Transformers: Principle and Basics With easy examples.

A transformer is a static electrical device that transfers power between two circuits through magnetic induction, changing the voltage level up or down without any moving parts. It consists of two coils of wire — the primary and the secondary — wound on a common iron core.

Electricity is generated, transmitted and used at different voltages: transformers step it up to hundreds of kilovolts for efficient transmission, then down to safe 230 V for homes.

Key Concepts

• Primary and secondary windings: the input coil (primary) creates a changing magnetic flux; the output coil (secondary) has voltage induced in it by that flux, following Faraday's law of electromagnetic induction.
• Turns ratio: the ratio of secondary turns to primary turns, N2/N1, which sets the voltage ratio.
• Step-up vs step-down: more secondary turns than primary gives higher voltage (step-up); fewer gives lower voltage (step-down). Power in roughly equals power out, so voltage up means current down.
• Core: laminated silicon-steel sheets that guide the magnetic flux while limiting eddy-current losses.
• Losses: copper loss (I²R heating, varies with load) and iron loss (hysteresis plus eddy currents, roughly constant).
• Why AC only: induction needs a changing magnetic field, so a transformer works on alternating current — steady DC produces no induced voltage.

Formulas

• Voltage ratio: V2 / V1 = N2 / N1. Current ratio (ideal): I2 / I1 = N1 / N2 — current transforms inversely to voltage. Assumes an ideal transformer with no losses.
• EMF equation: E = 4.44 × f × N × Φm, where E is the induced RMS voltage per winding, f the frequency in Hz, N the number of turns, and Φm the maximum core flux in webers.
• Efficiency: η = output power / (output power + copper loss + iron loss) × 100%. Large power transformers exceed 98% efficiency.

Worked Example

A single-phase transformer has 2000 primary turns and 100 secondary turns, fed with 230 V AC. It delivers 46 W to a load while total losses are 4 W. Find the secondary voltage and efficiency.

1. Turns ratio: N2/N1 = 100/2000 = 1/20 = 0.05.
2. Secondary voltage: V2 = 230 × 0.05 = 11.5 V — a step-down transformer, like a doorbell or charger transformer.
3. Secondary current: I2 = 46/11.5 = 4 A; primary current (ideal): I1 = 4 × 0.05 = 0.2 A — the high-voltage side carries far less current, which is why high-voltage transmission saves copper.
4. Efficiency: η = 46 / (46 + 4) × 100% = 46/50 × 100% = 92%.

Common Misconceptions

• "Transformers work on DC." They do not. A steady DC current makes a steady field, which induces nothing — DC just shorts the winding and burns it out.
• "A transformer creates power." It only converts voltage and current levels; output power can never exceed input power, and is always slightly less because of losses.
• "Bigger core always means better." An oversized core wastes iron and money; the core is sized for the flux at the operating voltage and frequency — too small saturates, too big is uneconomical.

Real-World Applications

• Power transmission: giant transformers step generator output up to 400 kV for long lines and back down near cities.
• Chargers and adapters: small transformers (or their electronic equivalents) drop mains to the few volts gadgets need.
• Welding machines: welding transformers deliver huge current at low voltage for the arc.

A transformer changes AC voltage levels by induction between two windings on a shared core, following V2/V1 = N2/N1 with current transforming inversely. It works only on AC and makes the journey from power station to wall socket possible.