Abstract : This paper introduces the features of the new Mitsubishi FR-F700 frequency converter, elaborates on the basic principle of constant pressure water supply control, and designs a control scheme for a frequency converter constant pressure water supply system. Performance and economic benefit analyses of the frequency converter constant pressure water supply system are conducted. Actual operation shows that the system is stable, reliable, and energy-saving. Keywords : Frequency converter; Constant pressure water supply; Energy saving Winter heating is very common in northern China. With the improvement of people's living standards, users' requirements for heating quality are also constantly increasing. The stability of the heating system pressure is directly related to the heating quality of the hot water boiler. To maintain the stability of the heating system pressure, the stability of the water supply system pressure is key. Constant pressure water supply refers to maintaining a constant return water pressure when the water consumption in the community heating system changes. This ensures both water supply capacity and system network pressure stability, and greatly improves heating quality. 1 Introduction to the New Mitsubishi FR-F700 Frequency Converter The new generation FR-F700 general-purpose frequency converter produced by Mitsubishi Electric Corporation is most suitable for use with fan and pump loads. Inheriting the excellent characteristics of FS00, it is easy to operate and has comprehensively improved various functions: (1) The newly developed energy-saving monitoring function makes the energy-saving effect clear at a glance. The V/F curve has been added with 5-point adjustable function. (2) It adopts a long life design, which is simple to maintain and safe to use. It also has the most advanced life diagnosis and early warning capabilities. (3) It has a built-in noise filter and a surge current absorption circuit. (4) It has added an RS-485 terminal and added support for Modbus-RTU (Binary) protocol. (5) It has a reverse start function, a regenerative braking avoidance function, and also added a PIE thermal resistance input. (6) Its power range: 0.75kW 630kW (three-phase 380V, FR-F740-CH series) (7) The built-in PID function is extremely convenient to use. 2. Principle and Control Scheme Design of Variable Frequency Constant Pressure Water Supply 2.1 Principle of Variable Frequency Constant Pressure Water Supply The variable frequency constant pressure water supply system uses a pressure transmitter (output signal 4-20mA) to measure the pressure in the water-steam drum. The output signal of the pressure transmitter serves as the system's feedback signal. The system's control objective is the pressure in the pump station's water-steam drum. The system's set water supply pressure value is compared with the actual feedback pressure value. The difference is input into a PID loop for processing, and then a signal indicating an increase or decrease in water flow is sent out to control the pump speed, completing closed-loop control of the water supply pressure. This achieves stable water supply pressure and energy savings when the pipeline flow changes. 2.2 System Design Scheme: This system is designed to meet the heating quality requirements of a centralized heating system in a certain residential area. Two 60kW motors are used as circulating pumps (one for standby and one for operation), and two 7.5kW motors are used as makeup water pumps (one for standby and one for operation). It is assumed that the circulating pumps are functioning normally. Calculations and experiments show that one 7.5kW motor is sufficient to meet the makeup water requirements. A new Mitsubishi FR-F700 series inverter, FR-F740-7.5K-CH, is used to achieve constant pressure (0.2MPa) in the makeup water steam drum. A YGS-3 pressure transmitter (0-0.4MPa range) two-wire instrument is selected. Since the YGS-3 two-wire pressure transmitter itself does not have a power supply, an external +24V power supply needs to be connected in series in the pressure transmitter circuit. The inverter uses external operation mode. Because PID regulation and a 420mA current signal input are required, the RT and AU terminals of the inverter need to be enabled, as shown in Figure 1. 2.2.1 The main circuit of the main circuit control scheme system is shown in Figure 1. The system has two operating modes: manual and automatic. IQF is the manual (power frequency) circuit, and 2QF is the automatic (variable frequency) circuit. Overload thermal protection is added in manual mode, and overcurrent protection is set by the inverter parameters in automatic mode. 2.2.2 Control Circuit The control circuit of the system is shown in Figure 2. When the control mode is switched to manual, it enters manual operation mode. At this time, any motor can be started directly at power frequency via the IST or 2ST buttons on the control cabinet. This operating mode is used when the inverter is damaged or the water pump is under inspection; in this case, the variable frequency function cannot be used. When the control mode is switched to automatic, it enters automatic operation mode. At this time, the inverter is started via the 3ST button on the control cabinet, and the inverter will automatically achieve constant pressure water replenishment according to the settings. 2.3 Setting parameters is very important during the commissioning process of the frequency converter. If the parameters are not set properly, they will not meet the production needs, resulting in failure of starting and braking or frequent tripping during operation. In severe cases, they may burn out the power module IGBT or rectifier bridge and other devices. Mitsubishi frequency converters have hundreds of parameters. During commissioning, most parameters can remain unchanged (the factory values ​​can be left as is). Only the original factory values ​​that are not suitable during use need to be reset. For example, external terminal operation, maximum frequency, upper and lower limit frequencies, start time, overcurrent protection, overvoltage protection, PID parameters, etc. must be adjusted. When the operation is not suitable, other parameters should be adjusted. According to the actual needs of the system, the parameters of the frequency converter are set as follows: (1) According to the user's requirements for operation instructions, select external start and stop, and select Pr79=2. (2) According to the actual needs of water replenishment, set the upper limit frequency Pr1=50Hz. (3) Since overcurrent protection is easily caused when the acceleration is too fast, set the acceleration time Pr7=10s. (4) When stopping the pump, the time should be longer rather than shorter, as rapid braking can easily cause "water hammer". Set the deceleration time Pr8=15s. (5) According to the matching of the rated current of the 7.5l motor, set the overcurrent protection P19=18A. (6) Since a 4-20mA feedback signal is used, set the input selection of terminal 4 Pr267=0. (7) Simple magnetic flux vector control, set Pr80=7.51. (8) PⅢ control automatic switching frequency, set Pr127=50Hz. (9) PⅢ action selection, set Pr128=21. (10) Proportional band, set Pr129=80%. (11) PⅢ integral time, set Pr130=0.5s. (12) PⅢ target setting Pr=50%. (13) Action selection of the second function signal (RT), set Pr155=0. (14) Input terminal (1iT) function selection, set Pr183=14. (15) Input terminal (AU) function selection, set Pr184=4. 3 Performance Analysis and Energy Saving Effect Analysis The water pump is designed for operation at the power frequency, with a synchronous speed of n=60f/p. The number of magnetic pole pairs p is fixed when manufactured by the manufacturer, and the speed can only be changed by changing the frequency. By changing the flow rate of the water pump through frequency conversion technology, the power consumption of the water pump is proportional to the cube of the speed, i.e., N=Kn[sup]3[/sup], where N is the power consumption of the water pump, n is the speed of the water pump during operation, and K is the proportional coefficient. It has been proven that using frequency conversion equipment can reduce the average operating speed of the water pump by 20% compared with the power frequency speed, thereby greatly reducing energy consumption and achieving an energy saving efficiency of 30% to 40%. 4 Conclusion The constant pressure water supply technology uses a frequency converter to change the power frequency of the motor. Compared with the method of controlling the outlet pressure of the water pump by adjusting the valve opening, it has the effect of reducing pipeline resistance and greatly reducing interception loss. Moreover, because it operates under frequency conversion, when the outlet flow of the water pump is less than the rated flow, the pump speed is reduced, which reduces bearing wear and heat generation and extends the mechanical service life of the pump and motor. In addition, because constant pressure automatic control is achieved, frequent operation by operators is not required. When the operation is normal, it can be unattended, which reduces the labor intensity of personnel and saves manpower. The actual operation effect of the system shows that the adoption of this design scheme has comprehensively improved the reliability, dynamic performance index and economic and technical index of the community heating water supply system. References: [1] Dai Baohua, Xie Jiankai, Kang Guocheng. A variable frequency speed regulation constant pressure water supply technology [J]. Journal of Wuhan University of Transportation Science and Technology, 2000, 24 (4): 467-468 [2] Hua Manxiang. Application of frequency converter in constant pressure water supply system [J]. Electrical Age, 2005, (3): 95 [3] Li Ming, Yang Dayong, Zhang Hongxing, et al. Intelligent control system for constant pressure water supply based on variable frequency speed regulation [J]. Coal Mine Machinery, 2005, (6): 93-94.