A transformer is a device that changes a high alternating voltage at a low current to a low alternating voltage at a high current or vice versa. It is a useful device used for:
electrical power transmissions from power stations to the consumer loads;
regulating voltages for proper operations of electrical appliances.
Closed core transformer
Consists of two coils of wires, the primary and the secondary, with the necessary number of turns. The coils are wound on a laminated soft-iron core which consists of thin sheets of soft-iron insulated from each other by a coat of lacquer. The insulation reduces heat loss due to induced currents.
Principle of Operation: this transformer transfers electricity supplied by from the primary coil to the secondary coil by electromagnetic induction between the two coils. At the primary coil, the applied alternating voltage sets up a changing magnetic field which induces an e.m.f. in the secondary coil. A step-up transformer is one in which the e.m.f. in the primary coil is smaller than the secondary coil. A step-down transformer is the opposite. For a stepup transformer the number of turns in the secondary coil needs to be greater than in the primary coil. And for a step-down transformer the opposite.
Power Transfer in a Transformer: in an efficient (100%) transformer, the power supplied to the primary coil is fully transferred to the secondary coil. So, by the Law of Conservation of Energy:
power in the primary coil = power in the secondary coil
Transmission of Electrical Power: During transmission of electricity, a huge amount of power is lost as joule heating in the grid cables. To minimise heat loss, the cables are made thick to create a lower resistance. There is a limit to the thickness of the cables because the thicker the heavier, creating more costs. Reducing the current in transfer also reduces heat loss. This is done by transformers to step-up the voltage at which electrical power is transmitted. A high voltage is used in electrical power transmission to reduce power loss, because a greater current would otherwise have to deliver the same amount of power.
