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Transformer, Transformer

Except for air-core transformers, the conductors are commonly wound around a single iron-rich core, or around separate but magnetically-coupled cores. A varying current in the first or "primary" winding creates a varying magnetic field in the core (or cores) of the transformer. This varying magnetic field induces a varying electromotive force or "voltage" in the "secondary" winding. This effect is called mutual induction.

While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household voltage. Transformers are essential for high voltage power transmission, which makes long distance transmission economically practical.

In the shell type transformer, the copper induction cables are passed through the core.

For example, transformers may need to be equipped with "volts per hertz" over-excitation relays to protect the transformer from overvoltage at higher than rated frequency.

Larger transformers are generally more efficient, and those rated for electricity distribution usually perform better than 98%.

However, any leakage flux that intercepts nearby conductive materials such as the transformer's support structure will give rise to eddy currents and be converted to heat.

An adjustable autotransformer is made by exposing part of the winding coils and making the secondary connection through a sliding brush, giving a variable turns ratio.

This provides a transformer with an inherent current limitation due to the loose coupling between its primary and the secondary windings. The output and input currents are low enough to prevent thermal overload under all load conditions even if the secondary is shorted.

It uses the leakage inductance of its secondary windings in combination with external capacitors, to create one or more resonant circuits. Resonant transformers such as the Tesla coil can generate very high voltages, and are able to provide much higher current than electrostatic high-voltage generation machines such as the Van de Graaff generator.

Instrument transformers isolate measurement, protection and control circuitry from the high currents or voltages present on the circuits being measured or controlled. current transformer is a transformer designed to provide a current in its secondary coil proportional to the current flowing in its primary coil.

Proper operation of over-current protection relays requires that current transformers provide a predictable transformation ratio even during a short-circuit.

A drawback of toroidal transformer construction is the higher cost of windings. As a consequence, toroidal transformers are uncommon above ratings of a few kVA. Small distribution transformers may achieve some of the benefits of a toroidal core by splitting it and forcing it open, then inserting a bobbin containing primary and secondary windings.

The air which comprises the magnetic circuit is essentially lossless, and so an air-core transformer eliminates loss due to hysteresis in the core material.

Since most cores are at least moderately conductive they also need insulation.Bottom: Lowest capacitance for one end of the secondary winding needed for low-power high-voltage transformers.Bottom left: Reduction of leakage inductance would lead to increase of capacitance.

A large bushing can be a complex structure since it must provide careful control of the electric field gradient without letting the transformer leak oil.

The transformer also electrically isolates the end user from contact with the supply voltage.

Audio transformers allowed telephone circuits to carry on a two-way conversation over a single pair of wires.

Source: Wikipedia > Transformer