Research on energy-saving control system of water pump based on frequency conversion technology

0 Introduction

Reference address of this article: http://www.eepw.com.cn/article/201809/388647.htm

At present, water pumps and fans are widely used in various sectors of the national economy, with a large number and high power consumption. According to relevant statistics, about 40% of the total electricity in the country is consumed by fans and water pumps. At present, there is still a lot of room for energy saving in fans and water pumps, and the focus of energy saving is how to maximize the operating efficiency of water pumps and fans. At present, about 70% of pumps and fans still use valves or windshields to adjust the flow in loads, causing the motor to be in a light-load or no-load operating state for a long time, resulting in a large amount of energy waste. For this reason, popularizing variable frequency speed regulation technology with huge energy-saving potential throughout the country has great social and economic value and practical significance.

1 Analysis of the principle of water pump energy saving

There are two ways to adjust the water flow. One is to adjust the flow by closing or opening the valve, while the speed remains unchanged (usually the rated speed), which is the valve control method. The other method is to adjust the flow by changing the speed of the water pump motor, while the valve opening remains unchanged (usually the maximum opening), which is the speed control method. The former is easy to understand and will not be described in detail. The latter uses the variable frequency speed regulation technology of the water pump motor to control the speed, thereby achieving the control of the water flow and effective energy saving effect. Next, the energy-saving principle of the speed control method will be explained.

1.1 Comparison of water supply power

(1) If the pump and fan can meet the three similarity conditions, namely, motion similarity, dynamic similarity, and geometric similarity, then the similarity law can be applied. For a pump, if the density ρ of the fluid is not changed but only the speed is changed, its performance parameters obey the proportional law:

It can be seen from the above formula that the pump flow rate, head and shaft power are proportional to its speed, the square of the speed and the cube of the speed respectively.

Figure 1 is the energy-saving principle diagram of variable speed regulation of water pumps. Curve ① in the figure is the head characteristic curve of the water pump at rated speed, which intersects with pipe resistance characteristic curve ② at point A, corresponding to flow rate QA. At this time, the water pump shaft power P is proportional to the area of ​​rectangle QA AHTAO. If the flow rate is to be halved, when valve control is used, the new pipe resistance characteristic curve ③ intersects with the head characteristic curve ① at point B. At this time, the water pump shaft power P is proportional to the area of ​​rectangle QB BHTBO. As can be seen from the figure, the difference between the two areas is not large. If the speed regulation method is used to reduce the water pump speed to curve ④, the pipeline characteristic is still curve ②, so the working point moves to point C. At this time, the area of ​​the rectangle QBCHTCO proportional to the water pump shaft power P is significantly reduced compared with QA AHTAO, which means that the shaft power has dropped a lot and the energy-saving effect is very obvious.

1.2 Comparison of water pump working efficiency

The working efficiency ηP of the water pump is equal to the ratio of the water supply power PG of the water pump and the shaft power PP of the water pump, which can be expressed as:

In the formula: The water supply power PG of the water pump is the power calculated based on the actual water supply flow and head, and is the output power of the water supply system. The shaft power PP of the water pump refers to the input power on the water pump shaft.

The approximate calculation formula for the relative working efficiency η- of the water pump is as follows:

In the formula: Q-, n-, η- are the relative values ​​of flow, speed, and efficiency (the percentage of the actual value to the rated value); C1, C2 are constants, and C1 - C2 = 1.

From the above formula, we can see that when the valve control method is used to reduce the flow rate, because the speed remains constant, that is, n- = 1, the value of Q- n- decreases, and the curve ① in Figure 2 is its efficiency curve. If the flow rate Q- = 0.6, its efficiency decreases to point B. It can be seen that as the flow rate of the pump decreases, its working efficiency has dropped significantly. If the speed control method is used, the valve opening is constant, and the flow rate is proportional to the speed, that is, Q- n- = constant. From Figure 2, we can see that when the flow rate Q- = 0.6, the efficiency is point C of curve ②, which is completely equal to the efficiency of point A on curve ① (when Q- = 1.0). It can be concluded that when the speed control method is used, the working efficiency of the pump always remains optimal.

2 Principle of variable frequency speed regulation operation

2.1 Principle of variable frequency speed regulation

The speed of AC asynchronous motor is:

Where: n0 is the synchronous speed; f is the power frequency; p is the number of motor pole pairs; s is the slip rate. Changing the power frequency will change the synchronous speed.

2.2 Basic requirements for variable frequency speed regulation

(1) Below the fundamental frequency f1N, the main magnetic flux -m remains unchanged.

3. Control method of water pump variable frequency speed regulation system

Figure 4 shows a diagram of a variable frequency constant pressure water supply system. The principle is that the actual pressure of the water supply network is measured by a pressure sensor, and the output voltage signal is sent to the signal processor. After A/D conversion, it is input to the PLC. In the PLC, the control program compares the pressure, that is, the comparison between the given pressure and the network pressure, and sends it back to the signal processor. After D/A conversion, it is output to the frequency converter, and the output frequency of the frequency converter is adjusted, thereby controlling the speed of the water pump motor to achieve the purpose of constant pressure. At the same time, the PLC outputs a control signal according to the pressure difference to execute the action of the relevant contactor.

5 Conclusion

Variable frequency speed regulation technology cannot be simply applied to all water supply systems, and its use in water pump energy saving is also limited. Variable frequency speed regulation is suitable for water supply systems where pipeline loss accounts for a large proportion of the total head, or where the flow rate changes frequently and with a large amplitude, or where the flow rate is significantly small [10]. Variable frequency speed regulation technology cannot be used for water supply systems where the static head accounts for a large proportion of the total head and the flow rate is relatively stable. Therefore, it is necessary to consider the actual situation and adopt corresponding energy-saving methods to be effective.