In recent years, variable frequency speed regulation has developed rapidly in water supply systems, but there is still a great deal of blindness in its practical application, resulting in unsatisfactory energy-saving effects. This article discusses some basic views on variable frequency speed regulation in water pump energy saving for discussion. [b]1 Variable Frequency Speed ​​Regulation and Water Pump Energy Saving[/b] Water pump energy saving is inseparable from the reasonable adjustment of the operating point. The adjustment methods are nothing more than the following two: adjustment of pipeline characteristic curve, such as valve closing adjustment; adjustment of water pump characteristic curve, such as water pump speed regulation, impeller cutting, etc. In terms of energy saving effect, changing the water pump performance curve is much more effective than changing the pipeline characteristic curve[1]. Therefore, changing the water pump performance curve has become the main way of water pump energy saving. Variable frequency speed regulation has obvious advantages in changing the water pump performance curve and automatic control, and is therefore widely used. However, it should be noted that there are many factors that affect the energy saving effect of variable frequency speed regulation. If it is selected blindly, it may be contrary to expectations. [b]2 Factors Affecting the Range of Variable Frequency Speed ​​Regulation[/b] Water pump speed regulation is generally a deceleration problem. When variable frequency speed regulation is adopted, the operating parameters of the pump and motor originally designed according to the power frequency state have changed significantly. In addition, factors such as pipeline characteristic curves and fixed-speed pumps running in parallel with the speed-regulating pump will have a certain impact on the speed regulation range. Speed ​​regulation beyond the range will make it difficult to achieve the purpose of energy saving. Therefore, variable frequency speed regulation cannot be unlimited. It is generally believed that variable frequency speed regulation should not be lower than 50% of the rated speed, preferably between 75% and 100%, and should be determined by calculation based on actual conditions. 2.1 The influence of pump process characteristics on speed regulation range Theoretically, the high-efficiency zone of pump speed regulation is the middle area OA1A2 of two similar working condition parabolas passing through the left and right ends of the high-efficiency zone of the power frequency (see Figure 1). In fact, when the pump speed is too low, the pump efficiency will drop sharply. Affected by this, the high-efficiency zone of pump speed regulation shrinks to PA1A2[2] (Obviously, if the operating condition point has exceeded this area, it is not advisable to use speed regulation to save energy.) In the figure, H0B is the pipeline characteristic curve, and the CB segment becomes the high-efficiency range of speed regulation operation. To simplify the calculation, point C is assumed to be located on curve OA1. Therefore, the efficiencies of point C and point A1 are theoretically equal. Point C becomes the left endpoint of the high-efficiency zone of the pump performance curve at minimum speed. Therefore, the minimum speed can be obtained as follows: Since the operating conditions of point C and point A1 are similar, according to the proportionality law, we have: (QC/Q1)2=HC/H1. Point C lies on the curve H=H0+S·Q2: HC=H0+S·QC2. Where HC and QC are unknowns, solving the equations gives: HC=H1×H0/(H1-S·Q12) QC=Q1×[H0/(H1-S·Q12)]1/2. According to the proportionality law, we have: nmin=n0×[H0/(H1-S·Q12)]1/2. 2.2 The Influence of Constant Speed ​​Pumps on Speed ​​Regulation Range In practice, water supply systems often use multiple pumps connected in parallel. Due to the high cost of investment, it is impossible to adjust the speed of all water pumps. Therefore, a combination of variable speed pumps and fixed speed pumps is generally used for water supply. In such a system, attention should be paid to ensuring that both variable speed pumps and fixed speed pumps can operate in the high-efficiency range and achieve system optimization. At this time, the fixed speed pump has a significant impact on the speed adjustment range of the variable speed pump running in parallel with it [2]. There are two main situations: 2.2.1 When the same type of water pumps are adjusted and fixed speed pumps are run in parallel, although the scheduling is flexible, the speed adjustment range is very small because it is impossible to take into account the high-efficiency working range of both the variable speed pump and the fixed speed pump. 2.2.2 When different types of water pumps are adjusted and fixed speed pumps are run in parallel, if the head at the right end of the high-efficiency range of the variable speed pump at the rated speed is equal to the head at the left end of the high-efficiency range of the fixed speed pump, the maximum range of speed adjustment can be achieved. However, at this time, the variable speed pump and the fixed speed pump are absolutely not allowed to be interchanged and run in parallel. 2.3 The Influence of Motor Efficiency on Speed ​​Regulation Range Under similar operating conditions, N∝n³, therefore, as the speed decreases, the shaft power will decrease sharply. However, if the motor output power deviates excessively from the rated power or the operating frequency deviates excessively from the power frequency, the motor efficiency will decrease too quickly, ultimately affecting the efficiency of the entire pump unit. Moreover, when a self-cooled motor operates continuously at low speed, insufficient airflow will also affect heat dissipation, threatening the safe operation of the motor. [b]3 The Influence of Pipeline Characteristic Curves on Speed ​​Regulation Energy Saving Effect[/b] Although changing the pump performance curve is the main way to save energy in pumps, the difference in speed regulation energy saving effect is very obvious in different pipeline characteristic curves. For the sake of intuitiveness, Figure 2 is used here for illustration. In three water supply systems with identical design operating conditions (i.e., the maximum design operating point is point A, and the flow rate needs to be adjusted to QB), the pump models are the same, but the pipeline characteristic curves are different: ①H=H1+S1·Q2 (H0=H1) ②H=H2+S2·Q2 (H0=H2, H1＞H2) ③H=S3·Q2 (H0=H3=0) Clearly, if valve-closed regulation is used, the operating point satisfying the flow rate QB for all three systems is point B, with a corresponding shaft power of NB. If speed regulation is used, the operating points satisfying the flow rate QB for the three systems are points C, D, and E, respectively, with corresponding operating speeds n1, n2, and n3, and corresponding shaft powers NC, ND, and NE. Since N∝Q·H, the shaft power at each point satisfies NB＞NC＞ND＞NE. It is evident that in systems with a pipeline characteristic curve of H=H0+S·Q2, the smaller H0 is, the better the energy-saving effect when speed regulation is used for energy saving. Conversely, when H0 is large enough, due to the decrease in motor efficiency and the efficiency of the speed regulation system itself, variable frequency speed regulation may not save energy or even increase energy waste. [b]4 Comparison of Energy-Saving Effects of Two Speed ​​Regulation Water Supply Methods[/b] In water supply systems, variable frequency speed regulation generally adopts the following two water supply methods: variable frequency constant pressure and variable flow water supply and variable frequency variable pressure and variable flow water supply. Among them, the former is more widely used, while the latter is more technically reasonable. Although it is more difficult to implement, it represents the development direction of water pump variable frequency speed regulation energy-saving technology. 4.1 Variable Frequency Constant Pressure (Variable Flow) Water Supply The so-called constant pressure water supply method is designed for the centrifugal pump's characteristic of "low head when the flow is large and high head when the flow is small". Through the self-controlled variable frequency system, the pump's operating head remains constant, which is equal to the design head, regardless of the flow rate change. If valve-closing regulation is used, when the flow rate changes from Q2 to Q1, the operating point changes from A1 to A2, resulting in a wasted head ΔH = H1 - H3 = ΔH1 + ΔH2. If variable frequency constant pressure water supply is used, the speed is automatically adjusted to n1, and the operating point is at point B1 (see Figure 3). Since variable frequency speed regulation is stepless, it can achieve continuous flow regulation. Therefore, the operating point of constant pressure water supply is always on the straight line H = H2. In terms of control method, only a pressure control value needs to be set at the water pump outlet, which is relatively simple and easy to implement. Obviously, constant pressure water supply saves ΔH1 but does not consider ΔH2. Therefore, it is not the most economical water supply regulation method, especially when the pipeline resistance is high and the pipeline characteristic curve is steep, the proportion of ΔH2 is even greater, and its limitations are obvious. 4.2 Variable Frequency Variable Pressure (AC) Water Supply The control principle of variable pressure water supply is the same as that of constant pressure water supply, only the pressure setting is different. It makes the pump head uncertain, instead moving along the pipeline characteristic curve (see Figure 3). When the flow rate changes from Q2 to Q1, the speed is automatically adjusted to n2, and the operating point is at point B2. At this time, the pump shaft power n2 is less than the constant pressure water supply pump shaft power N1. Variable pressure water supply theoretically avoids the waste of head when the flow rate decreases, and is obviously superior to constant pressure water supply. However, variable pressure water supply is essentially a form of constant pressure, but it changes the constant pump outlet pressure to a constant control point pressure. It generally has two forms: 4.2.1 Determining the pump head by flow rate Q. The flow meter feeds back the measured pump flow rate Q to the controller, and the controller determines the pump head H according to H=H0+S·Q2, and moves H along the designed pipeline characteristic curve by speed adjustment. However, the situation is more complicated in production practice. For a single pipeline water transmission system, a corresponding pipeline characteristic curve can be obtained. However, in a municipal water supply network, it is difficult to obtain a definite pipeline characteristic curve. In practice, the approximate pipeline characteristic curve can only be calculated based on the actual operation of the pipeline network and by making assumptions that are as close to reality as possible. 4.2.2 Determining the pump head from the pressure Hm at the most unfavorable point requires setting up a pressure remote transmission device at the most unfavorable point of the pipeline network and transmitting the signal back to the control room. The controller then uses this signal to make the pump operate at the head required to meet the pressure at the most unfavorable point. Since the most unfavorable point of the pipeline network is often far from the pumping station, remote signal transmission is not very convenient. Moreover, in municipal water supply systems, due to the influence of random factors such as pipeline network adjustments and changes in water usage, there will be some deviation between the actual most unfavorable point and the design most unfavorable point, which will bring difficulties to the implementation of variable pressure water supply. [b]5 Conclusion[/b] ① Variable frequency speed control is a widely used pump energy-saving technology, but it has relatively strict applicable conditions and cannot be simply applied to any water supply system. The specific energy-saving measures should be taken according to the actual situation. ② Variable frequency speed control is suitable for water supply systems with unstable flow, frequent and large fluctuations, significantly smaller flow rates, and a large proportion of pipeline losses to total head. ③ Variable frequency speed control is suitable for water supply systems with relatively stable flow, a single operating point, and a large proportion of static head to total head. ④ Variable frequency variable pressure water supply is superior to variable frequency constant pressure water supply.　[b]References:[/b] [1]Wang Xizhong, Jiang Zhijian, Gao Jingfeng. Development of variable frequency optimized pressure regulating energy-saving water supply device [J]. Water Supply and Drainage, 1998, 24(10): 64-67. [2]Gu Jinlong. Analysis of operating conditions of water pump set mixing water supply system [J]. Water Supply and Drainage, 1997, 23(12): 1-4.