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A transformer is a device that changes ac electric power at one voltage level to ac electric power at another voltage level through the action of a magnetic field. There are two or more stationary electric circuits that are coupled magnetically. It involves interchange of electric energy between two or more electric systems. Transformers provide much needed capability of changing the voltage and current levels easily. They are used to step-up generator voltage to an appropriate voltage level for power transfer. Stepping down the transmission voltage at various levels for distribution and power utilization. 1. Transformer Classification In terms of number of windings Conventional transformer: two windings Autotransformer: one winding Others: more than two windings In terms of number of phases Single-phase transformer Three-phase transformer Depending on the voltage level at which the winding is operated Step-up transformer: primary winding is a low voltage (LV) winding Step-down transformer: primary winding is a high voltage (HV) winding 2. Primary and Secondary Windings A two-winding transformer is shown below. It consists of two windings interlinked by a mutual magnetic field. Primary winding –energized by connecting it to an input source. Secondary winding –winding to which an electrical load is connected and from which output energy is drawn.