Pump frequency control application note

Abstract: Variable frequency speed regulation is widely used in energy saving of pumps and fans. However, in practical applications, due to the poor consideration of the factors affecting the energy saving effect, there is a big blindness in selection and use, which affects the energy saving efficiency Play 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. Key words: water pump; energy-saving; frequency conversion speed regulation In recent years, frequency conversion speed regulation has been developed rapidly in the water supply system, However, in practice, there is still a big blindness, resulting in unsatisfactory energy-saving effect. In this paper, for the frequency of energy-saving pump to talk about some of the simple views for consideration. 1 variable frequency speed control and pump energy saving pump can not be separated from the condition point reasonable regulation. 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 that affect the energy-saving effect of frequency control. If blindly selected, it may well be counterproductive. 2 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 in addition, such 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 of the speed range In theory, the pump speed efficient area through the frequency-efficient 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 / 22.2 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 lack of air volume affect the heat, threaten the safe operation of the motor.

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