We study the principle of operation, assemble and connect a frequency converter for asynchronous motors. Frequency converter for electric motor

Description:

Frequency converter complete with asynchronous electric motor allows you to replace the DC drive. DC motor speed control systems are quite simple, but weak point such an electric drive is an electric motor. It is expensive and unreliable. During operation, the brushes spark, and the collector wears out under the influence of electroerosion. Such an electric motor cannot be used in a dusty and explosive environment.

Asynchronous electric motors are superior to DC motors in many ways: they are simple in design and reliable, since they do not have moving contacts. Compared to DC motors, they have smaller dimensions, weight and cost at 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 by traditional methods (by changing the supply voltage, introducing additional resistances into the winding circuit).

The control of an asynchronous electric motor in the frequency mode until recently was a big problem, although the theory of frequency regulation was developed back in the thirties. The development of the frequency-controlled electric drive was held back by the high cost of frequency converters. The appearance of power circuits with IGBT transistors, the development of high-performance microprocessor control systems allowed various companies in Europe, the USA and Japan to create modern frequency converters at an affordable cost.

It is known that the regulation of the speed of rotation of actuators can be carried out using various devices: mechanical variators, hydraulic clutches, resistors additionally introduced 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, requires high costs during installation and operation.
Static frequency converters are the most advanced asynchronous drive control devices at present.

The principle of the frequency method for controlling the speed of an asynchronous motor is that by changing the frequency f1 supply voltage, can be in accordance with the expression

constant number of pole pairs p change the angular velocity magnetic field stator.

This method provides smooth speed control in a wide range, and the mechanical characteristics are highly rigid.

In this case, the speed control is not accompanied by an increase in the slip of the asynchronous motor, so the power loss during regulation is small.

To get high energy indicators 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 moment MS. At constant load torque Mc=const the voltage on the stator must be regulated proportionally to the frequency :

For the fan nature of the load moment, this state has the form:

When the load torque is inversely proportional to the speed:

Thus, for smooth stepless regulation of the rotational speed of the shaft of an asynchronous electric motor, the frequency converter must provide simultaneous regulation of the frequency and voltage on the stator of the asynchronous motor.

Benefits of using an adjustable electric drive in technological processes

The use of an adjustable electric drive ensures energy saving and allows obtaining new qualities of systems and objects. Significant energy savings are achieved through the regulation of any technological parameter. If it is a conveyor or conveyor, then you can adjust the speed of its movement. If it is a pump or a fan, you can maintain pressure or adjust the performance. If this is a machine, then you can smoothly adjust the feed rate or the main movement.

A special economic effect from the use of frequency converters is the use of frequency regulation at facilities that provide transportation of liquids. Until now, the most common way to control the performance of such objects is to use gate valves or control valves, but today frequency control of an asynchronous motor is becoming available, which drives, for example, the impeller of a pumping unit or a fan.

The prospects for frequency regulation are clearly visible from Figure 1

Thus, when throttling, the flow of a substance held back by a valve or valve does not 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 the loss of the transported substance.

The structure of the frequency converter

Most modern frequency converters are built according to the 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 mains voltage is converted in it into a direct current voltage.

The power three-phase pulse inverter consists of six transistor switches. Each motor winding is connected through the appropriate key to the positive and negative terminals of the rectifier. The inverter converts the rectified voltage into a three-phase alternating voltage of the desired 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 keys. Compared to thyristors, they have a higher switching frequency, which allows you to generate a sinusoidal output signal with minimal distortion.

How the frequency converter works

The frequency converter consists of an uncontrolled diode power rectifier B, an independent inverter, a PWM control system, a automatic regulation, inductor Lv and filter capacitor Cv (Fig. 2). Regulation of the 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, in motor windings, due to their filtering properties, sinusoidal currents flow.

In this case, the speed control is not accompanied by an increase in the slip of the asynchronous motor, so the power loss during regulation is 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 structure of the frequency converter

Most modern frequency converters built according to the scheme of double conversion. The input sinusoidal voltage with constant amplitude and frequency is rectified in the DC link B, smoothed by a filter consisting of a choke Lv and filter capacitor Cv, and then re-converted by the inverter AI into an alternating voltage of variable frequency and amplitude. Output frequency control 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 connection of each winding within the pulse repetition period is modulated according to a sinusoidal law. The largest pulse width is provided in the middle of the half-cycle, and decreases towards the beginning and end of the half-cycle. Thus, the PMS control system provides pulse-width modulation (PWM) of the voltage applied to the motor windings. The amplitude and frequency of the voltage are determined by the parameters of the modulating sinusoidal function. Thus, a three-phase alternating voltage of variable frequency and amplitude is formed at the output of the frequency converter.

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The adjustable electric drive is designed to control the motor by controlling the parameters. Speed ​​is directly proportional to frequency. Therefore, by varying the frequency, it is possible to maintain the speed of rotation of the motor shaft, set according to the technology. Step by step description The workflow for a variable frequency drive (VFD) looks something like this.

1. Step one. Diode power rectifier converts single or three-phase input current to direct current.
2. Step two. Frequency converter control of the torque and speed of rotation of the motor shaft.
3. Step three. Output voltage control, maintaining a constant U / f ratio.

The device that performs the inverse function of generating direct current to alternating current at the output of the system is called an inverter. Getting rid of bus ripple is achieved by adding a choke and a filter capacitor.

How to choose a variable frequency drive

The prevailing number of frequency converters are manufactured with a built-in electromagnetic compatibility (EMC) filter.

There are different types of control, such as sensorless and sensor vector, etc. According to the given priorities in making management decisions, the drives are selected according to:

• load type;
• voltage and rating of the motor;
• control mode;
• adjustments;
• EMC, etc.

If the VFD is intended for an asynchronous motor with a long service life, then it is recommended to choose a frequency converter with an overestimated output current. With the help of modern frequency converters, it is possible to control from a remote control, via an interface, or by a combined method.

Technical features of the use of a frequency electric drive

1. To ensure high performance, you can freely switch to any mode in the settings.
2. Almost all devices have diagnostic functions, which allows you to quickly fix the problem. However, it is recommended to check the settings first of all, to exclude the possibility of involuntary actions of employees.
3. An adjustable drive can synchronize conveyor processes or set a certain ratio of interdependent values. Equipment reduction leads to technology optimization.
4. In the auto-tuning state, the motor parameters are automatically stored in the memory of the frequency converter. Due to this, the accuracy of the torque calculation is increased, and the slip compensation is improved.

Application area

Manufacturers offer a wide range of drives used in areas where electric motors are involved. The ideal solution for all kinds of loads and fans. Mid-range systems are used in coal-fired power plants, mining, mills, utilities, etc. The rating range is as follows: 3 kV, 3.3 kV, 4.16 kV, 6 kV, 6.6 kV, 10 kV and 11 kV.

With the advent of an adjustable electric drive, the control of water pressure does not cause problems for the end user. The interface with a well-thought-out script structure is great for managing pumping equipment. Thanks to its compact design, the drive can be installed in a variety of cabinet designs. New generation products have the properties of advanced technology:

• high speed and accuracy of control in vector mode;
• significant energy savings;
• fast dynamic characteristics;
• large low frequency torque;
• double braking, etc.

Purpose and technical indicators

Complete VFD with voltage up to and above 1 kV (designed to receive and convert energy, protect electrical equipment from short-circuit currents, overload) allow:

• smoothly start the engine, and, consequently, reduce its wear;
• stop, maintain the engine speed.

Complete cabinet-type VFDs up to 1kV perform the same tasks in relation to motors with a power of 0.55 - 800 kW. The drive operates normally when the mains voltage is between -15% and +10%. During non-stop operation, a decrease in power occurs if the voltage is 85% -65%. Total power factor cosj = 0.99. The output voltage is automatically regulated by automatic transfer switch (ATS).

Benefits of using

In terms of optimization and potential benefits provide the opportunity to:

• control the process with high precision;
• remotely diagnose the drive;
• take into account engine hours;
• monitor the malfunction and aging of mechanisms;
• increase the resource of machines;
• significantly reduce the acoustic noise of the electric motor.

Conclusion

What is a CHRP? This is a motor controller that controls the electric motor by adjusting the frequency of the input network, and at the same time protects the unit from various faults (current overload, short circuit currents).

Electric actuators (performing three functions related to speed, control and braking) are an indispensable device for the operation of electric motors and other rotating machines. The systems are actively used in many areas of production: in the oil and gas industry, nuclear energy, woodworking, etc.

To date, dozens of brands of low-voltage frequency converters of foreign and 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, there is an extensive dealer network. The products of such companies from Europe as Emotron, Vacon, SSD Drives (Parker Corporation), Elettronica Santerno are still less known. There are also products of American manufacturers - General Electric Corporation, AC Technology International (part of the Lenze concern) and WEG (Brazil).

Asian companies are seriously competing with European and American manufacturers. 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 manufacturers the most famous is the Vesper company. We can also note the specialized converters of the brands APC, EPV (JSC Elektroapparat), REN2K or REMS (MKE).

Most manufacturers offer frequency converters capable of operating in open 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 operation mode. The control range can be 1:50.

However, leading manufacturers offer a more advanced sensorless vector control mode 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 control mode (RFC) without using a feedback sensor, which allows you to control the torque with accuracy sufficient for most tasks, expand the control range to 100, ensure high speed maintenance accuracy at low speed 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 complete them with CNC systems or motion controllers.

Increasingly, you can find the use of the 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 dedicated drive with a controlled rectifier is used for this. Leading companies such as Control Techniques offer Regen as one of the modes of operation of the Unidrive SP, thereby achieving significant energy savings and high efficiency systems.

The described assortment enables the engineer to choose a frequency converter that is suitable for the price 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. Supplier information can be found on our website:

Rockwell Automation, the undisputed leader in power electrical market, has released a new series of Allen-Bradley® PowerFlex® frequency drives in the power range from 0.25kW to 6770kW. The new high performance series combines a compact design, wide functionality and excellent performance characteristics. It is used in 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 the Architectural class drives can be easily adapted and built into the control systems of various power equipment due to flexible configuration changes. All models offer exceptional communication capabilities, a wide range of operator panels and programming tools that greatly facilitate operation and speed up equipment start-up.

PowerFlex® 4

The Powerflex 4 drive is the most compact and cost effective member of the family. As an ideal speed control device, this model provides application versatility while meeting manufacturers' and end users' requirements for flexibility, compactness and ease of operation.

The drive implements a volt-frequency control law with the possibility of slip compensation. An excellent addition to this model is the version of ultra-compact drives [email protected], with an extended operating power range up to 2.2 kW for a single-phase version and up to 11kW for a three-phase voltage of 400VAC. The proposed price scale for this model allows us to hope, if not for the 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 control the speed, torque, direction of rotation of asynchronous and synchronous AC motors. The unique design of the PowerFlex 7000 series is a patented PowerCage branded power package containing the main power components of the drives. The new modular design is simple and has a small number of components, which ensures high reliability and facilitates operation. The main advantages of medium voltage drives include: reduced operating costs, the ability to start large motors from small power supplies and improve the quality characteristics of controlled technological process and the equipment used.

Drives are available in three sizes depending on output power:

Frame A - Power range 150-900 kW at supply voltage 2400-6600 V

Case B - Power range 150-4100 kW at supply voltage 2400-6600V

Case C - Power range 2240-6770 kW at supply voltage 4160-6600 V

PowerFlex 7000 drives are available in 6-pulse, 18-pulse, or PWM options, giving the user significant flexibility in reducing the effects of line harmonics. In addition, it provides direct sensorless vector control for improved low speed control compared to drives using the V/f control method, as well as the ability to control motor torque, as is done in DC drives. As an operator panel, a module with a liquid crystal display of 16 lines and 40 characters is offered.

Higher moment of inertia without additional gearbox

The fast-response servomotors from Beckhoff of the AM3000 series, which are manufactured using new materials and technology, are mainly used in dynamic applications with high loads, such as for driving the axes of metalworking machines or devices without gears. Combined with a large rotor inertia, they offer the same advantages as the AM3xxx series motors, such as stator pole winding, which allows for a significant reduction in overall motor size. The flanges, connectors and shafts of the new AM3500 motor series are compatible with the well-proven AM3000 motors. The new AM3500 models are available with flange sizes 3 to 6 and have torques from 1.9 to 15 Nm. The rotation speeds of the motors are from 3000 to 6000 rpm. For feedback systems, there are coordinate transducers or absolute encoders (single or multi-turn). The case belongs to the protection class IP 64; options with protection class IP 65/67 are possible. This motor series complies with CE, UL and CSA safety regulations.

New generation of drives

The Emotron range has been expanded with NGD drives: FDU2.0, VFX2.0 (from 0.75 kW to 1.6 MW) and VSC/VSA (0.18 to 7.5 kW). Variable speed drives FDU2.0 (for centrifugal) and VFX2.0 (for reciprocating) allow the user to set operating parameters in the required units, have a removable control panel with a copy function, models up to 132 kW have a standard IP54 economy version (models from 160 to 800 kW can also be installed in special IP54 compact enclosures). Communication during the process takes place via fieldbus (Profibus-DP, DeviceNet, Ethernet), ports (RS-232, RS-485, Modbus RTU) and analog and digital outputs.

The VSA and VSC small vector drives are specially designed for speed control of three-phase induction motors not high power: 220V models are available from 0.18 to 2.2kW and 380V models from 0.75 to 7.5kW.

ATV61-ATV71 family

The frequency converter market in Russia is developing rapidly. Not surprisingly, it attracts numerous manufacturers, both large and little known. At the moment the Russian market is very segmented. But here's the paradox: despite the fact that there are currently more than 30 brands on the market, a significant share of the market belongs to 7 - 8 companies, and there are no more than two clear leaders. At the same time, the excellent technical characteristics of the equipment are not yet a guarantee of success. Leading positions in Russia were taken by companies investing significant funds in business development and business infrastructure.

The Schneider Electric company, whose interests in Russia are represented by ZAO Schneider Electric, in 2007 significantly expanded its product offer. Now the ATV61-ATV71 family has been expanded with a 690 V version, and there are many versions with an IP54 degree of protection. There is also a special model for lift and crane drive ATV71*383 with unique technology synchronous motor control. By the end of 2008, a device with a capacity of 2400 kW at 690V will appear in the Altivar line. Altivar 61 can now be used in step-up transformer applications.

The new economical Altivar 21 series has been specially developed for heating, air conditioning and ventilation systems in residential and public buildings. The Altivar 21 controls motors from 0.75 to 75 kW for 380 V and 200 … 240 V.

Altivar 21 has many application functions:

– built-in PI controller;

- "pickup on the fly";

– sleep/wake function;

– management of protections and signaling;

– resistance to mains interference, operation at temperatures up to + 50°C and voltage drop -50%.

With the new capacitorless technology, the Altivar 21 does not require harmonic reduction devices. The total coefficient is THDI 30%. The rejection of capacitors and the use of more powerful semiconductors increased the operating time.

Schneider Electric's leadership in the converter technology market is the result of serious work to improve the fault tolerance of converters. The MTTF setting for some models is up to 640,000 hours. Altivar works with voltage drops up to -50%, temperatures up to +50%, in chemically aggressive environments and with impulse noise in the network. This is a serious argument for repurchase. The buyer's confidence in the equipment and the company's reputation can hardly be overestimated.

Drives from SICK

Modern production requires the automation of many manual operations for setting various parameters on various machines and packaging machines. Often, the operator has a need to change the geometric parameters of the manufactured product or other similar tasks. In this case, positioning drives from SICK-Stegmann are the ideal low-cost device for this operation.

HIPERDRIVE® positioning drives are the result of integrating a brushless DC motor, gearbox, absolute multi-turn encoder, power and control electronics in one unit. 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 type 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, it is not economically justified. Servo-based systems typically also require an inverter, a brake, an absolute encoder.

The main advantages of these drives:

– Highly integrated device

Drive downsizing

Easy assembly and setup

According to the latest statistics, approximately 70% of all generated electricity in the world consumes an electric drive. And this percentage is growing every year.

With a properly selected method of controlling the electric motor, it is possible to obtain maximum efficiency, maximum torque on the shaft of the electric machine, and at the same time the overall performance of the mechanism will increase. Efficiently running electric motors consume a minimum of electricity and provide maximum efficiency.

For electric motors powered by a frequency converter, the efficiency will largely depend on the chosen method of controlling the electric machine. Only by understanding the merits of each method can drive engineers and designers get the best performance out of each control method.
Content:

Control methods

Many people working in the field of automation, but not closely involved in the development and implementation of electric drive systems, believe that the control of an electric motor consists of a sequence of commands entered using an interface from a control panel or a PC. Yes, in terms of the overall management hierarchy automated system this is correct, but there are still ways to control the electric motor itself. It is these methods that will have the maximum impact on the performance of the entire system.

For asynchronous motors connected to a frequency converter, there are four basic control methods:

• U / f - volt per hertz;
• U/f with encoder;
• Open-loop vector control;
• Closed-loop vector control;

All four methods use PWM pulse width modulation, which changes the width of a fixed signal by varying the pulse width to create an analog signal.

Pulse width modulation is applied to the frequency converter by using a fixed DC bus voltage. by quickly opening and closing (more correctly, switching) generate output pulses. By varying the width of these pulses, a "sine wave" of the desired frequency is obtained at the output. Even if the form of the output voltage of the transistors is pulsed, the current is still obtained in the form of a sinusoid, since the electric motor has an inductance that affects the shape of the current. All control methods are based on PWM modulation. The difference between the control methods is only in the method of calculating the applied voltage to the motor.

In this case, the carrier frequency (shown in red) represents the maximum switching frequency of the transistors. The carrier frequency for inverters is usually in the range of 2 kHz - 15 kHz. The frequency reference (shown in blue) is the output frequency reference signal. For inverters applicable in conventional systems electric drives, as a rule, lies in the range of 0 Hz - 60 Hz. When the signals of two frequencies are superimposed on each other, a transistor opening signal will be issued (indicated in black), which supplies power to the electric motor.

V/F control method

Volt-per-hertz control, most commonly referred to as V/F, is perhaps the easiest way to regulate. It is often used in simple electric drive systems due to its simplicity and the minimum number of parameters required for operation. This control method does not require mandatory installation of an encoder and mandatory settings for a frequency-controlled electric drive (but it is recommended). This results in lower costs for auxiliary equipment (sensors, feedback wires, relays, etc.). U / F control is quite often used in high-frequency equipment, for example, it is often used in CNC machines to drive spindle rotation.

The constant torque model has a constant torque over the entire speed range at the same U/F ratio. The variable torque ratio model has a lower supply voltage at low speeds. This is necessary to prevent saturation of the electric machine.

V/F is the only way to control the speed of an induction motor that allows the control of multiple drives from a single frequency converter. Accordingly, all machines start and stop at the same time and operate at the same frequency.

But this method management has several limitations. For example, when using the V/F control method without an encoder, there is absolutely no certainty that the shaft of an induction machine is rotating. In addition, the starting torque of the electric machine at a frequency of 3 Hz is limited to 150%. Yes, the limited torque is more than enough for most existing equipment. For example, almost all fans and pumps use a V/F control method.

This method is relatively simple due to its looser specification. Speed ​​control is typically in the range of 2% - 3% of the maximum output frequency. The speed response is calculated for frequencies above 3 Hz. The response speed of the frequency converter is determined by the speed of its response to a change in the reference frequency. The higher the response speed, the faster the response of the drive to a change in the speed reference.

The speed control range when using the V/F method is 1:40. Multiplying this ratio by the maximum operating frequency of the electric drive, we obtain the value of the minimum frequency at which the electric machine can operate. For example, if the maximum frequency is 60 Hz and the range is 1:40, then minimum value frequency will be 1.5 Hz.

The U/F pattern determines the ratio of frequency and voltage during the operation of a variable frequency drive. According to him, the curve for setting the rotation speed (frequency of the electric motor) will determine, in addition to the frequency value, the voltage value supplied to the terminals of the electric machine.

Operators and technicians can select the desired V/F regulation pattern with a single parameter in a modern frequency converter. The preset templates are already optimized for specific applications. There is also the possibility of creating your own templates, which will be optimized for a specific system of variable frequency drive or electric motor.

Devices such as fans or pumps have a load torque that depends on their rotational speed. The variable torque (figure above) of the V/F pattern prevents adjustment errors and improves efficiency. This regulation model reduces magnetizing currents at low frequencies by reducing the voltage on the electrical machine.

Constant torque machines such as conveyors, extruders and other equipment use the constant torque control method. With a constant load, full magnetizing current is required at all speeds. Accordingly, the characteristic has a direct slope in the entire speed range.

U/F control method with encoder

If it is necessary to improve the accuracy of speed control, an encoder is added to the control system. The introduction of speed feedback using an encoder allows you to increase the accuracy of regulation up to 0.03%. The output voltage will still be determined by the set V/F pattern.

This control method has not been widely used, since the advantages it presents compared to standard V/F functions are minimal. Starting torque, response speed and speed control range are all identical to standard V/F. In addition, with an increase in operating frequencies, problems may arise with the operation of the encoder, since it has a limited number of revolutions.

Open Loop Vector Control

Open Loop Vector Control (VU) is used for a wider and more dynamic speed control of an electrical machine. When starting from a frequency converter, motors can develop a starting torque of 200% of the rated torque at a frequency of only 0.3 Hz. This greatly expands the list of mechanisms where an asynchronous electric drive with vector control can be used. This method also allows you to control the machine torque in all four quadrants.

The torque is limited by the motor. This is necessary to prevent damage to equipment, machines or products. The value of the moments is divided into four different quadrants, depending on the direction of rotation of the electric machine (forward or backward) and depending on whether the electric motor implements . Limits can be set for each quadrant separately, or the user can set the total torque in the frequency converter.

The motor mode of the asynchronous machine will be provided that the magnetic field of the rotor lags behind the magnetic field of the stator. If the rotor magnetic field begins to outstrip the stator magnetic field, then the machine will enter the regenerative braking mode with energy return, in other words, the asynchronous motor will switch to the generator mode.

For example, a bottle capping machine may use a torque limit in quadrant 1 (forward with positive torque) to prevent over-tightening of the bottle cap. The mechanism produces a forward movement and uses positive moment to screw on the bottle cap. On the other hand, a device such as an elevator with a counterweight heavier than an empty car will use quadrant 2 (reverse rotation and positive torque). If the cab is raised top floor, then the torque will be opposite to the speed. This is necessary to limit the lifting speed and prevent the counterweight from free-falling, as it is heavier than the cab.

The current feedback in these inverters allows you to set limits on the torque and current of the motor, since as the current increases, so does the torque. The output voltage of the inverter may increase if the mechanism requires more torque, or decrease if its limit is reached. permissible value. This makes the vector control principle of an asynchronous machine more flexible and dynamic than the U/F principle.

Also frequency converters with open-loop vector control have a faster speed response - 10 Hz, which makes it possible to use it in mechanisms with shock loads. For example, crushers rock the load is constantly changing and depends on the volume and dimensions of the processed rock.

Unlike the V/F control pattern, vector control uses a vector algorithm to determine the maximum effective operating voltage of the motor.

The VU vector control solves this problem due to the presence of feedback on the motor current. As a rule, the current feedback is generated by the internal current transformers of the frequency converter itself. Based on the obtained current value, the frequency converter calculates the torque and flux of the electrical machine. The basic motor current vector is mathematically split into a magnetizing current vector (I d) and a torque vector (I q).

Using the data and parameters of the electric machine, the inverter calculates the vectors of the magnetizing current (I d) and torque (I q). For achievement maximum performance, the frequency converter must keep I d and I q separated by 90 0 . This is significant because sin 90 0 = 1 and the value 1 represents the maximum torque value.

In general, the vector control of an induction motor provides tighter control. The speed regulation is approximately ±0.2% of the maximum frequency, and the regulation range reaches 1:200, which allows you to keep the torque when working at low speeds.

Vector feedback control

Closed-loop vector control uses the same control algorithm as the VU without feedback. The main difference is the presence of an encoder, which allows the variable frequency drive to develop 200% starting torque at 0 rpm. This item is simply necessary to create an initial moment when starting off elevators, cranes and other lifting machines in order to prevent the load from sinking.

The presence of a speed feedback sensor allows you to increase the response time of the system more than 50 Hz, as well as expand the speed control range up to 1:1500. Also, the presence of feedback allows you to control not the speed of the electric machine, but the moment. In some mechanisms, it is the value of the moment that is of great importance. For example, winding machine, blocking mechanisms and others. In such devices, it is necessary to regulate the moment of the machine.

We produce and sell frequency converters:
Prices for frequency converters (21.01.16):
Frequency converters one phase in three:
Model Power Price
CFM110 0.25kW 2300UAH
CFM110 0.37kW 2400UAH
CFM110 0.55kW 2500UAH
CFM210 1.0 kW 3200UAH
CFM210 1.5 kW 3400UAH
CFM210 2.2 kW 4000UAH
CFM210 3.3 kW 4300UAH
AFM210 7.5 kW 9900 UAH

Frequency converters 380V three phases in three:
CFM310 4.0 kW 6800UAH
CFM310 5.5 kW 7500UAH
CFM310 7.5 kW 8500UAH
Contacts for orders of frequency converters:
+38 050 4571330
[email protected] website

A modern frequency-controlled electric drive consists of an asynchronous or synchronous electric motor and a frequency converter (see Fig. 1.).

An electric motor converts electrical energy into

mechanical energy and drives executive agency technological mechanism.

The frequency converter drives an electric motor and is an electronic static device. An electrical voltage with variable amplitude and frequency is generated at the output of the converter.

The name "variable frequency electric drive" is due to the fact that the motor speed control is carried out by changing the frequency of the supply voltage supplied to the motor from the frequency converter.

Over the past 10-15 years, the world has seen a widespread and successful introduction of a frequency-controlled electric drive to solve various technological problems in many sectors of the economy. This is primarily due to the development and creation of frequency converters based on a fundamentally new element base, mainly on IGBT insulated gate bipolar transistors.

This article briefly describes the currently known types of frequency converters used in a frequency-controlled electric drive, the control methods implemented in them, their features and characteristics.

In further discussions, we will talk about a three-phase frequency-controlled electric drive, since it has the greatest industrial application.

About management methods

In synchronous electric motor rotor speed in

steady state is equal to the frequency of rotation of the stator magnetic field.

In an asynchronous electric motor, the rotor speed

steady state differs from the rotational speed by the amount of slip.

The frequency of rotation of the magnetic field depends on the frequency of the supply voltage.

When the stator winding of an electric motor is supplied with a three-phase voltage with a frequency, a rotating magnetic field is created. The rotation speed of this field is determined by the well-known formula

where is the number of pairs of stator poles.

The transition from the field rotation speed, measured in radians, to the rotation frequency, expressed in revolutions per minute, is carried out according to the following formula

where 60 is the dimension conversion factor.

Substituting the field rotation speed into this equation, we obtain that

Thus, the rotor speed of synchronous and asynchronous motors depends on the frequency of the supply voltage.

The method of frequency regulation is based on this dependence.

By changing the frequency at the motor input with the help of a converter, we regulate the rotor speed.

In the most common frequency-controlled drive based on asynchronous squirrel-cage motors, scalar and vector frequency control are used.

With scalar control, according to a certain law, the amplitude and frequency of the voltage applied to the motor are changed. Changing the frequency of the supply voltage leads to a deviation from the calculated values ​​of the maximum and starting torques of the motor, efficiency, power factor. Therefore, in order to maintain the required performance characteristics of the engine, it is necessary to simultaneously change the voltage amplitude with a change in frequency.

In existing frequency converters with scalar control, the ratio of the maximum motor torque to the moment of resistance on the shaft is most often maintained constant. That is, when the frequency changes, the voltage amplitude changes in such a way that the ratio of the maximum motor torque to the current load torque remains unchanged. This ratio is called the overload capacity of the motor.

With a constant overload capacity, the rated power factor and efficiency engine over the entire speed control range practically do not change.

The maximum torque developed by the engine is determined by the following relationship

where is a constant coefficient.

Therefore, the dependence of the supply voltage on the frequency is determined by the nature of the load on the shaft of the electric motor.

For a constant load torque, the ratio U/f = const is maintained, and, in fact, the maximum motor torque is constant. The nature of the dependence of the supply voltage on the frequency for the case with a constant load torque is shown in fig. 2. The angle of inclination of the straight line on the graph depends on the values ​​of the moment of resistance and the maximum torque of the engine.

At the same time, at low frequencies, starting from a certain frequency value, the maximum motor torque begins to fall. To compensate for this and to increase the starting torque, an increase in the supply voltage level is used.

In the case of a fan load, the dependence U/f2 = const is realized. The nature of the dependence of the supply voltage on the frequency for this case is shown in Fig.3. When regulating in the region of low frequencies, the maximum torque also decreases, but for this type of load this is not critical.

Using the dependence of the maximum torque on voltage and frequency, it is possible to plot U against f for any type of load.

An important advantage of the scalar method is the possibility of simultaneous control of a group of electric motors.

Scalar control is sufficient for most practical applications of a variable frequency drive with a motor speed control range of up to 1:40.

Vector control allows you to significantly increase the control range, control accuracy, increase the speed of the electric drive. This method provides direct control of the motor torque.

The torque is determined by the stator current, which creates an exciting magnetic field. With direct torque control

it is necessary to change, in addition to the amplitude and phase of the stator current, that is, the current vector. This is the reason for the term "vector control".

To control the current vector, and, consequently, the position of the stator magnetic flux relative to the rotating rotor, it is required to know the exact position of the rotor at any time. The problem is solved either with the help of a remote rotor position sensor, or by determining the position of the rotor by calculating other engine parameters. The currents and voltages of the stator windings are used as these parameters.

Less expensive is a VFD with vector control without a speed feedback sensor, but vector control requires a large amount and high speed of calculations from the frequency converter.

In addition, for direct control of the torque at low, close to zero rotation speeds, the operation of a frequency-controlled electric drive without speed feedback is impossible.

Vector control with a speed feedback sensor provides a control range of up to 1:1000 and higher, speed control accuracy - hundredths of a percent, torque accuracy - a few percent.

In a synchronous variable frequency drive, the same control methods are used as in an asynchronous one.

However, in its pure form, frequency regulation of the speed of rotation of synchronous motors is used only at low powers, when the load moments are small, and the inertia of the drive mechanism is small. At high powers, only a drive with a fan load fully meets these conditions. In cases with other types of load, the motor may fall out of synchronism.

For high-power synchronous electric drives, a frequency control method with self-synchronization is used, which eliminates the loss of the motor from synchronism. The peculiarity of the method is that the frequency converter is controlled in strict accordance with the position of the motor rotor.

A frequency converter is a device designed to convert alternating current (voltage) of one frequency into alternating current (voltage) of another frequency.

The output frequency in modern converters can vary over a wide range and be both higher and lower than the mains frequency.

The circuit of any frequency converter consists of power and control parts. The power part of the converters is usually made on thyristors or transistors that operate in the electronic switch mode. The control part is executed on digital microprocessors and provides control of power
electronic keys, as well as solving a large number of auxiliary tasks (control, diagnostics, protection).

frequency converters,

applied in a regulated

electric drive, depending on the structure and principle of operation, the power drive is divided into two classes:

1. Frequency converters with a pronounced intermediate DC link.

2. Frequency converters with direct connection (without an intermediate DC link).

Each of the existing classes of converters has its own advantages and disadvantages, which determine the area of ​​rational application of each of them.

Historically, direct-coupled converters were the first to appear.

(Fig. 4.), in which the power part is a controlled rectifier and is made on non-lockable thyristors. The control system unlocks the groups of thyristors in turn and connects the stator windings of the motor to the mains.

Thus, the output voltage of the converter is formed from the "cut" sections of the sinusoids of the input voltage. In Fig.5. shows an example of output voltage generation for one of the load phases. At the input of the converter, a three-phase sinusoidal voltage acts ia, iv, ip. The output voltage uv1x has a non-sinusoidal "sawtooth" shape, which can be conventionally approximated by a sinusoid (thickened line). It can be seen from the figure that the frequency of the output voltage cannot be equal to or higher than the frequency of the supply network. It is in the range from 0 to 30 Hz. As a result, a small range of engine speed control (no more than 1: 10). This limitation does not allow the use of such converters in modern frequency-controlled drives with a wide range of technological parameters control.

The use of non-lockable thyristors requires relatively complex control systems, which increase the cost of the converter.

The “cut” sine wave at the output of the converter is a source of higher harmonics, which cause additional losses in the electric motor, overheating of the electric machine, torque reduction, and very strong interference in the supply network. The use of compensating devices leads to an increase in cost, weight, dimensions, and a decrease in efficiency. systems as a whole.

Along with the listed shortcomings of direct-coupled converters, they have certain advantages. These include:

Practically the highest efficiency relative to other converters (98.5% and above),

Ability to work with high voltages and currents, which makes it possible to use them in powerful high-voltage drives,

Relative cheapness, despite the increase in absolute cost due to control circuits and additional equipment.

Similar converter circuits are used in old drives and new designs are practically not developed.

The most widely used in modern variable frequency drives are converters with a pronounced DC link (Fig. 6.).

Converters of this class use double conversion of electrical energy: the input sinusoidal voltage with constant amplitude and frequency is rectified in the rectifier (V), filtered by the filter (F), smoothed, and then re-converted by the inverter (I) into an alternating voltage of variable frequency and amplitude. Double conversion of energy leads to a decrease in efficiency. and to some deterioration in weight and size indicators in relation to converters with direct connection.

To form a sinusoidal alternating voltage, autonomous voltage inverters and autonomous current inverters are used.

As electronic switches in inverters, lockable thyristors GTO and their advanced modifications GCT, IGCT, SGCT, and insulated gate bipolar transistors IGBT are used.

The main advantage of thyristor frequency converters, as in a direct-coupled circuit, is the ability to work with high currents and voltages, while withstanding continuous load and impulse effects.

They have a higher efficiency (up to 98%) in relation to converters on IGBT transistors (95 - 98%).

Thyristor-based frequency converters currently occupy a dominant position in a high-voltage drive in the power range from hundreds of kilowatts to tens of megawatts with an output voltage of 3-10 kV and higher. However, their price per kW of output power is the highest in the class of high voltage converters.

Until recently, frequency converters on GTOs were the main share in the low-voltage variable frequency drive. But with the advent of IGBT transistors, there was a " natural selection» and today, converters based on them are generally recognized leaders in the field of low-voltage frequency-controlled drives.

The thyristor is a semi-controlled device: to turn it on, it is enough to apply a short pulse to the control output, but to turn it off, you must either apply a reverse voltage to it or reduce the switched current to zero. For
This requires a complex and cumbersome control system in a thyristor frequency converter.

Insulated gate bipolar transistors IGBT differ from thyristors full controllability, simple low power control system, the highest operating frequency

As a result, IGBT-based frequency converters make it possible to expand the range of motor speed control and increase the speed of the drive as a whole.

For an asynchronous vector controlled drive, IGBT converters allow operation at low speeds without a feedback sensor.

Application of IGBT with more high frequency switching in conjunction with a microprocessor control system in frequency converters reduces the level of higher harmonics characteristic of thyristor converters. As a result, there are less additional losses in the windings and the magnetic circuit of the electric motor, a decrease in the heating of the electric machine, a decrease in torque ripples and the exclusion of the so-called “walking” of the rotor in the low-frequency region. Losses in transformers, capacitor banks are reduced, their service life and wire insulation are increased, the number of false alarms of protection devices and errors of induction measuring instruments are reduced.

Converters based on IGBT transistors compared to thyristor converters with the same output power are smaller in size, weight, increased reliability due to the modular design of electronic switches, better heat removal from the module surface and fewer structural elements.

They allow for more complete protection against surge current and overvoltage, which significantly reduces the likelihood of failures and damage to the drive.

At the moment, low-voltage IGBT converters have a higher price per unit of output power, due to the relative complexity of the production of transistor modules. However, in terms of price / quality ratio, based on the listed advantages, they clearly outperform thyristor converters, in addition, over the past years, there has been a steady decline in prices for IGBT modules.

The main obstacle to their use in high-voltage direct frequency conversion drives and at powers above 1 - 2 MW at the moment are technological limitations. An increase in the switching voltage and operating current leads to an increase in the size of the transistor module, and also requires more efficient heat removal from the silicon crystal.

New technologies for the production of bipolar transistors are aimed at overcoming these limitations, and the promise of using IGBTs is very high also in high-voltage drives. Currently, IGBT transistors are used in high-voltage converters in the form of several connected in series

Structure and principle of operation of a low-voltage frequency converter based on GBT transistors

A typical diagram of a low-voltage frequency converter is shown in fig. 7. At the bottom of the figure are graphs of voltages and currents at the output of each element of the converter.

The alternating voltage of the supply network (inv.) with a constant amplitude and frequency (UEx = const, f^ = const) is supplied to a controlled or uncontrolled rectifier (1).

Filter (2) is used to smooth out ripples of the rectified voltage (rect.). The rectifier and capacitive filter (2) form a DC link.

From the output of the filter, a constant voltage ud is fed to the input of an autonomous pulse inverter (3).

The autonomous inverter of modern low-voltage converters, as noted, is based on power bipolar transistors with an insulated gate IGBT. The figure in question shows a frequency converter circuit with an autonomous voltage inverter as the most widely used.

The inverter converts the direct voltage ud into a three-phase (or single-phase) pulsed voltage with variable amplitude and frequency. According to the signals of the control system, each winding of the electric motor is connected through the corresponding power transistors of the inverter to the positive and negative poles of the DC link.

The duration of connection of each winding within the pulse repetition period is modulated according to a sinusoidal law. The largest pulse width is provided in the middle of the half-cycle, and decreases towards the beginning and end of the half-cycle. Thus, the control system provides pulse-width modulation (PWM) of the voltage applied to the motor windings. The amplitude and frequency of the voltage are determined by the parameters of the modulating sinusoidal function.

At a high PWM carrier frequency (2 ... 15 kHz), the motor windings act as a filter due to their high inductance. Therefore, almost sinusoidal currents flow in them.

In converter circuits with a controlled rectifier (1), a change in the voltage amplitude uH can be achieved by controlling the value of the constant voltage ud, and a change in frequency can be achieved by the inverter operation mode.

If necessary, a filter (4) is installed at the output of the autonomous inverter to smooth out current ripples. (In IGBT converter circuits, due to the low level of higher harmonics in the output voltage, there is practically no need for a filter.)

Thus, a three-phase (or single-phase) alternating voltage of variable frequency and amplitude is formed at the output of the frequency converter (uout = var, tx = var).

In recent years, many companies have paid great attention, which is dictated by market needs, to the development and creation of high-voltage frequency converters. The required value of the output voltage of the frequency converter for a high-voltage electric drive reaches 10 kV and higher at a power of up to several tens of megawatts.

For such voltages and powers with direct frequency conversion, very expensive thyristor power electronic keys with complex control schemes. The converter is connected to the network either through an input current-limiting reactor or through a matching transformer.

The limiting voltage and current of a single electronic key are limited, therefore, special circuit solutions are used to increase the output voltage of the converter. It also reduces the overall cost of high voltage frequency converters by using low voltage electronic switches.

In frequency converters of various manufacturers, the following circuit solutions are used.

In the converter circuit (Fig. 8.), a double voltage transformation is carried out using a step-down (T1) and step-up (T2) high-voltage transformers.

Double transformation allows use for frequency regulation Fig 9. Relatively cheap

low-voltage frequency converter, the structure of which is shown in fig. 7.

Converters are distinguished by relative cheapness and ease of practical implementation. As a result, they are most often used to control high-voltage electric motors in the power range up to 1 - 1.5 MW. With a higher power of the electric drive, the transformer T2 introduces significant distortions in the process of controlling the electric motor. The main disadvantages of two-transformer converters are high weight and size characteristics, lower efficiency in relation to other circuits (93 - 96%) and reliability.

Converters made according to this scheme have a limited range of motor speed control both above and below the nominal frequency.

With a decrease in frequency at the output of the converter, the saturation of the core increases and the design mode of operation of the output transformer T2 is violated. Therefore, as practice shows, the regulation range is limited within Pnom>P>0.5Pnom. To expand the control range, transformers with an increased cross section of the magnetic circuit are used, but this increases the cost, weight and dimensions.

With an increase in the output frequency, losses in the core of the transformer T2 for remagnetization and eddy currents increase.

In drives with a power of more than 1 MW and a voltage of the low-voltage part of 0.4 - 0.6 kV, the cable cross-section between the frequency converter and the low-voltage winding of the transformers must be designed for currents up to kiloamperes, which increases the weight of the converter.

To increase the operating voltage of the frequency converter, electronic keys are connected in series (see Fig. 9.).

The number of elements in each arm is determined by the magnitude of the operating voltage and the type of element.

The main problem for this scheme is the strict coordination of the operation of electronic keys.

Semiconductor elements made even in the same batch have a spread of parameters, so the task of coordinating their work in time is very acute. If one of the elements opens with a delay or closes before the others, then the full tension of the shoulder will be applied to it, and it will fail.

To reduce the level of higher harmonics and improve electromagnetic compatibility, multipulse converter circuits are used. Coordination of the converter with the supply network is carried out using multi-winding matching transformers T.

In Fig.9. a 6-pulse circuit with a two-winding matching transformer is shown. In practice, there are 12, 18, 24-pulse circuits

converters. The number of secondary windings of transformers in these circuits is 2, 3, 4, respectively.

The circuit is the most common for high-voltage high power converters. The converters have one of the best specific weight and size indicators, the output frequency range is from 0 to 250-300 Hz, the efficiency of the converters reaches 97.5%.

3. Scheme of a converter with a multi-winding transformer

The power circuit of the converter (Fig. 10.) consists of a multi-winding transformer and electronic inverter cells. The number of secondary windings of transformers in known circuits reaches 18. The secondary windings are electrically shifted relative to each other.

This allows the use of low voltage inverter cells. The cell is made according to the scheme: uncontrolled three-phase rectifier, capacitive filter, single-phase inverter on IGBT transistors.

Cell outputs are connected in series. In the example shown, each motor supply phase contains three cells.

According to their characteristics, the converters are closer to the circuit with serial connection of electronic keys.