Application And Research On Programmable Logic Controller And Frequency Converter In Water Level Control System

Programmable logic controller and frequency converter are both the modern industry control products on the basis of computer technology. The common problem of modern industry control is how we manage to combine them and to apply the former to control the latter.The first part is concentrated on the primary introduction of these two products on their history, operation principle, inner structure and application area. Then through the research of application of these two products under the constant pressure water-supply, it is drawn that variable frequency converter is energy-saving for the following points.1.The remarkable energy-saving is presented by the least power loss of the conventional asynchronous motor at the rated point under the electricity provision from the converter.2.1t is demonstrated that the variable frequency equipment can make the pump run at the average rotation speed of 20% reduction of the work speed that causes a great reduction of energy consumption and energy saving efficiency can be up to 20%～40%.3.Manpower is saving from the reduction of the labor strength since the automatic control is realized under the constant pressure that makes operators free from frequency operation.4.Maintenance expense is saving since the machine life cycle time is prolonged for the pump and motor in which the shaft takes the less wear, tear and heat with the less rotation speed of the pump which input flow is less the output one when this variable pump works under the variable frequency condition.5.The energy can be saving from the compensation of power factor.6.The economy benefit is very obvious for the remarkable electricity energy can be saving with the system in operation when its rotation speed is dependant on the outer water supply because the variable pump is working by the variable frequency.7.It is helpful to reduce the resistance of pipeline and the loss of the flow. In general,frequency converter technology can make the pump rotation speed vary with the amount of the flow which can not only omit the reservoir of the skyscraper but also reduce the capacity of the pump therefore it can save energy finally. The water supply process is approximately a system of time lag of first order, real time control of pressure in pipe is adopted self-tuning fuzzy PID. The theory of fuzzy controller is analyzed in detail and a self-tuning fuzzy PID controller of pressure is designed, the Smith predictor is used to compensate time lag of automation system.