Note the application of frequency control of a pump Variable frequency speed control and pump energy-saving Pump energy can not be separated from the condition point reasonable adjustment. The adjustment is nothing more than the following two kinds: adjustment of the line characteristic curve, such as off valve adjustment; pump characteristic curve adjustment, such as pump speed, impeller cutting. In terms of energy saving, changing the performance curve of the pump is much more significant than changing the pipe characteristic curve [1]. Therefore, changing the pump performance curve becomes the main way of energy saving of the pump. The frequency control in changing the pump performance curve and automatic control obvious advantages, and thus widely used. But at the same time it should be noted that there are many factors affecting the energy-saving effect of frequency control. If blindly selected, it may well be counterproductive. Two factors affect the frequency control range Pump speed is generally slow down the problem. When the use of frequency control, the original design by the frequency of the state of the pump and motor operating parameters have undergone major changes, the other as pipeline characteristics curve, speed governor pump in parallel with the constant speed pump and other factors, will be Speed ​​range has a certain impact. Out of range speed control is difficult to achieve the purpose of energy conservation. Therefore, frequency conversion speed regulation can not be unlimited. Generally believed that the frequency control speed should not be lower than the rated speed of 50%, preferably at 75% to 100%, and should be combined with the actual calculation. 1 pump process characteristics of the speed range In theory, the pump speed control efficiency through the high frequency area around the end of the two similar conditions parabolic middle area OA1A2 (see Figure 1). In fact, when the pump speed is too small, the efficiency of the pump will drop sharply. Affected by this, the high-speed pump governor shrank to PA1A2 [2]. Obviously, if the operating point has exceeded this area, Energy saving.) The figure H0B for the pipeline characteristic curve, the CB section to become high-speed operation of the effective range. To simplify the calculation, it is assumed that point C lies on curve OA1, so the efficiency of points C and A1 is theoretically equal. Point C becomes the left endpoint of the pump's performance curve at its minimum speed. Therefore, the minimum rotation speed can be obtained as follows: Since the conditions at point C and point A1 are similar, according to the proportional law: (QC / Q1) 2 = HC / H1 The point C on the curve H = H0 + H0 + S · QC2 where HC and QC are unknowns, the solution equation is: HC = H1 × H0 / (H1-S · Q12) QC = Q1 × [H0 / (H1-S · Q12)] 1/2 According to the ratio Law: nmin = n0 × [H0 / (H1-S · Q12)] 1/2 2 speed pump on speed range In practice, the water supply system is often multiple pumps in parallel. Due to the expensive investment, it is impossible to speed up all the pumps, so the general speed pump, fixed speed pump mixed water supply. In such a system, care should be taken to ensure that both the variable speed pump and the fixed speed pump operate at the high efficiency stage and achieve the optimum system. At this moment, the constant speed pump has a great influence on the speed regulating range of the speed regulating pump running in parallel with it [2]. The main points of the following two situations: 2.1 with the same type of pump must be run side by side, although the scheduling flexibility, but can not take into account the speed pump and the constant speed pump efficient work section, therefore, in this case speed range is tiny. 2.2 different models of a pump must be run side by side, if it can reach the speed of the pump at the rated speed of the right end of the high end of the head and high-speed section of the pump head is equal. You can achieve the maximum range of speed operation. However, at this moment, the speed regulating pump and the constant speed pump must not be allowed to run in parallel after the exchange. 2.3 Effect of motor efficiency on speed range In the case of similar operating conditions, there is usually Nαn3, so the shaft power will drop rapidly as the speed decreases. However, if the motor output power is excessively offset from the rated power or the operating frequency is excessive Offset frequency, will make the motor efficiency drops too fast, eventually affecting the efficiency of the entire pump unit. And since the cold motor continuous low-speed operation, but also due to insufficient air volume affect the heat, threaten the safe operation of the motor. Effect of three-pipe characteristic curve on speed-saving energy-saving effect Although the change of pump performance curve is the main way of energy-saving of pump, the difference of speed-saving energy-saving effect in different pipeline characteristic curves is very obvious. For the sake of intuition, FIG. 2 is used here. In the same design conditions of the three water supply systems (ie, the maximum design conditions point A point, the flow must be adjusted to QB), the same pump type, but the pipeline characteristics are not the same curve, respectively: H = H1 + S1 · Q2 (H0 = H1) ② H = H2 + S2 · Q2 (H0 = H2, H1> H2) ③ H = S3 · Q2 (H0 = H3 = 0) Obviously, The system meets the conditions of the flow QB point B are points, the corresponding shaft power NB; if the use of speed operation, the three systems to meet the flow QB operating conditions were points C, D, E, the corresponding operation Speed ​​respectively n1, n2, n3, the corresponding shaft power is NC, ND, NE. Because of NαQ · H, the power of each point axis satisfies NB> NC> ND> NE. Can be seen in the pipeline characteristic curve H = H0 + S · Q2 in the system using energy-saving, H0 smaller, energy-saving effect is better. On the contrary, when H0 large to a certain extent, by the efficiency of the motor and speed control system itself, the impact of the use of variable frequency speed regulation may not even save energy and even increase energy waste. Four two kinds of speed-saving water supply energy-saving comparison In the water supply system, frequency control generally use the following two kinds of water supply: variable frequency constant pressure water supply and variable frequency variable pressure water supply. Among them, the former is more widely used, while the latter is technically more reasonable. Although the implementation is more difficult, it represents the development direction of energy-saving technology of pump frequency control. 1 variable frequency constant pressure (variable flow) water supply The so-called constant pressure water supply means that for the centrifugal pump, "when the flow is low, low head, small flow head lift" feature, through the automatic frequency control system, no matter how the flow changes, are to keep the pump running head The same, that is equal to the design head. If the use of off-valve regulation, when the flow from Q2 → Q1, then the working conditions from A1 into A2, waste head △ H = H1-H3 = △ H1 + △ H2. If the use of constant pressure water supply, the speed will be automatically adjusted to n1, working point at B1 (see Figure 3). As the variable frequency speed control is continuously variable speed, continuous flow adjustment can be achieved, so constant pressure water supply operating point is always in a straight line H = H2, in the control mode, just set a pressure control value at the pump outlet, compare Simple and easy. Obviously, constant pressure water supply saves △ H1 without considering △ H2. Therefore, it is not the most economical way to adjust the water supply. Especially in the case of large pipe resistance and steep curve of pipeline characteristic, the proportion of △ H2 is greater, and its limitation is obvious. 2 variable frequency (alternating current) water supply pressure water supply control principle and constant pressure water supply the same, but different pressure settings. Instead of lifting the pump head, it moves along the line characteristic (see Figure 3). When the flow from Q2 → Q1, the speed will be automatically adjusted to n2, working point at the B2 point. At this point pump shaft power n2 is less than constant pressure water pump shaft power N1. Transformer water supply theoretically avoided when the flow decreases head waste, obviously better than constant pressure water supply. But the transformer water supply is essentially a constant pressure, but the pump outlet pressure constant control point pressure becomes constant, it generally has 2 forms: 2.1 by the flow Q to determine the pump head flow meter will measure the pump flow Q feedback To the controller, the controller according to H = H0 + S · Q2 to determine the pump head H, H through the governor to move along the design line characteristics curve. However, the production practice is complicated. For a single pipeline water system, it is possible to get a corresponding line characteristics of the curve. In the municipal water supply network, it is difficult to get a certain pipeline characteristics curve. In practice, only based on the actual operation of the pipe network, through close to the actual assumptions as soon as possible to calculate the approximate pipeline characteristics curve. 2.2 by the most unfavorable point pressure Hm to determine the pump lift that is the most disadvantaged point in the pipe network to set the pressure remote devices, and back to the control room signal, the controller accordingly pump to meet the most unfavorable point pressure required lift operation, As the pipe network is often the most unfavorable point far away from the pump station, the far-end signal is not very convenient, and, in the municipal water supply system, due to pipe network adjustment, changes in water conditions and other random factors, will make the actual least Lee point and the design of the most disadvantageous point of some deviation, to the implementation of pressure water supply difficulties. V. Conclusion ①Frequency Control is a widely used energy-saving pump technology, but it has more stringent applicable conditions, can not be simply applied to any water supply system, the specific energy-saving measures taken, should be treated in accordance with the actual situation ② Frequency Governor for flow instability, frequent changes and large amplitude, regular flow was significantly smaller and the proportion of pipeline loss in the total head larger water supply system. ③ Frequency Control A suitable for more stable flow, a single operating point and the total head lift static lift a larger proportion of the water supply system. ④ variable frequency pressure water supply is better than constant pressure water supply.