In one line
A transformer works on mutual induction: an alternating current in the primary winding sets up an alternating flux in the iron core, and this changing flux induces an EMF in the secondary winding.
Explained simply
Hold two coils close together. If you switch a current on and off in one, a voltmeter on the other kicks each time. Now feed the first coil with AC, which keeps changing all the time, and the second coil gets a continuous AC voltage, with no wire connecting them. An iron core simply guides the magnetic flux so almost all of it links both coils.
Technically, a transformer is a static device that transfers electrical energy from one circuit to another by electromagnetic (mutual) induction, at the same frequency, usually changing the voltage level. By Faraday's law, the EMF in each winding is proportional to its number of turns and the rate of change of the common flux.
How it works
- AC voltage \( V_1 \) is applied to the primary winding of \( N_1 \) turns.
- A small magnetising current sets up an alternating flux \( \phi = \phi_m\sin\omega t \) in the laminated core.
- This flux links the primary, inducing a self-induced EMF \( E_1 \) that nearly balances \( V_1 \).
- The same flux links the secondary of \( N_2 \) turns and induces \( E_2 \) by mutual induction.
- When a load is connected, secondary current flows and power is delivered.
- On DC the flux is constant, so no EMF is induced in the secondary. Also, with no back EMF, only the small winding resistance limits the primary current, so a huge current flows and the winding can burn.
Key formula
EMF equation: \( E_1 = 4.44fN_1\phi_m, \quad E_2 = 4.44fN_2\phi_m \)
\( E_1, E_2 \) = RMS induced EMFs (V), \( f \) = supply frequency (Hz), \( N_1, N_2 \) = primary and secondary turns, \( \phi_m \) = maximum core flux (Wb). The 4.44 comes from form factor 1.11 × 4.
Quick example
A mobile charger's older linear adapters used a small transformer to step 230 V AC down to a few volts before rectification. Distribution transformers on electric poles step 11 kV down to 415 V for homes. In both, primary and secondary are electrically isolated and linked only magnetically.
Common mistakes
- Saying a transformer changes frequency; frequency stays the same.
- Saying a transformer works on DC with reduced output; it gives no output and the primary may burn.
- Calling it self-induction only; transfer between windings is by mutual induction.
How MU asks it
MU states numericals are not expected in this module, so questions are theory. Typical: "Explain the working principle of a single-phase transformer with a neat diagram" (5 marks), "Why can a transformer not be used on DC?" (3 marks), or "Derive the EMF equation" (5 marks). For full marks draw a core with two windings, mark \( V_1, I_1, \phi, E_2, \) load, explain mutual induction in steps, and mention 'static device, same frequency'.