Frequency converters

Since the late 1960s, frequency converters have changed dramatically, largely as a result of the development of microprocessor and semiconductor technologies and their falling costs.

However, the fundamental principles inherent in frequency converters remain the same.

Frequency converters include four main elements:

Rice. 1. Block diagram of frequency converter

1. The rectifier generates a pulsating DC voltage when it is connected to a single/three-phase AC power supply. There are two main types of rectifiers - controlled and uncontrolled.

2.An intermediate circuit of one of three types:

a) converting the rectifier voltage into D.C..

b) stabilizing or smoothing the pulsating DC voltage and supplying it to the inverter.

c) converting the constant DC voltage of the rectifier into a changing AC voltage.

3. An inverter that generates the voltage frequency of the electric motor. Some inverters can also convert constant DC voltage to varying AC voltage.

4. Electronic circuit control, which sends signals to the rectifier, intermediate circuit and inverter and receives signals from these elements. The construction of controlled elements depends on the design of the specific frequency converter (see Fig. 2.02).

Common to all frequency converters is that all control circuits control the semiconductor elements of the inverter. Frequency converters differ in the switching mode used to regulate the motor supply voltage.

In Fig. 2, which shows the various principles of construction/control of the converter, the following notations are used:

1- controlled rectifier,

2- uncontrolled rectifier,

3- intermediate circuit of varying direct current,

4- intermediate circuit constant voltage DC

5- intermediate circuit of varying direct current,

6- inverter with pulse amplitude modulation (PAM)

7- inverter with pulse width modulation (PWM)

Current inverter (IT) (1+3+6)

Converter with pulse amplitude modulation (PAM) (1+4+7) (2+5+7)

Pulse width modulation converter (PWM/VVCplus) (2+4+7)

Rice. 2. Various principles building/controlling frequency converters

For completeness, direct converters that do not have an intermediate circuit should be mentioned. Such converters are used in the megawatt power range to generate a low-frequency supply voltage directly from a 50 Hz network, with a maximum output frequency of about 30 Hz.

Rectifier

The mains supply voltage is three-phase or single-phase AC voltage with a fixed frequency (for example, 3x400 V/50 Hz or 1 x 240 V/50 Hz); The characteristics of these voltages are illustrated in the figure below.

Rice. 3. Single-phase and three-phase AC voltage

In the figure, all three phases are displaced in time, the phase voltage constantly changes direction, and the frequency indicates the number of periods per second. A frequency of 50 Hz means that there are 50 periods per second (50 x T), i.e. one period lasts 20 milliseconds.

The rectifier of the frequency converter is built either on diodes, or on thyristors, or on a combination of both. A rectifier built on diodes is uncontrolled, while a rectifier built on thyristors is controlled. If both diodes and thyristors are used, the rectifier is semi-controlled.

Uncontrolled rectifiers

Rice. 4. Diode operating mode.

Diodes allow current to flow in only one direction: from the anode (A) to the cathode (K). As with some other semiconductor devices, the diode current cannot be adjusted. The AC voltage is converted by the diode into a pulsating DC voltage. If an uncontrolled three-phase rectifier is powered by three-phase AC voltage, then in this case the DC voltage will pulsate.

Rice. 5. Uncontrolled rectifier

In Fig. Figure 5 shows an uncontrolled three-phase rectifier containing two groups of diodes. One group consists of diodes D1, D3 and D5. The other group consists of diodes D2, D4 and D6. Each diode conducts current for a third of the period's time (120°). In both groups, diodes conduct current in a certain sequence. The periods during which both groups work are shifted from each other by 1/6 of the time of the T period (60°).

Diodes D1,3,5 are open (conducting) when a positive voltage is applied to them. If the voltage of phase L reaches a positive peak value, then diode D is open and terminal A receives the voltage of phase L1. The other two diodes will be affected by reverse voltages of magnitude U L1-2 and U L1-3

The same thing happens in the group of diodes D2,4,6. In this case, terminal B receives a negative phase voltage. If at the moment phase L3 reaches the maximum negative value, diode D6 is open (conducting). Both other diodes are affected by reverse voltages of magnitude U L3-1 and U L3-2

The output voltage of the uncontrolled rectifier is equal to the difference in voltages of these two diode groups. The average value of ripple DC voltage is 1.35 x mains voltage.

Rice. 6. Output voltage of uncontrolled three-phase rectifier

Controlled rectifiers

In controlled rectifiers, diodes are replaced by thyristors. Like a diode, a thyristor passes current in only one direction - from the anode (A) to the cathode (K). However, in contrast to the diode, the thyristor has a third electrode called the “gate” (G). In order for the thyristor to open, a signal must be applied to the gate. If there is current flowing through the thyristor, the thyristor will pass it until the current becomes zero.

The current cannot be interrupted by applying a signal to the gate. Thyristors are used in both rectifiers and inverters.

A control signal a is supplied to the thyristor gate, which is characterized by a delay expressed in degrees. These degrees cause a delay between the moment the voltage crosses zero and the time when the thyristor is open.

Rice. 7. Thyristor operating mode

If the angle a is in the range from 0° to 90°, then the thyristor circuit is used as a rectifier, and if in the range from 90° to 300°, then as an inverter.

Rice. 8. Controlled three-phase rectifier

A controlled rectifier is basically no different from an uncontrolled rectifier, except that the thyristor is controlled by the signal a and begins to conduct from the moment when a conventional diode begins to conduct until the moment that is 30 ° later than the point where the voltage crosses zero.

Adjusting the value of a allows you to change the magnitude of the rectified voltage. The controlled rectifier generates a constant voltage, the average value of which is 1.35 x mains voltage x cos α

Rice. 9. Output voltage of controlled three-phase rectifier

Compared to an uncontrolled rectifier, a controlled one has more significant losses and introduces higher noise into the power supply network, since with a shorter transmission time of the thyristors, the rectifier takes more reactive current from the network.

The advantage of controlled rectifiers is their ability to return energy to the supply network.

Intermediate chain

The intermediate circuit can be thought of as a storage facility from which the electric motor can draw energy through an inverter. Depending on the rectifier and inverter, three principles for constructing an intermediate circuit are possible.

Inverters - current sources (1-converters)

Rice. 10. Variable DC intermediate circuit

In the case of inverters - current sources, the intermediate circuit contains a large inductance coil and is interfaced only with a controlled rectifier. The inductor converts the varying voltage of the rectifier into a varying direct current. The voltage of the electric motor is determined by the load.

Inverters - voltage sources (U-converters)

Rice. 11. DC voltage intermediate circuit

In the case of inverters - voltage sources, the intermediate circuit is a filter containing a capacitor, and can be interfaced with a rectifier of either of two types. The filter smoothes out the pulsating DC voltage (U21) of the rectifier.

In a controlled rectifier, the voltage at a given frequency is constant and is supplied to the inverter as a true DC voltage (U22) with varying amplitude.

In uncontrolled rectifiers, the voltage at the inverter input is a constant voltage with a constant amplitude.

Intermediate circuit of variable direct voltage

Rice. 12. Variable voltage intermediate circuit

In intermediate circuits of varying DC voltage, you can turn on a breaker in front of the filter, as shown in Fig. 12.

The chopper contains a transistor that acts as a switch, turning the rectifier voltage on and off. The control system controls the chopper by comparing the changing voltage after the filter (U v) with the input signal. If there is a difference, the ratio is adjusted by changing the time the transistor is on and the time it is off. This changes the effective value and magnitude of the constant voltage, which can be expressed by the formula

U v = U x t on / (t on + t off)

When the chopper transistor opens the current circuit, the filter inductor makes the voltage across the transistor infinitely large. To avoid this, the breaker is protected by a fast-switching diode. When the transistor opens and closes as shown in Fig. 13, the voltage will be highest in mode 2.

Rice. 13. The chopper transistor controls the intermediate circuit voltage

The intermediate circuit filter smoothes the square wave voltage after the chopper. The capacitor and filter inductor maintain a constant voltage at a given frequency.

Depending on the design, the intermediate circuit can also perform additional functions, which include:

Isolation of the rectifier from the inverter

Harmonic Reduction

Energy storage to limit intermittent load surges.

Inverter

The inverter is the last link in the frequency converter before the electric motor and the place where the final adaptation of the output voltage occurs.

The frequency converter provides normal operating conditions throughout the entire control range by adapting the output voltage to the load condition. This allows you to maintain optimal magnetization of the motor.

From the intermediate circuit the inverter receives

Variable direct current,

Varying DC voltage or

Constant DC voltage.

Thanks to the inverter, in each of these cases a changing quantity is supplied to the electric motor. In other words, the inverter always creates the desired frequency of the voltage supplied to the electric motor. If the current or voltage is variable, the inverter produces only the desired frequency. If the voltage is constant, the inverter creates both the desired frequency and the desired voltage for the motor.

Even though inverters operate in different ways, their basic structure is always the same. The main elements of inverters are controlled semiconductor devices, connected in pairs in three branches.

Currently, thyristors are in most cases replaced by high-frequency transistors, which are capable of opening and closing very quickly. The switching frequency usually ranges from 300 Hz to 20 kHz and depends on the semiconductor devices used.

The semiconductor devices in the inverter are opened and closed by signals generated by the control circuit. Signals can be generated in several different ways.

Rice. 14. Conventional variable voltage intermediate circuit current inverter.

Conventional inverters, which mainly switch the intermediate circuit current of varying voltage, contain six thyristors and six capacitors.

Capacitors allow the thyristors to open and close in such a way that the current in the phase windings is shifted by 120 degrees and must be adapted to the size of the electric motor. When current is periodically applied to the motor terminals U-V sequences, V-W, W-U, U-V..., an intermittent rotating magnetic field of the required frequency appears. Even if the motor current is almost rectangular shape, the motor voltage will be almost sinusoidal. However, when the current is turned on or off, voltage surges always occur.

The capacitors are separated from the load current of the electric motor by diodes.

Rice. 15. Inverter for variable or constant voltage of the intermediate circuit and the dependence of the output current on the switching frequency of the inverter

Inverters with variable or constant intermediate circuit voltage contain six switching elements and, regardless of the type of semiconductor devices used, operate almost identically. The control circuit opens and closes semiconductor devices using several in various ways modulation, thereby changing the output frequency of the frequency converter.

The first method is for varying voltage or current in the intermediate circuit.

The intervals during which individual semiconductor devices are open are arranged in a sequence used to obtain the required output frequency.

This semiconductor switching sequence is controlled by the magnitude of the varying intermediate circuit voltage or current. By using a voltage controlled oscillator, the frequency always tracks the voltage amplitude. This type of inverter control is called pulse amplitude modulation (PAM).

For a fixed intermediate circuit voltage, a different basic method is used. The motor voltage becomes variable by applying intermediate circuit voltage to the motor windings for longer or shorter periods of time.

Rice. 16 Modulation of pulse amplitude and duration

The frequency is changed by changing the voltage pulses along the time axis - positively during one half-cycle and negatively during the other.

Since this method changes the duration (width) of the voltage pulses, it is called pulse width modulation (PWM). PWM modulation (and related methods such as sine-wave controlled PWM) is the most common method of inverter control.

In PWM modulation, the control circuit determines when semiconductor devices switch at the intersection of a ramp voltage and a superimposed sinusoidal reference voltage (sine-controlled PWM). Other promising PWM modulation methods are modified pulse width modulation methods such as WC and WC plus, developed by Danfoss Corporation.

Transistors

Since transistors can switch at high speeds, the electromagnetic interference that occurs when the motor is “pulsed” (magnetized) is reduced.

Another advantage of high switching frequency is the flexibility of modulating the output voltage of the frequency converter, which allows the generation of sinusoidal motor current, while the control circuit must simply turn on and off the inverter transistors.

The inverter switching frequency is a double-edged sword, as high frequencies can cause the motor to heat up and generate large voltage peaks. The higher the switching frequency, the higher the losses.

On the other hand, low switching frequency can result in high acoustic noise.

High-frequency transistors can be divided into three main groups:

Bipolar transistors (LTR)

Unipolar MOSFETs (MOS-FETs)

Insulated Gate Bipolar Transistors (IGBTs)

Currently, IGBTs are the most widely used transistors because they combine the control properties of MOS-FET transistors with the output properties of LTR transistors; In addition, they have the appropriate power range, conductivity and switching frequency, which makes the control of modern frequency converters much easier.

With IGBTs, both the inverter elements and the inverter controls are placed in a molded module called an "intelligent power module" (IPM).

Pulse amplitude modulation (PAM)

Pulse amplitude modulation is used for frequency converters with variable intermediate circuit voltage.

In frequency converters with uncontrolled rectifiers, the amplitude of the output voltage is generated by the intermediate circuit breaker, and if the rectifier is controlled, the amplitude is obtained directly.

Rice. 20. Voltage formation in frequency converters with a breaker in the intermediate circuit

Transistor (chopper) in Fig. 20 is unlocked or locked by a control and regulation circuit. The switching times depend on the nominal value (input signal) and the measured voltage signal (actual value). The actual value is measured at the capacitor.

The inductor and capacitor act as a filter that smoothes out voltage ripple. The voltage peak depends on the opening time of the transistor, and if the nominal and actual value differ from each other, the breaker operates until the required voltage level is reached.

Frequency regulation

The frequency of the output voltage is varied by the inverter during a period, and the semiconductor switching devices are operated many times during a period.

The duration of the period can be adjusted in two ways:

1.directly by input signal or

2.using a varying DC voltage that is proportional to the input signal.

Rice. 21a. Frequency control using intermediate circuit voltage

Pulse width modulation is the most common method of generating three-phase voltage with the appropriate frequency.

With pulse-width modulation, the formation of the total voltage of the intermediate circuit (≈ √2 x U mains) is determined by the duration and switching frequency of the power elements. The repetition rate of PWM pulses between on and off moments is variable and allows voltage regulation.

There are three main options for setting switching modes in an inverter controlled by pulse width modulation.

1.Sinusoidal controlled PWM

2.Synchronous PWM

3.Asynchronous PWM

Each leg of a three-phase PWM inverter can have two different states (on and off).

The three switches form eight possible switching combinations (2 3), and therefore eight digital voltage vectors at the output of the inverter or at the stator winding of the connected electric motor. As shown in Fig. 21b, these vectors 100, 110, 010, 011, 001, 101 are located at the corners of the circumscribed hexagon, using vectors 000 and 111 as zero vectors.

In the case of switching combinations 000 and 111, the same potential is created at all three output terminals of the inverter - either positive or negative with respect to the intermediate circuit (see Fig. 21c). For an electric motor this means an effect close to short circuiting the terminals; voltage O V is also applied to the windings of the electric motor.

Sine wave controlled PWM

Sine-wave controlled PWM uses a sinusoidal reference voltage (Us) to control each inverter output. The duration of the sinusoidal voltage period corresponds to the desired fundamental frequency of the output voltage. A sawtooth voltage (U D) is applied to the three reference voltages, see fig. 22.

Rice. 22. Operating principle of sinusoidally controlled PWM (with two reference voltages)

When the ramp voltage and sinusoidal reference voltages intersect, the inverter semiconductors either open or close.

The intersections are determined by the electronic elements of the control board. If the ramp voltage is greater than the sinusoidal voltage, then as the ramp voltage decreases, the output pulses change from positive to negative (or from negative to positive), so that the output voltage of the frequency converter is determined by the intermediate circuit voltage.

The output voltage is varied by the ratio between the duration of the open and closed states, and this ratio can be changed to obtain the required voltage. Thus, the amplitude of negative and positive voltage pulses always corresponds to half the voltage of the intermediate circuit.

Rice. 23. Output voltage of sinusoidally controlled PWM

At low stator frequencies, the time in the closed state increases and may be so long that it becomes impossible to maintain the ramp voltage frequency.

This increases the period of no voltage and the motor will run unevenly. To avoid this, at low frequencies you can double the frequency of the ramp voltage.

The phase voltage at the output terminals of the frequency converter corresponds to half the intermediate circuit voltage divided by √ 2, i.e. equal to half the supply voltage. The line voltage at the output terminals is √ 3 times the phase voltage, i.e. equal to the supply voltage multiplied by 0.866.

A PWM controlled inverter that operates solely modulating the sine wave reference voltage can supply a voltage equal to 86.6% of the rated voltage (see Figure 23).

When using pure sine wave modulation, the output voltage of the frequency converter cannot reach the motor voltage because the output voltage will also be 13% less.

However, the required additional voltage can be obtained by reducing the number of pulses when the frequency exceeds about 45 Hz, but this method has some disadvantages. In particular, it causes a step change in voltage, which leads to unstable operation of the electric motor. If the number of pulses decreases, the higher harmonics at the output of the frequency converter increase, which increases losses in the electric motor.

Another way to solve this problem involves using other reference voltages instead of three sinusoidal ones. These stresses can be of any shape (eg trapezoidal or stepped).

For example, one common voltage reference uses the third harmonic of a sinusoidal reference voltage. It is possible to obtain such a switching mode for the semiconductor devices of the inverter, which will increase the output voltage of the frequency converter, by increasing the amplitude of the sinusoidal reference voltage by 15.5% and adding a third harmonic to it.

Synchronous PWM

The main difficulty in using the sinusoidally controlled PWM method is the need to determine optimal values commutation time and angle for voltage during a given period. These switching times must be set in such a way as to allow only a minimum of higher harmonics. This switching mode is maintained only for a given (limited) frequency range. Operation outside this range requires the use of a different switching method.

Asynchronous PWM

The need for field orientation and system responsiveness in terms of torque and speed control of three-phase AC drives (including servos) requires step changes in the amplitude and angle of the inverter voltage. Using the “normal” or synchronous PWM switching mode does not allow for stepwise changes in the amplitude and angle of the inverter voltage.

One way to meet this requirement is asynchronous PWM, which instead of synchronizing the modulation of the output voltage with the output frequency, as is usually done to reduce harmonics in an electric motor, modulates the vector voltage control loop, resulting in a synchronous coupling with the output frequency.

There are two main options for asynchronous PWM:

SFAVM (Stator Flow-oriented Asynchronous Vector Modulation = (synchronous vector modulation oriented to the stator magnetic flux)

60° AVM (Asynchronous Vector Modulation = asynchronous vector modulation).

SFAVM is a space vector modulation method that allows random but stepwise changes in the voltage, amplitude and angle of the inverter during the switching time. This achieves increased dynamic properties.

The main purpose of using such modulation is to optimize the stator magnetic flux using the stator voltage while reducing torque ripple, since the angle deviation depends on the commutation sequence and can cause an increase in torque ripple. Therefore, the commutation sequence must be calculated in such a way as to minimize vector angle deviation. Switching between voltage vectors is based on calculating the desired magnetic flux path in the motor stator, which in turn determines the torque.

The disadvantage of previous, conventional PWM power systems was deviations in the amplitude of the stator magnetic flux vector and the magnetic flux angle. These deviations adversely affected the rotating field (torque) in the air gap of the electric motor and caused torque pulsation. The influence of the U amplitude deviation is negligible and can be further reduced by increasing the switching frequency.

Motor voltage generation

Stable operation corresponds to regulation of the machine voltage vector U wt so that it describes a circle (see Fig. 24).

The voltage vector is characterized by the magnitude of the electric motor voltage and rotation speed, which corresponds to the operating frequency at the considered moment in time. The motor voltage is generated by creating average values ​​using short pulses from adjacent vectors.

The SFAVM method, developed by Danfoss Corporation, has, among others, the following properties:

The voltage vector can be adjusted in amplitude and phase without deviating from the set setting.

The commutation sequence always starts with 000 or 111. This allows the voltage vector to have three switching modes.

The average value of the voltage vector is obtained using short pulses of neighboring vectors, as well as zero vectors 000 and 111.

Control circuit

The control circuit, or control board, is the fourth main element of the frequency converter, which is designed to solve four important tasks:

Control of semiconductor elements of a frequency converter.

Data exchange between frequency converters and peripheral devices.

Data collection and generation of fault messages.

Performing protection functions for frequency converter and electric motor.

Microprocessors have increased the speed of the control circuit, significantly expanded the range of applications of drives and reduced the number of necessary calculations.

The microprocessor is built into the frequency converter and is always able to determine the optimal pulse combination for each operating condition.

Control circuit for AIM frequency converter

Rice. 25 Operating principle of a control circuit for an intermediate circuit controlled by a breaker.

In Fig. Figure 25 shows a frequency converter with AIM control and an intermediate circuit breaker. The control circuit controls the converter (2) and inverter (3).

Control is carried out based on the instantaneous value of the intermediate circuit voltage.

The intermediate circuit voltage drives a circuit that acts as an address counter in the data storage memory. The memory stores the output sequences for the inverter pulse pattern. When the intermediate circuit voltage increases, counting occurs faster, the sequence ends sooner, and the output frequency increases.

For chopper control, the intermediate circuit voltage is first compared with the nominal value of the reference voltage signal. This voltage signal is expected to give correct values output voltage and frequency. If the reference signal and the intermediate circuit signal are changed, the PI controller informs the circuit that the cycle time needs to be changed. This causes the intermediate circuit voltage to be adjusted according to the reference signal.

A common modulation method for controlling a power converter is pulse amplitude modulation (PAM). Pulse width modulation (PWM) is more modern method.

Field control (vector control)

Vector control can be organized in several ways. The main difference between the methods is the criteria that are used in calculating the values ​​of active current, magnetizing current (magnetic flux) and torque.

When comparing DC motors and three-phase asynchronous motors (Fig. 26), certain problems are revealed. At direct current, the parameters that are important for producing torque - magnetic flux (F) and armature current - are fixed with respect to the size and location of the phase and are determined by the orientation of the field windings and the position of the carbon brushes (Fig. 26a).

In a DC motor, the armature current and the current creating the magnetic flux are located at right angles to each other and their values ​​are not very large. In an asynchronous electric motor, the position of the magnetic flux (F) and rotor current (I,) depends on the load. Moreover, unlike a DC motor, phase angles and current cannot be directly determined from the stator size.

Rice. 26. Comparison of DC machine and AC asynchronous machine

However, using a mathematical model, it is possible to calculate the torque from the relationship between the magnetic flux and the stator current.

From the measured stator current (l s), a component (l w) is extracted, which creates a torque with magnetic flux (Ф) at right angles between these two variables (l in). This creates the magnetic flux of the electric motor (Fig. 27).

Rice. 27. Calculation of current components for field regulation

With these two current components, torque and magnetic flux can be independently influenced. However, due to the certain complexity of calculations based on the dynamic model of an electric motor, such calculations are only cost-effective in digital drives.

Since the excitation control, which is independent of the load, is separated from the torque control in this method, it is possible to dynamically control an induction motor in the same way as a DC motor - provided that the signal is available feedback. This method of controlling a three-phase AC motor has the following advantages:

Good response to load changes

Precise power control

Full torque at zero speed

Performance characteristics are comparable to those of DC drives.

Adjustment of V/f characteristics and magnetic flux vector

In recent years, speed control systems for three-phase AC motors have been developed based on two different principles controls:

normal V/f control, or SCALAR control, and magnetic flux vector control.

Both methods have their own advantages, depending on the specific requirements for drive performance (dynamics) and accuracy.

V/f control has a limited speed control range (approximately 1:20) and at low speed a different control principle (compensation) is required. Using this method, it is relatively easy to adapt the frequency converter to the motor, and the control is immune to instantaneous load changes over the entire speed range.

In flux-controlled drives, the frequency converter must be precisely configured for the motor, which requires detailed knowledge of its parameters. Additional components are also required to receive the feedback signal.

Some advantages of this type of control:

Fast response to speed changes and wide speed range

Better dynamic response to direction changes

A uniform control principle is ensured throughout the entire speed range.

For the user optimal solution there would be a combination best properties both principles. Obviously, both the property of resistance to step load/unload over the entire speed range, which is usually a strong point of V/f control, and fast response to changes in the speed reference (as in field control) are both required.

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 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 magnetic field 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.

Overall vector control asynchronous electric motor exercises 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.

Description:

A frequency converter combined with an asynchronous electric motor allows you to replace a DC electric drive. DC motor speed control systems are quite simple, but the weak point of such an electric drive is the electric motor. It is expensive and unreliable. During operation, the brushes spark, and the commutator wears out under the influence of electrical erosion. Such an electric motor cannot be used in dusty and explosive environments.

Asynchronous electric motors are superior to DC motors in many respects: they are simple in design and reliable, since they do not have moving contacts. They have smaller dimensions, weight and cost compared to DC motors for the same power. Asynchronous motors are easy to manufacture and operate.

The main disadvantage of asynchronous electric motors is the difficulty of regulating their speed traditional methods(by changing the supply voltage, introducing additional resistances into the winding circuit).

Control of an asynchronous electric motor in frequency mode has been a big problem until recently, although the theory of frequency control was developed back in the thirties. The development of variable frequency drives has been hampered by the high cost of frequency converters. The emergence of power circuits with IGBT transistors and the development of high-performance microprocessor control systems have allowed various companies in Europe, the USA and Japan to create modern frequency converters at an affordable price.

It is known that speed control actuators can be carried out using various devices: mechanical variators, hydraulic couplings, resistors additionally inserted into the stator or rotor, electromechanical frequency converters, static frequency converters.

The use of the first four devices does not provide High Quality speed control, uneconomical, expensive to install and operate.
Static frequency converters are the most advanced asynchronous drive control devices at present.

The principle of the frequency method of speed control of an asynchronous motor is that by changing the frequency f1 supply voltage, it is possible in accordance with the expression

without changing the number of pole pairs p, change the angular velocity of the stator magnetic field.

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

Speed ​​regulation is not accompanied by an increase in the slip of the asynchronous motor, so power losses during regulation are small.

To obtain high energy performance of an asynchronous motor - power factors, useful action, overload capacity - it is necessary to change the supplied voltage simultaneously with the frequency.

The law of voltage change depends on the nature of the load torque Ms. At constant load torque Mc=const The stator voltage must be regulated proportionally to the frequency :

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

With a load torque inversely proportional to speed:

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

Advantages of using adjustable electric drive in technological processes

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

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

The promise of frequency regulation is clearly visible from Figure 1

Thus, when throttling, the flow of a substance restrained by a gate or valve does not useful work. The use of an adjustable electric drive of a pump or fan allows you to set the required pressure or flow rate, which will not only save energy, but also reduce losses of the transported substance.

Frequency converter structure

Most modern frequency converters are built using a double conversion scheme. They consist of the following main parts: a DC link (uncontrolled rectifier), a power pulse inverter and a control system.

The DC link consists of an uncontrolled rectifier and a filter. The alternating voltage of the supply network is converted into direct current voltage.

The power three-phase pulse inverter consists of six transistor switches. Each winding of the electric motor is connected through a corresponding switch to the positive and negative terminals of the rectifier. The inverter converts the rectified voltage into a three-phase alternating voltage of the required frequency and amplitude, which is applied to the stator windings of the electric motor.

In the output stages of the inverter, power IGBT transistors are used as switches. Compared to thyristors, they have a higher switching frequency, which allows them to produce a sinusoidal output signal with minimal distortion.

Operating principle of frequency converter

The frequency converter consists of an uncontrolled diode power rectifier B, an autonomous inverter, a PWM control system, a automatic regulation, inductor Lв and filter capacitor Cв (Fig. 2). Regulation of output frequency fout. and voltage Uout is carried out in the inverter due to high-frequency pulse-width control.

Pulse-width control is characterized by a modulation period, within which the stator winding of the electric motor is connected alternately to the positive and negative poles of the rectifier.

The duration of these states within the PWM period is modulated according to a sinusoidal law. At high (usually 2...15 kHz) PWM clock frequencies, sinusoidal currents flow in the motor windings due to their filtering properties.

Speed ​​regulation is not accompanied by an increase in the slip of the asynchronous motor, so power losses during regulation are small. To obtain high energy performance of an asynchronous motor - power factors, efficiency, overload capacity - it is necessary to change the input voltage simultaneously with the frequency.

Frequency converter structure

Most modern frequency converters built using a double conversion scheme. The input sinusoidal voltage with constant amplitude and frequency is rectified in DC link B, smoothed by a filter consisting of a choke Lв and filter capacitor Cv, and then converted again by the inverter AIN into alternating voltage of variable frequency and amplitude. Output frequency regulation fout. and voltage Uout is carried out in the inverter due to high-frequency pulse-width control. Pulse-width control is characterized by a modulation period, within which the stator winding of the electric motor is connected alternately to the positive and negative poles of the rectifier.

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

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Content:

In asynchronous electric motors, there is a need to adjust the rotor speed. For this purpose, a variable frequency drive is used, the main element of which is a frequency converter. Its design includes a direct current bridge, which is also a rectifier that converts industrial alternating current into direct current. Another important part is the inverter, which reverse-converts direct current into alternating current with the required frequency and amplitude.

Operating principle of variable frequency drive

Asynchronous motors are widely used in industry and transport, being the main driving force of components, machines and mechanisms. They are highly reliable and relatively easy to repair.

However, these devices can only rotate at one frequency, which is the AC power supply. To operate in different ranges, special devices are used - frequency converters that adjust frequencies to the required parameters.

The operation of converters is closely related to the operating principle of an asynchronous motor. Its stator consists of three windings, each of which is connected electricity, creating an alternating magnetic field. Under the influence of this field, a current is induced in the rotor, which also leads to the appearance of a magnetic field. As a result of the interaction of the stator and rotor fields, the rotor begins to rotate.

When an induction motor starts, there is a significant current draw from the mains supply. Because of this, the mechanism drive experiences significant overload. There is a spasmodic desire of the engine to reach rated speed. As a result, the service life of not only the unit itself, but also those devices that it powers is reduced.

This problem is successfully solved by using a variable frequency drive, which allows you to change the frequency of the voltage supplying the motor. The use of modern electronic components makes these devices small-sized and highly efficient.

The operating principle of the frequency converter is quite simple. First, the mains voltage is supplied to the rectifier, where it is transformed into direct current. It is then smoothed by capacitors and sent to a transistor converter. Its transistors in the open state have extremely low resistance. They open and close at certain times using electronic control. A voltage similar to three-phase is formed when the phases are displaced relative to each other. The pulses are rectangular in shape, but this does not affect the operation of the engine at all.

Frequency converters are of great importance during operation. With this connection scheme, it is necessary to use a phase-shifting capacitor to create torque. The efficiency of the unit drops noticeably, but the frequency converter increases its performance.

Thus, the use of a variable frequency drive makes the control of three-phase AC motors more efficient. As a result, production improves technological processes, and energy resources are used more rationally.

Advantages and disadvantages of frequency control devices

These adjustment devices have undoubted advantages and provide a high economic effect. They are distinguished by high accuracy of adjustments and provide a starting torque equal to the maximum. If necessary, the electric motor can operate at partial load, which allows significant energy savings. Frequency regulators significantly extend the life of equipment. When the engine starts smoothly, its wear becomes much less.

The variable frequency drive can be remotely diagnosed via an industrial network. This allows you to keep track of engine hours worked, recognize phase failures in input and output circuits, and also identify other defects and malfunctions.

Various sensors can be connected to the control device, which make it possible to adjust certain quantities, for example, pressure. If the mains voltage suddenly disappears, a controlled braking and automatic restart system is activated. The rotation speed is stabilized when the load changes. Variable frequency drive is becoming an alternative replacement for circuit breaker.

The main drawback is the interference caused by most models of such devices. To ensure normal operation, it is necessary to install high-frequency interference filters. In addition, the increased power of variable frequency drives significantly increases their cost, so the minimum payback period is 1-2 years.

Application of adjusting devices

Frequency control devices are used in many areas - in industry and in everyday life. They are equipped with rolling mills, conveyors, cutting machines, fans, compressors, mixers, household washing machines and air conditioners. The drives have proven themselves well in urban trolleybus transport. Using Variable Frequency Drives in CNC Machine Tools program controlled allows you to synchronize movements in the direction of many axes at once.

These systems provide the maximum economic effect when used in various pumping equipment. The standard of any type is to adjust the chokes installed in the pressure lines and determine the number of operating units. Due to this, it is possible to obtain certain technical specifications, such as pipeline pressure and others.

Pumps have a constant speed and do not take into account the changing flow rate resulting from variable water consumption. Even in the case of minimal flow, the pumps will maintain a constant speed, leading to the creation of excess pressure in the network and causing emergency situations. All this is accompanied by significant wasteful energy consumption. This mainly happens at night when there is a sharp drop in water consumption.

With the advent of variable-frequency drives, it became possible to maintain constant pressure directly at consumers. These systems have proven themselves well in conjunction with asynchronous motors general purpose. Frequency control allows you to change the speed of rotation of the shaft, making it higher or lower than the nominal speed. A pressure sensor installed at the consumer transmits information to a variable frequency drive, which, in turn, changes the frequency supplied to the engine.

Modern control devices are compact in size. They are housed in a housing protected from dust and moisture. Thanks to the user-friendly interface, the devices can be operated even in the most difficult conditions, with a wide power range - from 0.18 to 630 kilowatts and a voltage of 220/380 volts.

The operating modes of centrifugal pumps are energetically most efficiently regulated by changing the rotation speed of their impellers. The rotation speed of the impellers can be changed if an adjustable electric drive is used as the drive motor.
Design and characteristics of gas turbines and engines internal combustion are such that they can provide a change in rotation speed in the required range.

It is convenient to analyze the process of regulating the rotation speed of any mechanism using the mechanical characteristics of the unit.

Let's consider the mechanical characteristics of a pumping unit consisting of a pump and an electric motor. In Fig. 1 presents mechanical characteristics centrifugal pump, equipped with a check valve (curve 1) and an electric motor with a squirrel-cage rotor (curve 2).

Rice. 1. Mechanical characteristics of the pump unit

The difference between the torque of the electric motor and the resistance torque of the pump is called dynamic torque. If the motor torque is greater than the resistance torque of the pump, the dynamic torque is considered positive; if less, it is considered negative.

Under the influence of a positive dynamic torque, the pumping unit begins to work with acceleration, i.e. accelerates. If the dynamic torque is negative, the pumping unit operates with a slowdown, i.e. slows down.

When these moments are equal, a steady state of operation occurs, i.e. the pump unit operates at a constant speed. This rotational speed and the corresponding torque are determined by the intersection of the mechanical characteristics of the electric motor and the pump (point a in Fig. 1).

If, during the regulation process, the mechanical characteristic is changed in one way or another, for example, to make it softer by introducing an additional resistor into the rotor circuit of the electric motor (curve 3 in Fig. 1), the rotational torque of the electric motor will become less than the resistance torque.

Under the influence of a negative dynamic torque, the pumping unit begins to work slower, i.e. slows down until the torque and the moment of resistance are again balanced (point b in Fig. 1). This point corresponds to its own rotation frequency and its own torque value.

Thus, the process of regulating the rotation speed of the pump unit is continuously accompanied by changes in the torque of the electric motor and the resistance moment of the pump.

The pump rotation speed can be controlled either by changing the rotation speed of an electric motor rigidly connected to the pump, or by changing the gear ratio of the transmission connecting the pump to the electric motor, which operates at a constant speed.

Regulating the speed of electric motors

Pumping units mainly use AC motors. The rotational speed of an AC motor depends on the frequency of the supply current f, the number of pole pairs p and slip s. By changing one or more of these parameters, you can change the rotation speed of the electric motor and the pump associated with it.

The main element frequency electric drive is . In the converter, the constant frequency of the supply network f1 is converted into a variable frequency f 2. The rotation speed of the electric motor connected to the output of the converter changes in proportion to the frequency f 2.

Using a frequency converter, the practically unchanged network parameters voltage U1 and frequency f1 are converted into variable parameters U2 and f 2 required for the control system. To ensure stable operation of the electric motor, limit its overload in current and magnetic flux, and maintain high energy performance, a certain ratio between its input and output parameters must be maintained in the frequency converter, depending on the type of mechanical characteristics of the pump. These ratios are obtained from the equation of the frequency regulation law.

For pumps the following ratio must be observed:

U1/f1 = U2/f2 = const

In Fig. Figure 2 shows the mechanical characteristics of an asynchronous electric motor with frequency regulation. As the frequency f2 decreases, the mechanical characteristic not only changes its position in the n - M coordinates, but also slightly changes its shape. In particular, the maximum torque of the electric motor is reduced. This is due to the fact that if the relation U1/f1 = U2/f2 = const is observed and the frequency f1 changes, the influence of the active stator resistance on the magnitude of the motor torque is not taken into account.

Rice. 2. Mechanical characteristics of a frequency drive at maximum (1) and low (2) frequencies

When frequency regulation takes into account this influence, the maximum torque remains unchanged, the shape of the mechanical characteristic is preserved, only its position changes.

Frequency converters with high energy characteristics due to the fact that the output of the converter provides a shape of the current and voltage curves that approaches sinusoidal. Recently, frequency converters based on IGBT modules (insulated gate bipolar transistors) have become most widespread.

The IGBT module is a highly efficient key element. It has a low voltage drop, high speed and low switching power. A frequency converter based on IGBT modules with PWM and a vector algorithm for controlling an asynchronous electric motor has advantages over other types of converters. It is characterized by a high power factor over the entire output frequency range.

The schematic diagram of the converter is shown in Fig. 3.

Rice. 3. Diagram of a frequency converter on IGBT modules: 1 - fan unit; 2 - power supply; 3 - uncontrolled rectifier; 4 - control panel; 5 - control panel board; 6 - PWM; 7 - voltage conversion block; 8 - control system board; 9 - drivers; 10 - inverter unit fuses; 11 - current sensors; 12 - asynchronous squirrel-cage motor; Q1, Q2, Q3 - switches of the power circuit, control circuit and fan unit; K1, K2 - contactors for charging capacitors and power circuit; C - capacitor block; Rl, R2, R3 - resistors for limiting the current of capacitor charging, capacitor discharge and drainage unit; VT - inverter power switches (IGBT modules)

At the output of the frequency converter, a voltage (current) curve is formed, slightly different from a sinusoid, containing higher harmonic components. Their presence entails an increase in losses in the electric motor. For this reason, when the electric drive operates at rotation speeds close to the rated speed, the electric motor is overloaded.

When operating at lower speeds, the cooling conditions for self-ventilated electric motors used to drive pumps worsen. In the normal control range of pumping units (1:2 or 1:3), this deterioration in ventilation conditions is compensated by a significant reduction in load due to a decrease in pump flow and pressure.

When operating at frequencies close to the nominal value (50 Hz), deterioration of cooling conditions in combination with the appearance of higher-order harmonics requires a reduction in the permissible mechanical power by 8 - 15%. Because of this, the maximum torque of the electric motor is reduced by 1 - 2%, its efficiency - by 1 - 4%, cosφ - by 5 - 7%.

To avoid overloading the electric motor, it is necessary to either limit the upper value of its rotation speed, or equip the drive with a more powerful electric motor. The last measure is mandatory when the pumping unit is intended to operate at a frequency f 2 > 50 Hz. The upper value of the engine speed is limited by limiting the frequency f 2 to 48 Hz. Increasing the rated power of the drive motor is carried out by rounding to the nearest standard value.

Group control of adjustable electric drives of units

Many pumping installations consist of several units. Usually, adjustable electric drive Not all units are equipped. Of the two or three installed units, it is enough to equip one with an adjustable electric drive. If one converter is constantly connected to one of the units, there is an uneven consumption of their motor resource, since the unit equipped adjustable drive, is used in work for a much longer time.

To distribute the load evenly between all units installed at the station, group control stations have been developed, with the help of which the units can be alternately connected to the converter. Control stations are usually manufactured for low-voltage (380 V) units.

Typically, low-voltage control stations are designed to control two or three units. Low-voltage control stations include circuit breakers that provide protection against phase-to-phase faults. short circuits and ground faults, thermal relays to protect units from overload, as well as control equipment (keys, etc.).

The switching circuit of the control station contains the necessary interlocks that allow connecting the frequency converter to any selected unit and replacing operating units without disrupting the technological operating mode of the pumping or blowing unit.

Control stations, as a rule, along with power elements ( automatic switches, contactors, etc.) contain control and regulating devices (microprocessor controllers, etc.).

At the customer's request, the stations are equipped with devices for automatic switching on of backup power (ABP), commercial metering of consumed electricity, and control of shut-off equipment.

If necessary, additional devices are introduced into the control station, ensuring the use of the device along with a frequency converter soft start units.

Automated control stations provide:

maintaining a given value of a process parameter (pressure, level, temperature, etc.);

control of operating modes of electric motors of regulated and unregulated units (control of current consumption, power) and their protection;

automatic switching on into operation of the backup unit in case of failure of the main one;

switching units directly to the network when the frequency converter fails;

automatic switching on of the backup (AVR) electrical input;

automatic restart (AR) of the station after loss and deep drops of voltage in the supply electrical network;

automatic change of station operating mode with stopping and starting of units at a given time;

automatic activation of an additional unregulated unit if the regulated unit, having reached the rated speed, did not provide the required water supply;

automatic alternation of operating units at specified intervals to ensure uniform consumption of motor resources;

operational control of the operating mode of the pumping (blowing) unit from the control panel or from the dispatch console.

Rice. 4. Group control station for variable-frequency electric drives of pumps

Efficiency of using variable-frequency electric drives in pumping units

The use of a variable-frequency drive allows significant energy savings, since it makes it possible to use large pumping units in low flow mode. Thanks to this, it is possible, by increasing the unit power of the units, to reduce their total number, and consequently, to reduce the overall dimensions of buildings, to simplify hydraulic diagram station, reduce the number of pipeline fittings.

Thus, the use of a controlled electric drive in pumping units allows, along with saving electricity and water, to reduce the number of pumping units, simplify the hydraulic circuit of the station, and reduce the construction volume of the building pumping station. In this regard, secondary economic effects arise: the costs of heating, lighting and building repairs are reduced; the given costs, depending on the purpose of the stations and other specific conditions, can be reduced by 20 - 50%.

IN technical documentation on frequency converters it is indicated that the use of an adjustable electric drive in pumping units allows saving up to 50% of the energy spent on pumping clean and Wastewater, and the payback period is three to nine months.

At the same time, calculations and analysis of the efficiency of an adjustable electric drive in existing pumping units show that in small pumping units with units with a power of up to 75 kW, especially when they operate with a large static component of pressure, the use of adjustable electric drives turns out to be inappropriate. In these cases, you can use more simple systems regulation using throttling, changing the number of operating pumping units.

Application of adjustable electric drive in automation systems pumping units, on the one hand, reduces energy consumption, on the other hand, it requires additional capital costs, therefore the feasibility of using an adjustable electric drive in pumping units is determined by comparing the given costs of two options: basic and new. Behind new option a pumping unit equipped with an adjustable electric drive is taken, and the base unit is a unit whose units operate at a constant speed.