiving a signal from a pressure sensor, the frequency generator reduces the performance of the pumping unit. As the engine speed decreases, the output voltage consumption decreases. Thus, standard water consumption by a pump requires 50Hz industrial frequency and 400V voltage. Using the power formula, you can calculate the ratio of power consumption.

By reducing the frequency to 40Hz, the voltage is reduced to 250V, which means that the number of pump revolutions is reduced and energy consumption is reduced by 2.56 times.

Rice. No. 6. Using a Speedrive frequency converter to regulate pumping units using the CKEA MULTI 35 system.

To improve energy efficiency, you must do the following:

• The frequency converter must match the parameters of the electric motor.
• The frequency generator is selected in accordance with the type of working equipment for which it is intended. Thus, the frequency converter for pumps operates in accordance with the parameters included in the program to control the operation of the pump.
• Precise settings of control parameters in manual and automatic mode.
• The frequency converter allows the use of energy saving mode.
• Vector control mode allows you to produce automatic setup engine control.

Single-phase frequency converter

The compact frequency conversion device is used to control single-phase electric motors for household equipment. Most frequency converters have the following design capabilities:

1. Most models use in their design latest technologies vector control.
2. They provide improved torque for single-phase motors.
3. Energy saving is set to automatic mode.
4. Some models of frequency converters use a removable control panel.
5. Built-in PLC controller (it is indispensable for creating data collection and transmission devices, for creating telemetry systems, and integrates devices with various protocols and communication interfaces into a common network).
6. Built-in PID controller (monitors and regulates temperature, pressure and technological processes).
7. The output voltage is adjusted automatically.

Fig. No. 7. Modern Optidrive converter with basic functional features.

Important: Single-phase frequency converter, powered by single-phase network voltage 220V, produces three linear voltages, the value of each of them is 220V. That is, the linear voltage between the 2 phases is directly dependent on the output voltage of the frequency converter itself.

The frequency converter does not serve for double voltage conversion; due to the presence of a PWM regulator in the design, it can increase the voltage value by no more than 10%.

Main task single-phase converter frequency - provide power to both single- and three-phase electric motors. In this case, the motor current will correspond to the connection parameters from a three-phase network and remain constant

Frequency regulation of single-phase asynchronous electric motors

The first thing we pay attention to when choosing a frequency converter for our equipment is the correspondence of the mains voltage and the rated value of the load current for which the motor is designed. The connection method is selected relative to the operating current.

The main thing in the connection diagram is the presence of a phase-shifting capacitor; it serves to shift the voltage supplied to the starting winding. It serves to start the engine, sometimes after the engine has started, the starting winding along with the capacitor is turned off, sometimes it remains on.

Connection diagram for a single-phase motor using a single-phase frequency converter without using a capacitor

The output linear voltage of the device at each phase is equal to the output voltage of the frequency converter, that is, there will be three line voltages at the output, each 220V. Only the starting winding can be used for starting.

Rice. No. 8. Connection diagram of a single-phase asynchronous motor through a capacitor

A phase-shifting capacitor cannot provide uniform phase shift within the frequency limits of the inverter. The frequency generator will provide a uniform phase shift. In order to exclude a capacitor from the circuit, you need to:

1. Starter capacitor C1 is removed.
2. We connect the output of the motor winding to the voltage output point of the frequency converter (direct wiring is used).
3. Point A joins SA; B connects to NE; W is connected to CC, so the motor will be connected directly.
4. To turn on in the reverse direction (reverse wiring), it is necessary to connect B to CA; And attach to SV; W connect with CC.

Rice. No. 9. Wiring diagram for a single-phase asynchronous motor without using a capacitor.

In the video - Frequency converter. to a single-phase 220V network.

The rotor of any electric motor is driven by forces caused by a rotating electromagnetic field inside the stator winding. Its speed of rotation is usually determined industrial frequency electrical network.

Its standard value of 50 hertz implies fifty oscillation periods within one second. In one minute, their number increases 60 times and amounts to 50x60=3000 revolutions. The rotor rotates the same number of times under the influence of an applied electromagnetic field.

If you change the value of the network frequency applied to the stator, you can adjust the rotation speed of the rotor and the drive connected to it. This principle is the basis for controlling electric motors.

Types of frequency converters

By design, frequency converters are:

1. induction type;

2. electronic.

Asynchronous electric motors, made and launched into generator mode, are representatives of the first type. They have low operating efficiency and are characterized by low efficiency. Therefore, they have not found wide application in production and are used extremely rarely.

The method of electronic frequency conversion allows you to smoothly regulate the speed of both asynchronous and synchronous machines. In this case, one of two control principles can be implemented:

1. according to a predetermined characteristic of the dependence of rotation speed on frequency (V/f);

2. vector control method.

The first method is the simplest and less advanced, and the second is used to accurately control the rotation speeds of critical industrial equipment.

Features of vector control of frequency conversion

The difference between this method is the interaction, the influence of the converter control device on the “spatial vector” of the magnetic flux, rotating with the frequency of the rotor field.

Algorithms for operating converters based on this principle are created in two ways:

1. touchless control;

2. flow control.

The first method is based on assigning a certain dependence of the inverter sequence alternation to pre-prepared algorithms. In this case, the amplitude and frequency of the voltage at the output of the converter are regulated by slip and load current, but without using feedback on the rotor rotation speed.

This method is used when controlling several electric motors connected in parallel to a frequency converter. Flux control involves monitoring the operating currents inside the motor, decomposing them into active and reactive components and making adjustments to the operation of the converter to set the amplitude, frequency and angle for the output voltage vectors.

This allows you to increase the accuracy of the engine and increase the limits of its regulation. The use of flow control expands the capabilities of drives operating at low speeds with large dynamic loads, such as crane lifting devices or industrial winding machines.

The use of vector technology allows dynamic adjustment of rotating torques to be applied.

Substitution scheme

Fundamentally simplified electrical diagram An asynchronous motor can be represented as follows.

Voltage u1 is applied to the stator windings, which have active R1 and inductive resistance X1. It, overcoming the resistance of the air gap Xv, is transformed into the rotor winding, causing a current in it that overcomes its resistance.

Vector diagram of equivalent circuit

Its construction helps to understand the processes occurring inside an asynchronous motor.

The stator current energy is divided into two parts:

iµ - flow-forming fraction;

iw is the torque-forming component.

In this case, the rotor has an active resistance R2/s, which depends on slip.

For touchless control the following are measured:

voltage u1;

current i1.

Based on their values, the following is calculated:

iµ - flow-forming current component;

iw is the torque-forming quantity.

The calculation algorithm has already included an electronic equivalent circuit of an asynchronous motor with current regulators, which takes into account the conditions of saturation of the electromagnetic field and losses of magnetic energy in steel.

Both of these components of the current vectors, differing in angle and amplitude, rotate together with the rotor coordinate system and are converted into stationary system stator orientation.

According to this principle, the parameters of the frequency converter are adjusted to the load of the asynchronous motor.

Operating principle of frequency converter

This device, also called an inverter, is based on a double change in the signal shape of the supply electrical network.

First, industrial voltage is supplied to a power rectifier unit with powerful diodes, which remove sinusoidal harmonics, but leave signal ripple. To eliminate them, a bank of capacitors with inductance (LC filter) is provided, providing a stable, smoothed shape to the rectified voltage.

The signal then goes to the input of the frequency converter, which is a bridge three-phase circuit of six series IGBT or MOSFET with reverse polarity breakdown protection diodes. The thyristors previously used for these purposes do not have sufficient speed and operate with great noise.

To enable the engine “braking” mode, a controlled transistor with a powerful resistor that dissipates energy can be installed in the circuit. This technique allows you to remove the voltage generated by the engine to protect the filter capacitors from overcharging and failure.

The method of vector control of the frequency of the converter allows you to create circuits that automatically regulate the signal by ACS systems. For this, a control system is used:

1. amplitude;

2. PWM (pulse width modeling).

The amplitude control method is based on changing the input voltage, and PWM is an algorithm for switching power transistors at a constant input voltage.

With PWM regulation, a signal modulation period is created when the stator winding is connected in strict order to the positive and negative terminals of the rectifier.

Since the generator clock frequency is quite high, in the winding of the electric motor, which has inductive reactance, they are smoothed to a sinusoid normal looking.

PWM control methods make it possible to eliminate energy losses as much as possible and provide high efficiency transformations due to simultaneous control of frequency and amplitude. They became available thanks to the development of control technologies for power turn-off thyristors of the GTO series or bipolar brands of IGBT transistors with an insulated gate.

The principles of their inclusion for controlling a three-phase motor are shown in the picture.

Each of the six IGBT transistors is connected in an anti-parallel circuit to its own reverse current diode. In this case, through the power circuit of each transistor passes active current asynchronous motor, and its reactive component is directed through diodes.

To eliminate the influence of external electrical noise on the operation of the inverter and motor, the design of the frequency converter circuit can be included, eliminating:

radio interference;

electrical discharges induced by operating equipment.

Their occurrence is signaled by the controller, and to reduce the impact, shielded wiring is used between the motor and the inverter output terminals.

In order to improve the accuracy of operation of asynchronous motors, the control circuit of frequency converters includes:

input of communication with advanced interface capabilities;

built-in controller;

memory card;

software;

LED information display displaying the main output parameters;

brake chopper and built-in EMC filter;

circuit cooling system based on blowing with long-life fans;

engine warm-up function via DC and some other possibilities.

Operational connection diagrams

Frequency converters are designed to work with single-phase or three-phase networks. However, if there are industrial DC sources with a voltage of 220 volts, then inverters can also be powered from them.

Three-phase models are designed for a network voltage of 380 volts and supply it to the electric motor. Single-phase inverters are powered by 220 volts and output three phases spaced apart in time.

The connection diagram of the frequency converter to the engine can be made according to the following diagrams:

stars;

triangle.

The motor windings are assembled into a “star” for the converter, powered from a three-phase 380 volt network.

The motor windings are assembled according to the “triangle” scheme when the converter feeding it is connected to a single-phase 220 volt network.

When choosing a method for connecting an electric motor to a frequency converter, you need to pay attention to the ratio of the power that a running motor can create in all modes, including slow, loaded starting, with the capabilities of the inverter.

You cannot constantly overload the frequency converter, and a small reserve of its output power will ensure its long-term and trouble-free operation.

Asynchronous motors are used in industry to power various mechanisms. But they have one significant drawback - upon startup, there is a short-term current jump of five to seven times. In addition to energy losses, industrial mechanisms suffer shock loads, which leads to their premature wear. Therefore, a frequency converter or inverter was developed that provides soft start and stopping of asynchronous motors.

Transducer Basics

The frequency converter not only provides smooth engine start-stop, but also changes the rotor speed, adjusting the frequency of the voltage at the motor input. In this case, the inverters change the frequency in a wide range from the frequency of the supply network. The magnitude of the supply voltage determines the rotation frequency of the magnetic field created by the stator. Let's denote voltage frequency, then the angular velocity of the motor’s magnetic field is determined by the following formula:

where is the number of pairs of stator poles. The law of proportionality depends on the load torque. If the load torque is constant, then the voltage on the stator is regulated according to the law

For fans the following relationship applies:

.

If the load torque is inversely proportional to the speed, then the voltage and frequency are related by the formula:

Based on the control principle, converters can be divided into types:

• with scalar control;
• with vector control.

Principle scalar control is to manage frequency of the supply current and the strength of this current. Scalar control provides for maintaining a given frequency and voltage ratio at a constant torque. Inverter with scalar control used for fans, compressors, pumps. It is possible to connect several motors to one converter.

The scalar mode allows you to adjust the motor speed in a narrow range and on average ranges from 1Hz to 100Hz. This means that the inverter converts the rotation frequency of the 50Hz electric current at the input into the rotation frequency of the electric current at the output in the range 1:100Hz.

An important characteristic of frequency converters is the range of maintaining speed while maintaining torque of the motor shaft.

Operating principle of the inverter with vector control is to manage the characteristics frequency, current and phases supply current. Since the rotation of the rotor lags behind the rotation of the stator magnetic field by 3-5% at maximum efficiency and, accordingly, maximum power and torque, the vector-controlled inverter regulates the phase rotation of the stator magnetic field in relation to the rotor rotation, so that it is always ahead by 3-5%.

When using a frequency converter implemented according to the vector principle feedback sensors required, which monitor the position of the electric motor rotor. With the use of sensors, the speed control range is increased and can reach output current readings from 1Hz to 800Hz, which is a range 1:800Hz. What is relevant for speed control in elevator mechanisms and machine tools.

The name “vector control” arose from the mathematical representation of the current created by the magnetic field of the stator in the form of a vector, the magnitude of which is equal to the magnitude of the current, and the coordinates depend on the phase of the current. Briefly, we can say that with vector control mode the engine develops maximum torque when the magnetic field vector is at an angle of 103 0 - 105 0 to the electric current in the rotor winding. Vector mode provides constant torque at low speeds, high control accuracy and the ability to quickly adjust speed by changing frequency.

The inverter uses the principle of converting mains voltage in two stages. At the first stage, the alternating voltage of the network (220 V/380 V) is rectified and smoothed using diodes and capacitors. As a result, at the first stage, a direct current voltage is obtained. At the second stage, rectangular pulses of a given frequency are formed. Through the inverter transistors, they are supplied to the stator windings, where, under the influence of a magnetic field, they are transformed into sinusoidal ones corresponding to alternating current.

Converters with voltage pulse-width modulation (PWM) form a sinusoidal curve, the parameters of which determine the amplitude and frequency of the voltage.

Types of frequency converters

According to their intended purpose, converters are produced for single-phase and three-phase voltage. By type of control - with scalar or vector control, as discussed above. According to the type of transformation, they are divided into two types:

• with an autonomous voltage inverter (AVI);
• autonomous current inverter (AIT).

Modern industry produces frequency converters in a wide range, different power and with different functions.

Types of input and output information

Frequency converters differ in the number of inputs and outputs. Input (output) signals are divided into following types, which are shown in Table 1.

Table 1

According to the method of connection to the network, frequency converters are divided into single-phase and three-phase. Single-phase frequency generators connect to home network 220 V, and at the output they form three phase voltage. They are connected to the engine via triangle pattern. In this case, it is necessary that the output current is no more than half the rated current.

Three-phase inverters connect to the network 380 V, connection is carried out according to the principle "star".

The frequency converter on the housing has a number of terminals for connection with appropriate markings. Let's look at their designations and functions.

There are separate digital outputs for connecting to control equipment (ACS). The number of outputs is determined by the inverter manufacturer; they are described in more detail in the operating instructions for a specific model.

Basic rules for selecting a converter

Depending on the power requirements and type of controlled mechanisms, a frequency converter is selected.

• Inverter power specified in the documentation must be equal or greater than the mechanical power of the electric motor. But at the same time, it is necessary to additionally focus on the type of connected mechanisms. For lifting devices, a converter is selected that has a power value higher than the rated value of the motor power. And for a centrifugal pump, a lower inverter power is allowed.
• If the connected load has a large inertia, then the converter power is selected depending on the required acceleration time. For fast acceleration you will need a converter with power more than the rated engine power by 10-15%.
• When choosing a frequency rated operating current must exceed nominal value motor current by 10% to prevent blocking due to overcurrent.

The main criterion for choosing a frequency converter if it is impossible to simultaneously satisfy current and voltage requirements is the choice of full rated power, which must exceed the rated power of the motor.

When choosing an inverter, you cannot ignore the number of input (output) signals and their type, which allows automation production process and its modernization. At the same time, it is advisable to focus principle - “there are never too many entries”.

As already discussed, the first choice is the control method: scalar or vector. The scalar method is used for simple mechanisms where it is necessary to ensure a given rotation speed (fans, compressors, etc.), where feedback sensors are not required. Vector control divided into voltage and current control. For high speed control requirements (from 1:800), special drives are additionally provided. And there is a need to install feedback sensors on the shaft

The work is based on the use of a feedback signal PID controller. PID controller stands for proportional-integral-differential controller. The deviation of a quantity (speed, voltage) from the setpoint (predetermined deviation) is measured and control system an adjustment signal is generated taking into account the statistical error. This system is used when operating pumps and machine tools.

Using a frequency converter allows you to protect the motor from overload ( idle speed) arising when the connected mechanisms malfunction. When an overload is detected, the converter generates an alarm and issues a Stop command.

Additional "Flying launch" function allows for engine start delay depending on the rotation conditions, when restarting the engine. This is especially true for mechanisms that allow rotation in one direction or the other.

The EMC filter reduces electromagnetic interference , providing protection for the converter and interference-sensitive machines.

Among the protection functions of the converter-motor system, we list the main ones, which are carried out using a frequency converter:

• from overcurrent;
• from overheating;
• from short circuit of output phases;
• from overvoltage;
• from malfunctions in the power system.

Different manufacturers equip inverters with different additional functions in agreement with the customer. Therefore, the choice of a frequency converter is determined by the connected equipment and the tasks that must be performed by the converter-motor system.

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 drilling machine 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 You can find out about the scheme in the above magazine. It should be noted that according to modern terminology, such generator-shapers are called controllers.

Figure 1.

Figure 2. Generator pulse timing diagrams.

Based on the considered controller by A. Dubrovsky from Novopolotsk, Vitebsk region. a variable frequency drive design was developed for a mains powered motor AC voltage 220V. 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 normal operation such a key scheme, it is necessary that by the time the lower key is opened, the upper key has already been closed. For this purpose, control controllers form 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 heating of the output transistors, 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 serves as an additional inverter: when there are 3 DD3.2 elements at the output high level The upper switch transistor 1VT3 is open.

To power each upper switch driver, a separate rectifier 1VD1, 1C1 is used. Each rectifier is powered by an individual transformer winding, which can be considered a drawback 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?

With a decrease in supply frequency AC voltage the inductive reactance of the motor windings drops (just remember the formula inductive reactance), which leads to an increase in current through the windings, and, as a consequence, to overheating of the windings. The stator magnetic circuit is also 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. Simpler and reliable way the use of 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). As a result of this connection regular generator square wave is converted 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 engine power is low; the engine is powered by rectangular pulses, so 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. Single phase output voltage waveform three-phase inverter with PWM.

Power part of the circuit

Modern branded frequency converters 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 available are from 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 one, 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 overload protection and short circuits. More information about these drivers can be found in technical descriptions Data Sheet 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 keys.

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 get impulse current gate voltage of at least 200mA, which allows for fairly reliable and fast control of the 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 almost short-circuited to the common wire (the resistance of the open section “drain-source” for 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 scheme power switches turned out to be so good that they are 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 on medium-integrated microcircuits; they have the following required functions, like, 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 circuits of three-phase inverter controllers 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 provides two simple circuits on microcontrollers PIC12F629 and PIC16F628.

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, special program, with which you can change the firmware parameters 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 low-cost systems adjustable three-phase drive containing all the necessary functions for control and protection. Such microcontrollers are increasingly used in various household appliances eg 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.