Transformer mmf balance

Why Transformer is constant flux machine?

• When the supply voltage V₁ is given to the transformer primary winding, primary current I₁ flows through the winding which produces maximum flux Φ_m in the core.
• When the secondary winding of the transformer is loaded, the secondary current I₂ flows through it.

• The secondary current I₂ sets up its own mmf N₂I₂ and its own flux Φ₂ which opposes the main or primary flux F_m.
• Therefore the resultant flux and primary induced emf is also decreased.

         Resultant flux Φ = Φ_m – Φ₂

• The supply system draws more current from the primary winding due to difference between supply voltage V₁ and induced emf E₁ increases until the original value of flux or main flux Φ_m is obtained.
• Let the additional primary current be I₂‘. This current is equal in magnitude with I₂ but anti phase with I₂.
• The current I₂‘ is known as reflected load current.
• The current I₂‘ sets up flux Φ₂‘ in magnitude. Therefore the mmf N₁I₂‘ and mmf N₂I₂ cancel each other.
• We can say that the magnitude and phase of load component of primary current depends upon the type of load.
• The net flux passing through the core remains constant whatever the load conditions if we neglect primary and secondary leakage fluxes