OPERATING PRINCIPLES OF T RANSFORMERS

A single-phase power system transformer consists basically of two windings wound onto an iron core. The iron core concentrates the flux and restricts it to a defined path. It also creates the maximum possible amount of flux for a given excitation. In order to maximize the mutual coupling the two windings are wound concentrically on to the same part of the iron core. Figure 6.1 shows the typical winding arrangement of a single-phase transformer. This is called shell-type construction.

Not all the flux created by one winding couples with the other winding. Furthermore the flux which does not couple both windings does not flow completely round the iron core, some of it flows in the air close to the windings. The common flux in the iron circuit is called the mutual or magnetizing flux. The flux that escapes into the air and does not couple the windings is called the leakage flux. One winding is referred to as the primary winding and is connected to the source of supply voltage. The second winding is the secondary winding and is connected to the load. The primary may be either the low or the high voltage winding.

The magnetizing flux is determined by the applied voltage to the primary winding. In power transformers the current drawn from the supply to magnetize the core is only a fraction of one percent of the rated primary winding current. The core design and type of iron is specially chosen to minimize the magnetizing current.

When current is drawn from the secondary winding the effect on the magnetizing flux is to reduce it. However, the magnetizing flux density must be maintained and this is achieved by the primary winding drawing more current from the supply.