Inspection of stator cores in large machines with a low yoke induction method-measurement and analysis of interlamination short-circuits

Interlamination short-circuits can cause major damage to electrical machines. Especially endangered are large turbomachines with high yoke width and correspondingly high interlamination voltages. These generate during operation, at certain interlamination short-circuit contact resistance's, high short-circuit currents and lead in the worst case to "core melting." There is a need to develop a safe measuring method, which enables on one hand all interlamination short-circuits to be registered, and on the other hand a quantitative assessment of the danger of the interlamination short-circuits for the machine. For nearly 20 years a measuring method with lower yoke induction has been used without disadvantages of the high induction method. By this method an interlamination short-circuit is detected with a measuring coil. The signal has been interpreted in terms of current. With this interpretation of the measuring signal it is not possible to take a meaningful consistent quantitative analysis of the interlamination short-circuit. For a correct analysis we introduce a calibration procedure and data processing algorithm. The method with introducing a calibration procedure permits a complete analysis of the lamination insulation, both quantitatively and qualitatively. The analysis of the measuring signal, the mechanism of "core melting" and the comparison of the measuring methods are treated in detail in this article.