**Abstract:**
The energy-saving principle, basic working mechanism, and system control process of a pump frequency conversion speed regulation system are discussed. Despite the progress made in this field, significant energy waste remains a critical issue. According to reports, China has approximately 42 million fans, pumps, and air compressors with an installed capacity of around 110 million kilowatts. However, the actual operating efficiency of these systems is only 30–40%, and their power consumption accounts for over 38% of the total generating capacity. This inefficiency arises because many fans and pumps operate at constant speeds, while the demand for airflow or water flow varies. Additionally, some companies design systems with oversized components, leading to unnecessary energy consumption. Therefore, improving the energy efficiency of pumps and fans plays a vital role in promoting sustainable economic development.
**1. Energy-Saving Principle of Pump Frequency Control**
Figure 1 illustrates valve control in a pump system. When the flow rate decreases from Q1 to Q2, the valve must be partially closed, increasing friction and shifting the pipeline curve from R to R', raising the head from Ha to Hb, and moving the operating point from A to B. In contrast, Figure 2 shows speed control. When the flow requirement drops from Q1 to Q2, the resistance curve R remains unchanged, but reducing the pump speed from n to n' shifts the performance curve from (QH) to (QH)', moving the operating point from A to C and lowering the head from Ha to Hc.
According to the centrifugal pump characteristic equation:
N = RQH / (102η)
Where N is the shaft power (kW), Q is the flow rate (m³/s), H is the head (m), R is the specific weight (kg/m³), and η is the pump efficiency (%).
At points B and C, the shaft power is calculated as:
Nb = RQ2Hb / (102η)
Nc = RQ2Hc / (102η)
The difference ΔN = Nb - Nc = R × Q2 × (Hb - Hc) / (102η) represents the power lost when using valve control. As the valve closes further, the loss increases. By reducing the motor speed instead of closing the valve, the same flow can be maintained without wasting energy, achieving a more efficient operation. This is the core principle behind energy-saving pump frequency control.
**2. Basic Principles of Frequency Control**
The fundamental principle of frequency control relies on the relationship between AC motor speed and frequency:
n = 60f(1 - s)/p
Where f is the motor power frequency (Hz), p is the number of pole pairs, and s is the slip. By adjusting the frequency f, the motor’s synchronous speed can be smoothly controlled. As the motor speed decreases, the shaft power and input power also drop, contributing to energy savings. This makes frequency control an effective method for reducing energy consumption in pump systems.
**3. Design of Pump Frequency Control Systems**
Currently, most domestic frequency control systems for pumps operate in open-loop mode, where the inverter frequency is manually adjusted based on process changes or external conditions. The system typically consists of four main components: (1) the control object, (2) the inverter, (3) the pressure transmitter (PT), and (4) the PID controller.
The control process involves measuring the outlet pressure via the PT, converting it into a 4–20 mA signal, and comparing it with the setpoint. The deviation is processed by the PID controller, which generates an adjustment signal sent to the inverter. The inverter then converts the 380V/50Hz AC input into variable voltage and frequency output (0–380V/0–400Hz) to control the pump motor.
**4. Key Considerations in Pump Frequency Control Applications**
Speed reduction is a common approach, but it must be carefully managed. When using frequency control, the original operating parameters of the pump and motor change significantly, including pipeline characteristics and the interaction between variable-speed and fixed-speed pumps. If the speed range is too wide, energy savings may not be achieved. Generally, the frequency control speed should not fall below 50% of the rated speed, ideally between 75% and 100%.
**4.1 Speed Range and Pump Characteristics**
In theory, the high-efficiency zone of a pump lies within the parabola formed by two similar operating conditions. However, if the speed is too low, the pump's efficiency drops sharply, making speed control less effective. If the operating point exceeds the optimal range, alternative methods should be considered.
**4.2 Operation with Fixed-Speed Pumps**
In real-world applications, multiple pumps often work in parallel. Since it is not always feasible to adjust all pumps, a combination of variable-speed and fixed-speed pumps is used. It is essential to ensure both types operate within their high-efficiency zones. If different types of pumps are used together, careful coordination is needed to maximize the speed control range.
**4.3 Motor Efficiency and Speed Range**
Under similar conditions, power consumption is proportional to the cube of the speed (N ∠n³). As speed decreases, the motor's power output drops rapidly. However, if the motor operates far from its rated power or experiences excessive frequency fluctuations, its efficiency may decline significantly, affecting the overall performance of the pump system. Moreover, prolonged low-speed operation can lead to overheating due to reduced cooling, posing risks to the motor's safe operation.
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