transformer It is an electrical device without any continuously moving parts which by electromagnetic induction transforms electrical energy from one circuit to another circuit at the same frequency, usually with changed values of voltage and current. It consists of two or more coils called the primary and the secondary windings, wound around the same core of ferromagnetic material. The core enhances the value of magnetic flux. An alternating current flowing through the primary winding of the transformer, produces a magnetic flux that alternates at the same frequency. This varying flux induces an emf of self induction on the primary, E₁. The flux also links the secondary coil producing induced emf E₂ on the secondary under no load condition that is secondary coil open. Since the main flux and its frequency are the same for both primary and the secondary windings, the emf’s induced in the two windings are respectively proportional to the number of turns in the primary, N₁ and the secondary, N₂.

E₁/E₂=N₁/N₂ (t9)

The ratio N₁ to N₂ is called the turns ratio or ratio of transformation. Now consider a transformer to be loaded with a resistor placed across the secondary winding. The voltage across the terminals of the secondary winding causes a current I₂ flows through the load resistance and the secondary coils. As the losses are small, the power taken by primary winding from the lines is equal to the power delivered to the external circuit placed across secondary winding.

I₁ E₁ @ I₂ E₂

or, I₁/I₂= N₂/N₁ (t10)

The currents in the two windings are approximately inversely proportional to the turn ratio.

Single phase transformers are generally either core type or shell type. In the core type the primary and the secondary windings are separate (see fig. t9). In shell type the primary and secondary windings are done together and laminations enclose the windings (fig. t10). The shell type has better mechanical protection to the windings and is used as large power transformers. On the other hand for the core type construction insulation between the primary and secondary is better. Hence it is better adapted for high voltages.

In large transformers, Joule’s heating in the windings (copper loss) and eddy current losses in the core (iron loss) produce a large amount of heat that must be removed quickly. Transformers can be cooled by circulating oil or air. Oil is preferred because it is better cooling medium than air and provides better insulation.

The iron loss in the core is reduced by constructing it with thin laminations, insulated from each other. They are oriented so that the insulation lies in the path of the eddy current. At high frequencies (~ 10⁷ Hz) the transformer cores are made of ferrites, materials with high resistivity. This reduces the magnitude of eddy current thereby reducing eddy current loss.

In three phase transformer, three single transformers are connected in D -D or Y-Y arrangement as shown in fig. t11 and fig. t12.