Transformer Core

• Core of the transformer is the main magnetic circuit and it offers low reluctance path for the flux. Special attention is given to core as it controls important operating characteristics such as no load current, losses, efficiency etc. hence high resistance and low hysteresis loss alloy steel is used the construction of core.
• Due to pulsating flux, eddy currents are introduced in the core and hence eddy current losses takes place in the transformer.
• As the eddy current loss is proportional to square of thickness of material, core of transformer is laminated and individual laminations are insulated from each other by varnish.
• To get the best results laminations used should be as thin as possible, however, due to mechanical consideration. (buckling effect during assembly) minimum thickness employed is 0.18 mm. Transformer grade steel having 3-5% silicon is used for the laminations.

Addition of silicon has three advantages :

1. It increases the resistivity of the core, hence reduces eddy current loss, temperature rise and       increases the efficiency of transformer.
2. It reduces area under hysteresis loop or in other words it reduces hysteresis loss.
3. It increases permeability of material which in turn reduces magnetizing current of the transformer. However high content of Si in the steel makes it hard and increases manufacturing cost. One more aspect is to be considered is the direction of grains, as the magnetic properties very considerably. Material offers higher permeability in the direction of grains, hence as for as possible rolling and cutting should be done in the direction of grain.
   

• Manufacturing process used such as hot rolling, cold rolling has impact on maximum permissible flux density.
• The hot rolled steel can have maximum flux density of 1.45 Wb/m”, while in case of cold rolled steel limit of maximum flux density is 1.8 Wb/m. Use of higher values of flux density reduces the amount of core material which in turn reduces weight, size and cost of transformer.
• Modern transformers uses cold rolled grain oriented steel. (i.e. CRGO steel) for the core of transformer which gives advantages of cold rolling as well as grain orientation advantages which include :