Alternator

Alternator

Following failure of original fitwater cooled alternator a similar sized air cooled 2MW unit was fitted.
After approximately 4 months run time the unit failed in service. Prior to this bearing and winding temperatures had been elevated but within limits.
failure took the form of drive end bearing failure. during failure the bearing had rotated on the inner anding surface. Severe brinelling damaged at single element pitch had occured indicating that a significant amount of axial movement had occured. Heating was to deep blue/black and a small fire had occured quickly stinguished by vessels staff.
Repair took the form of disassembly to alow access to the drive end bearing landing to ascertain whether in situ grinding was viable. Measurements taken either side of the landing indicated that a bend of some degre existed in the shaft and the unit was removed for better assesment.
Initial cause of failure was attributed to misalignement. The bedframe had to be substantially modified. FEA was carried out on the new structure and class approved. Close inspection was carried out on the structure of the assembly to ascertain if failure by deformation of fracture had occured. No obvious damage could be seen.
The flexible coupling was inspected and NDT carried out on major components.
Electrical tests were carried out on stator and rotor with no defects found
repair to bearing landing was carried out by machining and sleeving with thermally and galvanically compatible material. Repair by metal spraying was rejected.
After reassembly laser alignement revealed 11mm out of plane misalignement at NDE. The resin chocking was broken and the holes where suitably adjusted. After fitting of tapered dowels new resin chocking was used.
After reassembly the unit was run on test with progressively increasing load. After one hour failure again occured at the drive end bearing.
Investigation revealed that a sprious bearing had been supplied from local distributor. This was of poor quality and failure leading to plastic deformation of the rolling elements and distortion of the cageoccured.

reassembly with correct bearing allowed for proper testing and the unit being placed back in service