Operating principle of a frequency converter. What is Variable Frequency Drive

Frequency control of the angular speed of rotation of an electric drive with an asynchronous motor is currently widely used, as it allows smoothly changing the rotor speed over a wide range, both above and below the nominal values.

Frequency converters are modern, high-tech devices with a large control range and an extensive range of functions for controlling asynchronous motors. The highest quality and reliability make it possible to use them in various industries to control drives of pumps, fans, conveyors, etc.

According to the supply voltage, frequency converters are divided into single-phase and three-phase, and according to their design, into electric machine rotating and static. In electrical machine converters, variable frequency is obtained through the use of conventional or special electric machines. Changing the frequency of the supply current is achieved through the use of electrical elements that do not move.

Frequency converters for single-phase networks make it possible to provide electric drive for production equipment with a power of up to 7.5 kW. A design feature of modern single-phase converters is that there is one phase with a voltage of 220V at the input, and three phases with the same voltage value at the output, which allows you to connect to the device three-phase electric motors without the use of capacitors.

Frequency converters powered by three-phase network 380V are available in a power range from 0.75 to 630 kW. Depending on the power level, devices are manufactured in combined polymer and metal cases.

The most popular control strategy for asynchronous electric motors is vector control. Currently, most frequency converters implement vector control or even sensorless vector control (this trend is found in frequency converters that initially implement scalar control and do not have terminals for connecting a speed sensor).

Based on the type of load at the output, frequency converters are divided by type of design:

for pump and fan drives;

for general industrial electric drive;

used as part of electric motors operating with overload.

Modern frequency converters have a varied range of functional features, for example, have manual and automatic control speed and direction of rotation of the engine, as well as on the control panel. Equipped with the ability to regulate the output frequency range from 0 to 800 Hz.

The converters are capable of automatically controlling an asynchronous motor using signals from peripheral sensors and actuating the electric drive according to a specified timing algorithm. Support functions automatic recovery operating mode during a short-term power interruption. Control transient processes from a remote control and protect electric motors from overloads.

The relationship between the angular speed of rotation and the frequency of the supply current follows from the equation

ω o = 2πf 1 /p

When the voltage of the power supply U1 remains constant and the frequency changes, the magnetic flux of the asynchronous motor changes. Moreover, for best use magnetic system, when the supply frequency decreases, it is necessary to proportionally reduce the voltage, otherwise the magnetizing current and losses in the steel will increase significantly.

Similarly, when the supply frequency increases, the voltage should be proportionally increased in order to keep the magnetic flux constant, since otherwise (with a constant torque on the shaft) this will lead to an increase in the rotor current, overloading its windings with current, and a decrease in the maximum torque.

The rational law of voltage regulation depended on the nature of the moment of resistance.

At a constant static load torque (Mс = const), the voltage must be regulated in proportion to its frequency U1/f1 = const. For the fan load, the ratio takes the form U1/f 2 1 = const.

At a load torque inversely proportional to the speed U1/ √ f1= const.

The figures below show a simplified connection diagram and mechanical characteristics of an asynchronous motor with frequency control of angular velocity.

Frequency control of the speed of an asynchronous motor allows you to change the angular speed of rotation in the range - 20...30 to 1. Speed control asynchronous motor down from the main is carried out to almost zero.

When the frequency of the supply network changes, the upper limit of the rotation speed of an asynchronous motor depends on its mechanical properties, especially since at frequencies above the rated frequency the asynchronous motor operates with better energy performance than at lower frequencies. Therefore, if a gearbox is used in the drive system, this frequency control of the motor should be carried out not only downwards, but also upwards from the nominal point, up to maximum frequency rotation, permissible under the conditions of the mechanical strength of the rotor.

When the engine speed increases above the value specified in its passport, the frequency of the power source should not exceed the rated frequency by no more than 1.5 - 2 times.

The frequency method is the most promising for regulating an asynchronous motor with a squirrel-cage rotor. Power losses in this way of regulation are small, since they are not accompanied by an increase. The resulting mechanical characteristics are highly rigid.

Currently, the asynchronous electric motor has become the main device in most electric drives. Increasingly, an inverter with PWM control is used to control it. Such management provides a lot of advantages, but also creates some problems in choosing certain technical solutions. Let's try to understand them in more detail.

Frequency converter device

The development and production of a wide range of high-power high-voltage transistor IGBT modules has made it possible to implement multi-phase power switches controlled directly by digital signals. Programmable computing tools made it possible to generate numerical sequences at the inputs of switches that provide signals. The development and mass production of single-chip microcontrollers with large computing resources have made it possible to move to servo electric drives with digital controllers.

Power frequency converters are usually implemented according to a circuit containing a rectifier using powerful power diodes or transistors and an inverter ( managed switch) on IGBT transistors shunted by diodes (Fig. 1).

Rice. 1. Frequency converter circuit

The input stage rectifies the supplied sinusoidal mains voltage, which, after smoothing using an inductive-capacitive filter, serves as a power source for the controlled inverter, which is generated when commands are applied digital control signal c, which generates sinusoidal currents in the stator windings with parameters that ensure the required operating mode of the electric motor.

Digital control of the power converter is carried out using microprocessor hardware and corresponding to the assigned tasks software. computing device generates control signals for 52 modules in real time and also processes signals measuring systems, controlling the operation of the drive.

Power devices and control computing facilities are combined into a structurally designed industrial product called a frequency converter.

There are two main types of frequency converters used in industrial equipment:

branded converters for specific types equipment.

universal frequency converters are designed for multi-purpose control of IM operation in user-specified modes.

Installation and control of the operating modes of the frequency converter can be done using a control panel equipped with a screen to display the entered information. In a simple version of scalar frequency control, you can use a set of simple logical functions, available in the factory settings of the controller, and a built-in PID controller.

To implement more complex control modes using signals from feedback sensors, it is necessary to develop an ACS structure and an algorithm, which should be programmed using a connected external computer.

Most manufacturers produce a range of frequency converters that differ in input and output electrical characteristics, power, design and other parameters. To connect to external equipment (power supply, motor), additional external elements: magnetic starters, transformers, chokes.

Types of control signals

It is necessary to distinguish between different types of signals and use a separate cable for each of them. Different types of signals can influence each other. In practice, such a separation occurs often, for example, the cable from can be connected directly to the frequency converter.

Rice. 2. Example of connecting power circuits and control circuits of a frequency converter

You can select following types signals:

analog - voltage or current signals (0...10 V, 0/4...20 mA), the value of which changes slowly or rarely, usually these are control or measurement signals;

discrete signals voltage or current (0...10 V, 0/4...20 mA), which can take only two rarely changing values (high or low);

digital (data) - voltage signals (0...5 V, 0...10 V), which change quickly and with high frequency, usually these are signals from RS232, RS485, etc. ports;

relay - relay contacts (0...220 V AC) can switch on inductive currents depending on the connected load (external relays, lamps, valves, brakes, etc.).

Selecting the power of the frequency converter

When choosing the power of a frequency converter, it is necessary to base it not only on the power of the electric motor, but also on the rated currents and voltages of the converter and the motor. The fact is that the indicated power of the frequency converter only applies to its operation with a standard 4-pole asynchronous electric motor in standard applications.

Real drives have many aspects that can cause the drive's current load to increase, for example during start-up. In general, application frequency drive allows you to reduce current and mechanical loads due to soft starting. For example, the starting current is reduced from 600% to 100-150% of the rated value.

Drive operation at reduced speed

It must be remembered that although the frequency converter easily provides speed control of 10:1, when the engine operates at low speeds, the power of its own fan may not be enough. It is necessary to monitor the engine temperature and provide forced ventilation.

Electromagnetic compatibility

Since the frequency converter powerful source high-frequency harmonics, then a shielded cable of minimum length must be used to connect the motors. Such a cable must be laid at a distance of at least 100 mm from other cables. This minimizes interference. If you need to cross cables, the crossing is done at an angle of 90 degrees.

Power from emergency generator

Soft start, which is provided by the frequency converter, allows you to reduce required power generator Since with such a start the current is reduced by 4-6 times, the generator power can be reduced by a similar number of times. But all the same, a contactor must be installed between the generator and the drive, controlled from the relay output of the frequency drive. This protects the frequency converter from dangerous overvoltages.

Power supply of a three-phase converter from a single-phase network

Three-phase frequency converters can be powered from a single-phase network, but their output current should not exceed 50% of the rated current.

Saving energy and money

Savings occur for several reasons. Firstly, due to growth to values of 0.98, i.e. maximum power is used to perform useful work, the minimum goes into losses. Secondly, a coefficient close to this is obtained in all engine operating modes.

Without a frequency converter, asynchronous motors at low loads have a cosine phi of 0.3-0.4. Thirdly, there is no need for additional mechanical adjustments (flaps, throttles, valves, brakes, etc.), everything is done electronically. With such a control device, savings can reach 50%.

Sync multiple devices

Due to additional control inputs of the frequency drive, it is possible to synchronize processes on the conveyor or set the ratio of changes in some quantities depending on others. For example, make the rotation speed of the machine spindle dependent on the feed speed of the cutter. The process will be optimized because when the load on the cutter increases, the feed will be reduced and vice versa.

Protection of the network from higher harmonics

For additional protection, in addition to short shielded cables, line chokes and shunt capacitors are used. , in addition, limits the current surge when turned on.

Choosing the right protection class

For trouble-free operation of a frequency drive, a reliable heat sink is required. If you use high protection classes, for example IP 54 and higher, then it is difficult or expensive to achieve such heat dissipation. Therefore, you can use a separate cabinet with high class protection, where to install modules with a lower class and provide general ventilation and cooling.

Parallel connection of electric motors to one frequency converter

In order to reduce costs, one frequency converter can be used to control several electric motors. Its power must be selected with a margin of 10-15% of the total power of all electric motors. In this case, it is necessary to minimize the length of the motor cables and it is very advisable to install a motor throttle.

Most frequency converters do not allow motors to be disconnected or connected using contactors while the frequency drive is running. This can only be done via the drive stop command.

Setting the control function

To obtain maximum performance indicators of the electric drive, such as: power factor, coefficient useful action, overload capacity, smoothness of regulation, durability, you need to correctly choose the relationship between the change in operating frequency and voltage at the output of the frequency converter.

The voltage change function depends on the nature of the load torque. At a constant torque, the voltage on the stator of the electric motor must be regulated in proportion to the frequency (scalar regulation U/F = const). For a fan, for example, another ratio is U/F*F = const. If we increase the frequency by 2 times, then the voltage must be increased by 4 (vector regulation). There are drives with more complex functions regulation.

Advantages of using an adjustable electric drive with a frequency converter

In addition to increasing efficiency and energy saving, such an electric drive allows you to obtain new control qualities. This is expressed in the refusal of additional mechanical devices, creating losses and reducing the reliability of systems: brakes, dampers, throttles, valves, control valves, etc. Braking, for example, can be accomplished by reverse rotation of the electromagnetic field in the stator of the electric motor. Changing only functional dependence between frequency and voltage, we get a different drive without changing anything in the mechanics.

Reading Documentation

It should be noted that although frequency converters are similar to each other and having mastered one, it is easy to understand the other, nevertheless, it is necessary to carefully read the documentation. Some manufacturers impose restrictions on the use of their products, and if they are violated, they will remove the product from warranty.

Variable frequency drive (variable requency drive, VFD) is a system for controlling the rotor speed of an asynchronous (synchronous) electric motor. It consists of the electric motor itself and a frequency converter.

A frequency converter (frequency converter) is a device consisting of a rectifier (bridge direct current), converting alternating current of industrial frequency into direct current and an inverter (converter) (sometimes with PWM), converting direct current into alternating current of the required frequency and amplitude. Output thyristors (GTOs) or IGBTs provide the necessary current to power the motor. To avoid overloading the converter when the feeder is long, chokes are installed between the converter and the feeder, and to reduce electromagnetic interference, an EMC filter is installed. With scalar control, harmonic currents of the motor phases are formed. Vector control- a method of controlling synchronous and asynchronous motors, which not only generates harmonic currents (voltages) of the phases, but also provides control of the rotor magnetic flux (torque on the motor shaft).

Application of frequency drive

Frequency converters are used in:

ship electric drive high power
rolling mills ( synchronous work cages)
high-speed drive of vacuum turbomolecular pumps (up to 100,000 rpm)
conveyor systems
cutting machines
CNC machines - synchronization of the movement of several axes at once (up to 32 - for example in printing or packaging equipment) (servo drives)
automatically opening doors
mixers, pumps, fans, compressors
household air conditioners
washing machines
urban electric transport, especially trolleybuses.

The greatest economic effect comes from the use of VFDs in ventilation, air conditioning and water supply systems, where the use of VFDs has become virtually a standard.

Advantages of using VFD

High control accuracy
Energy savings in case of variable load (that is, operation of the electric motor at partial load).
Equal to the maximum starting torque.
Possibility of remote diagnostics of the drive via an industrial network
- phase failure detection for input and output circuits
- engine hour recording
- aging of main circuit capacitors
- fan malfunction
Increased equipment life
Reduced hydraulic resistance of the pipeline due to the absence of a control valve
Smooth engine start, which significantly reduces engine wear
A VFD usually contains a PID controller and can be connected directly to a sensor of the controlled variable (for example, pressure).
Controlled braking and automatic restart in case of failure mains voltage
Picking up a rotating electric motor
Rotation speed stabilization when load changes
Significant reduction in acoustic noise of the electric motor (using the Soft PWM function)
Additional energy savings from optimization of electrical excitation. engine
Allows you to replace a circuit breaker

Disadvantages of using a frequency drive

Most VFD models are a source of noise (requires installation of High Frequency Interference Filters)
Relatively high cost for high-power VFDs (payback minimum 1-2 years)

Application of frequency converters at pumping stations

The classic method of controlling the supply of pumping units involves throttling the pressure lines and regulating the number of operating units according to some technical parameter (for example, pressure in the pipeline). In this case, pumping units are selected based on certain design characteristics (usually with a performance reserve) and constantly operate at a constant speed, without taking into account changing costs caused by variable water consumption. At minimum flow, the pumps continue to operate at a constant speed, creating excess pressure in the network (the cause of accidents), while a significant amount of electricity is wasted. This, for example, happens at night, when water consumption drops sharply. The main effect is achieved not by saving energy, but by significantly reducing the cost of repairing water supply networks.

The advent of an adjustable electric drive made it possible to maintain constant pressure directly at the consumer. Variable-frequency electric drives with asynchronous electric motors for general industrial use are widely used in world practice. As a result of the adaptation of general industrial asynchronous motors To meet their operating conditions in controlled electric drives, special adjustable asynchronous motors are created with higher energy and weight-size-cost indicators compared to non-adapted ones. Frequency control of the shaft rotation speed of an asynchronous motor is carried out using electronic device, which is usually called a frequency converter. The above effect is achieved by changing the frequency and amplitude of the three-phase voltage supplied to the electric motor. Thus, by changing the parameters of the supply voltage (frequency control), you can make the motor rotation speed both lower and higher than the nominal one. In the second zone (frequency above the nominal), the maximum torque on the shaft is inversely proportional to the rotation speed.

The frequency conversion method is based on the following principle. Typically, the industrial network frequency is 50 Hz. For example, let's take a pump with a two-pole electric motor. Taking into account sliding, the engine rotation speed is about 2800 (depending on power) revolutions per minute and gives the output of the pumping unit the nominal pressure and performance (since these are its nominal parameters, according to the passport). If you use a frequency converter to reduce the frequency and amplitude of the alternating voltage supplied to it, the engine rotation speed will correspondingly decrease and, consequently, the performance of the pumping unit will change. Information about the pressure in the network enters the frequency converter unit from a special pressure sensor installed at the consumer; based on this data, the converter accordingly changes the frequency supplied to the engine.

The modern frequency converter has a compact design, dust- and moisture-proof housing, user-friendly interface, which allows it to be used in the most difficult conditions and challenging environments. The power range is very wide and ranges from 0.18 to 630 kW or more at standard food 220/380 V and 50-60 Hz. Practice shows that the use of frequency converters at pumping stations allows:

save energy (with significant changes in consumption) by adjusting the power of the electric drive depending on the actual water consumption (saving effect of 20-50%);
reduce water consumption by reducing leaks when the pressure in the main line is exceeded, when water consumption is actually small (by an average of 5%);
reduce costs (main economic effect) by emergency repairs equipment (the entire water supply infrastructure due to a sharp reduction in the number of emergency situations caused, in particular, by water hammer, which often happens when an unregulated electric drive is used (it has been proven that the service life of the equipment increases by at least 1.5 times);
achieve certain heat savings in hot water supply systems by reducing losses of heat-carrying water;
increase the pressure above normal if necessary;
comprehensively automate the water supply system, thereby reducing the wages of service and duty personnel, and eliminate the influence of the “human factor” on the operation of the system, which is also important.

According to available data, the payback period for a project to introduce frequency converters ranges from 3 months to 2 years.

Power loss when braking an electric motor

In many installations, an adjustable electric drive is tasked not only with smoothly regulating the torque and speed of rotation of the electric motor, but also with the task of slowing down and braking the elements of the installation. The classic solution to this problem is a drive system with an asynchronous motor with a frequency converter equipped with a brake switch with a braking resistor.

At the same time, in the deceleration/braking mode, the electric motor operates as a generator, converting mechanical energy into electrical energy, which is ultimately dissipated by the braking resistor. Typical installations in which acceleration cycles alternate with deceleration cycles are the traction drive of electric vehicles, hoists, elevators, centrifuges, winding machines, etc. The electric braking function first appeared on a DC drive (for example, a trolleybus). At the end of the twentieth century, frequency converters with a built-in recuperator appeared, which allow the energy received from the engine operating in braking mode to be returned back to the network. In this case, the installation begins to “make money” almost immediately after commissioning.

Operating principle of frequency converter

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 weak point Such an electric drive is an 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, the development of high-performance microprocessor systems control allowed various companies in Europe, the USA and Japan to create modern frequency converters at an affordable price.

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

The use of the first four devices does not provide high quality speed control, is uneconomical, and requires high installation and operation costs.
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, efficiency, overload capacity - it is necessary to change the input voltage simultaneously with the frequency.

The law of voltage change depends on the nature of the load 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 do any 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 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 rectified voltage into three-phase AC voltage 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, an automatic control system, a choke Lв and a 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) clock speeds

PWM, in the windings of the electric motor, due to their filtering properties, sinusoidal currents flow.

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. 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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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 direct current.

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 frequency of the electric motor voltage. Some inverters can also convert constant DC voltage to varying AC voltage.

4. An electronic control circuit that 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 is the case with some others 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 act reverse voltages values 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 negative phase voltage. If in this 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 current flows through the thyristor, the thyristor will pass it until the current becomes equal to 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 interference into the power supply network, since with a shorter transmission time of the thyristors, the rectifier takes more from the network reactive current.

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 only produces 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 in the sequence U-V, V-W, W-U, U-V..., an intermittent rotating magnetic field of the required frequency occurs. Even if the motor current has an 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, they work almost the same. The control circuitry opens and closes the semiconductor devices using several different modulation methods, 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. Thanks to the use of an oscillator, voltage controlled, frequency always tracks 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” because high frequencies can lead to heating of the electric motor and the appearance of large peak voltages. 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 proper power range, suitable conductivity and switching frequency, which makes control significantly easier modern converters frequencies.

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 time the transistor is turned on, and if the nominal and actual values \u200b\u200bdiffer from each other, the chopper 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 constant voltage, which is proportional 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.

Intersections are determined electronic elements control boards. 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 output voltage 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 specified 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 randomly, but abruptly change the voltage, amplitude and angle of the inverter during the commutation 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 the deviation of the vector angle. 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 work corresponds to the 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

Control circuit, or control board - fourth main element frequency converter, which is designed to solve four important problems:

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 voltage value of the intermediate circuit.

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 the correct 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 a 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, with the help mathematical model Torque can be calculated from the relationship between magnetic flux and 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 last years speed control systems developed three-phase motors AC 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. Also required additional components to receive a feedback signal.

Some advantages of this type of control:

Fast response to speed changes and wide range speeds

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.