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Reliable Diagnostics on Rotating Machines using FRA

The frequency response analysis (FRA) is a known highly sensitive method to detect mechanical and electrical defects in power transformers. Investigations already showed, that this technique is applicable to detect broken windings or shorted turns in rotor and stator of rotating machines. Here, the applicability of FRA to other defects was investigated, like broken rods of damping windings in synchronous machines or in a squirrel cage of induction motors, which often can be detected only by disassembling the machine. The reason lies in the design: damping windings as well as the squirrel cages have no accessible outer terminal, which can be used for electrical measurements. As the FRA method is capable to measure also capacitive or inductive couplings, it might be a possible low effort alternative. This investigation focuses on the reliability of failure detection. For a clear conclusion, numerous tests were performed on rotating machines of different types. It was found, that the frequency response can be measured on assembled rotating machines highly reproducible up to 1 MHz in different setups. Main factors to be considered are a good connection technique and a clear description of the measurement setup for later tests. A comparative analysis is only possible for the same phase configuration as well as the same rotation angle of the rotor. Like power transformers a time-based comparison is preferred. Different phases have not exactly the same frequency responses. For the investigation, described in this contribution, broken rods in a squirrel cage of a 55 kVA induction motor and in the damping winding of a 37 kVA synchronous machine were re-produced. It was found, that the FRA method is sensitive enough to identify even single defects.