Analysis on energy-saving application of frequency converter speed regulation technology in cement equipment

In production enterprises, fans and pumps are widely used. Their electricity consumption and throttling losses of valves, baffles and related equipment, as well as maintenance and repair costs account for 7% to 25% of production costs, which is a considerable production cost. With the continuous deepening of economic reform, market competition has continued to intensify; energy saving and consumption reduction have become one of the important means to reduce production costs and improve product quality. The variable frequency speed regulation technology developed in the early 1980s conforms to the requirements of industrial production automation and has ushered in a new era of intelligent motors. It has changed the old mode that ordinary motors can only run at a fixed speed, so that the motor and its drag load can adjust the speed output according to the production process requirements without any changes, thereby reducing the power consumption of the motor and achieving the purpose of efficient operation of the system. In the late 1980s, this technology was introduced into China and promoted. It has now been applied in motor transmission equipment in various industries such as electricity, metallurgy, petroleum, chemical industry, papermaking, food, and textiles. At present, variable frequency speed regulation technology has become a major development direction of modern power transmission technology. The advantages of excellent speed regulation performance, significant power saving effect, improvement of the operating conditions of existing equipment, improvement of system safety and reliability and equipment utilization, and extension of equipment service life have been fully reflected as the application field continues to expand.

Usually in industrial production and product processing manufacturing, fan equipment is mainly used in boiler combustion systems, drying systems, cooling systems, ventilation systems and other occasions. According to production needs, furnace pressure, wind speed, air volume, temperature and other indicators are controlled and adjusted to adapt to process requirements and operating conditions. The most commonly used control method is to adjust the size of the damper and baffle opening to adjust the controlled object. In this way, regardless of the size of the production demand, the fan must run at full speed, and the change in operating conditions causes the energy to be consumed by the throttling loss of the damper and baffle. In the production process, not only is the control accuracy limited, but it also causes a lot of energy waste and equipment loss. This leads to increased production costs, shortened equipment life, and high equipment maintenance and repair costs. Pump equipment also has a broad application space in the production field. Water pumping stations, water tank supply and drainage systems, industrial water (oil) circulation systems, and heat exchange systems all use centrifugal pumps, axial flow pumps, gear pumps, plunger pumps and other equipment. Moreover, according to different production needs, throttling devices such as regulating valves, reflux valves, and stop valves are often used to control signals such as flow, pressure, and water level. This not only causes a lot of energy waste and damages the sealing performance of pipelines, valves, etc.; it also accelerates the wear and cavitation of the pump chamber and valve body, which in severe cases damages the equipment, affects production, and endangers product quality. Most fans and pumps are directly driven by asynchronous motors , which have disadvantages such as large starting current, mechanical shock, and poor electrical protection characteristics. Not only does it affect the service life of the equipment, but it also cannot instantly protect the equipment when a mechanical failure occurs in the load. It is common for the pump to be damaged and the motor to be burned at the same time. In recent years, due to the urgent need for energy saving and the continuous improvement of product quality, the use of variable frequency speed regulators (referred to as variable frequency drives) is easy to operate, maintenance-free, high in control accuracy, and can achieve high functionality; therefore, the use of variable frequency drive solutions has begun to gradually replace the control solutions of dampers, baffles, and valves. The basic principle of variable frequency speed regulation technology is that the motor speed is proportional to the working power supply input frequency: n=60f(1-s)/p, (where n, f, s, and p represent the speed, input frequency, motor slip rate, and number of motor pole pairs, respectively); the purpose of changing the motor speed is achieved by changing the working power supply frequency of the motor. The frequency converter is a comprehensive electrical product based on the above principle, using AC-DC-AC power conversion technology, power electronics , microcomputer control and other technologies.

According to the basic laws of fluid mechanics, fans and pumps are all square torque loads, and their speed n has the following relationship with flow rate Q, pressure H and shaft power P: Q∝n, H∝n2, P∝n3; that is, flow rate is proportional to speed, pressure is proportional to the square of speed, and shaft power is proportional to the cube of speed.

Take a water pump as an example. Its outlet pressure head is H0 (the outlet pressure head is the static pressure difference between the pump inlet and the pipeline outlet), the rated speed is n0, the pipe resistance characteristic when the valve is fully open is r0, the corresponding pressure under rated working conditions is H1, and the outlet flow rate is Q1. The flow-speed-pressure relationship curve is shown in the figure below. In field control, the outlet valve of the water pump is usually used to control the flow rate. When the flow rate decreases from Q1 to Q2 by 50%, the valve opening decreases, causing the pipe network resistance characteristic to change from r0 to r1, and the system working point moves from the original point A to point B along the direction I; the pressure H1 changes to H2 under its throttling effect. The actual value of the water pump shaft power (kW) can be obtained by the formula: P=Q& mid dot;H／(ηc·ηb)×10-3. Among them, P, Q, H, ηc, and ηb represent power, flow, pressure, water pump efficiency, and transmission efficiency, respectively, and direct transmission is 1. Assuming that the total efficiency (ηc·ηb) is 1, when the pump moves from point A to point B, the power consumption saved by the motor is the area difference between AQ1OH1 and BQ2OH2. If the speed control method is used to change the speed n of the pump, when the flow rate decreases by 50% from Q1 to Q2, the pipe network resistance characteristic is the same curve r0, and the system working point will move from the original point A to point C along direction II, and the operation of the pump will become more reasonable. When the valve is fully open and there is only pipe network resistance, the system meets the flow requirements of the site, and the energy consumption is bound to decrease. At this time, the power consumption saved by the motor is the area difference between AQ1OH1 and CQ2OH3. Comparing the use of valve opening adjustment and pump speed control, it is obvious that the use of pump speed control is more effective and reasonable, and has a significant energy-saving effect. In addition, the system pressure H will increase when the valve is adjusted, which will threaten and damage the sealing performance of the pipeline and valve; while when the speed is adjusted, the system pressure H will decrease with the decrease of the pump speed n, so it will not have a negative impact on the system.