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The effect of grain size on core loss and permeability was studied in three heats of silicon-free motor lamination steel. Grain sizes in the final test specimens ranged from 7 to 1360 µm. The core loss and permeability were measured at 10 and 15 kG ac, and 15 kG dc. Regression equations were constructed which took account of the effects of major changes in grain size as well as of minor variations in thickness, resistivity, texture, and inclusions on core loss and permeability. At the commercially important induction of 15 kG, the ac core loss decreased and the permeability increased as the grain size increased from 7 to 90-160 µm. As the grain size increased further, the core loss increased gradually and the permeability decreased sharply. Similar results were obtained at 10 kG ac and 15 kG dc.