STUDY ON THE OPTIMAL ULTRASONIC FREQUENCY FOR STATOR INSULATION DIAGNOSIS

A new ultrasonic testing system has been developed for application to stator insulation of large generator. It provides A-scan signals and C-scan map for tested specimen. Experiments are performed to reference specimens, such as epoxy/mica composite laminated plates, and practical stator bar specimens with 0.5, 0.8, 1.2, 2.0 and 5.0 MHz transducer probes. The results show that 0.5 ~ 1.2 MHz is an appropriate frequency range for this unaged composite insulation, with which the testing system is found to be able to receive the reflection pulse at insulation-copper interface in stator bar, and the internal delamination can be detected clearly. Ultrasonic nondestructive testing method provides a useful tool to detect practical stator winding insulation.