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Transformer

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A transformer is a device that transfers energy from one electrical circuit to another by a magnetic coupling without requiring relative motion between its parts. A transformer comprises two or more coupled windings, and, in most cases, a magnetic core to concentrate magnetic flux [1]. A voltage applied to one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings. Varying the relative number of turns in the windings determines the ratio of their voltages, thus transforming the voltage from one circuit to another.

One of the simplest of electrical devices, the transformer is also one of the most efficient, with large units attaining efficiencies in excess of 99.75% [2, 3]. Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge gigawatt-rated units used to interconnect portions of national power grids. All operate with the same basic principles and with many similarities in their parts, though a variety of transformer designs exist to perform specialized roles throughout home and industry [1].

There are three main types of transformers: single phase or three phase; pole mounted or pad mounted; and underground transformers [4]. The most common type, a pole-mounted transformer, is a transformer mounted on an electrical service pole, usually at the level of the overhead cables but occasionally at ground level. Pole-mounted transformers are the common breadbox transformers used for converting distribution voltage from the primary distribution voltage (normally 4,160 to 23,000 volts) to the 120/240 volt power used by homes and low-volume commercial installations. Transformers are an important component of the electric distribution system because their arrangement allows the use of a multi-voltage level energy delivery system [4].

Transformers consist of two main components: an active component and a passive component. The active component consists of the core, windings, insulation, and often also the tap changer. These parts make the transformer function, allowing for multi-voltage transformations. The passive component includes those parts which are necessary to make the transformer function correctly such as shielding, cooling, the enclosure, and protective equipment [5].

In the US, a pole-mounted transformer in a suburban setting may supply 1 to 3 houses with electricity, whereas in the UK a typical urban or suburban low-voltage substation might be rated at 2 MW and supply a whole neighborhood. This is because the higher voltage used in Europe (380 volts) may be carried over a greater distance with acceptable power loss. An advantage of the North American setup is that failure or maintenance on a single transformer will only affect a few customers. Advantages of the UK setup are that the transformers may be fewer, larger and more efficient, and due to diversity there need be less spare capacity in the transformers, reducing power wastage. In North American city areas with many customers per unit area, network distribution will be used, with multiple transformers and low-voltage busses interconnected over several city blocks.

Transformers are classified according to their power level, application, frequency, voltage class, cooling type, purpose, and ratio of the number of turns in the coils. For example, a step-up transformer has more turns than the primary whereas a step-down transformer has fewer turns than the primary.