Machine Parts

When a transformer primary is energized from a voltage source and the secondary is opened circuited, a small current will flow in the primary. This current is usually less than 5 percent of the rated full load value of the transformer. Since no current flows in the secondary winding . The primary may be considered as a coil with a large reactance due to the iron core. Thus causes a small current . But when load is applied to the secondary terminals. In accordance with Lenz’s law, the current that flows through the secondary winding must act in such a direction as to oppose the flux set up by the primary current. When the flux is momentarily reduced, the induced emf in the primary winding is also reduced and therefore more current flows, in the primary winding. When more load current flows in the secondary the primary current will again increase.flux