In industrial production and product processing and manufacturing, fans and pumps are widely used; their power consumption and throttling losses of related equipment such as valves and baffles, as well as maintenance and repair costs account for 7% to 25% of production costs. It is a small production cost. With the deepening of economic reform and the intensification of market competition, energy saving and consumption reduction has become one of the important means to reduce production costs and improve product quality.
The frequency conversion speed regulation technology developed in the early 1980s is in line with the requirements of the development of industrial production automation, creating a new era of intelligent motors. Change the old mode that ordinary motors can only run at a constant speed, so that the motor and its drag load can adjust the speed output according to the production process requirements without any modification, thereby reducing the power consumption of the motor and achieving the purpose of efficient operation of the system .
At the end of the 1980s, this technology was introduced into my country and popularized. It has been practically used in motor transmission equipment in various industries such as electric power, metallurgy, petroleum, chemical industry, papermaking, food, textile and so on. At present, frequency conversion speed regulation technology has become a major development direction of modern electric drive technology. Excellent speed regulation performance, significant power saving effect, improve the operating conditions of existing equipment, improve the safety and reliability of the system and equipment utilization, prolong the service life of equipment and other advantages, which are fully reflected with the continuous expansion of application fields. .
Usually in industrial production and product processing and manufacturing, fan equipment is mainly used in boiler combustion systems, drying systems, cooling systems, ventilation systems and other occasions. 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, no matter the size of the production demand, the fan must run at full speed, and the change of the operating conditions causes the energy to be consumed by the throttling loss of the damper and the baffle. In the production process, not only the control accuracy is limited, but also a lot of energy waste and equipment loss. As a result, the production cost is increased, the service life of the equipment is shortened, and the maintenance and repair costs of the equipment remain high.
Pump equipment also has a broad application space in the production field. Centrifugal pumps, axial flow pumps, gear pumps, plunger Pumps and other equipment. Moreover, according to different production requirements, throttling equipment such as adjustment valves, return valves, and globe valves are often used to control signals such as flow, pressure, and water level. In this way, it not only causes a lot of energy waste and damage to the sealing performance of pipelines and valves, but also accelerates the wear and cavitation of pump chambers and valve bodies, which damages equipment, affects production, and endangers product quality in severe cases.
Most of the fans and pumps are directly driven by asynchronous motors, which have disadvantages such as large starting current, mechanical shock, and poor electrical protection characteristics. It not only affects the service life of the equipment, but also cannot act instantaneously to protect the equipment when the load has a mechanical failure, and the pump is often damaged and the motor is also burned.
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 frequency converters) is easy to operate, maintenance-free, has high control accuracy, and can achieve high functionality. The inverter-driven scheme began to gradually replace the control scheme of dampers, baffles and valves.
The basic principle of variable frequency speed regulation technology is based on the relationship between the motor speed and the input frequency of the working power supply: n =60 f(1-s)/p, (where n, f, s, p represent the speed, input frequency, Motor slip, motor pole pairs); the purpose of changing the motor speed is achieved by changing the working power frequency of the motor. The frequency converter is a comprehensive electrical product based on the above principles, which adopts AC-DC-AC power conversion technology, power electronics, microcomputer control and other technologies.
According to the basic laws of fluid mechanics, it can be known that 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 P: Q∝n, H∝n2, P∝n3; namely , the flow is proportional to the speed, the pressure is proportional to the square of the speed, and the shaft power is proportional to the cube of the speed.
Taking 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 opened is r0, and the rated working condition The corresponding pressure is H1, and the outlet flow is Q1. The flow-speed-pressure relationship curve is shown in the figure below.
In the field control, the water pump is usually used to control the flow by running the outlet valve at a constant speed. When the flow rate is reduced by 50% from Q1 to Q2, the valve opening is reduced so that the resistance characteristic of the pipe network changes from r0 to r1, and the operating point of the system moves from the original point A to point B along the direction I; H1 becomes H2. The actual value of the pump shaft power (kW) can be obtained by the formula: P =QH/(η cη b)×10-3. Among them, P, Q , H , η c , η b represent power, flow rate, pressure, pump efficiency, and transmission efficiency, respectively, and the direct transmission is 1. Assuming that the total efficiency (η cη b) 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 n of the pump is changed by means of speed regulation, when the flow rate is reduced by 50% from Q1 to Q2, the resistance characteristic of the pipe network is the same curve r0, and the operating point of the system will move from the original point A to the point C along the direction II. The operation of the water pump is also more reasonable. When the valve is fully open and there is only the resistance of the pipe network, the system meets the flow requirements of the site, and the energy consumption is bound to be reduced. At this time, the power consumption saved by the motor is the area difference between AQ1OH1 and CQ2OH3. Compared with the valve opening adjustment and the pump speed control, it is obvious that the use of the pump speed control is more effective and reasonable, and has a significant energy saving effect.
From the above comparison, it is not difficult to conclude that when the demand for the flow of the pump on site is reduced from 100% to 50%, the speed adjustment will save the power corresponding to BCH3H2 compared with the original valve adjustment, and the energy saving rate will be above 75%.
Similarly, if the frequency conversion speed regulation technology is used to change the speed of pumps and fans to control other process control parameters such as on-site pressure, temperature, water level, etc., the above comparison results can also be drawn according to the control characteristics of the system. . That is to say, the method of changing the speed of the motor by using the frequency conversion speed regulation technology is more energy-saving and economical than using valves and baffles to adjust, and the operating conditions of the equipment will also be significantly improved.
