Abstract : In the application and implementation of PLC and frequency converter in water supply pumping stations of water plants, PLC and frequency converter are the core of the control system. They not only successfully solve the problem of large pressure fluctuations in the water supply network by using frequency conversion speed regulation technology, but also realize the automatic control of the entire system through PLC. The PID function of PLC can control the frequency converter to adjust the speed of the water pump, and also enable the entire operating system to meet the energy-saving requirements.
Keywords: PLC; PID; frequency converter; energy saving
1 Overview
This article describes how to control a constant pressure water supply system in a water pumping station by combining the functions of a PLC and a frequency converter. The system uses frequency conversion speed regulation to automatically adjust the speed of the water pump motor, and utilizes the PLC's PID automatic adjustment function to control the frequency converter, forming a pressure-based closed-loop control system to achieve constant pressure water supply. The start and stop of the water pumps can be controlled automatically or manually. The system is reliable, has strong anti-interference capabilities, and is energy-efficient. However, several technical challenges remain in the implementation of automatic control. These challenges will be discussed in conjunction with some practical configuration examples.
2 System Composition
The PLC-inverter and soft starter control system consists of one PLC control station, two inverters, three soft starters, a pressure transmitter, a comprehensive protector, a level gauge, and five water pumps. The five water pumps have a flow rate of 12000 m³/h and a motor power of 132 kW. Two inverters control two pumps, and three soft starters control the other three. A pressure transmitter is installed on the main outlet pipe to detect the network pressure, and a level gauge is installed in the clear water storage tank to detect the tank level and send the signal to the PLC. Each water pump is equipped with one comprehensive protector (installed in the inverter or soft starter cabinet). The inverters are Siemens MICROMASTER (MM440), the PLC is a Siemens GE programmable logic controller, and the main outlet pipe is equipped with an E+H FT-1 pressure sensor and the clear water storage tank with an FT-1E level sensor.
3 System Control Principle
A pressure sensor installed on the water outlet pipeline converts the outlet pressure signal into a standard 4-20mA signal, which is then sent to the PLC. The PLC calculates and compares this signal with a given pressure, deriving a comparison parameter which is sent to the frequency converter. The frequency converter controls the motor speed, regulating the system's water supply to maintain the pressure on the water supply network at the given level. When water consumption exceeds the capacity of a single pump, the PLC controls a switch to add more pumps. Based on water consumption, the PLC controls the number of working pumps and the frequency converter adjusts the pump speed to achieve constant pressure water supply. When the water supply load changes, the input voltage and frequency of the motor also change accordingly, thus forming a closed-loop control system based on the set pressure. Furthermore, the system has multiple protection functions to ensure timely pump maintenance and normal system water supply. The frequency converter provides a variable frequency power supply to the motor, enabling stepless speed regulation and continuous variation of the network water pressure. The pressure sensor detects the network water pressure, and the pressure setting unit provides the system with the desired water pressure value to meet user needs. After the pressure setting signal and pressure feedback signal are input to the programmable logic controller (PLC), the PLC's internal PID control program calculates and outputs a speed control signal to the frequency converter. When the water supply equipment starts, the frequency converter pump starts first. When the water pressure in the pipeline reaches the set value, the output frequency of the frequency converter stabilizes at this value. When water consumption increases and water pressure decreases, the sensor sends this signal to the PLC. The PLC then sends an analog signal indicating increased water consumption, causing the frequency converter's output frequency to rise, the pump speed to increase, and the water pressure to rise. If water consumption increases significantly, causing the frequency converter's output frequency to reach its maximum value, but the pipeline water pressure still cannot reach the set value, the PLC sends a control signal to start one fixed-frequency pump, and so on for the other pumps. Conversely, when water consumption decreases and the frequency converter's output frequency falls below a certain value (generally around 30Hz), but the pipeline pressure is still higher than the set pressure, the PLC automatically starts a timer. If the pressure remains higher than the set value within a certain time period, the PLC automatically stops one fixed-frequency pump until the pressure decreases, and so on for the other pumps.
4. PLC Configuration and PID Programming
Based on user requirements, the PLC hardware configuration is as follows: The CPU module is the IC200CPUE05, a CPU with two serial ports and an Ethernet interface; digital inputs (DI) are IC200MDL640 modules; digital outputs (DO) are IC200MDL740 modules; analog inputs (AI) are IC200ALG240 modules; and analog outputs (AO) are IC200ALG230 modules. In actual field use, redundancy is provided for both digital inputs/outputs (DI/DO) and analog inputs/outputs (AI/AO) to accommodate temporary system expansion needs. The touchscreen is a Shenzhen Weintek 10.4-inch TFT 65,536-color MT6104T. The host computer monitoring system uses one engineer station and one operator station. Both industrial computers communicate with the PLC master station via Ethernet. The operator station's screen configuration software is GE Cimplicity configuration software used for user-developed secondary software.
The main functions of each correction element in PID control are as follows:
(1) Proportional element: Increasing the proportional gain is equivalent to increasing the open-loop gain of the system, which will improve the system response speed, reduce the steady-state error, and increase the overshoot. When the proportional gain is too large, it will make the closed-loop system unstable.
(2) Integral element: This is equivalent to adding an integral element to the system, and its main function is to eliminate the steady-state error of the system. The strength of the integral element depends on the integration constant. Increasing the derivative reduces the system overshoot, but slows down the response speed.
(3) Differential element: Its main function is to improve the system response speed and reduce overshoot. It counteracts the hysteresis effect of the system inertial element, thus significantly improving system stability. Excessive or insufficient differential will increase overshoot and lengthen the settling time. Since the control quantity generated by this element is related to the rate of signal change, the differential element has no effect on systems where the signal does not change significantly or changes slowly.
The GE Fanuc VersaMax PID self-tuning module works as follows: In this system, the PLC mainly sets the pressure value of the outlet water pipe through the analog input interface AI0001 (SP) and reads the actual pressure feedback signal of the pipe through the analog input interface AI0002 (PV). The difference between AI0001 (SP) and AI0002 (PV) is subtracted, and the resulting error is sent to the PID function module. By setting the proportional, integral, and derivative terms, the output term AQ0001 is made to achieve a stable output to the frequency converter, so that the frequency converter outputs a smooth frequency to the variable frequency water pump, preventing large fluctuations that could cause large oscillations in the pipe pressure.
Figure 1. PID self-tuning function module and settings
5. Water pump load variable frequency speed regulation energy saving principle
A water pump is a device that converts the shaft power of an electric motor into fluid. In the past, speed control was rarely used; instead, squirrel-cage induction motors were used for constant-speed operation. When the flow rate needed to be changed, throttle valves and baffles were adjusted. While this method was simple to control, it was inefficient, uneconomical, and had poor dynamic tracking performance. Variable frequency speed control (VFD) is an energy-saving method relative to valve regulation. Using a VFD, the valve is fully opened, and the motor speed is changed by altering the frequency of the motor power supply. According to fluid mechanics, the flow rate Q is directly proportional to the first power of the speed n, the air pressure H is directly proportional to the square of the speed n, and the power P is directly proportional to the cube of the speed n, i.e.: Q = Qe × (n/ne), H = He × (n/ne)², P = Pe × (n/ne)³, where Qe is the rated power of the water pump.
Figure 2. Characteristic curves of the water pump system
The constant flow rate is given by He, the rated pressure of the pump, Pe, the rated power of the pump, and ne, the rated speed of the pump.
As shown in the formula above, the pump flow rate can be adjusted by changing the rotational speed. In this case, the pump shaft output power is directly proportional to the cube of the rotational speed. This can be analyzed based on the pump system characteristic curve (as shown in Figure 2).
Assuming the optimal operating point of the water pump is point A, when the water supply needs to be reduced, the traditional valve regulation method increases system resistance to meet the requirement, shifting the pump's operating point from A to B. This method not only fails to save energy but also accelerates pump efficiency loss. Furthermore, inefficient operation causes higher structural vibrations, noise, and equipment damage. By adopting variable frequency drive (VFD) technology, the asynchronous motor speed is reduced, allowing the system to rebalance, shifting the operating point from A to C. As seen at point C, although the motor speed is reduced, the impact on pump efficiency is minimal. Based on this principle, when the pump flow rate varies over a large range, controlling the pump speed using VFD will achieve significant energy savings. The relationship between pump flow rate, speed, shaft power, and power supply frequency is shown in Table 1.
Table 1. Relationship between pump frequency, speed, flow rate, and frequency
Frequency (Hz) | Rotational speed (n) | Traffic (Q) | Shaft power (P) |
50 | 100% | 100% | 100% |
45 | 90% | 90% | 72.9% |
40 | 80% | 80% | 51.2% |
35 | 75% | 75% | 34.3% |
30 | 60% | 60% | 21.6% |
25 | 50% | 50% | 12.5% |
6. Energy-saving effect evaluation
Taking a water plant in Shenyang as an example, the system reports show that the average shaft power output is below 50%. Conservatively calculating with a 35% energy saving rate, the system's annual energy saving is: 132kW × 35% × 24 × 30 × 12 = 399168kWh. Based on an electricity price of 0.69 yuan, the annual electricity cost saving is: 399168 × 0.69 = 275425 yuan.
By applying variable frequency speed control technology, the water pump has changed its original operation mode and realized remote control. It can effectively regulate the water delivery process, make the system run stably, keep the water pump running efficiently, and the motor has achieved soft start without inrush current. The equipment failure rate has been greatly reduced, and maintenance costs have been greatly reduced.
The system utilizes variable frequency speed control technology, which significantly saves energy while allowing pumps that operate under light loads for extended periods to run at low speeds and low voltages. This results in less motor heat generation and lower temperature rise, extending the pump's lifespan. Variable frequency speed control also improves the power factor, reduces grid losses, increases efficiency, and lowers pump noise, thus improving the production environment. Furthermore, the frequency converter features comprehensive self-monitoring, fault diagnosis, and protection functions, effectively preventing accidents from escalating.
7. Conclusion
Variable frequency speed control for constant pressure water supply is widely used in water supply systems. This system features fast operation, high control precision, reasonable structure, complete functions, and high reliability of hardware and software configuration. It has high practical value and promising prospects for promotion and can meet the control requirements of industrial boilers. It saves money and personnel investment, improves the automatic control level of water plant supply, eliminates potential accident hazards, and greatly reduces the maintenance workload of the control system and the replacement quantity and cycle of equipment spare parts, resulting in considerable economic benefits.
References
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About the author: Yuan Baolong, male, born in 1970, is a senior engineer at the School of Information and Control Engineering, Shenyang Jianzhu University. His main research areas are control theory and control engineering.
Mailing Address: School of Information and Control Engineering, Shenyang Jianzhu University, No. 9, Hunnan East Road, Hunnan New District, Shenyang, 110168, China. Tel: 024-24693970 Email: [email protected]
