An Innovative Inductive Air-gap Monitoring System for Large Low Speed Hydro-generators, Tests in Operation

Large low speed hydro-generators (fig. 1) have a very small specific air-gap to stator bore diameter ratio making it virtually impossible to have a perfect centering of the rotor within the stator during the assembly process. Therefore the machines are operated with an eccentricity that, though small, is not negligible, and is the cause of undesirable effects: considerable unbalanced magnetic pulls, vibrations, additional losses. It is therefore important to assess the eccentricity and even more to check its trend in case of any stator or rotor deformations in order to guarantee a safe operation and to prevent any serious damage at an early stage. This paper describes a novel and patented [1] air-gap monitoring system for large low speed synchronous hydrogenerators. This new system is based on inductive sensors made of simple coils disFig. 1: Large low speed hydro-generator. tributed on the periphery of the stator, each one between two ventilation ducts. These coils may be put in place by the manufacturer; for generators already in operation, the coils are put in place from the back of the stator yoke using a simple method. It does not require an access to the rotor; of course the coils are easy to remove.