Dry Transformer Core Construction
The core is constructed of thin (<.30 mm) grain orientated laminate steel sheets. The lamination used is an inorganic material selected for its compatibility with the transformer oil and its corrosion and temperature resistant properties. Insulation mediums used in the past included varnish and paper. It is beneficial to construct the core with the maximum amount of laminate sheets possible, it is therefore imperative that the inorganic coating is as thin as possible (thus yielding a high core fill factor).
Each steel sheet is cold rolled during manufacture in order to align the magnetic domain with the rolling domain; this reduces eddy current losses and these losses can be further reduced with laser treatment. Thin sheets are required in order to reduce eddy current losses, typical sheet thicknesses range from 0.1 mm to 0.3 mm. It is important to remember that it is the iron within the steel (steel consists of carbon and iron) that gives the core its magnetic properties, without the iron the functioning of the transformer would not be possible.
Each of the core sheets is fully insulated in order to reduce eddy current losses, but the entire core is also earthed to one point. The purpose of the single point earth is to allow easy checking of the core for any unintentional earthing.
It is necessary to either glue or mechanical restrain the laminate sheets in order to prevent unwanted movement/displacement; smaller transformers typically use glue whilst larger units use steel straps, curved tie bolts or epoxy stockings.
Stacked Core – The type of core construction described above is for a ‘stacked core’, the laminated sheets are literally stacked on top of each other. The stacked core construction is used for almost all moderate to large size transformers.
Wound Core – The wound core is less common and is mainly used for single phase transformers.