Learning outcomes
- Describe transformer construction and operation.
- Use Vp/Vs = Np/Ns.
- Use ideal power equality.
- Distinguish step-up and step-down transformers.
- Explain high-voltage transmission and energy losses.
20.1 Transformer construction
A transformer consists of a primary coil and secondary coil wound on a laminated soft-iron core. The coils are electrically separate but magnetically linked through the core. An alternating current in the primary creates a changing magnetic flux in the core.
The changing flux links the secondary and induces an alternating e.m.f. A steady d.c. produces only a brief induction when switched on or off; it cannot operate an ordinary transformer continuously.
20.2 Role of the core
Soft iron magnetises and demagnetises readily, carrying changing magnetic flux efficiently between coils. The core is laminated into insulated sheets to reduce eddy currents and associated heating.
Other losses include resistance heating in windings, incomplete flux linkage, hysteresis in the core and sound. Real transformer efficiency is high but below 100%.
20.3 Turns and voltage ratio
For an ideal transformer, Vp/Vs = Np/Ns. If the secondary has more turns, Vs is greater and the transformer is step-up. If it has fewer turns, Vs is smaller and it is step-down.
Use consistent labels: p for primary input and s for secondary output. The equation uses turns numbers, not coil length or wire thickness.
20.4 Current and power
For an ideal transformer, input power equals output power: VpIp = VsIs. A step-up transformer raises voltage but lowers current; it does not increase power.
For a real transformer, output power is lower than input. Efficiency = output power/input power × 100%. Current may also be limited by wire ratings and load conditions.
20.5 High-voltage transmission
Electrical power is transmitted over long distances at high voltage. For a given power P = VI, increasing voltage reduces current. Cable heating loss is P_loss = I²R, so lower current greatly reduces energy wasted in transmission lines.
A step-up transformer near the power station raises voltage for transmission. Step-down transformers reduce it in stages for industry, distribution and consumers.
20.6 Why not eliminate resistance entirely?
Transmission cables must have finite resistance because practical materials and dimensions are limited. Thicker conductors reduce resistance but cost more, weigh more and require stronger supports. High voltage is therefore an economical method of reducing losses.
High voltage needs insulation, clearances and safety controls. Engineering balances loss reduction, equipment cost and reliability.
20.7 Charging and adapters
Low-voltage adapters use transformers where electrical isolation and voltage conversion are required, often followed by rectification and electronic regulation. High-frequency switch-mode supplies use more advanced circuits but still rely on induction principles.
Never connect experimental coils directly to mains. School transformer demonstrations use purpose-built low-voltage equipment.
Worked examples
Voltage ratio
A primary has 1200 turns and 240 V; secondary has 100 turns. Vs = 240 × 100/1200 = 20 V.
Current ratio
An ideal transformer steps 12 V, 3.0 A up to 120 V. Input power = 36 W, so secondary current = 36/120 = 0.30 A.
Efficiency
Input is 500 W and output 460 W. Efficiency = 460/500 × 100% = 92%.
Transmission loss
If current is reduced by a factor of 10, I²R loss falls by a factor of 100 for the same cable resistance.
Practical focus
Investigation
Use a low-voltage a.c. supply and two coils on a laminated iron core. Measure primary and secondary voltages for different turn combinations. Keep within equipment ratings. Compare measured ratio with turns ratio and discuss losses.
Examination guidance
- Transformers require changing current, normally a.c.
- Step-up voltage means step-down current for similar power.
- Use I²R to explain transmission losses.
- Laminations reduce eddy currents.
- Do not claim a transformer creates energy.
Check your understanding
- A transformer has Np = 500, Ns = 2000 and Vp = 12 V. Find Vs.
- Why is high voltage used for transmission?
- What is the purpose of core laminations?
Answers
- Vs = 48 V.
- It reduces current for the same power, greatly reducing I²R heating loss.
- To reduce eddy currents and heating.
