Variable-frequency drive (VFD) is a power control device that controls the AC motor by changing the working frequency of the motor by applying variable frequency technology and microelectronic technology. The frequency converter is mainly composed of rectification (AC to DC), filtering, inverter (DC to AC), braking unit, drive unit, and detection unit micro processing unit. The inverter adjusts the voltage and frequency of the output power supply by the internal IGBT breaking, and supplies the required power supply voltage according to the actual needs of the motor, thereby achieving the purpose of energy saving and speed regulation. In addition, the frequency converter 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. There are several ways to classify a frequency converter:
According to the main circuit working mode classification, it can be divided into voltage type inverter and current type inverter;
According to the switch mode classification, it can be divided into PAM control inverter, PWM control inverter and high carrier frequency PWM control inverter;
According to the working principle classification, it can be divided into V/f control inverter, slip frequency control inverter and vector control inverter;
According to the purpose classification, it can be divided into general-purpose inverter, high-performance special inverter, high-frequency inverter, single-phase inverter and three-phase inverter, hoist inverter, etc.
Selection of inverter selection:Inverter selection: The main points need to be determined:
(1) The purpose of using frequency conversion: constant voltage control or constant current control?
(2) The load type of the inverter (such as the vane pump or the volumetric pump, etc., pay special attention to the performance curve of the load, and the performance curve determines the method in time.
(3) Matching problem between inverter and load:
A. Voltage matching: The rated voltage of the inverter matches the rated voltage of the load.
B. Current matching: Ordinary centrifugal pump, the rated current of the inverter is consistent with the rated current of the motor. For special loads such as deep pumps, it is necessary to refer to the motor performance parameters to determine the current and overload capability with the maximum current.
C. Torque matching: This situation may occur when a constant torque load or a reduction gear is provided.
(4) When using a frequency converter to drive a high speed motor. Due to the low reactance of high-speed motors, the increase in higher harmonics leads to an increase in the output current value. Therefore, the selection of the inverter for high-speed motors is slightly larger than that of ordinary motors.
(5) If the inverter needs to run long cable, it is necessary to take measures to restrain the influence of the long cable to the ground coupling capacitor and avoid the insufficient output of the inverter. Therefore, in this case, the inverter capacity should be amplified by one gear or at the frequency conversion. An output reactor is installed at the output of the device.
(6) For some special applications, such as high temperature and high sea wave, the derating of the inverter will be caused at this time. The capacity of the inverter should be amplified. The selection of the inverter: First confirm the installation environment of the inverter:
A, working temperature. The inside of the inverter is a high-power electronic component that is highly susceptible to operating temperature. The product is generally required to be 0 to 55 degrees. However, in order to ensure safe and reliable work, you should consider leaving room for use, preferably below 40 degrees. In the control box, the inverter should generally be installed in the upper part of the cabinet, and strictly follow the installation requirements in the product manual. It is absolutely not allowed to install the heating element or the heat-prone component close to the bottom of the inverter.
B, ambient temperature. When the temperature is too high and the temperature changes greatly, the inside of the inverter is prone to condensation, and its insulation performance is greatly reduced, and may even cause a short circuit accident. Add desiccant and heater to the tank as necessary. In the water treatment room, the general water vapor is relatively large, and if the temperature changes greatly, this problem will be more prominent.
C, corrosive gas. If the concentration of corrosive gas is large, it will not only corrode the leads of the components, printed circuit boards, etc., but also accelerate the aging of the plastic devices and reduce the insulation performance.
D, vibration and shock. When the control cabinet equipped with the inverter is subjected to mechanical vibration and shock, it may cause poor electrical contact and poor contact between the module and the heat sink, resulting in burning of the module. Huai'an Thermal Power has had such problems. In addition to improving the mechanical strength of the control cabinet, away from the vibration source and the impact source, the anti-vibration rubber pad should be used to fix the control box and the AC contactor that generates vibration in the fixed box. Check and maintain in time after the equipment has been running for a period of time.
E, electromagnetic interference. Due to the need for rectification and frequency conversion, the frequency converter generates a lot of interference electromagnetic waves around. These high-frequency electromagnetic waves have certain interference to nearby instruments and instruments. Therefore, the instrument and electronic system in the cabinet should use a metal casing to shield the inverter from interference with the instrument. All components should be reliably grounded. In addition, shielded control cables should be used for wiring between electrical components, instruments and meters, and the shielding layer should be grounded. If the electromagnetic interference is not handled well, the whole system will not work, resulting in failure or damage of the control unit.
Select the type of inverter, according to the type of production machinery, speed range, static speed accuracy, starting torque requirements, it is most appropriate to decide which inverter to use. The so-called suitability is both easy to use and economical to meet the basic conditions and requirements of the process and production.
1. The motor that needs to be controlled and the inverter itself
(1) The number of poles of the motor. Generally, the number of poles of the motor is not more than (very suitable, otherwise the capacity of the inverter should be appropriately increased.
(2) Torque characteristics, critical torque, and acceleration torque. In the case of the same motor power, the inverter specifications can be derated in relation to the high overload torque mode.
(3) Electromagnetic compatibility. In order to reduce the main power interference, the reactor can be added to the intermediate circuit or the inverter input circuit, or the front isolation transformer can be installed. Generally, when the distance between the motor and the inverter is more than 50m, a reactor, a filter or a shielded protective cable should be connected in between.
2. Selection of inverter power The system efficiency is equal to the product of inverter efficiency and motor efficiency. Only when both are working at higher efficiency, the system efficiency is higher. From the perspective of efficiency, the following points should be noted when selecting the inverter power:
(1) When the power value of the inverter is equivalent to the motor power value, it is most suitable for the inverter to operate at a high efficiency value.
(2) When the power classification of the inverter is different from the motor power classification, the power of the inverter should be as close as possible to the power of the motor, but should be slightly larger than the power of the motor.
(3) When the motor is frequently started, braked or under heavy load and is working more frequently, a large-scale inverter can be selected to operate the inverter for a long time and safely.
(4) After testing, the actual power of the motor does have a surplus. You can consider the inverter with power less than the motor power, but pay attention to whether the instantaneous peak current will cause overcurrent protection.
(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 to achieve a higher energy-saving effect.
3, the selection of the inverter box structure The chassis structure of the inverter should be compatible with the environmental conditions, that is, temperature, humidity, dust, pH, corrosive gas and other factors must be considered. There are several types of structures that are available for users to choose from:
(1) The open type IPOO type has no chassis itself, and is suitable for being installed in the screen, disk or rack in the electric control box or in the electric room. Especially when multiple inverters are used intensively, this type is better, but the environmental conditions are required. Higher
(2) The closed type IP20 is suitable for general use, and may have a small amount of dust or a little temperature and humidity;
(3) The sealed IP45 type is suitable for environments with poor industrial site conditions;
(4) The sealed IP65 type is suitable for occasions with poor environmental conditions, such as water, dust and certain corrosive gases.
4, the determination of the capacity of the inverter reasonable capacity selection itself is a measure of energy saving. Based on available data and experience, there are three simple methods:
(1) The actual power of the motor is determined. First measure the actual power of the motor to select the capacity of the inverter.
(2) Formula method. When a frequency converter is used for multiple motors, it should be satisfied: at least consider the influence of the starting current of one motor to avoid the overcurrent trip of the frequency converter.
(3) Motor rated current method inverter. The selection process of the inverter capacity is actually the best matching process between the inverter and the motor. The most common and safer is to make the capacity of the inverter greater than or equal to the rated power of the motor, but the actual matching should consider the motor. The difference between the actual power and the rated power is usually the capacity of the equipment is too large, but the actual required capacity is small. Therefore, it is reasonable to select the inverter according to the actual power of the motor, avoiding the oversized inverter and increasing the investment. . For light loads, 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 matching the output power rating of the inverter.
5, the main power supply
1) Power supply voltage and fluctuations. Special attention should be paid to the inverter's low voltage protection setting value, because in actual use, the grid voltage is less likely to be lower.
2) Main power frequency fluctuation and harmonic interference. This interference increases the heat loss of the inverter system, resulting in increased noise and reduced output.
3) The power consumption of the inverter and the motor when it is working. When designing the system's main power supply, the power consumption factors of both should be taken into account.
The use of frequency converters for variable frequency speed control of mechanical equipment can achieve energy-saving effect. The use of multiple motors to operate at proportional speed can achieve the effect of labor-saving automation. The effect of increasing the speed of the motor can increase the output. High-frequency motors can be used for high-speed operation. To achieve the effect of increasing production, it can replace the DC motor in the harsh environment to reduce maintenance, choose the stepless speed to improve the quality of the product, and use the air conditioner compressor speed control for continuous temperature control, which can improve comfort. Effect.
1, the development of the inverter
With the continuous development of production technology, the weak links of DC drag gradually emerged. Due to the existence of the commutator, the maintenance of the DC motor is increased, and the stand-alone capacity, the maximum speed, and the use environment are limited. People began to turn to asynchronous motors with simple structure, reliable operation, convenient maintenance and low price. However, the speed regulation performance of asynchronous motors is difficult to meet the needs of production. Thus, since the 1930s, people have been working on the exchange of speed control technology, but progress has been slow. For a long period of time, DC speed regulation has dominated the field of electric transmission with its excellent performance. Since the 1960s, especially since the 1970s, the rapid development of power electronics technology, control technology and microelectronic technology has made AC speed regulation performance comparable to DC speed regulation. At present, the AC speed regulation has entered an era of gradually replacing DC speed regulation.
2, the application of the inverter
The frequency converter is mainly used for the adjustment of the speed of the AC motor (asynchronous motor or synchronous motor). It is recognized as the most ideal and most promising speed control scheme for AC motors. In addition to excellent speed regulation performance, the inverter has significant The energy-saving effect is the ideal speed control device for technological transformation and product upgrading. Since its introduction in China in the 1980s, frequency converters have been rapidly developed and widely used as an increasingly important automation device in energy-saving applications and speed process control.
(1) The initial use of inverters and energy-saving inverters is speed control, but currently more applications in China are energy-saving. China is a big energy consumer with low energy efficiency and insufficient energy reserves. Of the electricity consumption in China in 2003, 60-70% was power, and in the total capacity of 580 million kilowatts of motor, only less than 20 million kilowatts of motor were controlled by frequency conversion. According to analysis, in China, there are at least 180 million kilowatts of motors with variable load and energy saving potential. Therefore, the state strongly advocates energy-saving measures and has recommended the variable frequency speed control technology.
The application of frequency conversion speed regulation can greatly improve the control precision of the motor speed and make the motor run at the most energy-saving speed. Taking the fan pump as an example, according to the principle of fluid mechanics, the shaft power is proportional to the cube of the rotational speed. When the required air volume is reduced and the fan speed is reduced, its power drops in the third order of the speed. Therefore, the power saving effect of precise speed regulation is very impressive. Similarly, many variable load motors generally produce the capacity of the motor at the maximum demand, so the design margin is too large. In actual operation, the proportion of time spent on light load is very high. If the frequency conversion speed regulation is adopted, the working efficiency during light load operation can be greatly improved. Therefore, the energy saving potential of variable loads is enormous.
As an energy saving purpose, inverters are widely used in various industries. Taking the power industry as an example, due to the lack of electricity in China, power investment will continue to grow. At the same time, the national power reform program puts requirements on the cost control of power plants. The reduction of internal power consumption has become the focus of power plants. Great development potential, especially high-voltage inverters and high-power inverters.
(2) Inverter and process control (speed control) At present, the level of equipment control in China is relatively low compared with developed countries, and the manufacturing process and efficiency are not high. Therefore, it is very important to improve the level of equipment control. Because frequency conversion speed regulation has the advantages of wide speed regulation range, high speed regulation precision and good dynamic response, in many applications that require precise speed control, the frequency converter is playing a significant role in improving process quality and production efficiency.
(3) In addition to industry-related industries, frequency conversion appliances are attracting more and more attention in the ordinary households, saving electricity, improving the performance of home appliances, and protecting the environment. Frequency conversion appliances have become another broad market and application trend of inverters. Refrigerators, washing machines, household air conditioners, etc. with variable frequency control have great advantages in saving electricity, reducing voltage shock, reducing noise, and improving control precision.
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