Case Studies: Turbogenerator Rotor

Case Study: Rotor Forging Damage Repair

Inadvertent energization of this unit resulted in the CE retaining ring being welded to the forging and having to be machined off in order to remove it.
Closer view of the existing tooth tops and snap ring groove after removal of the damaged retaining rings.
Assessement of the the tooth top area of the forging showed numerous serious cracks.
Finite element plot of Von Mises stress in the new longer retaining ring design at 4,320 rpm.

A generator was inadvertantly energized for several hours in the standstill condition. Once exposed, it was clear that the tooth tops had been severely overheated. Various points of distorted, melted steel were evident and hardness checks showed some affected areas of the forging had experienced martinisitic transformation. NEC inspected the rotor and determine the full extent of the damage and whether the unit could be repaired.

  • Unit nameplate: WH, 10.5 MW, 13.8 kV and 3600 rpm.
  • Brass runners at the top of the rotor slots, which were part of the amortisseur circuit, melted and the liquid brass ran out of the snap ring holes.
  • The retaining rings became "welded" onto the rotor tooth tops. The only way to remove them from the rotor body was to machine them off. Once the rings were off the tooth tops could be examined more closely. Half were found with serious cracks or missing.
  • Further testing and design analysis was done. A "long ring modification" was found to be the best way to save the rotor forging. A new set of tooth tops and a new snap ring groove were machined into the forging. New retaining ring blanks were machined to a longer length to match up to the new snap ring groove. New snap rings were manufactured.
  • The rotor was also rewound with the old copper and old slot wedges were replaced with newly-manufactured ones.
  • The rotor was dynamicallly balance and shipped back to the owner's site.