Frequency generator for working with an asynchronous motor. Purpose and principle of operation of a frequency converter for asynchronous motors

For clarity, the diagram can be divided into three components or three interconnected blocks:

1. Rectifier.

2. Filter, the purpose of which is to smooth the output voltage.

3. Inverter, which is actually responsible for producing the required frequency.

Its use provides a significant reduction in the starting current when turning on the equipment, which significantly extends the operational life of the engine and the device where this engine is used. Naturally, by getting rid of high starting current readings in this way, it is also possible to save electricity, which was previously wasted when starting up the equipment. And this is especially true in conditions where frequent starts and stops of devices are provided.

Rice. 2. Components of a frequency converter

Modern purchased inverters are widely used in such areas as manufacturing, water supply, energy, agriculture and urban services, in electronics, and in automatic lines and complexes.

The cost of a branded frequency converter is too high in order to study its operating processes or use it in everyday life or in a home workshop. Therefore, homemade frequency generators are often used in such situations.

Assembling the device

It is worth paying attention to the fact that at home it is highly not recommended to use motors designed for power greater than 1 kW. These are the features of a home network.

Having the necessary motor, you will first need to connect its windings to each other using the “triangle” method.

Rice. 3. Three-phase motor

Rice. 4. Triangle connection

Diagram of the frequency converter itself.

Rice. 6. Frequency converter circuit

Power is supplied from a 27 Volt DC power supply. This can be either an adjustable power supply or a self-made power supply designed for a given voltage. Motor connection diagram;

Rice. 7. Motor connection diagram

The circuit is simple and proven and does not contain components that would be difficult to buy. But, unfortunately, it is not without its drawbacks and is only suitable for everyday use.
A more complex circuit to assemble, but also more effective, is presented below.

Rice. 8. Motor connection diagram

At the moment, this is the most discussed frequency converter circuit that you can make yourself. Microcontroller firmware abounds on thematic forums. You will need not only the ability to solder competently, but also to flash microcontrollers.

Rice. 9. Printed circuit board

A reliable 24 Volt power supply will be required. It is also proposed to make it yourself according to the diagram.

Rice. 10. Power supply circuit

Naturally, the device can be purchased ready-made. They can be branded or made by folk craftsmen who have positive recommendations.

Powerful asynchronous electric motors are of great importance for modern industry. To carry out their smooth start, frequency converters are used - small devices that control the value of starting currents and sometimes allow you to change the rotation speed.

Why do you need a frequency converter

An asynchronous motor is significantly superior to other types of electric machines in performance and power, but is not without characteristic disadvantages. For example, to control the speed of rotation of the rotor, the device must be equipped with additional elements. The same is true with starting - the starting current of an asynchronous motor exceeds the rated value by 5-7 times. Because of this, additional shock loads and energy losses arise, which in total only reduces the operating life of the unit.

To solve these problems, as a result of persistent research, a class of special devices designed for automatic electronic control of inrush currents was created - frequency converters.

A frequency converter for an electric motor reduces the starting current by 4-5 times and not only provides a smooth start, but also controls the rotor by adjusting the voltage and frequency. Using the device has other advantages:

allows you to save up to 50% of electricity during startup;
with its help, feedback is provided from adjacent drives.

In fact, this is not a converter, but a three-phase voltage generator of the required magnitude and frequency.

Operating principle

The basis of the frequency converter is a double conversion inverter. The principle of its operation is as follows:

input variable first current sinusoidal type with a voltage of 380 or 220 volts passes through a diode bridge and straightens up;
then fed to a group of capacitors for smoothing and filtering;
then the current is transmitted to the control chips and bridge switches from IGBT (insulated gate bipolar transistor, IGBT) transistors, forming from it three-phase pulse width sequence with specified parameters;
At the output, the generated rectangular pulses are converted into a sinusoidal voltage under the influence of the inductance of the windings.

The following diagram shows the working principle of an asynchronous electric motor frequency converter.

How to choose

For manufacturers of frequency converters and other electronic equipment, the main tool for conquering the market is price. In order to reduce it, they create devices with a minimum set of functions. Accordingly, the more versatile a particular model, the higher its price. For us, this is of great importance for the reason that for efficient and long-term operation of the engine, an inverter with certain functions may be required. Let's look at the main criteria you should pay attention to.

Control

According to the control method, frequency converters are divided into vector and scalar. The former are much more common today, but have a higher price compared to the latter. The advantage of vector control is its high control accuracy. Scalar control is very simple, it can only keep the ratio of output voltage and frequency at a given value. It is advisable to install such a converter on a small device without a high load on the motor, for example, a fan.

Power

Of course, the higher this value, the better. By the way, in this matter the numbers are not so important. Pay more attention to the manufacturer - the more “related” your equipment is to each other, the more efficiently it will work. In addition, the use of multiple converters from the same brand supports the principle of interchangeability and ease of maintenance. Consider whether there is an appropriate service center in your city.

Mains voltage

In this case, the same principle applies as in the previous section - the wider the operating voltage range, the better for us. Domestic electrical networks, unfortunately, are poorly familiar with the concept of “standard”, so it is better to protect the equipment as much as possible from possible surges. A voltage drop is unlikely to lead to serious consequences (the converter will most likely simply turn off), but a large increase is dangerous - it can damage the device as a result of the explosion of electrolytic network capacitors.

Frequency adjustment range

In this case, you should rely solely on the requirements of production and specific devices. For example, for equipment such as grinding machines, the maximum frequency value (from 1000 Hz) is important. The standard for the lower limit is considered to be a ratio of 1 to 10 relative to the upper limit. In practice, converters with a range from 10 to 100 Hz are most often used. Please note that only converter models with vector control have a wide adjustment range.

Control inputs

Discrete inputs are used to transmit control commands in converters. They are used to start the engine, stop it, brake it, reverse it, etc. Analog inputs are used for feedback signals that monitor and adjust the drive directly during operation. And digital ones are used to transmit high-frequency signals generated by encoders (rotation angle sensors).

In fact, the more inputs, the better, but a large number of them not only makes setting up the device difficult, but also increases its cost.

Number of output signals

The discrete outputs of the converter are necessary to output signals indicating the occurrence of problems, such as overheating of the device, deviation of the input voltage from the norm, accident, error, etc. Analog outputs are needed to provide feedback in complex systems. The selection principle is the same: look for a balance between the number of signals and the cost of the device.

Control bus

The connection diagram for the frequency converter will help in finding a suitable control bus - the number of outputs and inputs should be at least equal, but it is better to buy a bus with a small margin - this will make it much easier for you to further improve the device.

Overload capabilities

It is considered normal if the power of the frequency converter is 10-15% higher than the engine power. The current should also be slightly higher than the motor rating. However, such selection “by eye” is recommended only in cases where there is no necessary technical documentation for the engine. If available, carefully read the requirements and select the appropriate converter. If shock loads are important, the peak current of the inverter should be 10% greater than the specified value.

Self-assembly

Despite the fact that purchasing a reliable and durable frequency converter is a priority option, such a device can be assembled with your own hands. There are more than one diagram and instructions on how to do this on the World Wide Web. In fact, DIY can be a great alternative when you need a converter for a small home appliance. A homemade device will cope with its tasks no worse than a purchased one, and will cost much less. But it is better to abandon attempts to create a suitable converter for operating powerful asynchronous motors - here, no matter how hard you try, you will not be able to surpass professional devices in efficiency and quality.

So, let's take a closer look at how to assemble a frequency converter for an asynchronous motor with your own hands. Please note that the parameters of a single-phase home electrical network allow the use in this case of a motor with a power of no more than 1 kW.

For the engine to operate, we need a triangle connection diagram for the windings. To do this, you need to connect the terminals of the windings to each other in series, observing the principle of “the output of one winding to the input of another.”

In order to construct a converter with our own hands, we need the following components:
- any microcontroller similar to AT90PWM3B;
- three-phase bridge driver (analogue IR2135);
- 6 transistors IRG4BC30W;
- 6 buttons;
- indicator.
The design of the device we are creating includes two boards, one of which contains the driver, power supply, input terminals and transistors, and the second - an indicator and a microcontroller. To connect the boards to each other we will use a flexible cable.
To assemble the frequency converter, you must use a switching power supply. You can use a ready-made device, or assemble it yourself (we will not describe this process - this is a topic for a separate article).
To control the operation of the motor, it is necessary to supply an external control current, but we can use the IL300 microcircuit with linear decoupling.
Image
Transistors and a diode bridge are installed on a common radiator.
Optocouplers OS2-4 are used to duplicate control buttons.
Installing a transformer on a single-phase frequency converter for a low-power motor is not a necessary step. You can get by with a current shunt with a wire cross-section of 0.5 mm, and connect the DA-1 amplifier to it (by the way, it will also be used to measure voltage).
In our case, we are assembling a converter for an asynchronous motor with a power of 400 W with our own hands, so we will not install a temperature sensor - the circuit is quite complicated without it.
Upon completion of assembly, it is necessary to isolate the buttons using plastic pushers. The buttons are controlled using an optical coupler.

Please note that when using long wires, they must be fitted with noise suppression rings.

It allows you to adjust the motor rotation in a frequency range of 1:40.

Connection and setup

To connect a frequency converter, a general diagram for connecting an asynchronous electric motor. In the circuit, the converter is located immediately after the differential circuit breaker, designed for a current equal to the motor rating. When installing the converter in a three-phase network, you must use a three-phase machine with a common lever. This allows you to turn off all power at once if an overload occurs in one of the phases. The trip value must be selected in accordance with the current of one phase of the motor. And in a situation where the frequency converter is installed in a network with single-phase current, it is advisable to use an automatic machine designed for a triple phase value. One way or another, installation of the device must be done manually, without “cutting” into the “zero” gap and grounding.

In fact, setting up the inverter consists of choosing the scheme for connecting the phase wires to the terminals on the electric motor, but it often depends on what type of network they are connected to. For three-phase electrical networks at production facilities, the motor is connected as a “star” - this scheme provides for parallel connection of the winding wires. For household single-phase networks with a voltage of 220V, a “triangle” circuit is used (note that the output current should not exceed the nominal value by more than 50%).

The control panel should be located in any place most convenient for use. Its connection diagram is indicated in the technical documentation for the frequency converter. Before installation and before power is applied, the lever should be set to the off position. After moving the lever to the on position, the corresponding light indicator should light up. By default, you press the RUN key to start the device. To gradually increase engine speed, you need to slowly turn the remote control handle. When rotating in reverse, switch the mode using the reverse button. Now you can set the handle to the position that sets the required rotation speed. Please note that on the control panels of some frequency converters, instead of the mechanical speed, the frequency of the supply voltage is indicated.

To maximize the service life of the frequency converter, try to follow the following maintenance recommendations:

It is necessary to constantly clean the inside of the device from accumulated dust. Please note that due to its compaction, a vacuum cleaner cannot always cope with this task - it is much easier to blow the dust out with a small compressor.
Check the circuit components regularly and replace them in a timely manner. Remember that all elements have different service life: cooling fans are designed for 2-3 years, electrolytic capacitors - for 5, and fuses - for 10. The internal cables of the device should be replaced approximately once every 6 years.
The principle of timely response should also be applied to the consequences of periodic heating of parts of the device. This causes the thermal paste to dry out, which also leads to failure of the capacitors. Try to change it more often than once every 3 years.

Attention to the external conditions in which the frequency converter is installed also allows you to significantly extend its service life. It should be a well-ventilated place, without direct sunlight, without being in the immediate vicinity of flammable liquids and materials, without debris, metal and wood shavings, dust, oil drops, vibrations, pets, mice, cockroaches... The installation surface should be level and sustainable. In some cases, you should pay attention to the location of the converter relative to sea level - with every 100 meters of increase, the ambient temperature can be reduced by 0.5˚C relative to the norm (-10˚C - + 45˚C).

Currently, the asynchronous electric motor has become the main device in most electric drives. Increasingly, an inverter with PWM control is used to control it. Such management provides a lot of advantages, but also creates some problems in choosing certain technical solutions. Let's try to understand them in more detail.

Frequency converter device

The development and production of a wide range of high-power, high-voltage transistor IGBT modules has made it possible to implement multi-phase power switches controlled directly by digital signals. Programmable computing tools made it possible to generate numerical sequences at the inputs of switches that provide signals. The development and mass production of single-chip microcontrollers with large computing resources have made it possible to move to servo electric drives with digital controllers.

Power frequency converters, as a rule, are implemented according to a circuit containing a rectifier using powerful power diodes or transistors and an inverter (controlled switch) using IGBT transistors shunted by diodes (Fig. 1).

Rice. 1. Frequency converter circuit

The input stage rectifies the supplied sinusoidal network voltage, which, after smoothing using an inductive-capacitive filter, serves as a power source for a controlled inverter, which, under the action of digital control commands, generates a signal c, which generates sinusoidal currents in the stator windings with parameters that ensure the required operating mode of the electric motor.

Digital control of the power converter is carried out using microprocessor hardware and software corresponding to the assigned tasks. The computing device generates control signals for 52 modules in real time, and also processes signals from measuring systems that control the operation of the drive.

Power devices and control computing facilities are combined into a structurally designed industrial product called a frequency converter.

There are two main types of frequency converters used in industrial equipment:

branded converters for specific types of equipment.

universal frequency converters are designed for multi-purpose control of IM operation in user-specified modes.

Installation and control of the operating modes of the frequency converter can be done using a control panel equipped with a screen to display the entered information. In a simple version of scalar frequency control, you can use a set of simple logical functions available in the controller’s factory settings and a built-in PID controller.

To implement more complex control modes using signals from feedback sensors, it is necessary to develop an ACS structure and an algorithm, which should be programmed using a connected external computer.

Most manufacturers produce a range of frequency converters that differ in input and output electrical characteristics, power, design and other parameters. To connect to external equipment (power supply, motor), additional external elements can be used: magnetic starters, transformers, chokes.

Types of control signals

It is necessary to distinguish between different types of signals and use a separate cable for each of them. Different types of signals can influence each other. In practice, such a separation occurs often, for example, the cable from can be connected directly to the frequency converter.

Rice. 2. Example of connecting power circuits and control circuits of a frequency converter

The following types of signals can be distinguished:

analog - voltage or current signals (0...10 V, 0/4...20 mA), the value of which changes slowly or rarely, usually these are control or measurement signals;

discrete voltage or current signals (0...10 V, 0/4...20 mA), which can take only two rarely changing values (high or low);

digital (data) - voltage signals (0...5 V, 0...10 V), which change quickly and with high frequency, usually these are signals from RS232, RS485, etc. ports;

relay - relay contacts (0...220 V AC) can switch on inductive currents depending on the connected load (external relays, lamps, valves, brakes, etc.).

Selecting the power of the frequency converter

When choosing the power of a frequency converter, it is necessary to base it not only on the power of the electric motor, but also on the rated currents and voltages of the converter and the motor. The fact is that the indicated power of the frequency converter only applies to its operation with a standard 4-pole asynchronous electric motor in standard applications.

Real drives have many aspects that can cause the drive's current load to increase, for example during start-up. In general, the use of a frequency drive makes it possible to reduce current and mechanical loads due to soft starting. For example, the starting current is reduced from 600% to 100-150% of the rated value.

Drive operation at reduced speed

It must be remembered that although the frequency converter easily provides speed control of 10:1, when the engine operates at low speeds, the power of its own fan may not be enough. It is necessary to monitor the engine temperature and provide forced ventilation.

Electromagnetic compatibility

Since the frequency converter is a powerful source of high-frequency harmonics, a shielded cable of minimum length must be used to connect the motors. Such a cable must be laid at a distance of at least 100 mm from other cables. This minimizes interference. If you need to cross cables, the crossing is done at an angle of 90 degrees.

Power from emergency generator

The soft start provided by the frequency converter allows you to reduce the required generator power. Since with such a start the current is reduced by 4-6 times, the generator power can be reduced by a similar number of times. But all the same, a contactor must be installed between the generator and the drive, controlled from the relay output of the frequency drive. This protects the frequency converter from dangerous overvoltages.

Power supply of a three-phase converter from a single-phase network

Three-phase frequency converters can be powered from a single-phase network, but their output current should not exceed 50% of the rated current.

Saving energy and money

Savings occur for several reasons. Firstly, due to growth to values of 0.98, i.e. the maximum power is used to perform useful work, the minimum goes into losses. Secondly, a coefficient close to this is obtained in all engine operating modes.

Without a frequency converter, asynchronous motors at low loads have a cosine phi of 0.3-0.4. Thirdly, there is no need for additional mechanical adjustments (flaps, throttles, valves, brakes, etc.), everything is done electronically. With such a control device, savings can reach 50%.

Sync multiple devices

Due to additional control inputs of the frequency drive, it is possible to synchronize processes on the conveyor or set the ratio of changes in some quantities depending on others. For example, make the rotation speed of the machine spindle dependent on the feed speed of the cutter. The process will be optimized because when the load on the cutter increases, the feed will be reduced and vice versa.

Protection of the network from higher harmonics

For additional protection, in addition to short shielded cables, line chokes and shunt capacitors are used. , in addition, limits the current surge when turned on.

Choosing the right protection class

For trouble-free operation of a frequency drive, a reliable heat sink is required. If you use high protection classes, for example IP 54 and higher, then it is difficult or expensive to achieve such heat dissipation. Therefore, you can use a separate cabinet with a high protection class, where you can install modules with a lower class and provide general ventilation and cooling.

Parallel connection of electric motors to one frequency converter

In order to reduce costs, one frequency converter can be used to control several electric motors. Its power must be selected with a margin of 10-15% of the total power of all electric motors. In this case, it is necessary to minimize the length of the motor cables and it is very advisable to install a motor throttle.

Most frequency converters do not allow motors to be disconnected or connected using contactors while the frequency drive is running. This can only be done via the drive stop command.

Setting the control function

To obtain maximum performance indicators of an electric drive, such as: power factor, efficiency, overload capacity, smooth control, durability, you need to correctly choose the relationship between the change in operating frequency and voltage at the output of the frequency converter.

The voltage change function depends on the nature of the load torque. At a constant torque, the voltage on the stator of the electric motor must be regulated in proportion to the frequency (scalar regulation U/F = const). For a fan, for example, another ratio is U/F*F = const. If we increase the frequency by 2 times, then the voltage must be increased by 4 (vector regulation). There are drives with more complex control functions.

Advantages of using an adjustable electric drive with a frequency converter

In addition to increasing efficiency and energy saving, such an electric drive allows you to obtain new control qualities. This is expressed in the rejection of additional mechanical devices that create losses and reduce the reliability of systems: brakes, dampers, throttles, valves, control valves, etc. Braking, for example, can be accomplished by reverse rotation of the electromagnetic field in the stator of the electric motor. By changing only the functional relationship between frequency and voltage, we get a different drive without changing anything in the mechanics.

Reading Documentation

It should be noted that although frequency converters are similar to each other and having mastered one, it is easy to understand the other, nevertheless, it is necessary to carefully read the documentation. Some manufacturers impose restrictions on the use of their products, and if they are violated, they will remove the product from warranty.

Content:

Three-phase asynchronous motors have been used for a long time in industry and other areas of people's lives and activities. Among all the stages of the work process, the most attention is paid to ensuring smooth starting and braking of the unit. In order to fulfill this condition, it is necessary to use a frequency converter for a three-phase electric motor. In addition to its main name, a frequency converter is also known as an inverter, variable frequency drive or AC frequency converter.

The main function of a frequency converter is to regulate the rotation speed of asynchronous motors, with the help of which electrical energy is converted into mechanical energy. The initial movement is transformed into other types of movements necessary to perform a specific technological operation. The use of frequency converters makes it possible to increase the efficiency of the electric motor to 98%.

Design and principle of operation of the converter

The frequency converter regulates the rotation speed of three-phase asynchronous electric motors. The rotation obtained under the influence of electricity is converted into mechanical movement using special drive devices. The rotation speed can be adjusted by other devices. However, they all have serious disadvantages such as high cost, complex design and low quality. In addition, the range of adjustments for such devices is completely insufficient for normal operation.

All these problems are effectively solved using a frequency converter. In addition to ensuring smooth start and stop, this device also controls other processes occurring in the engine. The use of a frequency generator has minimized the risk of malfunctions and emergency situations. Speed and smooth adjustment are ensured by a specially designed frequency converter circuit for a three-phase motor. As a result of its use, the duration of continuous operation of the electric motor has significantly increased, significant energy savings and an increase have been achieved.

How does it become possible to control the rotation speed of an electric motor? First of all, the voltage coming from the network changes in frequency. Further, a normal three-phase voltage with the required amplitude and frequency is formed from it, which is consumed by the electric motor. Speed adjustment is carried out in a fairly wide range. If necessary, the frequency converter allows you to switch the rotor rotation to the opposite direction. All adjustments must be made taking into account the unit’s passport data, taking into account the maximum permissible speed and installed power.

The general structure of the frequency converter is shown in the diagram. The design of the device consists of three components:

Rectifier. When connected to a power source, it generates DC voltage. Depending on the modification, it can be controlled or uncontrolled.
Filter. Designed to smooth out rectified voltage, therefore its design includes capacitors.
Inverter. Directly generates voltage at the required frequency and supplies it to the engine.

The main classification of frequency generators is carried out depending on the type of rotation speed control. There are two main modes:

Scalar mode without feedback. In this case, the magnetic field of the stator is controlled.
Vector mode with or without feedback. Here the magnetic fields of the rotor and stator interact, which is taken into account during control. In this mode, torque is optimized at various speeds. This control method is considered more accurate and efficient. However, it requires special knowledge and skills and is more expensive to maintain.

Connecting and setting up the frequency converter

Connecting frequency converters is especially important for private owners of equipment with asynchronous motors. It is first recommended to install a circuit breaker that will de-energize the network in the event of a possible short circuit in one of the phases.

In the circuits, frequency converters for asynchronous motors are connected to electric motors in two ways - “triangle” and “star”. The first scheme is used for single-phase adjustable drives, without loss of power. Such frequency generators have a maximum power of 3 kW and are intended mainly for use in domestic conditions. The star circuit is used where there are three-phase industrial networks.

In order to limit the starting current and reduce the starting torque, starting engines with a power of over 5 kW is carried out according to a mixed scheme. The "star" is used at the moment of starting, when voltage is applied to the stator. Once the motor reaches rated speed, the power supply switches to another circuit - delta. This method is not used everywhere, but only where it is possible to connect both circuits at once.

The remote control is connected in accordance with the diagram supplied with the frequency converter. The control lever must be in the OFF position before installation and before power is applied. When the lever is moved to the ON position, this action is confirmed by an indicator light. On many models, the default startup is by pressing the RUN button. A gradual increase in engine speed is achieved by slowly turning the remote control handle. Once the required speed is reached, the handle is locked in this position. To switch the mode to reverse rotation, there is a reverse button.

Self-production of frequency converter

Recently, low-power asynchronous electric motors used in drives of various devices have become widespread in everyday life. Therefore, in order not to purchase expensive additional equipment for them, many home craftsmen provide frequency regulation of electric motors by making converters with their own hands. Thus, energy savings are achieved while maintaining engine power.

A single-phase home network allows you to connect an electric motor whose power does not exceed 1 kW. It is for such units that homemade frequency generators are mainly made. It is necessary to think in advance about the triangle connection diagram intended for a single-phase network. For this purpose, the terminals of the windings are connected in series to each other, according to the principle of connecting the output of one winding to the input of another. It is also recommended that the circuit of a frequency converter assembled by hand be drawn up in advance.

Before starting construction, you need to prepare all the necessary elements and materials. You can use any microcontroller - an analogue of the AT90РWМ3В model and a three-phase bridge driver similar to the IR2135 model. In addition, you need to stock up on 6 transistors like IRG4BC30W, 6 buttons and an indicator. All parts are located on two boards connected to each other by a flexible cable.

The design of the frequency converter is complemented. This part can be purchased ready-made or assembled with your own hands according to a separate scheme. Control of engine operation is carried out using an external control current or an IL300 microcircuit with linear decoupling. A common radiator is used to mount transistors and a diode bridge. The control buttons are duplicated by optocouplers OS2-4.

If the electric motor has low power, then it is not necessary to install a transformer on a single-phase frequency converter. Instead, you can use a current shunt, in which the wires have a cross-section of 0.5 mm. The DA-1 amplifier is also connected to it, which performs the additional function of measuring voltage.

Maintenance of the device during operation

First of all, you need to timely clean the inside of the device from dust. The basic procedure is performed using a vacuum cleaner, but complete cleaning cannot be done this way. The vacuum cleaner simply cannot cope with thick and dense layers of accumulated dust. Therefore, it is recommended to use a compressor or clean it manually.
Great importance is attached to the timely periodic replacement of elements, parts and assemblies. It is recommended to change cooling fans after 2-3 years of operation. There are deadlines for fuses, internal cables and other parts. If these terms are observed, the frequency converter for the electric motor will last much longer.
It is imperative to monitor the internal temperature and bus voltage. Too high a temperature leads to negative consequences when the capacitors are destroyed and the thermal conductive paste begins to dry out.
It is recommended to change the paste at least once every three years. The ambient temperature should not exceed 40 degrees, and humidity and dust concentration should not exceed acceptable limits.

Advantages of frequency drivers in asynchronous motors

Asynchronous motors have many advantages over DC devices. They are distinguished by their simplicity of design and high reliability. Therefore, asynchronous units are most often chosen for domestic and industrial purposes.

Currently, many users are abandoning mechanical current control during motor operation. This method does not guarantee proper quality of equipment operation. Instead, frequency converters have long been used. Electronic control allows you to significantly reduce energy consumption while maintaining the engine's own power.

Frequency converters should be operated in accordance with the technical specifications reflected in the equipment documentation. It is recommended to use homemade devices only in domestic conditions, and to use factory-made equipment in production. Repair and maintenance of inverters should only be performed by qualified personnel.

A simple frequency converter for an asynchronous electric motor.

The first was a restaurant - in winter, cold air should be blown in strictly measured doses on overheated visitors, and in summer, on the contrary, those frozen from cold ice cream should be smoothly warmed by hot air from the street. There is no way to do without an inverter.
The second one wants to shear shaggy sheep, but the problem is that the machine is three-phase. And there is only one in the field, and even that one is not 220V. We need an inverter again.
The third one is an emery stone, a drilling machine and a winding machine - I wanted to attach it to the engine.
In the end, looking around, I saw that everything...everything is made by Japanese, French, German inverters.... , but I don’t have my own screwdriver sharpener yet. And what’s more, all the decent companies have already written how to do this.

So, since the asynchronous motor is so widespread and the three-phase voltage system created by M. O. Dolivo-Dobrovolsky is so convenient. And the modern element base is so good. Then making a frequency converter is just a matter of personal desire and some financial capabilities. Perhaps someone will say, “Well, why do I need an inverter? I’ll install a phase-shifting capacitor and everything is decided.” But at the same time you can’t turn the speed and you’ll lose power and then it’s not interesting.

Let's take it as a basis - in everyday life there is a single-phase 220V network, the popular motor size is up to 1 kW. This means we connect the motor windings with a triangle. Further, it’s simpler, you will need a three-phase bridge driver IR2135 (IR2133), we choose this one because it is used in industrial equipment, it has an SD output and a convenient pin arrangement. IR2132 is also suitable, but it has a longer dead time and does not have an SD output. As a PWM generator, we will choose the AT90SPWM3B microcontroller - it is accessible, understandable to everyone, has a lot of capabilities and is inexpensive, there is a simple programmer - https://real.kiev.ua/avreal/. We will select 6 power transistors IRG4BC30W with some current reserve - the starting currents of the IM can exceed the nominal ones by 5-6 times. And for now we don’t install a “braking” key and a resistor, we will brake and magnetize the rotor with direct current before starting, but more on that later.... The entire work process is displayed on a 2-line LCD indicator. For control, 6 buttons are enough (frequency +, frequency -, start, stop, reverse, menu).
The result is this diagram.

I do not at all claim that the design is complete and I propose to take this design as a kind of basis for home electric drive enthusiasts. The boards shown here were made to fit the parts at my disposal.

Structurally, the inverter is made on two boards - the power part (power supply, driver and bridge transistors, power terminals) and the digital part (microcontroller + indicator). The boards are electrically connected by a flexible cable. This design is chosen for future migration to a TMS320 or STM32 or STM8 controller.
The power supply is assembled according to a classic design and does not need any comments. IL300 microcircuit is a linear opto-isolator for controlling current 4-20Ma. Optocouplers OS2-4 simply duplicate the “start, stop, reverse” buttons for galvanically isolated control. Optocoupler output OS-1 “user function” (alarm, etc.)
Power transistors and a diode bridge are mounted on a common radiator. Shunt 4 turns of manganin wire with a diameter of 0.5 mm on a 3 mm mandrel.
Let me immediately note that some nodes and elements are not at all necessary. In order to simply turn the motor, you do not need external current control of 4-20 Ma. There is no need for a current transformer; a current shunt is also suitable for evaluation measurements. No external alarm required. With an engine power of 400 W and a radiator area of 100 cm 2, there is no need for a temperature sensor.

IMPORTANT! - the control buttons on the board are isolated from the power supply only by plastic pushers. For safe control it is necessary to use optical isolation.

Possible changes in the circuit depending on the firmware.
The DA-1 amplifier can be connected to a current transformer or to a shunt. The DA-1-2 amplifier can be used to measure network voltage or to measure the resistance of a thermistor if the PD-1 temperature sensor is not used.
In the case of long connecting wires, it is necessary to at least put an interference suppression ring on each wire. There is interference. For example, until I did this, my mouse froze.
I also think it is important to note the reliability of the insulation of the blood pressure because... When switching power transistors, voltage surges on the windings can reach values of 1.3 Upit.

General view.

A little about management.

Having read books with long formulas mainly describing how to make a sine wave using PWM. And how to stabilize the rotation speed of the motor shaft using a tachometer and PID controller. I came to the conclusion that the AD has a fairly rigid characteristic over the entire range of permissible loads on the shaft.
Therefore, for personal needs, the management described by law is quite suitable Kostenko M.P. or as it is also called scalar. Sufficient for most practical cases of using a variable frequency electric drive with an engine speed control range of up to 1:40. Those. Roughly speaking, in the simplest case we make an ordinary 3-phase socket with variable frequency and voltage changing in direct proportion. With small “buts”, in the initial sections of the characteristic it is necessary to perform IR compensation, i.e. at low frequencies you need a fixed voltage. Three times “but” to mix the 3rd harmonic into the voltage supplying the engine. The physical principles of AD will do the rest for us. You can read more about this in the document AVR494.PDF
Based on my personal observations and modest experience, these methods are most often used in drives with power up to 15 kW without any special frills.
Further I will not delve into the theory and description of mathematical models of blood pressure. This was explained quite well even without me by professors back in the 60s.

But in no case should you underestimate the complexity of blood pressure management. All my simplifications are justified only by the non-commercial use of the inverter.

Board of power elements.

The V-1.0 program for AT90SPWM3B implements
1- Frequency control of AD. Voltage waveform is a sinusoid with 3rd harmonic.
2- Reference frequency 5 Hz -50 Hz in 1 Hz steps. PWM frequency 4 kHz.
3- Fixed acceleration and deceleration time
4- Reverse (only via STOP button)
5- Acceleration to a given frequency in 1 Hz steps
6 - Indication of ADC channel 6 readings (8 bits, window filter aperture 4 bits)
I use this channel to measure shunt current.
7 - Indication of operating mode START, STOP, RUN, RAMP, and Frequency in Hz.
8- Processing the alarm signal from ms IR2135