Bump testing of generator stator end-windings

Diagnosing vibration problems of stator end-windings

Electromagnetic forces cause vibration of stator end-windings. This vibration can cause a number of problems, including: erosion of coil insulation, precipitating a ground fault; breakage of coil strands, leading to overheating; and weakened end-windings, decreasing rigidity of structure. Over time, stator vibration can loosen the end-windings.

 

Stator vibration, which is caused by double-frequency operating forces, can loosen the end turns over time. KEMA’s bump testing service helps clients identify stator vibration problems before they loosen the end-windings and cause conditions requiring extensive out-of-service repairs. 

 

Bump testing—identifying problems before the become costly repairs

Bump testing can reveal if a generator’s stator windings are resonating at the double frequency, making them prone to vibration problems. Stator windings whose end turns vibrate chronically may have to be retrofitted with a new bracing system.

 

The KEMA bump test tests a total of three “rings”: an inner ring close to the core, a middle ring, and the outer ring close to the bar-to-bar connections. Much information is gained from data-trending, follow-up tests, and comparison of spectrum changes. Analog signal sampling takes place with 5kHz (up to 25kHz possible). The frequency response spectra are typically monitored in the range of 30 to 240Hz.

 

KEMA developed the Bumptest software in-house and can customize the package to meet specific needs. Therefore, the system is capable of diagnosing critical vibrations in all kinds of generators.