Dodge

Scalar and vector control. Principles of vector control of an asynchronous motor. As applied to asynchronous motors

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

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. The functional diagram of 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.

Dmitry Levkin

main idea vector control is to control not only the magnitude and frequency of the supply voltage, but also the phase. In other words, the magnitude and angle of the spatial vector are controlled. Vector control has higher performance compared to. Vector control eliminates almost all the disadvantages of scalar control.

Advantages of vector control:

high accuracy of speed control;
smooth start and smooth rotation of the engine over the entire frequency range;
quick response to load changes: when the load changes, there is practically no change in speed;
increased control range and control accuracy;
losses due to heating and magnetization are reduced, and .

The disadvantages of vector control include:

the need to set parameters;
large speed fluctuations at constant load;
high computational complexity.

General functional diagram of vector control

The general block diagram of a high performance AC speed control system is shown in the figure above. The basis of the circuit is the magnetic flux linkage and torque control circuits together with an evaluation unit, which can be implemented in various ways. In this case, the external speed control loop is largely unified and generates control signals for the torque controllers M * and the magnetic flux linkage Ψ * (via the flow control unit). Motor speed can be measured by a (speed/position) sensor or obtained through an estimator allowing implementation.

Classification of vector control methods

Since the seventies of the twentieth century, many methods of torque control have been proposed. Not all of them are widely used in industry. Therefore, this article discusses only the most popular management methods. The torque control methods discussed are presented for control systems with sinusoidal back EMF.

Existing torque control methods can be classified in various ways.

linear (PI, PID) regulators;
nonlinear (hysteresis) regulators.

Speed control range	Speed error 3,%	Torque rise time, ms	Starting torque	Price	Description
1:10 1	5-10	Not available	Short	Very low	It has a slow response to load changes and a small speed control range, but is easy to implement.
>1:200 2	0		High	High	Allows you to smoothly and quickly control the main engine parameters - torque and speed. For this method to work, information about the rotor position is required.
>1:200 2	0		High	High	A hybrid method designed to combine the advantages of...
>1:200 2	0		High	High	It has high dynamics and a simple circuit, but a characteristic feature of its operation is high current and torque ripples.
>1:200 2	0		High	High	It has an inverter switching frequency lower than other methods and is designed to reduce losses when controlling high-power electric motors.

Note:

No feedback.
With feedback.
In steady state

Among vector control, the most widely used are (FOC - field oriented control) and (DTC - direct torque control).

Linear torque regulators

Linear torque controllers work in conjunction with pulse width modulation (PWM) of the voltage. The regulators determine the required stator voltage vector averaged over the sampling period. The voltage vector is finally synthesized by the PWM method; in most cases, space vector modulation (SVM) is used. Unlike nonlinear torque control circuits, where signals are processed using instantaneous values, in linear torque control circuits, a linear controller (PI) operates with values averaged over the sampling period. Therefore, the sampling frequency can be reduced from 40 kHz in nonlinear torque controller circuits to 2-5 kHz in linear torque controller circuits.

(POA, English field oriented control, FOC) is a control method that controls a brushless alternating current (,) like a direct current machine with independent excitation, implying that the field and can be controlled separately.

Field-oriented control, proposed in 1970 by Blaschke and Hasse, is based on an analogy with mechanically switched control. In this motor, the field and armature windings are separated, the flux linkage is controlled by the field current, and the torque is independently controlled by current regulation. Thus, the flux linkage and torque currents are electrically and magnetically separated.

General functional diagram of sensorless field-oriented control 1

On the other hand, brushless AC motors ( , ) most often have a three-phase stator winding, and the stator current vector I s is used to control both flux and torque. Thus, the field current and armature current merged into the stator current vector and cannot be controlled separately. Disconnection can be achieved mathematically - by decomposing the instantaneous value of the stator current vector I s into two components: the longitudinal component of the stator current I sd (creating the field) and the transverse component of the stator current I sq (creating torque) in a rotating dq coordinate system oriented along the rotor field (R -FOC – rotor flux-oriented control) - picture above. Thus, control of a brushless AC motor becomes identical to control and can be achieved using a PWM inverter with a linear PI regulator and space vector voltage modulation.

In field-oriented control, torque and field are controlled indirectly by controlling the stator current vector components.

The instantaneous values of the stator currents are converted to the dq rotating coordinate system using the Park transformation αβ/dq, which also requires information about the rotor position. The field is controlled through the longitudinal current component I sd , while the torque is controlled through the transverse current component I sq . The inverse Park transform (dq/αβ), a mathematical coordinate transformation module, allows one to calculate the reference components of the voltage vector V sα * and V sβ * .

To determine the rotor position, either a rotor position sensor installed in the electric motor or a sensorless control algorithm implemented in the control system is used, which calculates information about the rotor position in real time based on the data available in the control system.

A block diagram of direct torque control with space-vector modulation with torque and flux linkage adjustment with feedback operating in a rectangular coordinate system oriented along the stator field is shown in the figure below. The outputs of the PI torque and flux linkage controllers are interpreted as the reference components of the stator voltage V ψ * and V M * in the dq coordinate system oriented along the stator field (English stator flux-oriented control, S-FOC). These commands (constant voltages) are then converted into a fixed coordinate system αβ, after which the control values V sα * and V sβ * are sent to the space vector modulation module.

Functional diagram of direct torque control with space vector voltage modulation

Please note that this circuit can be considered as a simplified stator field-oriented control (S-FOC) without a current control loop or as a classic circuit (PUM-TV, English switching table DTC, ST DTC) in which the switching table is replaced by a modulator (SVM ), and the hysteresis torque and flux controller are replaced by linear PI controllers.

In direct torque control with space vector modulation (DTC-FCM), the torque and flux linkage are directly controlled in a closed loop, so accurate estimation of the motor flux and torque is necessary. Unlike the classic hysteresis algorithm, it operates at a constant switching frequency. This significantly improves the performance of the control system: it reduces torque and flow pulsations, allowing you to confidently start the engine and operate at low speeds. But at the same time, the dynamic characteristics of the drive are reduced.

Direct self-government

A patent application for the direct self-government method was filed by Depenbrock in October 1984. The block diagram of direct self-government is shown below.

Based on the stator flux linkage commands ψ s * and the current phase components ψ sA , ψ sB and ψ sC , the flux linkage comparators generate digital signals d A , d B and d C , which correspond to the active voltage states (V 1 – V 6). The hysteretic torque controller has an output signal d M, which determines the zero states. Thus, the stator flux linkage regulator sets the time interval of active voltage states that move the stator flux linkage vector along a given path, and the torque regulator determines the time interval of zero voltage states that maintain the torque of the electric motor in a tolerance field determined by hysteresis.

Direct self-government scheme

non-sinusoidal forms of flux linkage and stator current;
the stator flux linkage vector moves along a hexagonal trajectory;
there is no supply voltage reserve, the inverter’s capabilities are fully used;
the inverter switching frequency is lower than that of direct torque control with a switching table;
excellent dynamics in the constant and weakened field ranges.

Note that the performance of the direct self-control method can be reproduced using a circuit with a flux hysteresis width of 14%.

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

scalar control

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.

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.

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.