Transformers: Principle, Working and Losses Explained

A transformer is a device that changes the voltage of an alternating current supply, raising it in a step-up transformer or lowering it in a step-down transformer, without any moving parts. It works purely on mutual induction between two coils wound on a common laminated iron core. Your phone charger, the pole-mounted box in your street and the substation outside your town all contain transformers.

Transformers matter because they make the entire electricity grid possible. Power is transmitted over long distances at very high voltage to reduce losses, then stepped down in stages to the 230 V used at home. Without transformers, electricity could not travel economically from a distant power station to your wall socket, and no electronic device could be charged safely.

Key Concepts

• Primary coil: the coil connected to the AC input supply.
• Secondary coil: the coil from which the changed AC output is taken.
• Mutual induction: the changing flux produced by the primary links the secondary and induces an EMF in it.
• Turns ratio: the ratio of secondary turns to primary turns, which fixes the voltage ratio of an ideal transformer.
• Laminated core: the core is made of thin insulated iron sheets to reduce eddy current losses.
• Efficiency: the ratio of output power to input power; real transformers reach 95 to 99 percent but are never perfect.

Formulas

For an ideal transformer, Vs / Vp = Ns / Np, where Vp and Vs are the primary and secondary voltages in volts and Np and Ns are the numbers of turns. If there are no losses, input power equals output power, so Vp Ip = Vs Is, giving Is / Ip = Np / Ns. A step-up transformer raises voltage but lowers current in the same ratio.

Real efficiency is η = (Vs Is) / (Vp Ip) × 100 percent. The main losses are copper loss P = I²R in the windings, eddy current loss in the core, hysteresis loss from repeated magnetisation, and flux leakage where some primary flux fails to link the secondary.

Worked Example

A transformer has 2000 primary turns and 100 secondary turns. It is connected to a 230 V AC supply. Find the secondary voltage, and the primary current when the secondary supplies 2 A, assuming an ideal transformer.

1. Voltage ratio: Vs = Vp × (Ns / Np) = 230 × (100 / 2000) = 230 / 20 = 11.5 V.
2. Output power = Vs × Is = 11.5 × 2 = 23 W.
3. Ideal input power = output power, so Ip = 23 / 230 = 0.1 A.

The transformer steps 230 V down to 11.5 V, a typical value for charging circuits and doorbells, and it draws only 0.1 A from the mains while delivering 2 A at the lower voltage. Stepping voltage down automatically steps the available current up.

Common Misconceptions

• A transformer can work on DC. Wrong. Steady DC produces constant flux, so no EMF is induced in the secondary. Worse, the primary then acts like a low-resistance wire and can overheat or burn out.
• A step-up transformer creates extra power. Wrong. It raises voltage but reduces current in the same ratio. Output power can never exceed input power; in real transformers it is slightly less because of losses.
• The iron core carries current from primary to secondary. Wrong. The core carries only magnetic flux. Energy is transferred through the changing magnetic field, and the two coils are electrically insulated from each other.

Real-World Applications

Grid substations step voltage up to lakhs of volts for transmission and down again for cities and homes. Phone chargers, laptop adapters, microwave ovens and doorbells use small step-down transformers. Welding machines use transformers that deliver very high current at low voltage. Instrument transformers safely measure high voltages and currents in power stations, and isolation transformers protect patients and technicians from electric shock. Audio and radio circuits use small transformers to match impedances between stages.

A transformer changes AC voltage through mutual induction, with the turns ratio fixing the voltage and current ratios. Hysteresis, eddy currents, copper heating and flux leakage cause small losses, which good design with laminated cores and thick low-resistance windings keeps to a few percent.