Alternator repair

Following failure of original fit water 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 landing surface. Severe brinelling damaged at single element pitch had occurred indicating that a significant amount of axial movement had occurred.

Heating was to deep blue/black and a small fire had occurred quickly extinguished by vessels staff.

Repair took the form of disassembly to allow 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 degree existed in the shaft and the unit was removed for better assessment.

Initial cause of failure was attributed to misalignment. 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 occurred. 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 alignment revealed 11mm out of plane misalignment at NDE. The resin chocking was broken and the holes where suitably adjusted. After fitting of tapered dowels new resin chocking was used.

The unit was run on test with progressively increasing load. After one hour failure again occurred at the drive end bearing.

Investigation revealed that a spurious 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 cage.

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