With a transformer alternating currents can be transformed, meaning either amplified or reduced.

In principle the transformer consists of two adjacent coils. One coil generates a magnetic field upon an electric current. For a sinusoidal electric current source a corresponding magnetic field is built up with sinusoidal alternating flux Φ. If this magnetic field penetrates the second electromagnetic coil, a voltage is induced. The secondary coil should not have a conductive connection with the primary coil. Hence, both electromagnetic coils are separated by galvanic means. They are only coupled via the magnetic field. Such a setup describes a transformer.

The electric energy of the primary coil is converted to magnetic energy. In the secondary coil, again electric energy is generated from the magnetic field by induction. The prerequisite for high efficiency is stable magnetic coupling. Isolation transformers provide a basis for galvanic separation of alternating current voltage from the electric circuit.

Transformers are important for data transfer and the field of measurement and control technique, i.e. audio frequency transmittance. A real transformer is different from an ideal transformer in the sense, that it is prone to errors. These are caused by ohmic resistances of tapings (taping resistance R1, R2), the non-ideal flux linkage of single coils (leakage inductance LS1, LS2), the magnetising current demand of the core material and its iron losses (equivalent resistance for the iron losses RFe).

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