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Reducing Building Factor by Using Step Lap (SL) Laminations

A proper value of the Building factor (BF) is important while calculating the No-Load Loss (NLL) of any power or distribution transformer. It is an empirically derived factor which is based on the experience of the Transformer Manufacturer (TM) and ranges from 1.08 to 1.35 for three phase, three limb cores. Step Lap (SL) construction of transformer core instead of conventional but type construction (or Non Step Lap (NSL) type) which is still widely used by TMs in India, has been successfully used by various TMs world over, to reduce the Building factor in Transformer cores by 5 to 8%, reduce the No Load current and the noise level relative to conventionally stacked NSL cores. This paper explains how SL laminations reduce the No Load Loss in a transformer by considering the specifics of magnetic Flux transfer in joints areas of a SL core versus a NSL transformer core. By 1947, the first catalogue containing design curves and other essential information on grain oriented steels was published.

The magnetic circuit is one of the most important active parts of any transformer. While the basic principle of transformation of energy has remained the same for over a century, since the first transformer was built, transformers have become more efficient due to improvements in materials and more sophisticated production processes (better manufacturing technology). However, the first Conventional Grain Oriented Steel (CGOS) grades known popularly today as M3, M4 and M5 were developed in the late sixties and the Hi-B Grain Oriented Steel grades (HG-OS) were developed in the early seventies whilst laser scribed material in the mid-eighties.

This paper examines one such production process which reduces the NLL, No Load current and noise level of a transformer and hence improves it’s efficiency.

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