Magnetic Field Analysis And Thermal Field Calculation Of An Induction Motor With Broken-bars Fault

At recent years, the heating and cooling problem of electrical motors has aroused more and more experts' interests, which has become one of the most important problems during the process of Electrical Motor Design. Also, the temperature-rise is a vital factor that brings in the broken bar fault of motors. In passed, analysis of induction motors with broken-bar fault was mainly based on electromagnetism analysis; while the thermal analysis of the motor with broken-bar fault is not paid on enough attention. Because of the heavy influence of broken-bar fault on the motor's temperature-rise, a new study path using the variation of heat distribution inner electrical motors to study the broken-bar fault of motor is broken in this paper. From the variations of the motor thermal field, some valuable idea can be proposed to the broken bars fault diagnosis.The finite element models of a squirrel-cage induction motor in healthy and faulty states are developed firstly, and the process of solving the electromagnetic field inner an electrical machine using the FEM is introduced briefly. The variations of the magnetic field inner the motor operating with broken bars are studied using the FEM. Then the influences of the broken bars on the tooth magnetic field distributions are emphatically analyzed. In addition, the effects of the broken-bar fault on the bars'current distributions are also discussed.Aiming at the problem of determining the forced boundary condition difficultly when traditionally solving the electrical machine stator thermal field or rotor thermal field singly, the effective thermal conductivity of the air-gap is introduced and an innovational thought is advanced to calculate the stator-rotor coupled thermal field over the whole cross section of a squirrel-cage induction motor. So the decoupling between thermal field and fluid field of the stator and rotor can be implemented. So the complex exchange of heat between the stator and rotor is greatly simplified. Then the two dimensional finite element model of the thermal field over the whole cross section of a squirrel-cage induction motor is developed.Based on the losses of the motor calculated on normal full load, the steady stator-rotor coupled thermal fields of the motor under healthy state and broken bars fault are calculated respectively. Finally, some work is given to the analysis of thermal sensitivity to the stator losses, the height of the cooling ribs and the stator windings impregnation quality respectively, which could help optimum design of induction motors in future applications. The validity of the thermal field model and the correctness of the calculated values are verified by experimental results derived from a prototype motor. The thermal field calculation model of motor can be used in the analysis of other electrical motors without axial cooling whether there are with broken-bar fault or not.