What Is a Transformer ?

In its most elementary form, a transformer consists of two coils wound of wire and inductively coupled to each other. When alternating current at a given frequency flows in either coil, an alternating voltage of the same frequency is induced in the other coil. The value of this voltage depends on the degree of coupling and the flux linkages in the two coils. The coil connected to a source of alternating voltage is usually called the primary coil, and the voltage across this coil is the primary voltage. Voltage induced in the secondary coil may be greater than or less than the primary voltage, depending on the ratio of primary to secondary turns. A transformer is termed a step-up or a step-down transformer accordingly.

Most transformers have stationary iron cores, around which the primary and secondary coils are placed. Because of the high permeability of iron, most of the flux is confined to the core, and a greater degree of coupling between the coils is thereby obtained. So tight is the coupling between the coils in some transformers that the primary and secondary voltages bear almost exactly the same ratio to each other as the turns in the respective coils or windings. Thus the turns ratio of a transformer is a common index of its function in raising or lowering voltage. This function makes the transformer an important adjunct of modern electrical power systems. Raising the voltage makes possible the economical transmission of power over long distances; lowering the voltage again makes this power available in useful form. It is safe to say that, without transformers, modern industry could not have reached its present state of development.