Application of VFD (Variable-frequency Drive) in cement plants

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Looking back at the development history of China's cement industry, it has gradually transformed from an industry with small scale, backward technology, and waste of resources to a modern management enterprise with large-scale group, centralized computer control, and energy saving and efficiency improvement. Along with this change, whether starting from the macro perspective of the general trend of energy conservation vigorously promoted by national policies, or starting from the company's own needs to reduce power consumption costs and increase product competitiveness, energy conservation has become an important factor in the design and construction of cement plants. An indispensable link. In the cement production process, electricity consumption is very large, and electricity costs account for a large proportion of cement production costs. In the cement plant's process equipment configuration, the fan power in the raw meal preparation and clinker burning sections accounts for about 40% of the total equipment power. Therefore, the power consumption of the fan directly affects the production cost of the cement company. Whether the power consumption of fans, especially large fans, can be controlled well is crucial to reducing cement production costs and improving the economic benefits of the enterprise. Practice has proved that using VFD (Variable-frequency Drive) to control the fan to adjust the air volume can achieve significant energy saving effects.

Currently, the newly built new dry-process production lines have large scale, high technical requirements, and large investments. Therefore, the high-temperature fans, circulation fans, and exhaust gas fans on the production lines are usually high-power and high-voltage motors. The application of high-voltage VFD (Variable-frequency Drive) is inevitable. More and more. So in practical applications, how to choose based on the actual situation of the project? What issues need to be paid attention to when formulating plans and designing construction drawings? The above issues will be discussed below based on the energy-saving principles, categories and application methods of high-voltage VFD (Variable-frequency Drive). Explore.

1. Energy-saving principle of high-voltage VFD (Variable-frequency Drive)

The so-called "energy saving" is not only saving energy consumption, but also not wasting energy. In the simplest words, it is: "I will provide you with as much as you need!". It can be known from the basic laws of fluid mechanics: Fans and pumps are all square torque loads, and their rotational speed n has the following relationship with flow rate Q, pressure H and shaft power lP: Q∝n H∝n2 P∝n3. That is, the flow rate is proportional to the rotation speed, the pressure is proportional to the square of the rotation speed, and the shaft power is proportional to the cube of the rotation speed. In actual production, the system air volume is often adjusted by adjusting the speed of the high-temperature fan. As the rotational speed decreases, the shaft power of the fan decreases in a cubic relationship with the rotational speed while maintaining the efficiency of the fan (the premise is that the wind resistance remains unchanged), and the electric energy consumed by the motor decreases sharply. For example, when the air volume drops to 80% and the rotational speed also drops to 80%, the shaft power will drop to 51% of the rated power; if the air volume drops to 50%, the shaft power will drop to 13% of the rated power, and its power saving potential Very big. When the imported guide vanes are used for adjustment, the decrease in air volume leads to a decrease in fan efficiency and an increase in wind pressure. The farther the operating conditions deviate from the rated operating conditions, the lower the efficiency. Therefore, although the air volume has decreased, the fan shaft power and the electric energy consumed by the motor have not changed much. This is the energy-saving basis for the fan frequency conversion speed regulation.

As for the methods of fan speed regulation, fluid coupling speed regulation and liquid resistance speed regulation are currently used more frequently. Fluid coupling is a transmission device that uses liquid (mostly oil) as the working medium and uses liquid to transmit energy. By changing the fullness of the liquid in the working chamber of the hydraulic coupling, the torque transmitted by the hydraulic coupling and the rotational speed of the output shaft can be changed, making the rotational speeds of the motor end and the fan end of the hydraulic coupling inconsistent, thus causing the motor speed to differ when the motor speed is different. The speed of the fan is adjusted under changing conditions to achieve the purpose of adjusting the air volume. Since the hydraulic coupling will produce slip power loss, volume loss, and mechanical loss during the adjustment process, the heat generated by these losses requires a large amount of cooling medium to cool, and the transmission efficiency of the hydraulic coupling is equal to the speed ratio, the lower the speed, The efficiency of the hydraulic coupling is lower. Therefore, the energy-saving effect of the hydraulic coupling is not ideal. It mainly has the following shortcomings: low efficiency, large losses, low speed regulation accuracy, slow speed response, unstable speed, large slip, sometimes lost rotation, the need to be equipped with corresponding oil systems and adjustment systems, and low reliability.

The liquid resistance speed regulator changes the resistance in the motor rotor circuit by adjusting the distance between the two plates in the liquid resistor, thereby changing the slip rate to achieve the purpose of changing the motor speed. Since different resistors are connected in series to the rotor coil of a wound motor, the corresponding slip rates are different. The greater the resistance, the lower the motor speed; when the resistance is zero, the motor reaches full speed. This is the basic principle of the liquid resistance start speed regulator. Since the liquid resistance speed regulator will produce slip power loss and heat loss caused by the resistance being energized during the adjustment process, the energy-saving effect of the liquid resistance speed regulator is not ideal. Its main disadvantages are: speed adjustment range