Frequency control is widely used in the energy saving of pumps and fans. However, in practical applications, due to the poor consideration of the factors that affect its energy saving effect, there is a big blindness in selection and use, which affects its energy-saving benefits. Taking water pump as an example, this paper analyzes symptomatic problems and discusses some main factors affecting its speed regulation range and energy saving effect. Based on this, it points out the applicable range of frequency control. 1. Frequency control and pump energy saving pump can not do without the reasonable adjustment of operating conditions. 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. 2. Factors affecting the frequency control range Pump speed is generally a deceleration 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. 2.1 pump process characteristics on the speed range In theory, the pump speed control efficiency through the frequency-efficient area around the end of two similar conditions parabolic middle area OA1A2. 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. 2.2 constant speed pump on the speed range In practice, the water supply system is often more than one pump parallel water supply. 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: 2.2.1 with the same type of pump a certain parallel operation, although the scheduling flexibility, but can not take into account the high-speed pump and constant speed pump efficient work section, so speed operation in this case The range is very small. 2.2.2 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. 3. The effect of pipeline characteristic curve on energy-saving effect Although the change of pump performance curve is the main way of energy-saving of water pump, the difference of energy-saving effect in different pipeline characteristics 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. 4. Comparison of energy-saving effect of two speed-regulated water supply ways In the water supply system, frequency-conversion speed regulation generally adopts the following two kinds of water supply modes: variable frequency constant pressure variable-flow water supply and variable frequency variable pressure variable-flow 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. 4.1 Variable frequency constant pressure (variable flow) water supply The so-called constant pressure water supply, that is, for the centrifugal pump "when the flow is large, 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. 4.2 Frequency conversion (AC) water supply 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 theory to avoid the loss of head when the flow is reduced, obviously better than the 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 2 forms: 4.2.1 by the flow Q to determine the pump head flow meter The measured 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 along the design of pipe Road characteristic curve moves. 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. 4.2.2 by the most unfavorable point pressure Hm to determine the head of the pump that is required to set the most adverse points in the pipe network pressure remote devices, and back to the control room signal, the controller accordingly pump to meet the most unfavorable point pressure required lift In operation, due to the pipe network is often the most unfavorable point far away from the pump station, the far signal is not very convenient, and, in the municipal water supply system, due to pipe network adjustments, changes in water conditions and other random factors, will make the actual Some of the most unfavorable points and the most unfavorable points in design have some deviations, which has brought difficulties to the implementation of the transformer water supply.
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