Magazine | Issue 1 2025 Rotor windings in rotating electric machines are subjected to high mechanical forces that often cause unexpected outages. This makes turbo generators particularly vulnerable due to the high speeds of their rotors. These systems are usually essential components in energy generation and industrial production, so outages can incur significant additional costs. Short circuits between a rotor pole’s windings are one of the leading causes of these outages, making their reliable diagnosis crucial to prevention measures. In the world of transformers, frequency response analysis (SFRA, Sweep Frequency Response Analysis) is a widely used, highly sensitive method for carrying out diagnosis measurements. This method can also detect interturn short circuits in a rotating machine’s rotor poles. Until now, there hasn’t been an objective data-based evaluation method that ensures precise diagnostics – although they’re essential for accurately evaluating rotor windings. This prompted EMIS, a German service provider in electrical engineering, automation, and energy supply, to explore the potential advantages of applying frequency response analysis to the rotors of the turbo generators they service. This investigation was undertaken as part of an internal master’s thesis project. Pascal Fröhlich, who worked at the company while pursuing his master’s degree, conducted the research with the encouragement of Johannes Ohde, Head of the Electric Machines department. Both were kind enough to answer our questions regarding their work. Johannes, what kind of rotors are we talking about here? Johannes Ohde: Although EMIS specializes in turbo generator maintenance, the scope of their expertise also covers an entire range of machines. We wanted the thesis to focus on high-speed machines, with 1,500 or 3,000 rpm speeds as we usually work in the 50 Hz power system. Pascal measured various rotors for his thesis, including salient pole and non-salient pole machines. Seven pieces of equipment find their way into the thesis with nominal powers ratings from 1.8 MVA up to 588 MVA. «Existing methods are highly subjective interpretations, so we were looking for a more sensitive method with objective evaluation criteria.» Johannes Ohde, Head of the Electric Machines department, EMIS Electrics GmbH 37
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