Transformers A2 Unit 4

Transformers A2 Unit 4 A transformer is a device that converts one oscillating potential difference supply to another with a different potential difference. All transformers consist of a primary (input) coil and a secondary (output) coil.

Transformer Rule Faraday’s law of electromagnetic induction tells us that the emf induced in a circuit, ߝ = −ܰ

݀߶ ݀‫ݐ‬

This means that ܸଵ = −ܰଵ

݀߶ ݀‫ݐ‬

ܸଶ = −ܰଶ

݀߶ ݀‫ݐ‬

and

Therefore ݀߶ ܸଶ −ܰଶ ݀‫ݐ‬ = ݀߶ ܸଵ −ܰଵ ݀‫ݐ‬ which gives us the Transformer Rule,

ࢂ૛ ࡺ૛ = ࢂ૚ ࡺ૚ nb: • • •

ܸଵ and ܸଶ are often referred to as Primary and Secondary voltages, so ܸଵ = ܸ௉ and ܸଶ = ܸௌ Step-up transformers increase the voltage; so ܸଶ > ܸଵ and ܰଶ > ܰଵ Step-down transformers decrease the voltage; so ܸଶ < ܸଵ and ܰଶ < ܰଵ

Transformer Efficiency Transformers are nearly 100% efficient. Those used in practical applications, for electricity distribution, are around 98% efficient, and researchers have achieved efficiencies of 99.85% using superconducting wires. The efficiency of a transformer, ߟ (“eta”), can be calculated by the ratio of power delivered to the primary coil (input) to the power provided by the secondary coil (output).

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Transformers A2 Unit 4 Using ܲ = ܸ‫ܫ‬, ߟ=

‫ܫ‬ଶ ܸଶ ‫ܫ‬ଵ ܸଵ

Transformers are so efficient because: • Low-resistance resistance windings mean less power wasted due to heating effect of current. • Laminated core reduces reduce eddy currents. • Soft iron core is easily magnetised and demagnetised.

Practical Uses Transformers are used extensively across the country, in the National Grid.

A laminated core (layers of soft iron) is used to reduce eddy currents, currents improving efficiency. A current is induced in the core itself (an eddy current), current) resulting in a heating effect. However, the laminated core prevents this current from fr flowing in one direction,, thus reducing this effect. effect

Electricity generated at about a 25kV. By increasing this to about 400kV before longlong distance transmission, the current is hugely reduced. This reduces the heating effect in these wires, and therefore increases efficiency. A step-down down transformer is then used at the other end to reduce reduce the voltage to 230V.

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