**Abstract:**
The energy-saving principle, fundamental working mechanism, and system control process of a pump frequency conversion speed regulation system are discussed. Despite technological progress, energy waste remains significant. According to reports, China has approximately 42 million fans, pumps, and air compressors with a total 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 is largely due to motors running at constant speeds while the demand for airflow or water flow fluctuates. Additionally, many companies design systems with oversized components, leading to unnecessary energy waste. Improving energy efficiency in fans and pumps is therefore crucial for sustainable economic development.
**1. Energy-Saving Principle of Pump Frequency Control**
Figure 1 illustrates valve control, where reducing flow from Q1 to Q2 requires closing the valve, increasing friction and shifting the pipeline curve from R to R', causing the operating point to move from A to B. In contrast, Figure 2 shows speed control, where reducing flow from Q1 to Q2 involves lowering the pump speed from n to n'. The performance curve shifts from (QH) to (QH)', and the operating point moves from A to C, resulting in a drop in head from Ha to Hc.
Using the centrifugal pump characteristic formula:
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 of the fluid (kg/m³), and η is the pump efficiency (%).
At points B and C, the shaft power is:
Nb = RQ2Hb / (102η)
Nc = RQ2Hc / (102η)
The difference ΔN = Nb - Nc = R × Q2 × (Hb - Hc) / (102η).
This indicates that using a valve to control flow results in wasted power, which increases as the valve closes further. By reducing the motor speed instead, the same flow can be achieved with significantly less energy consumption, demonstrating the energy-saving principle of frequency control.
**2. Basic Principles of Frequency Control**
Frequency control operates based on the relationship between AC motor speed and supply frequency:
n = 60f(1-s)/p
Where f is the motor's 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 changed, leading to reduced shaft power and lower input power. This is the core concept behind energy-efficient pump frequency control.
**3. Design of Pump Frequency Control Systems**
Currently, most domestic frequency control systems for pumps operate in an open-loop configuration. This means that the inverter’s frequency is manually adjusted based on process changes or external conditions. A typical system consists of four main components: (1) the control object, (2) the inverter, (3) a pressure transmitter (PT), and (4) a PID controller.
The control process works as follows: The pressure transmitter measures the outlet pressure and converts it into a 4–20mA signal sent to the PID controller. The controller compares this signal with the setpoint and calculates the deviation. Based on pre-defined control rules, it generates an adjustment signal, which is then sent to the inverter. The inverter adjusts the AC input (380V/50Hz) to a variable output (0–380V/0–400Hz), directly controlling the pump motor.
**4. Key Considerations in Pump Frequency Control Applications**
One important factor is the pump speed range. When using frequency control, the original motor and pump parameters may change significantly, affecting the system’s performance. For example, pipeline characteristics, parallel operation with fixed-speed pumps, and other factors can influence the effective speed range. Therefore, frequency control should not be used beyond certain limits. It is generally recommended that the speed not fall below 50% of the rated speed, ideally between 75% and 100%. Proper calculations should guide the selection of the speed range.
**4.1 Speed Range and Pump Characteristics**
In theory, the high-efficiency zone of a pump occurs between the two similar curves. However, if the pump speed is too low, its efficiency drops sharply. Similarly, if the operating point exceeds the high-efficiency area, speed control may not be suitable for energy savings.
**4.2 Fixed-Speed Pumps and Variable-Speed Pumps**
In real-world applications, multiple pumps often operate in parallel. Due to cost constraints, not all pumps can be variable-speed. In such cases, both variable-speed and fixed-speed pumps must operate within their high-efficiency zones. The presence of fixed-speed pumps can limit the effective speed range of the variable-speed pump.
**4.3 Motor Efficiency and Speed Range**
Under similar conditions, power is proportional to the cube of the speed (N ∠n³). As speed decreases, power consumption drops rapidly. However, if the motor runs far below or above its rated speed, efficiency may decline significantly, affecting the overall performance of the pump system. Moreover, prolonged low-speed operation can lead to overheating due to insufficient cooling, posing risks to the motor’s safe operation.
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