What Is a Frequency Transducer?

A variable frequency drive (VFD) is a power control device that applies variable frequency technology and microelectronics technology to control the AC motor by changing the frequency of the motor's working power. ^[1]

The inverter is mainly composed of rectification (AC to DC), filtering, inverter (DC to AC), braking unit, driving unit, and detection unit micro-processing unit. The inverter relies on the opening and closing of the internal IGBT to adjust the voltage and frequency of the output power, and provides the required power voltage according to the actual needs of the motor, thereby achieving the purpose of energy saving and speed regulation. In addition, the inverter has many protection functions , Such as overcurrent, overvoltage, overload protection and so on. With the continuous improvement of industrial automation, inverters have also been widely used. ^[2]

Chinese name: Inverter
Foreign name: Variable-frequency Drive, AC Drive

Short name: VFD
Category: Motor control equipment
Application: Frequency conversion technology and microelectronic technology

Development history of inverter

The birth of variable frequency technology is the widespread demand for stepless speed regulation of AC motors. Traditional DC speed control technology has limited applications due to its large volume and high failure rate. ^[3]

After the 1960s, thyristors and their upgraded products were widely used in power electronic devices. But its speed regulation performance is far

Not enough. In 1968, high-tech companies represented by Danfoss began mass production of inverters, opening a new era of inverter industrialization. ^[3]

Beginning in the 1970s, the research on the PWM-VVVF speed regulation achieved a breakthrough. The improvement of microprocessor technology after the 1980s made it easy to implement various optimization algorithms. ^[3]

In the mid-to-late 1980s, VVVF inverter technology in developed countries such as the United States, Japan, Germany, and the United Kingdom was put into practical use, and products were put on the market and widely used. The earliest inverter was probably developed by a Japanese who bought a British patent. However, the United States and Germany rely on the advantages of electronic component production and electronic technology, and high-end products quickly seize the market. ^[3]

After entering the 21st century, domestic inverters have gradually risen, and now they have gradually seized the high-end market. Shanghai and Shenzhen have become the forefront of the development of domestic inverters, and a number of well-known domestic inverters have emerged, such as Huichuan inverters, INVT inverters, Anbangxin inverters, and Ou Rui inverters. Among them, Anbangxin Inverter was established in 1998 and is one of the earliest manufacturers of inverters in China. For more than ten years, Anbang believers have built a solid foundation with strong cultural heritage to support growth. The company passed the ISO9000 quality system certification of the TUV organization earlier and was awarded the "National High-tech Enterprise". Top Ten Domestic Brands. " ^[3]

Inverter composition

Inverter main circuit

The main circuit is the power conversion part that provides voltage and frequency regulation power to the asynchronous motor. The main circuit of the inverter can be roughly divided into two categories

Frequency conversion power analyzer (5 photos)

: Voltage type is an inverter that converts the DC of the voltage source to AC, and the filtering of the DC circuit is a capacitor. The current type is a frequency converter that converts the DC of a current source into an AC, and its DC loop filter is an inductor. It consists of three parts, a "rectifier" that converts a commercial frequency power supply into DC power, a "flat wave circuit" that absorbs voltage ripples generated by converters and inverters, and an "inverter" that converts DC power to AC power. Transformer ". ^[4]

Inverter rectifier

A diode converter is widely used, which converts a power frequency power source into a DC power source. It is also possible to use two sets of transistor converters to form a reversible converter. Because its power direction is reversible, regenerative operation can be performed. ^[4]

Inverter smooth wave circuit

The DC voltage rectified by the rectifier includes a pulsating voltage that is 6 times the frequency of the power supply. In addition, the pulsating current generated by the inverter also changes the DC voltage. In order to suppress voltage fluctuations, inductors and capacitors are used to absorb the pulsating voltage (current). If the device capacity is small, if there is a margin in the components of the power supply and the main circuit, the inductor can be omitted and a simple smooth wave circuit can be used. ^[4]

Inverter inverter

In contrast to the rectifier, the inverter converts DC power to AC power of the required frequency, and when the 6 switching devices are turned on and off at a determined time, a 3-phase AC output can be obtained. Take the voltage type PWM inverter as an example to show the switching time and voltage waveform. ^[4]

The control circuit is a circuit that provides control signals to the main circuit that supplies power to the asynchronous motor (voltage and frequency are adjustable). It has a frequency and voltage "computing circuit", a main circuit "voltage and current detection circuit", and a motor "speed detection Circuit ", a" driving circuit "that amplifies the control signal of the arithmetic circuit, and a" protection circuit "for the inverter and the motor. ^[4]

(1) Calculation circuit: Compare the external speed and torque commands with the current and voltage signals of the detection circuit to determine the output voltage and frequency of the inverter. ^[4]

(2) Voltage and current detection circuit: Detect voltage and current from the main circuit potential. ^[4]

(3) Drive circuit: A circuit that drives the main circuit device. It is isolated from the control circuit to make the main circuit device on and off. ^[4]

(4) Speed detection circuit: Take the signal of the speed detector (tg, plg, etc.) installed on the asynchronous motor shaft machine as the speed signal, and send it to the calculation circuit. According to the instruction and operation, the motor can run at the instruction speed. ^[4]

(5) Protection circuit: Detect the voltage and current of the main circuit. When an abnormality such as overload or overvoltage occurs, in order to prevent the inverter and asynchronous motor from being damaged. ^[4]

Function of inverter

Frequency converter energy saving

Inverter energy saving is mainly reflected in the application of fans and pumps. After the frequency conversion speed regulation is adopted for fans and pumps, the power saving rate is 20% to 60%. This is because the actual power consumption of fans and pumps is basically proportional to the third power of the speed. When the average flow rate required by the user is small, the speed of the fans and pumps is reduced by frequency conversion, and the energy saving effect is very obvious. However, the traditional fans and pumps use baffles and valves to adjust the flow, the motor speed is basically unchanged, and the power consumption does not change much. According to statistics, the electricity consumption of fans and pump motors accounts for 31% of the national electricity consumption and 50% of the industrial electricity consumption. It is of great significance to use variable frequency speed control devices on such loads. At present, the more successful applications are constant pressure water supply, various types of fans, central air conditioning and variable frequency speed regulation of hydraulic pumps. ^[5]

Application of frequency converter in automation system

Because the inverter has a built-in 32-bit or 16-bit microprocessor, it has a variety of arithmetic and logic operations and intelligent control functions. The output frequency accuracy is 0.1% ~ 0.01%, and it has perfect detection and protection links. Widely used in the system. For example: winding, stretching, metering, guide wire in the chemical fiber industry; flat glass annealing furnace, glass kiln stirring, edger, bottle making machine in the glass industry; electric arc furnace automatic feeding, batching system and intelligent control of elevator Wait. Application of frequency converters for improving process level and product quality in the control of CNC machine tools, automotive production lines, papermaking and elevators. ^[5]

Application of frequency converter in improving process level and product quality

The frequency converter can also be widely used in various mechanical equipment control fields such as transmission, lifting, extrusion and machine tools. It can improve the technological level and product quality, reduce the impact and noise of the equipment, and extend the service life of the equipment. The use of variable frequency speed control simplifies the mechanical system, makes operation and control more convenient, and some can even change the original process specifications, thereby improving the function of the entire equipment. For example, the setting machine used in the textile and many industries, the temperature inside the machine is adjusted by changing the amount of hot air sent. The circulating fan is usually used to transport hot air. Because the speed of the fan is not changed, the amount of hot air can only be adjusted by the damper. If the damper adjustment fails or is not adjusted properly, it will cause the setting machine to lose control, which will affect the quality of the finished product. The circulating fan starts at high speed, and the wear between the transmission belt and the bearing is very severe, which makes the transmission belt a consumable. After the frequency conversion is adopted, the temperature adjustment can be realized by the inverter automatically adjusting the speed of the fan, which solves the problem of product quality. In addition, the inverter can easily start the fan at low frequency and low speed and reduce the wear between the transmission belt and the bearing. It can also extend the service life of the equipment and save energy by 40%. ^[5]

Inverter realizes motor soft start

The hard start of the motor will not only cause serious impact on the power grid, but also have excessive requirements on the capacity of the power grid. The large current and vibration generated during startup will cause great damage to the baffles and valves, which is extremely detrimental to the service life of the equipment and pipelines. After using the inverter, the soft start function of the inverter will change the starting current from zero, and the maximum value does not exceed the rated current, which reduces the impact on the power grid and the requirements for power supply capacity, and extends the service life of equipment and valves. , While also saving equipment maintenance costs. ^[6]

Inverter classification

1. Classified by input voltage level

The inverter can be divided into low-voltage inverter and high-voltage inverter according to the input voltage level. Common low-voltage inverters in China include single-phase 220 V inverter, three-phase 220 V inverter and i-phase 380 V inverter. High voltage inverters usually have 6 kV and 10 kV transformers, and the control mode is generally changed according to the high-low-high inverter or high-high inverter. ^[7]

2. Classification by transform frequency method

Inverters are divided into AC-AC inverters and AC-DC inverters according to the frequency conversion method. AC-AC inverter can directly convert industrial frequency AC power into AC whose frequency and voltage can be controlled, so it is called direct inverter. The AC-DC-AC inverter is first referred to as an indirect inverter because it first converts power frequency AC power to DC power through a rectifier and then converts DC power to AC power with adjustable frequency and voltage. ^[7]

3 Classified by the nature of the DC power supply

In the AC-DC-AC inverter, according to the process of converting the main circuit power to the DC power, the nature of the DC power is divided into a voltage inverter and a current inverter. ^[7]

Inverter setting method

Common frequency setting methods of the inverter include: operator keyboard setting, contact signal setting, analog signal setting, pulse signal setting and communication mode setting. These frequency setting methods have their own advantages and disadvantages, and must be selected and set according to actual needs. At the same time, different frequency setting methods can be selected for superposition and switching according to functional needs. ^[6]

Inverter control mode

The low-voltage general-purpose frequency conversion output voltage is 380 650V, the output power is 0.75 400kW, and the working frequency is 0 400Hz. Its main circuits are all AC-DC-AC circuits. Its control method has experienced the following four generations. ^[8]

(SPWM) Inverter sinusoidal pulse width modulation (SPWM) control method

Its characteristics are simple control circuit structure, low cost, good mechanical characteristics and hardness, which can meet the requirements of smooth speed regulation of general transmission, and has been widely used in various fields of the industry. However, at this low frequency, due to the lower output voltage, the torque is significantly affected by the stator resistance voltage drop, which reduces the maximum output torque. In addition, its mechanical characteristics are not as hard as those of a DC motor. The dynamic torque capability and static speed regulation performance are not satisfactory. The system performance is not high. The control curve will change with the change of the load. The torque response is slow and the motor turns. The torque utilization rate is not high. At low speeds, the performance is degraded and the stability is deteriorated due to the existence of the stator resistance and the dead zone effect of the inverter. Therefore, people have also researched vector control frequency conversion speed regulation. ^[8]

(SVPWM) Inverter voltage space vector (SVPWM) control method

It is based on the premise that the three-phase waveform is generated as a whole, and the purpose is to approximate the ideal circular rotating magnetic field trajectory of the motor air gap, to generate a three-phase modulation waveform at a time, and control it by approaching the polygon with an inscribed polygon. It has been improved after practical use, that is, the introduction of frequency compensation can eliminate the error of speed control; estimate the amplitude of the magnetic flux through feedback to eliminate the influence of stator resistance at low speed; and close the output voltage and current to improve the dynamic accuracy and stability. However, there are many links in the control circuit and no torque adjustment is introduced, so the system performance has not been fundamentally improved. ^[8]

(VC) Inverter vector control (VC) mode

The method of vector control of variable frequency speed regulation is to convert the stator currents Ia, Ib, Ic of the asynchronous motor in the three-phase coordinate system into three-phase to two-phase conversion, which is equivalent to the AC current Ia1Ib1 in the two-phase stationary coordinate system, and Directed rotation transformation according to the rotor magnetic field is equivalent to the direct current Im1 and It1 in the synchronous rotating coordinate system (Im1 is equivalent to the excitation current of the DC motor; It1 is equivalent to the armature current proportional to the torque), and then imitates the DC motor The control method obtains the control amount of the DC motor, and realizes the control of the asynchronous motor through the corresponding inverse transformation of the coordinates. The essence is equivalent to an AC motor as a DC motor, which independently controls the two components of speed and magnetic field. By controlling the rotor flux linkage, and then decomposing the stator current, two components of torque and magnetic field are obtained, and orthogonal or decoupling control is achieved through coordinate transformation. The proposed vector control method has epoch-making significance. However, in practical applications, because the rotor flux linkage is difficult to accurately observe, the system characteristics are greatly affected by motor parameters, and the vector rotation transformation used in the equivalent DC motor control process is more complicated, making the actual control effect difficult to achieve the ideal analysis. result. ^[8]

(DTC) Inverter direct torque control (DTC) method

In 1985, Professor DePenbrock of Ruhr University in Germany first proposed the direct torque control frequency conversion technology. This technology largely solves the above-mentioned shortcomings of vector control, and has developed rapidly with novel control ideas, simple and clear system structure, and excellent dynamic and static performance. This technology has been successfully applied to high-power AC drives for electric locomotive traction. Direct torque control analyzes the mathematical model of the AC motor directly in the stator coordinate system, and controls the magnetic flux and torque of the motor. It does not require the AC motor to be equivalent to a DC motor, so many complicated calculations in vector rotation transformation are omitted; it does not need to imitate the control of the DC motor, and it does not need to simplify the mathematical model of the AC motor for decoupling. ^[8]

Inverter matrix AC-AC control mode

VVVF frequency conversion, vector control frequency conversion, direct torque control frequency conversion are all one of AC-DC-AC frequency conversion. Its common disadvantages are low input power factor, large harmonic current, large energy storage capacitors for DC circuits, and regenerative energy cannot be fed back to the grid, that is, four-quadrant operation cannot be performed. For this reason, matrix-type AC-AC frequency conversion came into being. Because the matrix type AC-AC frequency conversion eliminates the intermediate DC link, thereby eliminating the bulky and expensive electrolytic capacitors. It can achieve a power factor of l, the input current is sinusoidal and can operate in four quadrants, and the system has a high power density. Although this technology is not yet mature, it still attracts many scholars to study it in depth. Its essence is not to control the current, flux, etc. indirectly, but to realize the torque directly as the controlled quantity. The specific method is: ^[8]

1. Control the stator flux to introduce the stator flux observer to realize the speed sensorless method; ^[8]

2. Automatic identification (ID) relies on accurate motor mathematical model to automatically identify motor parameters; ^[8]

3. Calculate the actual value corresponding to the stator impedance, mutual inductance, magnetic saturation factor, inertia, etc. Calculate the actual torque, stator flux, and rotor speed for real-time control; ^[8]

4. Realize Band-Band control Band-Band control according to magnetic flux and torque generates PWM signals to control the switching state of the inverter. ^[8]

Matrix AC-AC frequency converter has fast torque response (<2ms), high speed accuracy (± 2%, no PG feedback), high torque accuracy (<+ 3%); it also has high starting Torque and high torque accuracy, especially at low speed (including 0 speed), can output 150% to 200% torque. ^[8]

Inverter selection

The type of inverter is selected. According to the requirements of the type of production machinery, speed range, static speed accuracy, and starting torque, it is determined which inverter is most suitable for the control method. The so-called appropriateness is both easy to use and economical, so as to meet the basic conditions and requirements of the process and production ^[9] .

The motor that the inverter needs to control and the inverter itself

1) The number of poles of the motor. Generally, the number of motor poles is not more than (very appropriate, otherwise the capacity of the inverter must be appropriately increased.

2) Torque characteristics, critical torque, acceleration torque. Under the same motor power, the inverter specifications can be derated compared to the high overload torque mode. 3) Electromagnetic compatibility. In order to reduce the interference of the main power supply, a reactor can be added to the intermediate circuit or the input circuit of the inverter when used, or a front isolation transformer can be installed. Generally, when the distance between the motor and the inverter exceeds 50m, a reactor, a filter or a shielded protective cable should be connected in series between them ^[9] .

Selection of inverter power

The system efficiency is equal to the product of the inverter efficiency and the motor efficiency. The system efficiency is higher only if both are working at higher efficiency. From the perspective of efficiency, when selecting the inverter power, pay attention to the following points: ^[9]

1) The inverter power value is most suitable when it is equivalent to the motor power value, so as to facilitate the inverter to run at a high efficiency value. ^[9]

2) When the power classification of the inverter is not the same as the motor power classification, the power of the inverter should be as close to the power of the motor as possible, but it should be slightly greater than the power of the motor. ^[9]

3) When the motor is frequently started, braked, or under heavy load and operated more frequently, a higher-level inverter can be selected to use the inverter for long-term and safe operation. ^[9]

4) After testing, the actual power of the motor does have a surplus. You can consider using an inverter with a power smaller than the motor power, but pay attention to whether the instantaneous peak current will cause an overcurrent protection action. ^[9]

5) When the power of the inverter and the motor are not the same, the settings of the energy saving program must be adjusted accordingly in order to achieve a higher energy saving effect ^[9] .

Selection of inverter box structure

The cabinet structure of the inverter must be adapted to the environmental conditions, that is, factors such as temperature, humidity, dust, pH, and corrosive gases must be considered. The following types of structures are commonly available for users: ^[9]

1) The open type IPOO has no chassis itself, and is suitable for installation in the electric control box or the screen, panel, or rack in the electrical room. Especially when multiple inverters are used in a concentrated manner, this type is better, but the environmental conditions require High; ^[9]

2) The closed IP20 type is suitable for general use, where there is a small amount of dust or a little temperature and humidity; ^[9]

3) The sealed IP45 type is suitable for environments with poor industrial site conditions; ^[9]

4) The sealed IP65 type is suitable for occasions with poor environmental conditions, water, dust and certain corrosive gases ^[9] .

Determination of inverter capacity

Reasonable capacity selection is an energy saving measure. According to the available information and experience, there are three simpler methods: ^[9]

1) The actual power of the motor is determined. First measure the actual power of the motor to select the capacity of the inverter. ^[9]

2) Formula method. When one inverter is used for multiple motors, it should be satisfied that at least the influence of the starting current of one motor must be considered to avoid overcurrent tripping of the inverter. ^[9]

3) Motor rated current method inverter. ^[9]

The selection process of the inverter capacity is actually the best matching process between the inverter and the motor. The most common and safer method is to make the inverter's capacity greater than or equal to the rated power of the motor. How much the actual power differs from the rated power is usually because the selected capacity of the equipment is too large, and the actual required capacity is small. Therefore, it is reasonable to choose the inverter according to the actual power of the motor. Avoid using too large inverters and increasing investment. . For light load, the inverter current should generally be selected according to 1.1N (N is the rated current of the motor), or according to the maximum motor power indicated by the manufacturer in the product that matches the output power rating of the inverter ^[9] .

Inverter main power

1) Power supply voltage and fluctuation. Special attention should be paid to adapt to the low-voltage protection setting value of the inverter, because in actual use, the possibility of low grid voltage is greater. ^[9]

2) Main power frequency fluctuations and harmonic interference. This kind of interference will increase the heat loss of the inverter system, resulting in increased noise and reduced output. ^[9]

3) When the inverter and the motor are working, their own power consumption. When designing the system main power supply, the power consumption factors of both should be taken into account ^[9] .

Development direction of inverter

The substrate of power electronic devices has been changed from Si (silicon) to SiC (silicon carbide), so that new power electronic components have the advantages of high voltage resistance, low power consumption, and high temperature resistance; and manufactured drive devices with small size and large capacity; Permanent magnet motors are also being developed. With the rapid popularization of IT technology, inverter-related technologies have developed rapidly, and they will mainly develop in the following areas in the future: ^[10]

Intelligent inverter network

The intelligent inverter does not need to set many parameters when it is used. It has the function of fault self-diagnosis, which has high stability, high reliability and practicality. The use of the Internet can realize the linkage of multiple inverters, and even a comprehensive inverter management and control system based on the factory. ^[10]

Specialization and integration of frequency converters

Specializing in the manufacture of inverters can make the inverters perform better in a certain field, such as fans, pump inverters, elevator-specific inverters, crane-specific inverters, and tension-control inverters. In addition, the inverter has a tendency to be integrated with the motor, so that the inverter becomes a part of the motor, which can make the size smaller and the control more convenient. ^[10]

Inverter energy saving and environmental protection

Protecting the environment and manufacturing "green" products is a new concept for human beings. The electric drive device should focus on the consideration of energy saving and low pollution of the inverter's energy conversion process, so as to reduce the noise of the inverter during use and the pollution of power harmonics to the power grid to a minimum. ^[10]

Inverter adapts to new energy

Fuel cells that use solar energy and wind power now stand out with their low prices, and they are likely to catch up later. The biggest feature of these power generation equipment is that the capacity is small and scattered. In the future, the inverters will need to adapt to such new energy sources, which must be efficient and low power consumption. Now power electronics technology, microelectronics technology and modern control technology are developing at an amazing speed, and the frequency conversion speed regulation transmission technology has also made rapid progress. This progress is concentrated in the large capacity and frequency conversion of AC speed control devices. High-performance, multi-functional, and miniaturized structure. ^[10]