What happens inside a transformer when a load is connected to its secondary winding?

In one line

When load current flows in the secondary, its MMF tries to reduce the core flux, so the primary automatically draws an extra current whose MMF cancels it, keeping the flux almost constant and transferring power.

Explained simply

It is like a self-adjusting water tank: whenever someone opens a tap downstairs (load on the secondary), the inlet valve upstairs opens wider by just the right amount to keep the tank level (flux) steady. The transformer needs no controller for this; physics does it automatically.

Technically, the secondary current \( I_2 \) produces an MMF \( N_2I_2 \) that opposes the main flux (Lenz's law). If flux fell, \( E_1 \) would fall below \( V_1 \), and more primary current would flow. The primary draws a load component \( I_2' \) such that \( N_1I_2' = N_2I_2 \), so the net MMF and flux return to the no-load value.

How it works

1. Load is connected; \( I_2 \) flows at the load power factor.
2. \( N_2I_2 \) opposes the main flux and momentarily weakens it.
3. \( E_1 \) dips slightly, so the primary draws extra current \( I_2' = I_2N_2/N_1 = KI_2 \), in anti-phase with \( I_2 \).
4. Total primary current \( \bar I_1 = \bar I_0 + \bar I_2' \) (phasor sum).
5. Since \( V_1 \) and \( f \) are fixed, \( \phi_m = V_1/(4.44fN_1) \) stays almost constant from no load to full load. Hence iron loss is practically constant.
6. Resistances and leakage reactances cause internal drops, so \( V_2 \) falls a little as load increases. The percentage change is called voltage regulation.

Key formula

\( N_1I_2' = N_2I_2, \quad \bar I_1 = \bar I_0 + \bar I_2', \quad \phi_m \approx \dfrac{V_1}{4.44fN_1} \)

\( I_2 \) = secondary load current (A), \( I_2' \) = load component of primary current (A), \( I_0 \) = no-load current (A), \( I_1 \) = total primary current (A), \( \phi_m \) = maximum flux (Wb).

Quick example

When more appliances are switched on in a building, the building's distribution transformer secondary current rises, and the substation's feeder (primary) current rises proportionally, even though the two windings never touch electrically.

Common mistakes

• Thinking the core flux increases with load; it stays nearly constant.
• Adding \( I_0 \) and \( I_2' \) arithmetically instead of as phasors.
• Saying the primary current is unchanged on load.

How MU asks it

MU states numericals are not expected in this module, so questions are theory. Typical: "Explain the working of a transformer on load with the help of a phasor diagram" (5–8 marks). For full marks explain the MMF balance in steps, write \( N_1I_2' = N_2I_2 \), show \( \bar I_1 = \bar I_0 + \bar I_2' \) on a phasor diagram (for a lagging load), and state that flux and iron loss stay nearly constant.