Study On Simulation Models And Diagnosis Methods For Multiple Faults Of Squirrel Cage Induction Motor

This subject is an important part of major projects in Shanxi Province whose name is “Based on the Internet of things in the coal mine production equipment condition monitoring and life cycle management system development"(item serial number: 20131101029),the first subtopic of “condition monitoring and development of life cycle management system of mine main fan”.In this paper,the fault diagnosis of motor in mine main fan is studied.The sensor technology,simulation modeling,fault diagnosis technology and so on are used to realize the effective monitoring and reliable diagnosis of single and compound faults of squirrel cage induction motor.The type of high voltage motor of mine main fan is squirrel cage induction motor,which is not only used for mine ventilation,but also widely used in industrial and agricultural production.The bad variable driving load and terrible working conditions due to the faults of squirrel cage induction motor.In squirrel cage induction motor,the rotor broken bar fault rate is as high as 10%,and most of the mechanical faults will cause the air gap eccentricity.Because of the assembly error and processing defects,even the new squirrel cage induction motor will be accompanied by a certain degree of static eccentricity.Static eccentricity produces an asymmetric magnetic pull,which results in dynamic eccentricity.Rotor broken bars,bearing defects and other failures will directly cause dynamic eccentricity.Therefore,the common eccentric fault type is mixed eccentricity.When there is the rotor broken bar fault,in a slight degree it would be accompanied by the static eccentricity fault,and the mixed eccentricity fault will occur in a serious degree.The low characteristic frequencies caused by rotor broken bar and eccentricity fault in stator current are very close,and difficult to distinguish.Broken rotor bars,air-gap eccentricity and the compound fault of broken rotor bar and eccentricity have been studied and diagnosed.In this paper,each single fault is analyzed in depth,and the fault types are clearly distinguished.On the other hand,the mechanism of compound faults is analyzed,and the characteristic frequencies are deduced.The broken rotor bars fault and eccentric fault directly lead to the change of the air gap magnetic field,which causes the oscillation of stator current,and the corresponding characteristic frequencies will be generated in the stator current.Based on this mechanism,a number of scholars have summed up the fault characteristic frequency formula of broken rotor bars and three kinds of eccentric faults in stator current after years of research and experiments.While,the characteristic frequencies is not simply the superposition of two single fault characteristic frequencies in the compound fault of broken rotor bar and static eccentric compound fault and compound fault of broken rotor bar and mixed eccentricity.When two kinds of faults occur at the same time,the air gap magnetic field will form a new equilibrium state,and the flux distribution inside the motor will be adjusted accordingly.The distorted magnetic field will produce some harmonics in the stator currents.With that,it is necessary to recalculate the harmonics under complex faults.According to this train of thought,the squirrel cage induction motor is regarded as the combination of rotor and stator circuits with mutual movement.In this paper,the multi-loop theory model of the induction motor is established according to the circuit principle.The key of the multi-loop theory model is to calculate the inductance parameters of the motor accurately.The modified winding function approach(MWFA)is used to calculate the inductance parameters,and the formulas are derived for calculating the characteristic harmonic frequency of the stator current under the compound fault.In order to further understand the change of the magnetic field inside the motor,and to verify the theoretical analysis results,the simulation model of squirrel cage induction motor is carried out based on the finite element simulation software of Ansoft.Fault motor simulation models with the four single faults and two compound faults are established separately.The magnetic field distribution of the motor under different faults and the simulated stator currents are obtained.The spectrum analysis of the simulated stator current is carried out to verify the results of the mechanism analysis.On the basis of the theory and simulation analysis,the fault experimental platform is built.Experiments of squirrel cage induction motor with different faults were carried out.The experimental results are in agreement with the theoretical and simulation results.Fault analysis is only the means,diagnosis is the purpose.For multi fault diagnosis of squirrel cage induction motor,a diagnosis method based on empirical mode decomposition and hidden markov model is proposed in this paper.Firstly,decompose the stator currents under each fault by EMD,and select the effective intrinsic mode function to form the feature vector.Then use the feature vector of each fault to train the corresponding HMM model.Finally,calculate the maximum likelihood probability value of test data in each HMM model,then the biggest one corresponds the fault type.In view of the broken rotor bar,static eccentricity,dynamic eccentricity,mixed eccentricity,compound fault of broken rotor bar and static eccentric compound fault and compound fault of broken rotor bar and mixed eccentricity of squirrel cage induction motor,this paper has carried on mechanism analysis,simulation,experiment and fault diagnosis,which realizes the reliable diagnosis of single and compound faults.