The Study On Protection System Of The Three Phase Motor

With the development of electronics and microcomputer,microprocessor-based protection has advanced. But much of than are just the republication of the electromechanical and electronic protection. Many advantages of the computer,such as nunerical evaluation data storage and logical decision,isn't used. And most of the microprocessor-based protection is running isolated. There is little data exchange with outside. It's a new domain how connect motors with network economically and reliably and exerts its ability to analysis the data. There are many tries in this paper on these aspects.This paper studied the basic principle of the asynchronous motor deeply,analyses the faults and educed the ways to obtain the failure characteristics. Because of the separat between the protection and the monitoring,there are many disadvantages in avoiding the failuer. This paper designed a system to combine the advantages of the protection and the monitoring in software and hardware. This thesis analyzes failure characteristics of the induction motor systematically. A simple and easy operating criterion is put forward in accordance with the relation between positive-sequence component and negative-sequence component of stator current when the motor is in unbalance and single phase broken.Based on analyzing the traditional inverse time-lag overloading protection of motors,an inverse time-lag operating equation with accumulating stator current is achieved,according to the differential equation of the heating and cooling while the current flows into the conductor. Then the equation is further simplified to recurrence formula,which represents the thermal condition of the induction motor really,and reflects that the higher temperature rises,the more rapidly the motor cools. In order to consider the different heating of positive-sequence current and negative-sequence current of the motor stator generally,the equivalent current is adopted instead of the traditional phase current.The way how to gather positive-sequence current and negative-sequence current is brought forward,and the recurrence Fourier-algorithm is adopted to calculate the effective value of current and voltage,the effective value of positive-sequence current and negative-sequence current.In this thesis,according to the protection projects of various motor failure,the software and hardware for the control system is designed and the interference problem is solved. A new digital and multifunctional protection device for induction motor,which takes advantage of the 80C196KC single chip microcomputer as the key component and uses the MAX 197 conversion chip in the A/D conversion,can gather and process data,control synchronously,protect,display and warn automatically.