发电机转子小齿裂纹的超声回波特性及在机检测方法

Ultrasonic Echo Characteristics and On-machine Detection Method of Small Tooth Cracks in Generator Rotors

发电机转子小齿在护环约束下具有极高的预应力,然而由于其频繁启停的特性使得电机转速变化进而致使护环离心力改变最终引起小齿齿根的交变集中应力,因而小齿齿根的疲劳裂纹是不拔环条件下发电机检测的重要内容。针对护环-小齿双层结构下小齿裂纹反射回波特性不明、信号衰减严重、检测精度低的问题,基于超声有限元声学仿真手段,探究了不同几何特征下裂纹的超声回波特性,确认了超声信号的产生机理及演变规律,构建了表征裂纹有无的尾波指标;进而规划了超声裂纹检测的扫描与判定准则,最后通过自搭建的小齿-护环高保真模拟结构试验台验证了该方法对模拟裂纹状态定性及定位检测的有效性,从而为发电机转子小齿内部裂纹缺陷的超声原位快速检测提供了一种新的思路。

The small teeth of the generator rotor have extremely high prestress under the restraint of the retaining ring.However,because of its frequent start and stop,the centrifugal stress of the retaining ring changes with the rotation speed,which causes the alternating concentrated stress of the tooth root of the small tooth.Therefore,the fatigue crack of the small tooth root is an important concern of the regular inspection of the generator.Shooting at the problems including the unknown reflection echo characteristics,serious signal attenuation and low detection accuracy of the small tooth crack under the l retaining-ring double-layer structure,the ultrasonic echo characteristics of cracks with different geometrical characteristics are explored through the finite element acoustic simulation model,the generation mechanism and evolution law of ultrasonic signal are confirmed and the coda index characterizing the existence of cracks is constructed.Correspondingly,the scanning strategy and the judging criteria for the ultrasonic-based crack detection are figured out.Eventually,the effectiveness of the method for the qualitative and localization detection of simulated cracks is verified by a small tooth-retaining ring simulation bench.This method provides a new way for the in-situ rapid detection of cracks in small teeth of generator rotors.