By Kevin Eaton
During the 19th century, when three-phase transformer technology was beginning to come to fruition, it was commonly acknowledged that three identical core legs proved to be the best method for electrical performance for a transformer. The technical challenges in producing a core from a manufacturing perspective — and the unpractical cost — made this method not a viable solution for mass supply to the electrical industry.
What evolved during that time, and what is still dominating the market space today, is a linear stacked three-legged core. A linear core for a three-phase transformer consists of essentially five main components that are combined: three individual core legs and the top and bottom yokes which are all made up of stacked straight sheets of electrical steel.
Upon assembly, small gaps remain at the joint locations, which cause an increase in core losses and magnetizing current. The use of overlapping layers (the step lap method) does reduce these effects and is the typical construction style of the linear core provided for the dry-type and conventional distribution market space.
Since 2010, the Department of Energy (DOE) has continued to move toward increasing efficiency level requirements (per the Code of Federal Regulations at 10 CFR 431.196) for all distribution transformers, including dry-type transformers rated 2500 kVA or less. The combination of the increase in efficiency level requirements and the advancements in transformer manufacturing capabilities has allowed the original symmetrical transformer design to become a viable and cost-effective solution for the electrical transformer market space.
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