Transformer
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A transformer is a device that transfers electrical energy from one circuit to another through a shared magnetic field. A changing current IP in the first circuit (the primary) creates a changing magnetic field; in turn, this magnetic field induces a voltage VS in the second circuit (the secondary). The secondary circuit mimics the primary circuit, but it need not carry the same current and voltage as the primary circuit. Instead, an ideal transformer keeps the product of the current and the voltage the same in the primary and secondary circuits.
Transformers allow an alternating voltage to be stepped up or stepped down. If the secondary voltage VS is stepped up by a given factor, the secondary current IS is stepped down by the same factor, keeping their product ISVS constant. The ratio of the primary and secondary voltages is a constant for a given transformer, being equal to the ratio of the number of turns of wire N that each circuit has within the shared magnetic field:
A key application of transformers is to reduce the current before transmitting electrical energy over long distances through wires. Since the energy lost over the transmission is proportional to the square of the current, transformers allow electricity to be transmitted more efficiently. By transforming electrical power to a high-voltage, low-current form before transmission and back again afterwards, the transformer reduces the energy lost, enabling the economic transmission of power over long distances. Transformers have thus shaped the electricity supply industry, permitting generation to be located remotely from points of demand.[1] All but a fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer.[2]
A transformer usually comprises two or more coupled windings, and, in most cases, a core to contain and concentrate magnetic flux. It is also one of the most efficient devices,[3] with large units attaining efficiencies in excess of 99.75%.[4] In other words, only 0.25% of the energy that enters a transformer is lost as heat; the remaining 99.75% of the energy is passed from the primary to the secondary circuit. Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge giga VA-rated units used to interconnect portions of national power grids. All operate with the same basic principles, though a variety of transformer designs exist to perform specialized roles throughout home and industry.
