Transformer On Load

Figure 1.25(a) shows the transformer during no-load condition. The flux Φ is set up in the core. When the secondary is loaded shown in Figure 1.25(b), the secondary current will set up its own flux (Φ₂), which opposes Φ. The resultant flux becomes Φ – Φ₂. The value of e₁ will decrease because magnitude of Φ decreases. Hence, v₁ becomes greater than e₁ and the primary winding draws more current from the source. Let the additional current drawn by the primary be I₂^′. This current I₂^′ will set up its own flux (Φ₂^′) in the same direction of Φ and it will oppose Φ₂ shown in Figure 1.25(c). The resultant flux will be Φ at any load condition if and only if

Φ₂^′=Φ₂
i.e., N₁I₂^′=N₂I₂
i.e.,