Study On Characteristics Of The Vibration Performance Caused By Broken-bar Fault In A Squirrel Cage Induction Motor

Squirrel-cage induction motors are widely used in industrial and agriculturalproduction and people’s daily life, guaranteeing the safe and reliable operation of themotor is very important. Broken rotor bar fault is a kind of common fault types,domestic and foreign researchers have used the characteristics quantities of statorcurrent to realize fault diagnosis and do the online monitoring. Broken-bar fault willinduce distortion of multiple physical fields inside the motor, such aselectromagnetic field, temperature field and stress field. In this paper, changingcharacteristics of the electromagnetic force wave caused by broken-bar fault werestudied and the vibration characteristics caused by broken-bar fault were calculatedto find out the fault features.Taking a squirrel-cage induction motor Y80-2as example, the2-D models forfinite element method of the healthy and faulty motor were established, then thetransient electromagnetic field was calculated, the time-space varied air-gapmagnetic flux density wave was obtained, the results were compared with the testdata. Based on the Maxwell stress equation, the electromagnetic force wave in theair-gap of healthy motor and broken motors was calculated using the air-gapmagnetic flux density. The orders, frequencies and magnitudes of theelectromagnetic force wave were calculated and analyzed combining with2-D fastFourier transform and the technology of discrete spectrum correction. The orders,frequencies of the magnetic flux density wave and electromagnetic force wave werecalculated by the traditional formulas, a comparison is made between the analyticand numerical calculation, and some variation laws of the electromagnetic force areobtained.Modal tests were conducted using the hammering method. The stators with nowindings, with no varnished windings, with varnished windings and with windings,which were cut off the ends, as well the whole motors were as the test targets.Meanwhile it is ensured that the stators of all the test models are the same. The second order radial natural frequency of the stator was recognized through themagnitudes and phases of the acceleration frequency response functions derivedfrom the modal tests. Based on the test results, the difference between the naturalfrequencies of the stator with varnished and no varnished was analyzed; as well theinfluence that the end windings made on the natural frequency of stator wasdiscussed. The modal of the stators with no windings and with windings that werecut off the ends was calculated respectively with numerical method. At the same time,the3D whole domain FEM calculation model of the prototype motor is established,and a constraint is imposed on the connection of rotor and bearing, so fullconsideration is made into the influence that rotor made on the motor modal. Thecalculated and test results were compared. Finally, the effects that rotor, end caps andmotor shell made on the stator modal were discussed.The transient dynamic FEM calculation model of the motor is established, theelectromagnetic force results derived from the transient electromagnetic field isregarded as the load so that the transient dynamics calculation of the healthy andfaulty motors is done. The vibration displacement of one point on the top of motorshell is extracted, and then the frequency spectral analysis is done to obtain thecharacteristics of the vibration performance caused by broken bar.