An electrical transformer essentially consists of a magnetic core upon which are wound two (or more) coils of insulated copper wire, suitably located with respect to one another and usually termed the primary and secondary respectively. The ratios of these windings determining if it is a step-up or step-down transformer with respect to the input voltage or an isolating transformer if the ratio is 1:1.
Power-transformers used in iron cored transformers are given insulating coatings to develop adequate magnetic insulation between layers in the stacked core, thus reducing the interlamination eddy current loss to an acceptable level; this keeps the total watts loss of the stack to a minimum. The use of grain oriented steel enables cores to be used at higher flux densities, thus giving higher efficiency, smaller size or reduced weight, compared to transformers using lower grade steel.
Stamped power-transformers come in a variety of shapes and sizes, the most popular type being ‘E’ and ‘I’ power-transformers. This shape gives excellent economy with acceptable losses per kilo of material used for a given transformer design. There are several grades of this type of power transformers available which can be used to either reduce the physical size or improve efficiency.
When a voltage is applied to the primary windings (with the secondary open circuit) a small current will flow, this magnetises the core and supplies the iron loss and is called the no-load current. The iron losses are constant in the transformer; the losses that vary with load are the copper losses. When a load is connected to the secondary, increased current flows in the windings and copper losses become more significant. The ability of the transformer to dissipate the losses determines the eventual temperature rise. Other losses are present such as stray eddy currents in the core clamps, bolts, dielectric loss in insulation, etc., although normally these are of a smaller nature.