Constant pressure water-supplyWith the progress development of social economic and the construct scale of city continuously enlarge, increasing of population and standard of living of people continuously improved, more and more demanding to quantity, quality and stability of water supply bring forward in city. The automatic degree of waterworks in town is much lower than the correspondence in city. Especially in some old waterworks, the equipment of automatic control system is dropped behind. The control of unit ismainly depended on manual operating of watches. The process of control is very complicated and that manual control cant meet the change of pressure in the pipes and the water level of the pure pond. It also cant act the proper feedback in time. For the sake of water supply, the electromotor unit usually works in the state of over charge. Not only efficiency of electromotor is low and electricity consuming is bigger, but also the pressure of pipes is overload in long term. On the basis of analyzing status in quo of the waterworks in our country, this paper designs a suit of constant pressure water supply automatic control system by using variable frequency speed-regulating technology based on PLC. The system is made up of PLC, transducer, units of pumps and electromotors, pressure sensor, industrial control computer and console. The system is used a transducer to make four electromotors starting, running and timing. The two high-power electromotors(220Kw) and low-power electromotors(160Kw) is partly circulated. To connect control computer with PLC and realize supervisal, a real time supervisal control software of water supply system is developed. The software has the function of data connection, communication, state control of facilities and data management. To tune up the frequency of power supply by the transducer in the system, this adjusting make the speed of electromotor and performance curve of pump change. At last, it make the status of a pump alter. Through analyzing the figure of energy wasting of pump, the quality in pipes is direct proportion of the speed of pump if the speed of pump is reduced. The power of electromotor fell the cube of the speed of pump. So the energy wasting of constant pressure water supply based on variable frequency speed regulating technology is envident less than the traditional mode isused throttle to supply water. Through using STEP7 MicroWin program software of SIMENS Company in German in the control system, a PID controller of pressure control within PLC isdesigned. The controller can compare the measure value and the value in advance of pressure. Throughreal-time controlling the output voltage and frequency of transducer, the output quality of pump is changed along with the changing of pumps speed. It makes the pressure of pipe self-regulating and steady in the scheduled value. There are some drawbacks such as conversion current is great, easy to burn the fuse and switch if off in variable frequency to working frequency of high-power electromotor. In the former researching, the conversion of variable frequency to working frequency is only emphasized the speediness. The conversion is usually failed because the key is not the speediness of conversion. Through analyzing and researching the problem of variable frequency to working frequency of high-power electromotor in theory, the key of conversion is pointed out in the paper. On the basisof analyzing equivalent circuit diagram and vector-diagram of induction-motor, the phase of working frequency power whether or not consistent with the output phase of variable frequency power at the moment of conversion is the key to decide whether the conversion is successfully. With the application of phase detector in industrial experiment on the spot, when the phase of working frequency power supply is consistent with the output phase of variable frequency power supply, PLC sends out the instruction to switch off the output of transducer. The electromotor is connected with the industrial frequency voltage after it was disconnected from the output of transducer. The conversion time is less than half hundred ms. The switch off current of variable frequency to working frequency is around one and half times of rating current of electromotor when the measures is made. It is succeed to solve the problem of variable frequency to workingfrequency and realizing the smooth conversion of high-power electromotor. It reduced the impact on power supply and equipments, too.The operation and experiment demonstrates that the system make the work of water supply automated in some degree. It is good for energy saving and improving the management of waterworks. Its also facility to mount and maintain, reliable to operating, and precise to supervise and control. The supervisory control software has abundant functions, kind interface and convenience to using.恒压供水随着社会经济