Generator Rotor Rim to Spider Arm Attachment Data Collection
The U. S. Bureau of Reclamation, Technical Service Center conducted tests designed to measure generator rotor rimmovement on two, seventy-five megawatt hydroelectric generators. Tests focused on evaluating the generator rotor rim to spider armattachments and the measurement of movement of the rotor rim as a result of centrifugal and torque loading and thermal expansion. Onone unit, tests were conducted prior to and after repairs were made on the rotor to ‘reshrink' the rotor rim. Minute measurements of radial and tangential movement and changes in rotor shape relative to increased generator speed were recorded as the rim lost pre-tensioning with the rotor spider.
Need and Benefit
On certain hydro-generators, the rotor rim laminations often relax with age and the tight shrink fit between the rotor rim and rotor spider loosens. This relaxing is often accelerated by the forces created during load rejections, overspeed events, and thermal cycling. When the rim keys, that align and keep the rim preloaded, loosen to the point that the rim ‘floats' during normal operating speed or below, the accelerated cyclic loading and unloading can lead to damaging consequences to the rotor. Detrimental issues that can occur include irregular air-gap, rotor arm fatigue leading to rotor cracks, loss of rotor center, irregular magnetic forces between the rotor poles and stator winding, poor balance and damage to the rotor spider rim ledge. In recent years, hydroelectric generator manufacturer's and utilities have expressed increased concerns with rotor pole and rotor rim attachments cracking due to cyclic fatigue. This has generated the need for better insight into better methodology that can be used for stress analysis, fracture mechanics andmaterial degradation so that the remaining life of the rim and attachments can be predicted. By using actual data , assessments of stresses and cyclic loading of components can be more accurate. This successful research focused on the ability to measure within the rotor and in real time, generator rotor spider arm to rotor rim movement on an operating hydroelectric generator. Reclamation research engineers instrumented and developed test methodsandtechniques that were used in testing a generator with a suspected loose rotor rim. This report includes a travel report that details the methodology and findings from the test effort. The knowledge gained from the research will help to better assess rotor spider arm to rim fixationconditions in Reclamation's aging generators. Real-time measurements were taken on two hydroelectric generators. Research tests demonstrated the ability to obtain high precision measurements of rotor rim movement from the rotating rotor platform. When applied across Reclamation, data collected from this new test method allows for improved operation and maintenance that will help extend the life of these critical assets. This research was conducted on two hydroelectric generators at one of Reclamation powerplants and funded through Reclamation's Science and Technology program.
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Bureau of Reclamation Review
The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.
Generator Rotor Rim to Spider Arm Attachment Data Collection (final, PDF, 11.2MB)
By John Germann
Report completed on March 30, 2020