Research On Parameters Identification And Variable Frequency Driving System Of Asynchronous Motor

Ac asynchronous motor has the advantages of simple structure,convenient manufacturing,low price,reliable operation and can be used in the worse environment.Benefit from the universal application of vector control theory.It can be widely used in production machinery and living instruments.The key of vector control strategy is the orientation of the magnetic field,and the accuracy of the motor parameters is an important factor affecting the orientation of the magnetic field.If some parameters of the motor are setting not accurate or changing during operation,it will lead to the deviation of the magnetic field orientation,thus affect the performance of vector control system.Therefore,the parameter identification of asynchronous motor is very important.This paper mainly studies the problems of parameter identification and variable frequency driving of ac asynchronous motor in vector control system.In there three kinds of parameter identification methods and two kinds of variable frequency driving methods were discussed.First,The simplified circuits of dc,block and no-load test in traditional parameter identification of asynchronous motors are analyzed systematically.In order to eliminate the influence of other harmonics and improve the precision of parameter identification.The PI current control and the least square method are added on the original basis.In addition,the data processing method of discrete Fourier transform is used in the block and no-load experiments.However,the accuracy of this method is limited,This parameter identification method can only be used to determine the initial value of each parameter of vector control.Second,on the basis of vector control of rotor field orientation,The second derivative of the current in the motor model needs to be discretized,when using recursive least square method for parameter identification.Considering this operation not only increases the amount of computation but also amplifies the measurement noise.Therefore,an improved linear model of asynchronous motor is proposed.In the model input term,only the first derivative of the current needs to calculate,and the others can be observed directly.The second order butterworth filter is used to filter the measurment noise.Moreover,the improved euler method is used to directly solve the state equation of butterworth filter,need not to do discretization processing can directly obtain the first derivative value of the current and the current value after filtering.This parameter identification method is accurate and simple,but it is susceptible to noise and speed fluctuation.Third,when the time of asynchronous motor parameter identification,higher requirements for current stability are presented.Therefore,using the parameter adaptive PID algorithm instead of the original PI in asynchronous motor vector control current loop control,this algorithm adopts the recursive least square method with forgetting factor to estimate the PID parameters online.The PID parameters can be adjusted in real time according to different situations.Fourth,the rotor flux cannot be directly observed,when using the motor model under the rotor magnetic field orientation ?-? coordinate system for genetic algorithm parameter identification.Therefore,a coordinate system of stator current orientation is proposed.In this coordinate system,the magnetic chain model can be expressed by direct observation term,through the coordinate transformation substitute into the original motor model,elimination of the magnetic chain input term,avoid the error caused by the observation magnetic chain.Due to the genetic algorithm is easy to arise the problems of slow convergence and local optimum in the process of optimization.Therefor,the Newton iteration method is applied to the genetic algorithm.and make once iteration for identification parameters.Greatly improved the convergence speed and search ability of genetic algorithm.This parameter identification method is accurate,fast and robust.Fifth,when the asynchronous motor is in variable frequency control,The traditional flux weakening control method cannot fully utilize the ability of voltage output torque.Therefore,proposes a flux weakening control strategy based on current boundary limit condition.This method can make the output torque current close to the torque current constraints condition and then the stator current vector close to the current limiting circle,after adjusting exciting current.The maximum excitation current can be used as the initial value of excitation current in the constant torque zone.Improved the output torque capability of the constant torque region.In the flux weakening region,it is not necessary to consider the problem of voltage margin,which can make full use the ability of voltage output and improve the output capability of the torque of the flux weakening region.In view of the non-linearity of the exciting current regulation process,the fuzzy PI controller was introduced,which makes the regulation process more precise and has better dynamic and static characteristics.