Study Of Magnetic Field And Thermal Field In Induction Motor With Broken Bars Fault

Induction motors are commonly used in a variety of industrial applications, and some induction motors are key elements in assuring the continuity of the process and production chains of many industries, such as electric power, ferrous metallurgy and petrochemical industry. However, the rotor bars breaking in the motor occurs frequently. Rotor failures are caused by inadequate casting and fabrication procedure or a combination of various stresses which act on the rotor, such as electromagnetic, thermal, environmental and mechanical. A sudden motor failure may reduce productivity and affect the production line safety and product quality in an industrial system, in which the electric motor is the prime driver. Hence, early detection and on-line monitoring of broken bars would help to avoid catastrophic failures and reduce repair cost. In recent years the broken bar detection and on-line monitoring problem of electrical motors have aroused more and more experts' interests. The aims of this paper are to study the magnetic field and thermal field of broken bar fault based on the fundamental theory. According to the variation of motor operating performance and parameter, some valuable ideas and methods can be proposed to the broken bars faults on-line diagnosis.Based on the multi-loop theory, the finite element model of induction motor with broken bars was defined. Under rated load, the quantitative evaluation of magnetic field in induction motor with broken bars was made, which contains the effect on distribution of magnetic field, flux density waveform on stator and rotor and its harmonic component. The typical time variation of stator core loss before and after broken bars fault was studied, and the iron core loss distribution on rotor tooth adjacent to broken bars was analyzed in detail. Meanwhile, the 3thharmonic component of air gap flux density was extracted, from this data, the faulty signatures were extracted. Furthermore, this method which could help to develop diagnostics of broken bars and performance evaluation of induction motor and provide new method has great potential in future applications.Inquiring into the effect on the motor operating performance based on time-stepping coupled finite element method, and the variation of the starting current, torque and speed with the evolution of the fault were studied, so a series time- varying of data could be obtained, such as, rotor current, end-ring current, magnetic force distribution on the rotor bars. The rotor parameters of healthy and faulty motor can also be got in response to above analysis, and the causal relation between the change of rotor parameter and the increase of rotor bar current adjacent to broken bar was explained. The position of induction motor rotor broken-bar and its effects were studied in this paper. The relationships between the broken-bars position and fault feature in stator current were analyzed, and the rules were shown. In addition, the obvious faulty feature may be observed by analyzed the harmonic component of phase current and voltage, which can provide another theory for the broken bar diagnostic method.Based on the magnetic field result of the locked bar condition before and after broken bars fault, the fault predisposing cause which is locked rotor and frequent starting was pointed out. The different of magnetic field distribution, torque, stator and rotor current between locked rotor and rated load was analyzed in detail in this paper, and its effects on the faulty tendency was explained. The variation of air-gap flux density of radial and tangential component at locked rotor was given, and faulty effects on the component were studied.The two dimensional finite element model of the thermal field over the whole cross section of a squirrel-cage induction motor with one broken bar and two adjacent broken bars were developed based on the healthy motor. The effective thermal conductivity of the air-gap was introduced and an innovational thought was advanced to calculate the stator-rotor coupled thermal field over the whole cross section of a squirrel-cage induction motor. So the complex exchange of heat between the stator and rotor was greatly simplified. The relationship between thermal field and broken bar's quantity and position was researched according to the analysis of thermal field. The validity of the thermal field model and the correctness of the calculated values were 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, and it could provide theoretical basis for the broken bar fault from the perspective of the variation of thermal field.