A brief analysis of the application status and prospects of frequency converters
Abstract: Frequency converters have good development and application prospects. This article outlines the development and application of frequency converters in our country and the work we should do in this technology in the future.
Keywords: frequency converter; application; prospect
Foreword
In recent years, with the rapid development of power electronics technology, computer technology, and automatic control technology , AC transmission and control technology has become one of the most rapidly developing technologies. Electric transmission technology is facing a historical revolution, that is, AC speed regulation replacing DC speed regulation and computer digital control technology replacing analog control technology have become development trends. Motor AC frequency conversion speed regulation technology is a major means today to save energy, improve process flow, improve product quality and environment, and promote technological progress. Variable frequency speed regulation is recognized at home and abroad as the most promising speed regulation method due to its excellent speed regulation and starting and braking performance, high efficiency, high power factor and power saving effect, wide range of applications and many other advantages. It is of very positive significance to have an in-depth understanding of the trends of AC transmission and control technology.
1. Energy-saving principle of frequency converter speed regulation operation
The device that realizes frequency conversion speed regulation is called a frequency converter. Frequency converters generally consist of rectifiers, filters, drive circuits, protection circuits and controllers (MCU/DSP). First, the single-phase or three-phase AC power supply is passed through the rectifier and filtered by the capacitor to form a DC voltage with a basically fixed amplitude and is applied to the inverter. The on-off control of the inverter power component is used to obtain the output end of the inverter. A rectangular pulse waveform of a certain shape. Here, the voltage amplitude of the rectangular pulse is controlled by changing the width of the rectangular pulse; the output frequency is controlled by changing the modulation period, so that the output voltage and frequency are controlled simultaneously on the inverter, thereby satisfying the U/f coordinated control of variable frequency speed regulation. requirements. The advantage of PWM is that it can eliminate or suppress low-order harmonics, allowing the load motor to operate under a nearly sinusoidal alternating voltage, with small torque pulses and a wide speed range.
The motor speed using PWM control is limited by the upper speed limit. For example, for compressors, it generally does not exceed 7000r/rain. The compressor speed using PAM control method can be increased by about 1.5 times, which greatly improves the ability of rapid speed increase and deceleration. At the same time, because PAM has a shaping effect on the current waveform when adjusting the voltage, it can achieve higher efficiency than PWM. In addition, it also has unparalleled advantages in anti-interference and PWM, which can suppress the generation of high-order harmonics and reduce pollution to the power grid. After adopting the variable frequency speed regulation technology of this control method, the motor stator current dropped by 64%, the power frequency dropped by 30%, and the glue discharging pressure dropped by 57%. According to motor theory, the speed of an asynchronous motor can be expressed as:
n=60·f 8 (1-8)/p
f s is the motor stator frequency (that is, the power grid frequency), P the number of winding pole pairs of the motor stator, s is the slip rate. It can be seen from the above formula that as long as the slip rate is not too large, the rotation speed n can be approximately considered to be proportional to f s, which means that by continuously and smoothly changing the power frequency, a wide range of continuous smooth speed regulation of the AC motor can be achieved. For example, a motor with a rated speed of 3000 rpm is powered by a frequency converter. If the starting frequency is set to 5HZ, then the frequency converter can run at any frequency between 5-50HZ, and the motor can run at 30o-3000 At any speed between rpm, the motor is started by the mains power, which ensures a balanced start, large torque and energy saving.
The 50HZ380V mains power goes through the rectification and filtering link to become direct current, and then goes through the inverter link to become alternating current with adjustable frequency and amplitude. In the main circuit of the frequency converter, the electric energy undergoes AC-DC-AC conversion, so this type of frequency converter is called AC-DC-AC type frequency converter.
2. Overview of the development and application of frequency converter technology in my country
(1) Development of frequency converters
With the continuous development of production technology, DC drive The weak links are gradually revealed. Due to the existence of the commutator, the maintenance of the DC motor is increased, and the capacity, maximum speed and use environment of the single machine are limited. People began to turn to asynchronous motors with simple structure, reliable operation, easy maintenance and low price. However, the speed regulation performance of asynchronous motors is difficult to meet the needs of production. Therefore, since the 1930s, people have devoted themselves to the research of AC speed regulation technology, but the progress has been slow. For a long period of time, DC speed regulation has dominated the field of electrical transmission with its excellent performance. Since the 1960s, especially since the 1970s, the rapid development of power electronics technology, control technology and microelectronics technology has made the performance of AC speed regulation comparable to that of DC speed regulation. At present, AC speed regulation has entered an era of gradually replacing DC speed regulation.
(2) Application of frequency converters in my country
Various frequency converters are mainly used to adjust the speed of AC motors (asynchronous motors or synchronous motors) and are recognized as the most ideal and most useful AC motors. In addition to excellent speed regulation performance, Qiantu's speed regulation scheme also has significant energy-saving effects. It is an ideal speed regulation device for enterprise technological transformation and product upgrading.
Since being introduced to China in the 1980s, frequency converters have developed rapidly and been widely used as an increasingly important automation equipment in energy-saving applications and speed process control.
1. Frequency converter and energy saving
The original use of frequency converter was speed control, but currently it is mostly used in China for energy saving. China is a large energy-consuming country with low energy utilization rate and insufficient energy reserves. In 2003, 60-70% of China's electricity consumption was power electricity. Of the total motor capacity of 580 million kilowatts, less than 20 million kilowatts were equipped with variable frequency control. According to analysis, in China, there are at least 180 million kilowatts of motors with variable loads and energy-saving potential. Therefore, the country vigorously promotes energy-saving measures and emphatically recommends variable frequency speed regulation technology.
The application of variable frequency speed regulation can greatly improve the control accuracy of the motor speed, allowing the motor to run at the most energy-saving speed. Taking the fan water 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 decreases and the fan speed decreases, its power decreases according to the cube of the speed. Therefore, the power saving effect of precise speed regulation is very considerable. Similarly, many variable load motors generally produce motor capacity based on maximum demand, so the design margin is relatively large. In actual operation, the proportion of light-load operation time is very high. If 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 huge.
For energy saving purposes, frequency converters are widely used in various industries. Take the electric power industry as an example. Due to the large-scale power shortage in China, electric power investment will continue to grow. At the same time, the national electric power reform plan has put forward requirements for cost control of power plants. Reducing internal power consumption has become the focus of power plants. Therefore, frequency converters have an important role in the electric power industry. Huge development potential, especially for high-voltage inverters and high-power inverters.
2. Frequency converter and process control (speed control)
At present, China's equipment control level is still relatively low compared with developed countries, and the manufacturing process and efficiency are not high, so Improving the level of equipment control is critical. Since variable frequency speed regulation has the advantages of wide speed regulation range, high speed regulation accuracy, and good dynamic response, in many applications that require precise speed control, frequency converters are playing a significant role in improving process quality and production efficiency.
3. Frequency conversion home appliances
In addition to industrial-related industries, in ordinary households, saving electricity bills, improving the performance of home appliances, protecting the environment, etc. have received more and more attention. Frequency conversion home appliances have become the Another broad market and application trend of devices. 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 accuracy.
3. Current status and development prospects of domestic frequency conversion technology
There are already many frequency converter manufacturers in China, but most of their products are V/F control and voltage space vector There should be no problem in controlling the frequency converter when used on loads that do not require high speed regulation accuracy and dynamic performance. Most industrial applications have this kind of load. The most important requirement for inverters in this application is reliability. The main reason why domestic inverters do not occupy a high share of the domestic market is that the product quality is not good enough. V/F control and voltage space vector control inverters are technically much simpler than vector control inverters. Since most domestic manufacturers are produced in manual workshops, the technology is poor, the detection methods are limited, and the quality is consistent. and stability are difficult to guarantee. It is also a V/F controlled inverter. The quality of foreign products is better than that of domestic products. This may be due to the gap in production technology. The biggest gap is in the manufacturing industry of semiconductor power devices, which is still a blank in China.
Another aspect of inverter technology is application technology. Over the years, the State Economic and Trade Commission has been committed to the development, promotion and application of frequency converter technology in conjunction with relevant national departments. It has provided key support in technology development and technological transformation, organized the evaluation and recommendation work of frequency conversion speed regulation technology, and promoted and applied frequency converter technology. Speed ??regulation technology is a key investment direction for the energy-saving technological transformation of fans and water pumps. At the same time, units are encouraged to carry out the same loan and repayment method, focusing on development, demonstration projects, and promotion and application. It also established the fan and water pump energy-saving center to carry out information consultation and training. From 1995 to 1997, my country invested 350 million yuan in the transformation of frequency conversion speed regulation technology for fans and water pumps in three years. The total transformation capacity reached 1 million kilowatts, which can save 700 million kilowatt hours of electricity annually. The average investment payback period is about 2 years. According to relevant data, the application of variable frequency speed regulation technology in our country has achieved considerable results, with sales of billions of yuan every year, indicating that the frequency converter application in our country has been very extensive. From simple manual control to multi-machine control based on RS-485 network, it can be connected with computers and PLC to form a complex control system. In large-scale comprehensive automation systems, advanced control and optimization technologies, large-scale complete sets of special systems, such as continuous casting and rolling production lines, high-speed paper production lines, cable and optical fiber production lines, chemical fiber production lines, building materials production lines, etc., the function of the frequency converter is electrical transmission control, and its control It has very high requirements on complexity, control accuracy and dynamic response, and has completely replaced DC speed control technology. In recent years, in terms of function, the frequency converter has used advanced control theory to develop control functions such as coiling, lifting, master-slave, etc., making the construction of application systems more convenient and easier, and raising the application technology of frequency converters to a new level. level.
IV. Conclusion
The technology of variable frequency speed regulation is becoming more and more widely penetrated into the industry. Its significant advantages of energy saving, labor saving, and ease of forming an automatic control system are also an effective way to transform and tap potential and increase benefits by applying frequency conversion speed regulation technology. Especially in enterprises with a lot of high-energy consumption and low-output equipment, the use of frequency conversion speed regulation devices will enable the enterprises to gain huge economic benefits. At the same time, this is also the need for the sustainable development of the national economy.