The sewage booster pumping station is equipped with four AS submersible sewage pumps, each with a motor power of 85kW and a voltage of 380V. The original design used a KF-90K-308 high-performance, high-power frequency converter to regulate the speed of one of the pump motors. Based on the sewage tank level, it adjusts the pump speed by changing the frequency of the power supply to the sewage pump motor, thereby controlling the drainage volume. Therefore, the frequency converter must be controlled by the water level feedback signal to form an automatic water level control system. A commonly used closed-loop control scheme is a linear feedback control system. While this system can achieve constant liquid level automatic control, it has the following drawbacks: ① High cost, due to the high price of linear liquid level (pressure) sensors and regulators; ② Low reliability, because the linear liquid level (pressure) sensor must be submerged and in full contact with the water to measure the water level (or pressure), the sensor's measuring orifice is easily blocked by sediment, leading to measurement errors and causing the controlled water level to deviate from the expected value; ③ Complex on-site debugging, as the nonlinearity and large inertia of the controlled object make parameter configuration of the regulator difficult, potentially affecting the system's stability and dynamic performance. In summary, linear feedback control is unreasonable for such a nonlinear, high-inertia process; it is economically unfeasible, performs poorly, and is inconvenient to debug. This paper introduces a simple, reliable, and low-cost nonlinear feedback variable frequency speed control system designed for this sewage lifting pump station, successfully overcoming the problems of linear feedback control [table][tr][td][/td][/tr][/table]. In this system, a Japanese-made frequency converter controls the pump motor, and a set of level sensors forms a nonlinear feedback control system to keep the water level within the limit loop. A set of level sensors is installed in the sewage tank, which can measure four levels (as shown in Figure 1). During normal operation, the water level will fluctuate between 2SL and 3SL. When there is a large disturbance, 1SL or 4SL achieves self-tuning of the operating frequency and water level alarm function. The output signal of this level sensor is sent to the electrical control unit to realize relay-type nonlinear feedback characteristics, and combined with the frequency converter control terminals to form a nonlinear feedback four-level water level control system (see Figure 2). The main circuit of the high-performance, high-power frequency converter uses an IPM (Intelligent Power Module) power semiconductor switching device with drive circuits, current protection, and temperature protection circuits. The basic control circuit uses a high-speed 32-bit RISC (Reduced Instruction Set Computing) as the CPU and employs ultra-dense LSI (Large-Scale Integrated Circuit). Its main features are: wide speed range, high speed control accuracy, high efficiency, high power factor, low output current fluctuation, low input-side grid pollution, convenient parameter setting, and comprehensive protection functions; it has overvoltage, undervoltage, overcurrent, overload, short circuit, overheating, and power phase loss protection functions; and it has a fault self-diagnosis function. In addition, this frequency converter also has a multi-speed setting function, and each speed can be arbitrarily set. 1. Working Principle According to the principle of variable frequency speed control of electric motors, the synchronous speed n0 of an asynchronous motor is directly proportional to the power supply frequency f1. Therefore, changing f1 changes n0, thus achieving speed control. From fluid mechanics, we know that flow rate is proportional to the first power of rotational speed, and power is proportional to the cube of rotational speed: n′=n（f′/f） (1) Q′=Q（n′/n）2 (2) N′=N（n′/n）3 (3) Based on the water level provided by the process (which is determined by factors such as the city's maximum and minimum sewage volume, the depth and volume of the culvert, and the sewage pump flow rate), the four speed levels are represented by 1SP, 2SP, 3SP, and 4SP, respectively, and the corresponding frequency values ​​are 368z (40%Pe), 422z (60%Pe), 464z (80%Pe), and 50z (100%Pe). The correspondence between water level and inverter operating frequency is shown in Table 1. Table 1. Correspondence between Water Level and Inverter Operating Frequency Water Level | Actuating Component | Motor | Set Speed ​​| Inverter Output Frequency (Hz) | Motor Output Power | High Water Level 1SL | 4SP | 50 | 100%Pe | High Water Level 2SL | 3SP | 46.4 | 80%Pe | Normal Water Level 2SP | 42.2 | 60%Pe | Low Water Level 3SL | 1SP | 36.8 | 40%Pe | Low Water Level 4SL | Shutdown | 0 | 0 The working principle of this control system is described as follows: Under normal conditions, the water level fluctuates within the limit loop defined by 2SL and 3SL, and the inverter output frequency is 422Hz. When the sewage water level increases above the high water level, the inverter automatically adjusts to an output frequency of 46.4Hz. If the high water level persists for more than the set time limit T1 and does not drop back to the normal limit loop, the first working pump is automatically activated. If the high water level continues to rise (after time limit T2), the system automatically activates the second working pump. If the water level continues to rise and reaches the ultra-high water level, the inverter output frequency automatically adjusts to 50Hz and issues an ultra-high water level alarm signal. When the water level falls back and moves away from the ultra-high water level, the inverter output frequency automatically decreases to control the water level within the normal limit loop. If the water level continues to fall and deviates from the normal operating loop, the inverter output frequency decreases to 368Hz, and the second working pump stops operating. If the low water level lasts for more than the set time limit T3, the first working pump stops operating successively. If an ultra-low water level occurs, the inverter pump also stops operating and issues an ultra-low water level alarm signal. When the water level disturbance disappears and the water level returns to within the normal limit loop, the alarm signal is automatically cleared. The main features of this control system are: ① Suitable for controlling processes with unclear mathematical models or large inertia and nonlinearity. ② Simple structure and low cost; the sensor and control unit are composed of a float level controller and a relay, respectively, making it easy to implement. ③ High reliability and strong anti-interference ability; the float controller is not affected by debris in the sewage or corrosion, and all input and output signals are not easily interfered with. ④ It has significant energy-saving effects. ⑤ It has parameter self-tuning function. This design applies advanced frequency conversion technology to the field of automatic control of sewage lifting, which conforms to the 2000 technological progress development plan of the urban water supply industry and is a project promoted by the Ministry of Construction and the State Planning Commission's Energy Conservation Office. 2. Actual Operation Results The system was put into actual operation in May 1998. Operational results prove that the system is convenient to use, safe and reliable, has a high power factor, operates ahead of schedule, and has a very significant energy-saving effect. According to the system's operation records in manual control mode, the frequency converter operated at 368Hz for 8 hours, at 422Hz for 75 hours, at 50Hz for 75 hours, and was shut down for 1 hour. Although the initial investment in this system is relatively large, due to its significant energy-saving effect (saving approximately 200,000 kWh per year), the investment can be recovered within one year.