Scalar frequency control applied to asynchronous motors. Advantages of vector control of an asynchronous motor Frequency converter vector control

Dmitry Levkin

Scalar control(frequency) - brushless control method alternating current, which consists in maintaining a constant voltage/frequency ratio (V/Hz) throughout the entire operating speed range, while only the magnitude and frequency of the supply voltage are controlled.

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.

For the purpose of adjustment angular velocity rotor rotation, as well as the torque on the shaft of modern brushless motors, either vector or scalar control of the electric drive is used.

Scalar control of an asynchronous motor has become most widespread, when to control, for example, the rotation speed of a fan or pump, it is enough to keep the rotor rotation speed constant; for this, a feedback signal from a pressure sensor or a speed sensor is sufficient.

The principle of scalar control is simple: the amplitude of the supply voltage is a function of frequency, and the ratio of voltage to frequency is approximately constant.

The specific type of this dependence is associated with the load on the shaft, but the principle remains the same: we increase the frequency, and the voltage increases proportionally depending on the load characteristic of this engine.

As a result, the magnetic flux in the gap between the rotor and stator is maintained almost constant. If the voltage-to-frequency ratio deviates from the nominal one for a given motor, then the motor will either be overexcited or underexcited, which will lead to losses in the motor and disruptions in the working process.

Thus, scalar control makes it possible to achieve an almost constant torque on the shaft in the operating frequency range, regardless of frequency, however, at low speeds the torque still decreases (to prevent this from happening, it is necessary to increase the voltage-to-frequency ratio), so for each motor there is a strictly defined operating scalar control range.

In addition, it is impossible to build a scalar speed control system without a speed sensor mounted on the shaft, because the load greatly affects the lag of the actual rotor rotation speed from the supply voltage frequency. But even with a speed sensor, with scalar control it will not be possible to regulate the torque with high accuracy (at least in a way that would be economically feasible).

This is where the shortcomings of scalar control lie, explaining the relative small number of areas of its application, limited mainly to conventional asynchronous motors, where the dependence of slip on load is not critical.

To get rid of these shortcomings, back in 1971, Siemens engineers proposed using vector motor control, in which control is carried out with feedback by the magnitude of the magnetic flux. The first vector control systems contained flux sensors in the motors.

Today, the approach to this method is somewhat different: a mathematical model of the engine allows you to calculate the rotor rotation speed and shaft torque depending on the current phase currents (on the frequency and magnitude of the currents in the stator windings).

This more progressive approach makes it possible to independently and almost inertia-free regulate both the torque on the shaft and the speed of rotation of the shaft under load, because the control process also takes into account the phases of the currents.

Some more precise vector control systems are equipped with speed feedback circuits, and control systems without speed sensors are called sensorless.

So, depending on the area of ​​application of a particular electric drive, its vector control system will have its own characteristics and its own degree of adjustment accuracy.

When the requirements for speed control accuracy allow a deviation of up to 1.5%, and the adjustment range does not exceed 1 to 100, then a sensorless system is quite suitable. If speed control accuracy is required with a deviation of no more than 0.2%, and the range is reduced to 1 in 10,000, then feedback from the speed sensor on the shaft is necessary. The presence of a speed sensor in vector control systems allows you to accurately adjust the torque even when low frequencies up to 1 Hz.

So, vector control provides the following advantages. High precision control of the rotor rotation speed (and without a speed sensor on it) even under conditions of dynamically changing load on the shaft, and there will be no jerks. Smooth and even shaft rotation at low speeds. High efficiency due to low losses under conditions of optimal supply voltage characteristics.

Vector control is not without its drawbacks. Complexity of computational operations. The need to set initial data (variable drive parameters).

For a group electric drive, vector control is fundamentally unsuitable; scalar control is better suited here.

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, efficiency will largely depend on the chosen control method electric 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 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 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 a frequency converter by using fixed voltage tires direct current. by quick opening and closings (more correctly, switching) generate output pulses. By varying the width of these pulses, a “sine wave” is obtained at the output. required frequency. 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 mandatory installation encoder and mandatory settings for variable frequency drive (but 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 maximum value frequency 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. At constant load it is necessary full current magnetization 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 may arise with the operation of the encoder, since it has limited quantity rpm

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 rate of ascent and prevent free fall counterweight, 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 change upward if the mechanism requires the application of greater torque, or decrease if its maximum torque is reached. permissible value. This makes the principle of vector control 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.

Any change or maintenance of a constant speed of the electric drive provides targeted regulation of the torque developed by the engine. The torque is formed as a result of the interaction of the flow (flux linkage) created by one part of the motor with the current in the other part and is determined by the vector product of these two spatial torque-generating vectors. Therefore, the magnitude of the torque developed by the engine is determined by the modules of each vector and the spatial angle between them.

When building scalar control systems Only the numerical values ​​(modules) of the torque-generating vectors were controlled and regulated, but their spatial position was not controlled. Vector control principle lies in the fact that the control system controls the numerical value and position in space relative to each other of the torque-generating vectors. Hence, the task of vector control is to determine and forcefully establish instantaneous current values ​​in the motor windings in such a way that the generalized vectors of currents and flux linkages occupy a position in space that ensures the creation of the required electromagnetic torque.

Electromagnetic torque generated by the motor:

where m is the design factor; , 2 - spatial

vectors of currents or flux linkages that form torque; X- spatial angle between moment-generating vectors.

As follows from (6.53), the minimum values ​​of currents (flux linkages) forming the torque will be for the required torque value if the vectors X and 2 are perpendicular to each other, i.e. X = °.

In vector control systems, there is no need to determine the absolute spatial position of the vectors, and 2 in relation to the stator or rotor axes. It is necessary to determine the position of one vector relative to another. Therefore, one of the vectors is taken to be base, and the position of the other controls the angle X.

Based on this, when constructing vector control systems, it is advisable to proceed from a mathematical description of electromagnetic and electromechanical processes expressed in coordinates tied to the base vector (coordinates And- v). Such a mathematical description is given in § 1.6.

If we take as the base vector and direct the coordinate axis And along this vector, then, based on (1.46), we obtain the following system of equations:

In these equations? v = , since the vector coincides with the coordinate axis And.

In Fig. Figure 6.31 shows a vector diagram of currents and flow linkages in the axes And- v ^coordinate orientation And along the rotor coupling vector. From the vector diagram it follows that

Rice. B.31. Vector diagram of flux linkages and currents in axes u-v at M

With constant (or slow change) p rotor clutch d"V u /dt= resulting in i and = And Г = yji u +i v = i v

In this case, the rotor current vector G perpendicular to the rotor flux linkage. Since the rotor leakage flux 0 is significantly less than the flux in the machine gap H, t then, if the rotor flux linkage is constant, we can assume that the projection of the stator current vector onto the coordinate axis v i v equal to |/"| or /

The advantage of the adopted coordinate system u-v for constructing a system of vector control of torque and speed of an asynchronous motor is that the motor torque (6.54) is defined as the scalar product of two mutually perpendicular vectors: the rotor flux linkage *P and the active component of the stator current. This definition of torque is typical, for example, for DC motors independent excitation, most convenient for constructing an automatic control system.

Vector control system. The structural diagram of such management is based on the following principles:

• ? a two-channel control system consists of a channel for stabilizing the rotor flux linkage and a channel for regulating speed (torque);
• ? both channels must be independent, i.e. changes in the regulated values ​​of one channel should not affect the other;
• ? the speed (torque) control channel controls the stator current component /v. The algorithm for the operation of the torque control loop is the same as in systems of slave speed control of DC motors (see § 5.6) - the output signal of the speed controller is a reference to the motor torque. By dividing the value of this task by the rotor flux linkage module And we get the task for the stator current component i v (Fig. 6.32);
• ? each channel contains an internal circuit of currents /v and i and with current regulators that provide the required quality of regulation;
• ? obtained current values i v and i and through coordinate transformations are converted into values i a and / p of a two-phase fixed coordinate system a - (3 and then in the task of real currents in the stator windings in a three-phase coordinate system a-b-c;
• ? The signals of speed, rotor rotation angle, and currents in the stator windings necessary for calculations and feedback formation are measured by appropriate sensors and then, using inverse coordinate transformations, are converted into the values ​​of these quantities corresponding to the coordinate axes u-v.

Rice.

Such a control system provides high-speed control of torque, and, consequently, speed in the widest possible range (over 10,000:1). In this case, the instantaneous torque values ​​of an asynchronous motor can significantly exceed the nominal value of the critical torque.

In order to make the control channels independent of each other, it is necessary to introduce cross compensating signals e K0MPU and e compm at the input of each channel (see Fig. 6.32). We find the value of these signals from the stator circuit equations (6.54). Having expressed and CHK 1y through the corresponding currents and inductances (1.4) and taking into account that when the axis is oriented And along the rotor flux linkage vector Х / |у =0 we obtain:

Where do we find it from?

Where dissipation coefficient.

Substituting (6.55) into (6.54) and taking into account that in the control system under consideration d x V 2u /dt = 0, we get

or

new time constants; e and e v - EMF of rotation along the axes u-v

To set independent quantities i and and /v needs to be compensated e and And e v introduction of compensating voltages:

To implement the principles of vector control, it is necessary to directly measure or calculate using a mathematical model (estimate) the module and angular position of the rotor flux linkage vector. Functional diagram vector control of an asynchronous motor with direct measurement of the flow in the air gap of the machine using Hall sensors is shown in Fig. 6.33.

Rice. B.ZZ. Functional diagram of direct vector control of an asynchronous motor

The circuit contains two control channels: a control (stabilization) channel for the rotor flux linkage *P 2 and a speed control channel. The first channel contains an external rotor flux linkage loop containing a PI flux linkage controller RP and flux linkage feedback, the signal of which is generated using Hall sensors that measure the flow in the machine gap X? T along the axes ai(3. The real flux values ​​are then recalculated in the PP block into the values ​​of the rotor flux linkage along the axes a and p and using the vector filter VF, the modulus of the rotor flux linkage vector is found, which is supplied as a negative feedback signal to the flux linkage regulator RP and is used in as a divider in the speed control channel.

In the first channel, the internal current circuit is subordinated to the flux linkage circuit i and, containing a PI current regulator PT1 and feedback on the actual value of the current / 1i, calculated from the real values ​​of the stator phase currents using the phase converter PF2 and the coordinate converter KP1. The output of the current regulator PT1 is the voltage setting Ulu, to which the compensation signal of the second channel is added e kshpi(6.57). The received voltage setting signal is converted by means of coordinate KP2 and phase PF2 converters into specified values ​​and voltage phases at the output of the frequency converter.

The rotor flux linkage control channel ensures that the flux linkage Ch* 2 remains constant in all drive operating modes at the level of the specified value x P 2set. If it is necessary to weaken the field, H*^ can vary within certain limits with a small rate of change.

The second channel is designed to regulate the speed (torque) of the engine. It contains an external speed loop and a subordinate internal current loop / 1у. The speed command comes from the intensity generator, which determines the acceleration and the required speed value. Speed ​​feedback is implemented via a DS speed sensor or a rotor angular position sensor.

The PC speed controller is adopted as proportional or proportional-integral, depending on the requirements for the electric drive. The output of the speed controller is the command for the torque developed by the L/R engine. Since the torque is equal to the product of the current by the rotor flux linkage H / 2, then by dividing the torque setting value in the DB division block M back on Ch / 2, we obtain the current setting value, which is supplied to the input of the current regulator PT2. Further signal processing is similar to the first channel. As a result, we obtain a task for the motor supply voltage by phase, which determines the value and spatial position at each moment of time of the generalized stator voltage vector!? Note that the signals relating to variables in the - coordinates are direct current signals, and the signals reflecting currents and voltages in the air coordinates are alternating current signals that determine not only the module, but the frequency and phase of the corresponding voltage and current.

The considered vector control system is currently implemented in digital form on the basis of microprocessors. Various structural vector control schemes have been developed and are widely used, differing in detail from the one under consideration. Thus, at present, the actual values ​​of flux linkages are not measured by magnetic flux sensors, but are calculated using a mathematical model of the motor, based on measured phase currents and voltages.

In general, vector control can be assessed as the most effective way to control AC motors, providing high accuracy and speed of control.

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.

AC electric drive is electromechanical system, which 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.

Within technological process Often you have 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 technical specifications 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 – optimal choice for cases where there is no variable load, and there is also 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 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

A frequency converter current/voltage 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 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. For cost-effective production, long equipment life and economical consumption energy, these factors are of great importance.