Vector control
Vector control is a method of controlling synchronous and asynchronous motors, not only generating harmonic currents (voltages) of the phases (scalar control), but also providing control of the rotor magnetic flux. The first implementations of the vector control principle and high-precision algorithms require the use of rotor position (speed) sensors.
In general, under " vector control" refers to the interaction of the control device with the so-called "spatial vector", which rotates with the frequency of the motor field.
Mathematical apparatus of vector control
Wikimedia Foundation. 2010.
See what “Vector control” is in other dictionaries:
Tracing paper with him. Vektorregelung. A method of controlling the rotation speed and/or torque of an electric motor using the influence of an electric drive converter on the vector components of the electric motor stator current. In Russian-language literature in ... Wikipedia
The solution to the optimal control problem of mathematical theory, in which the control action u=u(t) is formed in the form of a function of time (thereby it is assumed that during the process no information other than that given at the very beginning enters the system... ... Mathematical Encyclopedia
- (frequency controlled drive, PNC, Variable Frequency Drive, VFD) system for controlling the rotor speed of an asynchronous (or synchronous) electric motor. It consists of the electric motor itself and a frequency converter... Wikipedia
This term has other meanings, see CNC (meanings). This page is proposed to be merged with CNC. Explanation of reasons and discussion on the Wikipedia page: Toward unification/25 f... Wikipedia
Stator and rotor of an asynchronous machine 0.75 kW, 1420 rpm, 50 Hz, 230-400 V, 3.4 2.0 A An asynchronous machine is an alternating current electric machine ... Wikipedia
- (DPR) part of an electric motor. In commutator electric motors, the rotor position sensor is a brush commutator unit, which is also a current switch. In brushless electric motors, the rotor position sensor can be of different types... Wikipedia
DS3 DS3 010 Basic data Country of construction ... Wikipedia
An asynchronous machine is an alternating current electric machine, the rotor speed of which is not equal to (less than) the rotation speed of the magnetic field created by the stator winding current. Asynchronous machines are the most common electrical... ... Wikipedia
The most well-known method of saving energy is reducing the speed of the AC motor. Since power is proportional to the cube of the shaft speed, a small reduction in speed can lead to significant energy savings. Everyone understands how relevant this is for production. But how to achieve this? We will answer this and other questions, but first, let’s talk about the types of control of asynchronous motors.
The AC electric drive is an electromechanical system that serves as the basis for most technological processes. An important role in it belongs to the frequency converter (FC), which plays the main “playing of the main violin of the duet” – the asynchronous motor (IM).
A bit of elementary physics
From school, we have a clear idea that voltage is the potential difference between two points, and frequency is a value equal to the number of periods that the current manages to pass through literally in a second.
As part of the technological process, it is often necessary to change the operating parameters of the network. For this purpose, there are frequency converters: scalar and vector. Why are they called that? Let's start with the fact that the special features of each type become clear from their name. Let us remember the basics of elementary physics and allow ourselves to call the IF shorter for simplicity. “Vectornik” has a certain direction and obeys the rules of vectors. “Scalarnik” has none of this, so the algorithm for controlling it is naturally very simple. It seems the names have been decided. Now let's talk about how various physical quantities from mathematical formulas are related to each other.
Remember that as soon as the speed decreases, the torque increases and vice versa? This means that the greater the rotation of the rotor, the greater the flux will go through the stator, and, consequently, a greater voltage will be induced.
The same principle lies in the principle of operation in the systems we are considering, only in the “scalar” the magnetic field of the stator is controlled, and in the “vector” the interaction of the magnetic fields of the stator and rotor plays a role. In the latter case, the technology makes it possible to improve the technical parameters of the operation of the propulsion system.
Technical differences between converters
There are many differences, let’s highlight the most basic ones, and without a scientific web of words. For a scalar (sensorless) frequency driver, the U/F relationship is linear and the speed control range is quite small. By the way, this is why at low frequencies there is not enough voltage to maintain torque, and sometimes it is necessary to adjust the voltage-frequency characteristic (VFC) to the operating conditions, the same thing happens at a maximum frequency above 50 Hz.
When rotating the shaft in a wide speed and low-frequency range, as well as meeting the requirements for automatic torque control, the vector control method with feedback is used. This reveals another difference: the scalar usually does not have such feedback.
Which emergency situations to choose? The application of one or another device is mainly guided by the scope of use of the electric drive. However, in special cases, the choice of the type of frequency converter becomes choiceless. Firstly: there is a clear, noticeable difference in price (scalar ones are much cheaper, there is no need for expensive computing cores). Therefore, cheaper production sometimes outweighs the decision-making process. Secondly: there are areas of application in which only their use is possible, for example, in conveyor lines, where several electric motors are synchronously controlled from one (VFD).
Scalar method
An asynchronous electric drive with scalar speed control (i.e., VFC) remains the most common today. The basis of the method is that the motor speed is a function of the output frequency.
Scalar motor control is the optimal choice for cases where there is no variable load and there is no need for good dynamics. The scalar does not require any sensors to operate. When using this method, there is no need for an expensive digital processor, as is the case with vector control.
The method is often used for automatic control of fans, compressors and other units. Here it is required that either the rotation speed of the engine shaft is maintained using a sensor, or another specified indicator (for example, the temperature of the liquid, controlled by an appropriate tracking device).
With scalar control, the frequency-amplitude change in the supply voltage is determined by the formula U/fn = const. This allows for constant magnetic flux in the motor. The method is quite simple, easy to implement, but not without some significant drawbacks:
- It is not possible to simultaneously control torque and speed, so the value that is most significant from a technological point of view is selected;
- narrow speed control range and low torque at low speeds;
- poor performance with dynamically changing load.
What is the vector method?
Vector method
It arose in the process of improvement, and is used when it is necessary to realize maximum speed, regulation in a wide speed range and controllability of the torque on the shaft.
In the latest models of electric drives, a mathematical model of the engine is introduced into the control system (CS) of this type, which is capable of calculating the engine torque and shaft rotation speed. In this case, only the installation of stator phase current sensors is required.
Today they have a sufficient number of advantages:
- high accuracy;
- without jerking, smooth rotation of the blood pressure;
- wide range of regulation;
- quick response to load changes;
- ensuring the operating mode of the engine, in which losses due to heating and magnetization are reduced, and this leads to a cherished increase in efficiency!
The advantages are, of course, obvious, but the vector control method is not without its disadvantages, such as computational complexity and the need to know the technical indicators of the motor. In addition, larger amplitudes of speed fluctuations are observed than in the “scalar” under constant load. The main task in the manufacture of a frequency converter (“vector”) is to provide high torque at low rotation speed.
The diagram of a vector control system with a pulse-width modulation unit (PWM) looks something like this:
In the diagram shown, the controlled object is an asynchronous motor connected to a sensor (DS) on the shaft. The depicted blocks are actually links in the control system chain implemented on the controller. The BZP block sets the values of the variables. Logical blocks (BRP) and (BVP) regulate and calculate the variables of the equation. The controller itself and other mechanical parts of the system are located in the electrical cabinet.
Option with frequency microcontroller
The current/voltage frequency converter is designed for smooth regulation of basic quantities, as well as other indicators of equipment operation. It functions as a "scalar" and a "vector" at the same time, using mathematical models programmed in the built-in microcontroller. The latter is mounted in a special panel and is one of the nodes of the information network of the automation system.
The block controller/frequency converter is the latest technology; in the circuit with them, inductors are used, which reduce the intensity of input noise. It should be noted that special attention is paid to this issue abroad. In domestic practice, the use of EMC filters still remains a weak link, since there is not even a sensible regulatory framework. We use the filters themselves more often where they are not needed, and where they are really needed, for some reason they are forgotten about.
Conclusion
The fact is that an electric motor in normal operation from the network tends to have standard parameters; this is not always acceptable. This fact is eliminated by introducing various gear mechanisms to reduce the frequency to the required one. Today, two control systems have been formed: a sensorless system and a sensor system with feedback. Their main difference is the accuracy of control. The most accurate, of course, is the second.
The existing framework is expanded through the use of various modern IM control systems, providing improved quality of regulation and high overload capacity. These factors are of great importance for cost-effective production, long service life of equipment and economical energy consumption.
Dmitry Levkin
Scalar control(frequency) - a method of controlling brushless alternating current, which consists of maintaining a constant voltage/frequency ratio (V/Hz) throughout the entire operating speed range, while only controlling the magnitude and frequency of the supply voltage.
The V/Hz ratio is calculated based on the rating (and frequency) of the AC motor being monitored. By keeping the V/Hz ratio constant, we can maintain a relatively constant magnetic flux in the motor gap. If the V/Hz ratio increases then the motor becomes overexcited and vice versa if the ratio decreases the motor is in an underexcited state.
Changing the motor supply voltage with scalar control
At low speeds it is necessary to compensate for the voltage drop across the stator resistance, so the V/Hz ratio at low speeds is set higher than the nominal value. The scalar control method is most widely used to control asynchronous electric motors.
As applied to asynchronous motors
In the scalar control method, the speed is controlled by setting the stator voltage and frequency so that the magnetic field in the gap is maintained at the desired value. To maintain a constant magnetic field in the gap, the V/Hz ratio must be constant at different speeds.
As the speed increases, the stator supply voltage must also increase proportionally. However, the synchronous frequency of an asynchronous motor is not equal to the shaft speed, but depends on the load. Thus, a scalar open-loop control system cannot accurately control speed when a load is present. To solve this problem, speed feedback, and therefore slip compensation, can be added to the system.
Disadvantages of Scalar Control
- Method scalar control relatively simple to implement, but has several significant disadvantages:
- firstly, if a speed sensor is not installed, you cannot control the shaft rotation speed, since it depends on the load (the presence of a speed sensor solves this problem), and in the case of a change in load, you can completely lose control;
- secondly, it cannot be controlled. Of course, this problem can be solved using a torque sensor, but the cost of installing it is very high, and will most likely be higher than the electric drive itself. In this case, torque control will be very inertial;
- it is also impossible to control torque and speed at the same time.
Scalar control is sufficient for most tasks in which an electric drive is used with an engine speed control range of up to 1:10.
When maximum speed is required, the ability to regulate over a wide speed range and the ability to control the torque of the electric motor is used.
According to the latest statistics, approximately 70% of all electricity generated in the world is consumed by electric drives. And every year this percentage is growing.
With a correctly selected method of controlling an 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 operating electric motors consume a minimum of electricity and provide maximum efficiency.
For electric motors powered by an inverter, the efficiency will largely depend on the chosen method of controlling the electrical machine. Only by understanding the merits of each method can engineers and drive system designers get the maximum performance from 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 electric motor control consists of a sequence of commands entered using an interface from a control panel or PC. Yes, from the point of view of the general hierarchy of control of an automated system, this is correct, but there are also 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 main control methods:
- U/f – volts 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 width of the pulses 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) they generate output pulses. By varying the width of these pulses at the output, a “sinusoid” of the desired frequency is obtained. Even if the shape 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 control methods lies only in the method of calculating the voltage supplied to the electric 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 command signal. For inverters used in conventional electric drive systems, as a rule, it ranges from 0 Hz to 60 Hz. When signals of two frequencies are superimposed on each other, a signal will be issued to open the transistor (indicated in black), which supplies power voltage to the electric motor.
U/F control method
Volt-per-Hz control, most commonly referred to as U/F, is perhaps the simplest control method. 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 the mandatory installation of an encoder and mandatory settings for a variable-frequency electric drive (but is recommended). This leads to 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 constant torque over the entire speed range with 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 electrical machine.
U/F is the only way to regulate the speed of an asynchronous electric motor, which allows the control of several electric drives from one frequency converter. Accordingly, all machines start and stop simultaneously and operate at the same frequency.
But this control method has several limitations. For example, when using the U/F control method without an encoder, there is absolutely no certainty that the shaft of an asynchronous machine rotates. In addition, the starting torque of an electric machine at a frequency of 3 Hz is limited to 150%. Yes, the limited torque is more than enough to accommodate most existing equipment. For example, almost all fans and pumps use the U/F control method.
This method is relatively simple due to its looser specification. Speed regulation 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 changes in the reference frequency. The higher the response speed, the faster the electric drive will respond to changes in the speed setting.
The speed control range when using the U/F method is 1:40. By 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 value is 60 Hz and the range is 1:40, then the minimum frequency value will be 1.5 Hz.
The U/F pattern determines the relationship between frequency and voltage during operation of a variable frequency drive. According to it, the rotation speed setting curve (motor frequency) will determine, in addition to the frequency value, also the voltage value supplied to the terminals of the electric machine.
Operators and technicians can select the desired U/F control pattern with one parameter in a modern frequency converter. Pre-installed templates are already optimized for specific applications. There are also opportunities to create your own templates that will be optimized for a specific variable frequency drive or electric motor system.
Devices such as fans or pumps have a load torque that depends on their rotation speed. The variable torque (picture above) of the U/F pattern prevents control errors and improves efficiency. This control model reduces magnetizing currents at low frequencies by reducing the voltage on the electrical machine.
Constant torque mechanisms such as conveyors, extruders and other equipment use a constant torque control method. With constant load, full magnetizing current is required at all speeds. Accordingly, the characteristic has a straight slope throughout the entire speed range.
U/F control method with encoder
If it is necessary to increase the accuracy of rotation speed control, an encoder is added to the control system. The introduction of speed feedback using an encoder allows you to increase the control accuracy to 0.03%. The output voltage will still be determined by the specified U/F pattern.
This control method is not widely used, since the advantages it provides compared to standard U/F functions are minimal. Starting torque, response speed and speed control range are all identical to standard U/F. In addition, when operating frequencies increase, problems with the operation of the encoder may arise, since it has a limited number of revolutions.
Open-loop vector control
Open-loop vector control (VC) is used for broader and more dynamic speed control of an electrical machine. When starting from a frequency converter, electric motors can develop a starting torque of 200% of the rated torque at a frequency of only 0.3 Hz. This significantly 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's torque in all four quadrants.
The torque is limited by the motor. This is necessary to prevent damage to equipment, machinery or products. The value of torques is divided into four different quadrants, depending on the direction of rotation of the electric machine (forward or reverse) and depending on whether the electric motor implements . Limits can be set for each quadrant individually, or the user can set the overall torque in the frequency converter.
The motor mode of an 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 regenerative braking mode with energy release; in other words, the asynchronous motor will switch to generator mode.
For example, a bottle capping machine may use torque limiting in quadrant 1 (forward direction with positive torque) to prevent overtightening of a bottle cap. The mechanism moves forward and uses the positive torque to tighten the bottle cap. But a device such as an elevator with a counterweight heavier than the empty car will use quadrant 2 (reverse rotation and positive torque). If the cabin rises to the 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, since it is heavier than the cabin.
Current feedback in these frequency converters allows you to set limits on the torque and current of the electric motor, since as the current increases, the torque also increases. The output voltage of the inverter may increase if the mechanism requires more torque, or decrease if its maximum permissible value is reached. This makes the vector control principle of an asynchronous machine more flexible and dynamic compared to the U/F principle.
Also, frequency converters with vector control and open loop have a faster speed response of 10 Hz, which makes it possible to use it in mechanisms with shock loads. For example, in rock crushers, the load is constantly changing and depends on the volume and dimensions of the rock being processed.
Unlike the U/F control pattern, vector control uses a vector algorithm to determine the maximum effective operating voltage of the electric motor.
Vector control of the VU solves this problem due to the presence of feedback on the motor current. As a rule, current feedback is generated by the internal current transformers of the frequency converter itself. Using 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 vector of magnetizing current (I d) and torque (I q).
Using the data and parameters of the electrical machine, the inverter calculates the vectors of the magnetizing current (I d) and torque (I q). To achieve maximum performance, the frequency converter must keep I d and I q separated by an angle of 90 0. This is significant because sin 90 0 = 1, and a value of 1 represents the maximum torque value.
In general, 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 can maintain torque when running at low speeds.
Vector feedback control
Feedback vector control uses the same control algorithm as open-loop VAC. The main difference is the presence of an encoder, which allows the variable frequency drive to develop 200% starting torque at 0 rpm. This point is simply necessary to create an initial moment when moving off elevators, cranes and other lifting machines, in order to prevent subsidence of the load.
The presence of a speed feedback sensor allows you to increase the system response time to more than 50 Hz, as well as expand the speed control range to 1:1500. Also, the presence of feedback allows you to control not the speed of the electric machine, but the torque. In some mechanisms, it is the torque value that is of great importance. For example, winding machine, clogging mechanisms and others. In such devices it is necessary to regulate the torque of the machine.
To implement the ability to regulate torque and speed, modern electric drives use the following frequency control methods, such as:
- Vector;
- Scalar.
The most widespread are asynchronous electric drives with scalar control. It is used in drives of compressors, fans, pumps and other mechanisms in which it is necessary to maintain at a certain level either the rotation speed of the electric motor shaft (a speed sensor is used), or some technological parameter (for example, pressure in a pipeline, using an appropriate sensor).
The operating principle of scalar control of an asynchronous motor is that the amplitude and frequency of the supply voltage change according to the law U/f^n = const, where n>=1. How this dependence will look in a particular case depends on the requirements imposed by the load on the electric drive. As a rule, frequency acts as an independent influence, and the voltage at a certain frequency is determined by the type of mechanical characteristic, as well as the values of the critical and starting torques. Thanks to scalar control, a constant overload capacity of an asynchronous motor is ensured, independent of the voltage frequency, and yet at fairly low frequencies a significant reduction in the torque developed by the motor can occur. The maximum value of the scalar control range at which it is possible to regulate the rotation speed of the electric motor rotor without losing the moment of resistance does not exceed 1:10.
Scalar control of an induction motor is quite simple to implement, but there are still two significant drawbacks. Firstly, if a speed sensor is not installed on the shaft, then it is impossible to regulate the value of the shaft rotation speed, since it depends on the load acting on the electric drive. Installing a speed sensor easily solves this problem, but another significant drawback remains - the inability to regulate the torque value on the motor shaft. You can, of course, install a torque sensor, but the cost of such sensors, as a rule, exceeds the cost of the electric drive itself. Moreover, even if you install a torque control sensor, the process of controlling this very torque will turn out to be incredibly inertial. Another “but” - scalar control of an asynchronous motor is characterized by the fact that it is impossible to simultaneously regulate speed and torque, so it is necessary to regulate the value that is most important at a given time due to the conditions of the technological process.
In order to eliminate the shortcomings of scalar motor control, back in the 71st year of the last century, SIEMENS proposed the introduction of a vector motor control method. The first electric drives with vector control used motors that had built-in flow sensors, which significantly limited the scope of such drives.
The control system of modern electric drives contains a mathematical model of the engine, which allows one to calculate the rotation speed and shaft torque. Moreover, only motor stator phase current sensors are installed as necessary sensors. The specially designed structure of the control system ensures independence and virtually inertia-free control of the main parameters - shaft torque and shaft rotation speed.
To date, the following vector control systems for asynchronous motors have been formed:
- Sensorless – there is no speed sensor on the motor shaft,
- Systems with speed feedback.
The use of vector control methods depends on the application of the electric drive. If the speed measurement range does not exceed 1:100, and the accuracy requirements vary within ±1.5%, then a sensorless control system is used. If the speed measurement is carried out in the range of values reaching 1: 10000 or more, and the level of accuracy must be quite high (±0.2% at speeds below 1 Hz), or it is necessary to position the shaft or control the torque on the shaft at low speeds , then a system with speed feedback is used.
Advantages of the vector method of controlling an asynchronous motor:
- High level of accuracy when regulating shaft speed, despite the possible absence of a speed sensor,
- The engine rotates at low frequencies without jerking, smoothly,
- If a speed sensor is installed, it is possible to achieve the nominal value of the torque on the shaft even at zero speed,
- Quick response to possible load changes - sudden load surges have virtually no effect on the speed of the electric drive,
- High level of motor efficiency due to reduced losses due to magnetization and heating.
Despite the obvious advantages, the vector control method also has certain disadvantages - greater complexity of calculations; knowledge of the motor parameters is required for operation. In addition, fluctuations in the speed value at a constant load are much greater than with the scalar control method. By the way, there are areas where electric drives are used exclusively with a scalar control method. For example, a group electric drive in which one converter powers several motors.
