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The purpose of this paper is to investigate the distribution of in‐plane eddy currents in stator core packets of turbine generators, and to reveal the loss reduction effect by the slits in the stator teeth.

The in‐plane eddy currents are calculated by a 3D finite element method that considers lamination of electrical steel sheets. First, this method is applied to a simple model that simulates the stator core of the turbine generators. The calculated losses are compared with the measured losses in order to confirm the validity. Next, the same method is applied to a 250 MVA class turbine generator.

The validity of the calculation method is confirmed by the measurement of the simple model. By applying this method to the turbine generator, it is clarified that the considerable in‐plane eddy currents are generated not only at the end stator packets, but also at the top of the teeth of the interior packets due to the duct space. It is also clarified that the in‐plane eddy‐current loss decreases as nearly half by the slits of the stator teeth.

A reliable calculation method for the in‐plane eddy‐current loss in the turbine generators is developed. The results obtained by this method are valuable for the design of the generator from the viewpoint of heat conduction.