Regulation by a frequency drive allows, using a special converter, to flexibly change the operating modes of the electric motor: start, stop, accelerate, brake, change the rotation speed.
Changing the frequency of the supply voltage leads to a change in angular velocity magnetic field stator. When the frequency decreases, the motor decreases and the slip increases.
Operating principle of the drive frequency converter
The main disadvantage of asynchronous motors is the difficulty of speed control traditional ways: by changing the supply voltage and introducing additional resistances into the winding circuit. The frequency drive of the electric motor is more advanced. Until recently, converters were expensive, but the advent of IGBT transistors and microprocessor control systems allowed foreign manufacturers to create affordable devices. The most advanced now are static
The angular velocity of the stator magnetic field ω 0 changes in proportion to the frequency ƒ 1 in accordance with the formula:
ω 0 = 2π׃ 1 /p,
where p is the number of pole pairs.
The method provides smooth speed control. In this case, the sliding speed of the engine does not increase.
To obtain high energy performance of the motor - efficiency, power factor and overload capacity, together with the frequency, the supply voltage is changed according to certain dependencies:
- constant load torque - U 1 / ƒ 1 = const;
- fan character of the load torque - U 1 / ƒ 1 2 = const;
- load torque, inversely proportional to speed - U 1 /√ ƒ 1 = const.
These functions are implemented using a converter that simultaneously changes the frequency and voltage on the motor stator. Electricity is saved through regulation using the required technological parameter: pump pressure, fan performance, machine feed speed, etc. In this case, the parameters change smoothly.
Methods of frequency control of asynchronous and synchronous electric motors
In a variable frequency drive based on asynchronous motors with squirrel-cage rotor Two control methods are used - scalar and vector. In the first case, the amplitude and frequency of the supply voltage change simultaneously.
This is necessary to maintain the operating characteristics of the engine, most often a constant ratio of its maximum torque to the moment of resistance on the shaft. As a result, efficiency and power factor remain unchanged throughout the entire rotation range.
Vector control consists of simultaneously changing the amplitude and phase of the current on the stator.
The type frequency drive only works when light loads, with growth higher acceptable values synchronicity may be disrupted.
Advantages of frequency drive
Frequency regulation has a whole range of advantages compared to other methods.
- Automation of engine operation and production processes.
- Smooth start, eliminating typical errors that occur during engine acceleration. Increasing the reliability of frequency drives and equipment by reducing overloads.
- Increased operating efficiency and overall drive performance.
- Creating a constant electric motor speed regardless of the nature of the load, which is important during transient processes. The use of feedback makes it possible to maintain a constant engine speed under various disturbing influences, in particular under variable loads.
- The converters are easily integrated into existing technical systems without significant rework and stopping technological processes. The range of capacities is large, but as they increase, prices increase significantly.
- The ability to abandon variators, gearboxes, chokes and other control equipment or expand the range of their application. This results in significant energy savings.
- Elimination of the harmful effects of transient processes on technological equipment, such as water hammer or increased fluid pressure in pipelines with a decrease in its consumption at night.
Flaws
Like all inverters, frequency converters are sources of interference. They need to install filters.
Brand values are high. It increases significantly with increasing power of the devices.
Frequency control when transporting liquids
At facilities where water and other liquids are pumped, flow control is mostly done using gate valves. Currently, a promising direction is the use frequency drive a pump or fan that drives their blades.
Application frequency converter as an alternative to the throttle valve, it provides an energy-saving effect of up to 75%. The valve, holding back the flow of liquid, does not perform useful work. At the same time, the loss of energy and matter during its transportation increases.
The frequency drive makes it possible to support the consumer constant pressure when fluid flow changes. A signal is sent from the pressure sensor to the drive, which changes the engine speed and thereby regulates its speed, maintaining a given flow rate.
Pumping units are controlled by changing their performance. The power consumption of the pump is a cubic function of the performance or speed of rotation of the wheel. If the speed is reduced by 2 times, the pump performance will drop by 8 times. Having a daily schedule of water consumption allows you to determine energy savings for this period if you control a frequency drive. Due to it, it is possible to automate the pumping station and thereby optimize the water pressure in the networks.
Operation of ventilation and air conditioning systems
Maximum air flow in ventilation systems is not always necessary. Operating conditions may require reduced performance. Traditionally, throttling is used for this, when the wheel speed remains constant. It is more convenient to change the air flow due to a variable frequency drive when seasonal and climatic conditions, release of heat, moisture, vapors and harmful gases.
Energy savings in ventilation and air conditioning systems are achieved no lower than in pumping stations, since the power consumption of shaft rotation is a cubic function of the speed.
Frequency converter device
A modern frequency drive is designed using a double converter circuit. It consists of a rectifier and a pulse inverter with a control system.
After rectifying the mains voltage, the signal is smoothed by a filter and fed to an inverter with six transistor switches, where each of them is connected to the stator windings of an asynchronous electric motor. The block converts the rectified signal into a three-phase signal of the required frequency and amplitude. The power IGBT transistors in the output stages have a high switching frequency and provide a clear, distortion-free square wave signal. Due to the filtering properties of the motor windings, the shape of the current curve at their output remains sinusoidal.
Methods for adjusting signal amplitude
The output voltage value is regulated by two methods:
- Amplitude - change in voltage value.
- Pulse width modulation is a method of converting a pulse signal in which its duration changes, but the frequency remains unchanged. Here the power depends on the pulse width.
The second method is used most often in connection with the development of microprocessor technology. Modern inverters are made based on turn-off GTO thyristors or IGBT transistors.
Capabilities and applications of converters
The frequency drive has many capabilities.
- Regulation of the frequency of three-phase supply voltage from zero to 400 Hz.
- Acceleration or braking of the electric motor from 0.01 sec. up to 50 min. according to a given law of time (usually linear). During acceleration, it is possible not only to reduce, but also to increase dynamic and starting torques by up to 150%.
- Reversing the engine with specified modes of braking and acceleration to the desired speed in the other direction.
- The converters feature configurable electronic protection against short circuits, overloads, ground leaks, and open motor power lines.
- The digital displays of the converters display data about their parameters: frequency, supply voltage, speed, current, etc.
- The converters adjust the voltage-frequency characteristics depending on the required load on the motors. The functions of control systems based on them are provided by built-in controllers.
- For low frequencies, it is important to use vector control, which allows you to work with the full torque of the motor, maintain a constant speed when loads change, and control the torque on the shaft. A variable frequency drive works well if the motor's nameplate data is entered correctly and after it has been successfully tested. Well-known products from the companies HYUNDAI, Sanyu, etc.
The areas of application of the converters are as follows:
- pumps in hot and cold water and heat supply systems;
- slurry, sand and pulp pumps of processing plants;
- transportation systems: conveyors, roller tables and other means;
- mixers, mills, crushers, extruders, dispensers, feeders;
- centrifuges;
- elevators;
- metallurgical equipment;
- drilling equipment;
- electric drives of machine tools;
- excavator and crane equipment, manipulator mechanisms.
Manufacturers of frequency converters, reviews
The domestic manufacturer has already begun to produce products that are suitable for users in terms of quality and price. The advantage is the ability to quickly get the desired device, as well as detailed advice on setup.
The company "Effective Systems" produces serial products and pilot batches of equipment. The products are used for household use, small business and industry. The manufacturer "Vesper" produces seven series of converters, among which there are multifunctional ones suitable for most industrial mechanisms.
The leader in the production of frequency converters is the Danish company Danfoss. Its products are used in ventilation, air conditioning, water supply and heating systems. The Finnish company Vacon, part of the Danish company, produces modular designs, from which you can assemble the necessary devices without unnecessary parts, which allows you to save on components. Converters from the international concern ABB are also known, used in industry and in everyday life.
Judging by the reviews, to solve simple typical problems you can use cheap domestic converters, but for complex ones you need a brand with significantly more settings.
Conclusion
The frequency drive controls the electric motor by changing the frequency and amplitude of the supply voltage, while protecting it from faults: overloads, short circuits, breaks in the supply network. These perform three main functions related to acceleration, braking and speed of engines. This makes it possible to increase the efficiency of equipment in many areas of technology.
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 with the help of 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.
IN industrial equipment Two main types of frequency converters are used:
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 simple version For 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. Various types 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) may include inductive currents depending on the connected load (external relays, lamps, valves, braking devices, 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.
Three-phase converter power supply from 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 high class protection, where to install modules with a lower class and implement 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 total power 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 the electric drive, such as: power factor, coefficient useful action, overload capacity, smoothness of regulation, 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
Except 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.
At the moment, dozens of brands of low-voltage frequency converters from foreign and foreign countries are represented on the Russian market. Russian manufacturers. Among them are leading European companies: Siemens, ABB, SEW Eurodrive, Control Techniques (Emerson Corporation), Schneider Electric, Danfoss, K.E.B., Lenze, Allen-Breadly (Rockwell Automation Corporation), Bosch Rexroth. The products of these manufacturers are widely represented, and there is an extensive dealer network. So far, the products of such companies from Europe as Emotron, Vacon, SSD Drives (Parker Corporation), Elettronica Santerno are less known. There are also products from American manufacturers - General Electric Corporation, AC Technology International (part of the Lenze concern) and WEG (Brazil).
Serious competition with European and American manufacturers made up of companies from Asia. First of all, these are companies from Japan: Mitsubishi Electric, Omron-Yaskawa, Panasonic, Hitachi, Toshiba, Fuji Electric. Korean and Taiwanese brands are widely represented - LG Industrial Systems, HYUNDAI Electronics, Delta Electronics, Tecorp, Long Shenq Electronic, Mecapion.
Among domestic producers the most famous is the Vesper company. You can also note specialized converters of the brands ACh, EPV (JSC Elektroapparat), REN2K or REMS (MKE).
Most manufacturers offer frequency converters that can operate in open-loop and closed loop control (vector control), with sets of programmable inputs and outputs, with a built-in PID controller. Even in the cheapest Korean or Taiwanese frequency converters you can find the so-called sensorless, i.e. without rotor position sensor, vector operating mode. The control range can be 1:50.
However, leading manufacturers offer a more advanced vector control mode without a feedback sensor, based on advanced control algorithms. One of the pioneers in this area was ABB, which proposed DTR (Direct torque control) - a method of controlling speed and torque without a feedback sensor. The English company Control Techniques has implemented a rotor flux linkage (RFC) control mode without using a feedback sensor, which allows you to control the torque with an accuracy sufficient for most tasks, expand the control range to 100, ensure high accuracy of maintaining speed at low speeds and achieve the same overload current , as in closed-loop modes.
Large manufacturers offer multifunctional devices with a whole range of options (expansion modules, braking resistors, built-in controllers, filters, chokes, etc.) or equip them with CNC systems or motion controllers.
Increasingly, you can see the use of a drive in regenerative mode, i.e. with the ability to return the energy released during braking back to the network (elevators, escalators, cranes). Typically, a specialized drive with a controlled rectifier is used for this. Leading companies, such as Control Techniques, offer regen as one of the operating modes of the Unidrive SP frequency converter, thereby achieving significant energy savings and high efficiency systems.
The described range allows the engineer to choose a suitable frequency converter with a wide range of built-in functions and programs. At the same time, leading European brands, for example from the UK and Germany, successfully compete on price with greater functionality and quality
We bring to your attention a description of some products available on the Russian market. Information about suppliers can be found on our website:
Rockwell Automation, the undisputed leader in the power electrical market, has released a new series of Allen-Bradley® PowerFlex® variable frequency drives ranging from 0.25kW to 6770kW. The new highly efficient series combines a compact design with wide functionality and excellent performance characteristics. It is used in the food, paper, textile industries, metalworking, woodworking, pumping and ventilation equipment, etc. The palette contains two classes of drives – Component and Architectural. Models from the Component class are designed to solve standard control tasks, and Architectural class drives, due to flexible configuration changes, can be easily adapted and integrated into the control systems of various power equipment. All models offer exceptional communication capabilities, a wide range of operator panels and programming tools, which greatly facilitates operation and speeds up equipment startup.
PowerFlex® 4
The Powerflex 4 drive is the most compact and inexpensive member of this family. The ideal speed control device, this model provides versatility while meeting manufacturers' and end-users' requirements for flexibility, compactness and ease of use.
The drive implements a voltage-frequency control law with the possibility of slip compensation. An excellent addition to this model is the version of the ultra-compact drive Power@Flex4M, with an extended operating power range of up to 2.2 kW for single-phase versions and up to 11 kW for three-phase voltage 400VAC. The proposed price scale for this model allows us to hope, if not for a hit of the season, then for its fairly wide popularity.
PowerFlex® 7000
The PowerFlex 7000 Series drives are the third generation of medium voltage drives from Rockwell Automation. Designed to regulate speed, torque, direction of rotation of asynchronous and synchronous AC motors. The unique design of the PowerFlex 7000 Series features a patented PowerCage design of power cages containing the main drive components. The new modular design is simple and has a small number of components, which ensures high reliability and ease of operation. The main advantages of medium voltage drives include: reduced operating costs, the ability to start large motors from small power supplies and improved quality characteristics of the controlled process and equipment used.
Depending on the output power, the drives are available in three sizes:
Housing A – Power range 150-900 kW with supply voltage 2400-6600 V
Housing B – Power range 150-4100 kW with supply voltage 2400-6600V
Housing C – Power range 2240-6770 kW at supply voltage 4160-6600 V
PowerFlex 7000 drives are available with 6-pulse, 18-pulse, or PWM options, giving the user significant flexibility in reducing the impact of utility line harmonics. In addition, it provides direct sensorless vector control for improved zone control low speeds, compared to drives using the U/f control method, as well as the ability to regulate motor torque, as is done in drives DC. A module with a liquid crystal display with 16 lines and 40 characters is offered as an operator panel.
Higher moment of inertia without additional gearbox
The low-inertia servomotors from Beckhoff AM3000 series, which are manufactured using new materials and technology, are mainly used in dynamic applications with high loads, for example to drive the axes of metalworking machines or gearless devices. Combined with high rotor inertia, they offer the same advantages as the AM3xxx series motors, such as the pole stator winding, which allows the overall dimensions of the motor to be significantly reduced. The flanges, connectors and shafts of the new AM3500 series motors are compatible with the well proven AM3000 motors. The new AM3500 models are available in flange sizes 3 – 6 and have torques from 1.9 to 15 Nm. Motor rotation speeds range from 3000 to 6000 rpm. For feedback systems, coordinate converters or absolute position sensors (single- or multi-turn) are available. The housing is rated IP 64; Options with protection class IP 65/67 are available. This series of motors meets CE, UL and CSA safety standards.
New generation of drives
The Emotron line has been expanded with NGD drives: FDU2.0, VFX2.0 (power from 0.75 kW to 1.6 MW) and VSC/VSA (0.18–7.5 kW). Drives with variable speed FDU2.0 (for centrifugal mechanisms) and VFX2.0 (for piston mechanisms) allow the user to set operating parameters in the required units, have a removable control panel with a function for copying settings, models up to 132 kW have a standard economical IP54 design (models from 160 to 800 kW can also be installed in special compact enclosures IP54). Data exchange during the process is carried out using Fieldbus (Profibus-DP, DeviceNet, Ethernet), via ports (RS-232, RS-485, Modbus RTU), as well as analogue and digital outputs.
Small size VSA and VSC vector drives are specially designed for speed control of three-phase asynchronous motors. high power: 220 V input models are available from 0.18 to 2.2 kW, and 380 V models are available from 0.75 to 7.5 kW.
Family ATV61-ATV71
The frequency converter market in Russia is developing at a rapid pace. It is not surprising that it attracts numerous manufacturers, both large and little-known. At the moment, the Russian market is very segmented. But here’s a paradox: despite the fact that there are currently more than 30 brands on the market, a significant market share belongs to 7–8 companies, and no more than two clear leaders. At the same time magnificent technical specifications equipment is not a guarantee of success. Leading positions in Russia were taken by companies that invest significant funds in business development and business infrastructure.
The Schneider Electric company, whose interests in Russia are represented by JSC Schneider Electric, significantly expanded its product offering in 2007. Now the ATV61-ATV71 family has been replenished with a modification for a voltage of 690 V, and many versions with a degree of protection IP54 have appeared. There is also a special model for elevator and crane drive ATV71*383 with unique control technology synchronous motor. By the end of 2008, a device with a power of 2400 kW at 690V will appear in the Altivar line. Altivar 61 can now operate in step-up transformer applications.
The new economical Altivar 21 series is designed specifically for heating, air conditioning and ventilation systems in residential and public buildings. Altivar 21 controls motors from 0.75 to 75 kW at voltages of 380 V and 200 ... 240 V.
Altivar 21 has many application functions:
– built-in PI regulator;
– “pick up on the fly”;
– sleep/wake function;
– protection and alarm management;
– resistance to network interference, operation at temperatures up to + 50°C and voltage drop of -50%.
With new capacitorless technology, Altivar 21 does not require harmonic mitigation devices. The total coefficient is THDI 30%. The abandonment of capacitors and the use of more powerful semiconductors increased the operating time.
Schneider Electric's leadership in the converter market is the result of serious work to improve the fault tolerance of converters. The MTTF for some models is up to 640,000 hours. Altivar operates under voltage drops of up to -50%, temperatures up to +50%, in chemically aggressive environments and with impulse noise in the network. This is a serious argument for repeat purchase. The buyer's trust in the equipment and the company's reputation cannot be overestimated.
Drives from SICK
Modern production requires automation of many manual setup operations various parameters on various machines and packaging machines. Often the operator needs to change geometric parameters manufactured product or other similar tasks. In this case, positioning drives from SICK-Stegmann are the ideal low-cost device for this type of operation.
HIPERDRIVE® – positioning drives are the result of the integration of a brushless DC motor, gearbox, absolute multi-turn encoder, power and control electronics in one device. Among other things, the drives have a Profibus or DeviceNet network interface. This device is aimed at performing point-to-point positioning tasks and is a black box device that is easy to operate.
Currently, servo drives are used for such tasks. But the use of such systems has a number of disadvantages. First of all, this is not economically justified. Servo-based systems usually also require an inverter, a brake, and an absolute encoder.
The main advantages of these drives:
– Highly integrated device
Reducing drive size
Easy assembly and setup
The operating principle of a frequency converter or, as it is often called, an inverter: alternating voltage industrial network is rectified by a block of rectifying diodes and filtered by a bank of high-capacity capacitors to minimize the ripple of the resulting voltage. This voltage is supplied to a bridge circuit that includes six controlled IGBT or MOSFET transistors with diodes connected in antiparallel to protect the transistors from breakdown by reverse polarity voltage that occurs when working with the motor windings. In addition, the circuit sometimes includes an energy “draining” circuit - a transistor with a high-power dissipation resistor. This circuit is used in braking mode to suppress the generated voltage by the motor and protect the capacitors from overcharging and failure.
The block diagram of the inverter is shown below.
A frequency converter combined with an asynchronous electric motor allows you to replace a DC electric drive. DC motor speed control systems are quite simple, but the weak point of such an electric drive is the electric motor. It is expensive and unreliable. During operation, the brushes spark, and the commutator wears out under the influence of electrical erosion. This electric motor cannot be used in dusty or explosive environments.
Asynchronous electric motors are superior to DC motors in many respects: they are simple in design and reliable, since they do not have moving contacts. They have smaller dimensions, weight and cost compared to DC motors for the same power. Asynchronous motors are easy to manufacture and operate.
The main disadvantage of asynchronous electric motors is the difficulty of regulating their speed using traditional methods (changing the supply voltage, introducing additional resistances into the winding circuit).
Control of an asynchronous electric motor in frequency mode has been a big problem until recently, although the theory of frequency control was developed back in the thirties. The development of variable frequency drives has been hampered by the high cost of frequency converters. The emergence of power circuits with IGBT transistors and the development of high-performance microprocessor control systems have allowed various companies in Europe, the USA and Japan to create modern frequency converters at an affordable price.
Speed control actuators can be done using various devices: mechanical variators, hydraulic couplings, resistors additionally inserted into the stator or rotor, electromechanical frequency converters, static frequency converters.
The use of the first four devices does not provide high quality speed control, uneconomical, expensive to install and operate. Static frequency converters are the most advanced asynchronous drive control devices at present.
Principle frequency method speed regulation of an asynchronous motor is that by changing the frequency f1 of the supply voltage, it is possible in accordance with the expression
constant number of pole pairs p change angular velocity stator magnetic field.
This method provides smooth speed control over a wide range, and the mechanical characteristics are highly rigid.
Speed regulation is not accompanied by an increase in the slip of the asynchronous motor, so power losses during regulation are small.
To obtain high energy performance of an asynchronous motor - power factors, efficiency, overload capacity - it is necessary to change the input voltage simultaneously with the frequency.
The law of voltage change depends on the nature of the load torque Ms. At a constant load torque Mc=const, the voltage on the stator must be regulated in proportion to the frequency:
For the fan nature of the load torque, this state has the form:
With a load torque inversely proportional to speed:
Thus, for smooth stepless regulation of the shaft speed of an asynchronous electric motor, the frequency converter must provide simultaneous regulation of the frequency and voltage on the stator winding of the asynchronous motor.
Advantages of using a variable speed drive in technological processes
The use of a controlled electric drive ensures energy saving and allows obtaining new qualities of systems and objects. Significant energy savings are achieved by regulating any technological parameter. If it is a conveyor or conveyor, then you can regulate the speed of its movement. If it is a pump or fan, you can maintain pressure or regulate performance. If this is a machine tool, then you can smoothly adjust the feed speed or main movement.
A special economic effect from the use of frequency converters comes from the use of frequency regulation at facilities that transport liquids. Until now, the most common way to regulate the performance of such objects is the use of gate valves or control valves, but today frequency control of an asynchronous motor driving, for example, the impeller of a pumping unit or fan, is becoming available. When using frequency regulators, smooth adjustment of the rotation speed is ensured, which in most cases eliminates the use of gearboxes, variators, chokes and other control equipment.
When connected through a frequency converter, the engine starts smoothly, without starting currents and shocks, which reduces the load on the engine and mechanisms, thereby increasing their service life.
The prospects of frequency regulation are clearly visible from the figure
Thus, when throttling, the flow of a substance restrained by a gate or valve does not do any useful work. The use of an adjustable electric drive of a pump or fan allows you to set the required pressure or flow rate, which will not only save energy, but also reduce losses of the transported substance.
Frequency converter structure
Most modern frequency converters are built using a double conversion scheme. They consist of the following main parts: a DC link (uncontrolled rectifier), a power pulse inverter and a control system.
The DC link consists of an uncontrolled rectifier and a filter. The alternating voltage of the supply network is converted into direct current voltage.
The power three-phase pulse inverter consists of six transistor switches. Each winding of the electric motor is connected through a corresponding switch to the positive and negative terminals of the rectifier. The inverter converts the rectified voltage into a three-phase alternating voltage of the required frequency and amplitude, which is applied to the stator windings of the electric motor.
In the output stages of the inverter, power IGBT transistors are used as switches. Compared to thyristors, they have a higher switching frequency, which allows them to produce a sinusoidal output signal with minimal distortion.
Operating principle of frequency converter
The frequency converter consists of an uncontrolled diode power rectifier B, an autonomous inverter, a PWM control system, an automatic control system, a choke Lв and a filter capacitor Св. Regulation of output frequency fout. and voltage Uout is carried out in the inverter due to high-frequency pulse-width control.
Pulse-width control is characterized by a modulation period, within which the stator winding of the electric motor is connected alternately to the positive and negative poles of the rectifier.
The duration of these states within the PWM period is modulated according to a sinusoidal law. At high (usually 2...15 kHz) PWM clock frequencies, sinusoidal currents flow in the motor windings due to their filtering properties.
Thus, the output voltage waveform is a high-frequency bipolar sequence of rectangular pulses (Fig. 3).
The pulse frequency is determined by the PWM frequency, the duration (width) of the pulses during the period of the output frequency of the AU is modulated according to a sinusoidal law. The shape of the output current curve (current in the windings of an asynchronous electric motor) is almost sinusoidal.
Regulation of the inverter output voltage can be done in two ways: amplitude (AP) by changing the input voltage Uv and pulse width (PWM) by changing the switching program of valves V1-V6 at Uv = const.
The second method has become widespread in modern frequency converters due to the development of modern element base (microprocessors, IBGT transistors). With pulse-width modulation, the shape of the currents in the stator windings of an asynchronous motor turns out to be close to sinusoidal due to the filtering properties of the windings themselves.
This control allows for high converter efficiency and is equivalent to analog control using frequency and voltage amplitude.
Modern inverters are made on the basis of fully controlled power semiconductor devices - turn-on GTO - thyristors, or bipolar IGBT transistors with an insulated gate. In Fig. Figure 2.45 shows a 3-phase bridge circuit of an autonomous inverter using IGBT transistors.
It consists of an input capacitive filter Cf and six IGBT transistors V1-V6 connected back-to-back reverse current diodes D1-D6.
By alternately switching valves V1-V6 according to the algorithm specified by the control system, the constant input voltage Uв is converted into an alternating rectangular-pulse output voltage. The active component of the asynchronous electric motor current flows through the controlled switches V1-V6, and the reactive component of the current flows through the diodes D1-D6.
I – three-phase bridge inverter;
B – three-phase bridge rectifier;
Sf – filter capacitor;
Option for connecting a frequency converter from Omron. 
Connecting frequency converters in compliance with EMC requirements 
EMC-compliant installation and connection are described in detail in the relevant device manuals.
Technical information converters
The operating modes of centrifugal pumps are energetically most efficiently regulated by changing the rotation speed of their impellers. The rotation speed of the impellers can be changed if an adjustable electric drive is used as the drive motor.
Device and characteristics gas turbines and internal combustion engines are such that they can provide a change in rotation speed in the required range.
It is convenient to analyze the process of regulating the rotation speed of any mechanism using the mechanical characteristics of the unit.
Let's consider the mechanical characteristics of a pumping unit consisting of a pump and an electric motor. In Fig. 1 presents mechanical characteristics centrifugal pump, equipped with a check valve (curve 1) and an electric motor with a squirrel-cage rotor (curve 2).
Rice. 1. Mechanical characteristics of the pump unit
The difference between the torque of the electric motor and the resistance torque of the pump is called dynamic torque. If the motor torque is greater than the resistance moment of the pump, the dynamic torque is considered positive; if less, it is considered negative.
Under the influence of a positive dynamic torque, the pumping unit begins to work with acceleration, i.e. accelerates. If the dynamic torque is negative, the pumping unit operates with a slowdown, i.e. slows down.
When these moments are equal, a steady state of operation occurs, i.e. the pump unit operates at a constant speed. This rotational speed and the corresponding torque are determined by the intersection of the mechanical characteristics of the electric motor and the pump (point a in Fig. 1).
If, during the regulation process, the mechanical characteristic is changed in one way or another, for example, to make it softer by introducing an additional resistor into the rotor circuit of the electric motor (curve 3 in Fig. 1), the rotational torque of the electric motor will become less than the resistance torque.
Under the influence of a negative dynamic torque, the pump unit begins to work slower, i.e. slows down until the torque and the moment of resistance are again balanced (point b in Fig. 1). This point has its own rotation frequency and torque value.
Thus, the process of regulating the rotation speed of the pump unit is continuously accompanied by changes in the torque of the electric motor and the resistance moment of the pump.
Regulation of the pump rotation speed can be carried out either by changing the rotation speed of the electric motor rigidly connected to the pump, or by changing gear ratio transmission connecting the pump to an electric motor that operates at a constant speed.
Regulating the speed of electric motors
Pumping units mainly use AC motors. The rotational speed of an AC motor depends on the frequency of the supply current f, the number of pole pairs p and slip s. By changing one or more of these parameters, you can change the rotation speed of the electric motor and the pump associated with it.
The main element of a frequency electric drive is. In the converter, the constant frequency of the supply network f1 is converted into a variable frequency f 2. The rotation speed of the electric motor connected to the output of the converter changes in proportion to the frequency f 2.
Using a frequency converter, the practically unchanged network parameters voltage U1 and frequency f1 are converted into variable parameters U2 and f 2 required for the control system. To ensure stable operation of the electric motor, limit its overload in current and magnetic flux, and maintain high energy performance, a certain ratio between its input and output parameters must be maintained in the frequency converter, depending on the type of mechanical characteristics of the pump. These ratios are obtained from the equation of the frequency regulation law.
For pumps the following ratio must be observed:U1/f1 = U2/f2 = const
In Fig. Figure 2 shows the mechanical characteristics of an asynchronous electric motor with frequency regulation. As the frequency f2 decreases, the mechanical characteristic not only changes its position in the n - M coordinates, but also slightly changes its shape. In particular, the maximum torque of the electric motor is reduced. This is due to the fact that if the relation U1/f1 = U2/f2 = const is observed and the frequency f1 changes, the influence of the active stator resistance on the magnitude of the motor torque is not taken into account.
Rice. 2. Mechanical characteristics of a frequency electric drive at maximum (1) and low (2) frequencies
When frequency regulation takes into account this influence, the maximum torque remains unchanged, the shape of the mechanical characteristic is preserved, only its position changes.
Frequency converters have high energy characteristics due to the fact that the output of the converter provides a shape of the current and voltage curves that approaches sinusoidal. IN lately The most widely used frequency converters are IGBT modules (insulated gate bipolar transistors).
The IGBT module is highly efficient key element. It has low voltage drop, high speed and low power switching A frequency converter based on IGBT modules with PWM and a vector algorithm for controlling an asynchronous electric motor has advantages over other types of converters. It is characterized by a high power factor over the entire output frequency range.
The schematic diagram of the converter is shown in Fig. 3.
Rice. 3. Diagram of a frequency converter on IGBT modules: 1 - fan unit; 2 - power supply; 3 - uncontrolled rectifier; 4 - control panel; 5 - control panel board; 6 - PWM; 7 - voltage conversion block; 8 - control system board; 9 - drivers; 10 - inverter unit fuses; 11 - current sensors; 12 - asynchronous squirrel-cage motor; Q1, Q2, Q3 - switches of the power circuit, control circuit and fan unit; K1, K2 - contactors for charging capacitors and power circuit; C - capacitor block; Rl, R2, R3 - resistors for limiting the current of capacitor charging, capacitor discharge and drainage unit; VT - inverter power switches (IGBT modules)
At the output of the frequency converter, a voltage (current) curve is formed, slightly different from a sinusoid, containing higher harmonic components. Their presence entails an increase in losses in the electric motor. For this reason, when the electric drive operates at rotation speeds close to the rated speed, the electric motor is overloaded.
When operating at lower speeds, the cooling conditions for self-ventilated electric motors used to drive pumps worsen. In the usual control range of pumping units (1:2 or 1:3), this deterioration in ventilation conditions is compensated by a significant reduction in load due to a decrease in pump flow and pressure.
When operating at frequencies close to the nominal value (50 Hz), deterioration of cooling conditions in combination with the appearance of higher-order harmonics requires a reduction in the permissible mechanical power by 8 - 15%. Because of this, the maximum torque of the electric motor is reduced by 1 - 2%, its efficiency - by 1 - 4%, cosφ - by 5 - 7%.
To avoid overloading the electric motor, it is necessary to either limit the upper value of its rotation speed, or equip the drive with a more powerful electric motor. The last measure is mandatory when the pumping unit is intended to operate at a frequency f 2 > 50 Hz. The upper value of the engine speed is limited by limiting the frequency f 2 to 48 Hz. Increasing the rated power of the drive motor is carried out by rounding to the nearest standard value.
Group control of adjustable electric drives of units
Many pumping installations consist of several units. As a rule, not all units are equipped with an adjustable electric drive. Of the two or three installed units, it is enough to equip one with an adjustable electric drive. If one converter is constantly connected to one of the units, there is an uneven consumption of their motor life, since a unit equipped with an adjustable drive is used for a much longer time.
To distribute the load evenly between all units installed at the station, group control stations have been developed, with the help of which the units can be alternately connected to the converter. Control stations are usually manufactured for low-voltage (380 V) units.
Typically, low-voltage control stations are designed to control two or three units. Low-voltage control stations include automatic switches that provide protection against interphase short circuits and ground faults, thermal relays to protect units from overload, as well as control equipment (keys, etc.).
The switching circuit of the control station contains the necessary interlocks that allow connecting the frequency converter to any selected unit and replacing operating units without disrupting the technological operating mode of the pumping or blowing unit.
Control stations, as a rule, along with power elements (circuit breakers, contactors, etc.) contain control and regulating devices (microprocessor controllers, etc.).
At the customer's request, the stations are equipped with devices for automatic switching on of backup power (ABP), commercial metering of consumed electricity, and control of shut-off equipment.
If necessary, additional devices are introduced into the control station, ensuring the use of a soft start device for units along with a frequency converter.
Automated control stations provide:
maintaining a given value of a process parameter (pressure, level, temperature, etc.);
control of operating modes of electric motors of regulated and unregulated units (control of current consumption, power) and their protection;
automatic switching on into operation of the backup unit in case of failure of the main one;
switching units directly to the network when the frequency converter fails;
automatic switching on of the backup (AVR) electrical input;
automatic restart (AR) of the station after loss and deep drops in the supply voltage electrical network;
automatic change of station operating mode with stopping and starting of units at a given time;
automatic activation of an additional unregulated unit if the regulated unit, having reached the rated speed, did not provide the required water supply;
automatic alternation of operating units at specified intervals to ensure uniform consumption of motor resources;
operational control of the operating mode of the pumping (blowing) unit from the control panel or from the dispatch console.
Rice. 4. Group control station for variable-frequency electric drives of pumps
Efficiency of using variable-frequency electric drives in pumping units
The use of a variable-frequency drive allows you to significantly save energy, since it makes it possible to use large pumping units in low flow mode. Thanks to this, it is possible, by increasing the unit power of the units, to reduce their total number, and therefore reduce overall dimensions buildings, simplify the hydraulic circuit of the station, reduce the number of pipeline fittings.
Thus, the use of a controlled electric drive in pumping units allows, along with saving electricity and water, to reduce the number of pumping units, simplify the hydraulic circuit of the station, and reduce the construction volume of the building pumping station. In this regard, secondary economic effects arise: the costs of heating, lighting and building repairs are reduced; the given costs, depending on the purpose of the stations and other specific conditions, can be reduced by 20 - 50%.
IN technical documentation on frequency converters it is indicated that the use of an adjustable electric drive in pumping units allows saving up to 50% of the energy spent on pumping clean and waste water, and the payback period is three to nine months.
At the same time, calculations and analysis of the efficiency of an adjustable electric drive in existing pumping units show that in small pumping units with units with a power of up to 75 kW, especially when they operate with a large static component of pressure, the use of adjustable electric drives turns out to be inappropriate. In these cases, simpler control systems can be used using throttling and changing the number of operating pumping units.
The use of an adjustable electric drive in automation systems for pumping units, on the one hand, reduces energy consumption, on the other hand, it requires additional capital costs, therefore the feasibility of using an adjustable electric drive in pumping units is determined by comparing the given costs of two options: basic and new. For new option accepted pumping unit, equipped with an adjustable electric drive, and for the basic one - an installation whose units operate at a constant speed.