Abstract: This paper introduces and analyzes the load characteristics and variable flow regulation methods of centrifugal water pumps, and compares their energy consumption. It also briefly introduces the characteristics of variable frequency speed regulation of AC motors, pointing out the economic advantages of retrofitting centrifugal water pumps with frequency converters.
Keywords: centrifugal water pump; load characteristics; electric motor energy saving; variable frequency speed control
Abstract: This article describes and analyzes the load characteristics of centrifugal pumps and variable flow regulator method, compared the size of its energy consumption. A brief introduction of the variable frequency AC motor speed control features, pointed out that the centrifugal pump inverter transformation of the economy.
Key word: Centrifugal water pump; Load characteristics; Electric motor energy conservation; Frequency conversion velocity modulation.
Foreword
Centrifugal water pumps play a significant role in my country's current industrial and agricultural production and daily life. These pumps are driven by three-phase asynchronous motors, and their flow rate and pressure are mostly controlled by regulating flow through pipe valves. While this method of artificially increasing pipe resistance meets the flow control needs of production and daily life, it wastes a large amount of electrical energy and is not an economical operating method. In today's context of increasingly scarce electrical energy, finding and popularizing an economical and convenient water pump operating method is of great significance for energy conservation.
1. Operating characteristics of centrifugal water pumps
1.1 Working principle of centrifugal water pump
A centrifugal water pump is a pumping machine that utilizes the centrifugal motion of water. It consists of a pump casing, impeller, pump shaft, and pump frame. Before starting, the pump should be filled with water. After starting, the rotating impeller drives the water inside the pump to rotate at high speed. The water undergoes centrifugal motion, being thrown outwards and forced into the outlet pipe. After the water is thrown out, the pressure near the impeller decreases, forming a low-pressure area near the shaft. The pressure here is much lower than atmospheric pressure, and the outside water, under the action of atmospheric pressure, forces open the foot valve and enters the pump through the inlet pipe. The incoming water is then thrown out again by the high-speed rotation of the impeller and forced into the outlet pipe. Driven by the motor, the impeller rotates continuously at high speed, and water is continuously pumped from a lower to a higher level.
1.2 Pump Load Characteristic Analysis
To adapt to changes in user water consumption and adjust the outlet water flow rate, two methods are commonly used to achieve continuous flow regulation. One method is to use control valves or throttle valves to throttle the flow and change the outlet water flow rate; the other method is to control the pump speed, adjusting the pump speed to change the outlet water flow rate. Figure 1 shows the full head characteristic (H-Q) curve of the pump during speed regulation.
Figure 1 HQ curve during pump speed adjustment
In the above figure, curve n0 represents the head-flow curve of the water pump at its rated speed when the valve opening in the pipeline remains constant. R1 represents the relationship curve between the total head and the flow rate when the pump speed remains constant, also known as the pipe resistance characteristic curve. H0 represents the required head equal to the actual head when the water supply Q is close to 0. Its physical meaning is: if the total head is less than the actual head, the system will not be able to supply water.
As shown in the figure above, the pump's head characteristic curve and the pipe resistance characteristic curve of the pipeline network intersect at a point. This point indicates that the pump meets both the head and pipe resistance characteristics when it is working, and the water supply system is operating in a balanced state with stable operation.
When using pipeline valve control, if the flow requirement decreases from QA to QB, the valve opening must be reduced. At this time, the resistance of the water supply pipeline increases, the pipe resistance characteristic curve shifts from R1 to R2, the head increases from HA to HB, and the operating point shifts from point A to point B.
When using pump speed control, if the flow requirement decreases from QA to QB, the resistance curve R of the pipeline remains unchanged because the valve opening remains constant. At this time, the pump's characteristics depend on its rotational speed. If the speed is reduced from n0 to n1, the operating point shifts from point A to point C, and the head decreases from HA to HC.
According to the formula for the characteristic curve of a centrifugal pump:
Formula 1
Where: P—is the shaft power (KW) at the pump's operating point;
Q – Water pressure or flow rate at the operating point ( m² /s);
H—Head (m) at the operating point;
ρ — density of the output medium (kg/ m³ );
η — the efficiency (%) of the pump at the operating point;
From Formula 1, the shaft power of the water pump at point B when using valve adjustment and the shaft power of the water pump at point C when using speed adjustment can be obtained as follows:
The difference in pump shaft power between the two operating points is: (The output medium flow rate Q is equal at operating points B and C)
Formula 2
As can be seen from Formula 2, when the same flow rate is required, if valve regulation is used to control the flow rate, a power of ΔP is wasted compared to pump speed regulation. Furthermore, this loss increases as the valve is closed further.
Based on the similarity principle of water pumps, we know that when the pump speed changes, the flow rate is directly proportional to the rotational speed, the head is directly proportional to the square of the rotational speed, and the shaft power is directly proportional to the cube of the rotational speed. From this proportional relationship, it can be seen that for the same pump, when the rotational speed changes, the pump's main performance parameters will change according to the above proportional laws, while maintaining a basically constant efficiency during the change. Therefore, by adjusting the rotational speed to regulate the flow rate, the electrical power consumed by the motor will be greatly reduced. Thus, this is a good method that can significantly save energy.
The main parameters and shaft power of the water pump changed using the variable frequency speed control method, as shown in the table below:
2. Variable Frequency Speed Control Principle
Currently, the vast majority of centrifugal water pumps used in my country are driven by three-phase asynchronous motors. To adjust the pump speed, simply adjust the motor speed.
According to the principles of electrical machinery, the speed of an AC asynchronous motor can be expressed by the following formula:
Formula 3
Where s is the slip of the motor (a constant for motors);
p — the number of pole pairs in the stator winding of the motor (a constant for motors);
f — the power supply frequency (Hz) of the motor.
Therefore, by adjusting the frequency of the power supply to the asynchronous motor, the speed of the motor can be controlled.
The frequency converters currently in widespread use are specialized devices that integrate power electronics, microelectronics, and automatic control technologies to directly or indirectly change the power supply frequency before outputting it. Due to the software structure and manufacturing principles of frequency converters, we do not need to consider the relationship between frequency and voltage too much when using them; we only need to match and select the appropriate settings based on the load characteristics of the controlled motor.
When using a frequency converter to drive and control a water pump, its speed can be adjusted at any time to meet the needs of the water supply system. It also enables soft start, soft stop, and stepless speed regulation, reducing the motor starting current to approximately 1.5 times the rated current. This significantly reduces the impact on the motor's electrical components and bearings, effectively preventing water hammer in the pipeline, avoiding sudden changes in flow rate, and reducing the probability of pipe bursts and leaks. More importantly, it achieves a substantial reduction in shaft power proportional to the cube of the power supply frequency, greatly reducing the motor's energy consumption. The frequency converter's built-in PID control function, combined with sensors, transmitters, and PLCs, can easily achieve automated control of the water supply system.
However, the following two points should also be noted: When using frequency conversion speed regulation, the efficiency of the water pump and motor should be fully considered to avoid low efficiency due to excessively low speed; at the same time, the temperature rise and heat dissipation of the motor should be considered. If it is a self-cooled motor, the speed should be adjusted at more than 70% of the rated speed.
3. Conclusion
Analysis revealed that among the variable flow control methods for centrifugal water pumps, using a frequency converter to adjust the pump motor speed is the most economical and simple to implement. Variable frequency control of water pumps is worthy of widespread promotion in today's increasingly energy-constrained environment.
References
[1] Siemens (China) Co., Ltd. MM430 Fan and Pump Load Dedicated Frequency Inverter User Manual. 2005.
[2] Sun Dianzhong, et al. Motor Control. Changchun: Jilin Science and Technology Press. 2002.
