Process factors of transformer cores made from 0.18 mm, 0.23 mm, and 0.28 mm thick grain oriented steel

Regular grain oriented (RGO) silicon steels (0.18, 0.23, and 0.28 mm thick) with forsterite coating were formed into 37.2 and 63.6 kg uncut toroids, gapped toroids, and gapped, pressed rectangular cores. During the core (or toroid) manufacturing process, samples of the steel were obtained for Epstein testing at the front and back ends of each core or toroid. The cores/toroids and Epsteins were stress relief annealed at 800° for 4 hours in an 85% nitrogen - 15% hydrogen atmosphere. Additionally, gapped rectangular cores were made from forsterite-coated and stress-coated 0.18, 0.23, and 0.28 mm RGO silicon steels and 0.23 mm stress-coated high-permeability oriented (HGO) silicon steels. Process Factors (PF) were calculated by the equation: PF (%) = % For pressed rectangular cores made from RGO steel the process factors improve (i.e. become more negative) with decreasing steel thickness at a given test induction. Comparing gapped rectangular cores, gapped toroids, and uncut toroids, the most negative process factors were obtained in the uncut toroid, reaching as low as about -10% for the 0.18 mm and 0.23 mm RGO steels. For pressed rectangular cores, the high permeability oriented (HGO) silicon steels experienced more positive process factors than the regular grain oriented (RGO) silicon steels. At 1.7T the 29.0Kg cores made from 0.23 mm stress-coated HGO steel had an average process factor of +8.2% while the cores made from stress-coated RGO steel had an average process factor of -1.1%. The differences in core loss seen between Epstein tests of regular grain oriented (RGO) steels and high-permeability grain oriented (HGO) steels are not as large in transformer cores. For transformer manufacturers designing to high loss evaluations (i.e., lower design inductions), RGO steels may be the steels of choice, particularly 0.18 mm thick steel at inductions below 1.5T.