Study On Constant Pressure Water Supply Control System Of Variable Frequency

With rapid development of social economy and continuous enlargement of urban construction scale, as well as increasing population and continuously improved living standard, more and more demands to quantity, quality and stability of water supply are put forward. Meanwhile, energy resources are becoming much scarce presently. So it is inevitable tendency to design water supply system which has high function and saves on energy well, with help of advanced techniques of automation, control and communication. At the same time, this system can be applied to different water supply fields.According to running property curves of pipe nets and water pumps, principle of variable frequency speed regulation of water supply system is dwelt on firstly. Then, this dissertation analyzes theory of VF constant pressure water supply and construction of the system and proposes several different control schemes. Constant pressure water supply control system of variable frequency speed regulation is composed of PLC, transducer, sets of pump motors, pressure sensors, industrial control computer and control housing, etc. This system uses one transducer to propel four motors, accomplishing their starting, running and speed regulation. The two higher motors and lower ones are running respectively in an alternate mode. Designs of hardware and software of the system are also made in this dissertation.As for high-power motors, converting current tends to be so heavy in the course of transformation from variable frequency to work frequency that fusible cutouts are prone to be burnt out. In terms of these problems of high-power motors in course of transformation from variable frequency to work frequency, further studies are performed theoretically. According to analyses of equivalent circuit and vectorgraph of the induction motor, it is the phases of work frequency power and variable frequency output power, in the instantaneous occasion of transformation from variable frequency to work frequency, that determine whether the transformation from variable frequency to work frequency can be accomplished or not. Integrating with phase-and-frequency detector, as phase of work frequency power is in accordance with that of variable frequency output power, PLC sends instructions and cuts off output of the transducer. When the motor is cut off from output interface of the transducer, it is connected to work frequency power immediately. Thus damages to the motor devices and electric nets are reduced greatly.In the end, by taking a Fuji POD as an instance, characteristics of POD and its applications in constant pressure water supply system are presented in this dissertation respectively.