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
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