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A transformer is a device that transfers electrical energy from

one circuit to another through inductively coupled conductors

(transformer's coils).

A varying current in the first orprimarywinding creates avarying magnetic flux in the transformer's core and thus a

varying magnetic field through thesecondarywinding. This varying

magnetic field induces a varying electromotive force (EMF), or voltage ,

in the secondary winding. This effect is called inductive coupling.

Principle

The transformer is based on two principles: first, that an electriccurrent can produce a magnetic field (electromagnetism) and secondthat a changing magnetic field within a coil of wire induces a voltageacross the ends of the coil (electromagnetic induction). Changing thecurrent in the primary coil changes the magnetic flux that is developed.The changing magnetic flux induces a voltage in the secondary coil.

An ideal transformer is shown in following figure.

Current passing through the primary coil creates a magnetic field. Theprimary and secondary coils are wrapped around a core of veryhigh magnetic permeability, such as iron, so that most of the magnetic

flux passes through both the primary and secondary coils. If a load isconnected to the secondary winding, the load current and voltage will be
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in the directions indicated, given the primary current and voltage in thedirections indicated (each will be alternating current in practice).

Induction lawThe voltage induced across the secondary coil may be calculatedfrom Faraday's law of induction, which states that:

where Vs is the instantaneous voltage, Ns is the number of turns inthe secondary coil and is the magnetic flux through one turn of thecoil.

If the turns of the coil are oriented perpendicularly to the magneticfield lines, the flux is the product of the magnetic flux densityB andthe areaA through which it cuts. The area is constant, being equal tothe cross-sectional area of the transformer core, whereas themagnetic field varies with time according to the excitation of theprimary.

Since the same magnetic flux passes through both the primary andsecondary coils in an ideal transformer, the instantaneous voltageacross the primary winding equals

Taking the ratio of the two equations forVs and Vp gives the basicequation for stepping up or stepping down the voltage

Np/Ns is known as the turns ratio, and is the primary functionalcharacteristic of any transformer

Construction

A constant-voltage transformer consists essentially of threeparts as shown in the above figure:
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1)The primary coil which carries the alternating current fromthe supply lines,2) the core of magnetic material in which is produced analternating magnetic flux

3) the secondary coil in which is generated an emf by thechange of magnetism in the core which it surrounds.

Sometimes the transformer may have only one winding, whichwill serve the dual purpose of primary and secondary coils.

The high-tension winding is composed of many turns ofrelatively fine copper wire, well insulated to withstand thevoltage impressed on it. The low-tension winding is composed

of relatively few turns of heavy copper wire capable of carryingconsiderable current at a low voltage.

1) Cores

The core is the magnetic circuit upon which the windings

are wound.

Different types of transformer cores are present

depending upon their type and use.

Laminated steel cores

Transformers for use at power or audio frequencies typically havecores made of high permeabilitysilicon steel.The steel has apermeability many times that offree space and the core thusserves to greatly reduce the magnetizing current and confine theflux to a path which closely couples the windings.

a) Solid cores

Powdered iron cores are used in circuits such as switch-mode powersupplies that operate above mains frequencies and up to a few tens ofkilohertz. These materials combine high magnetic permeability with highbulk electrical resistivity. For frequencies extending beyond the VHFband, cores made from non-conductive magnetic ceramic materialscalled ferrites are common.
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Toroidal cores

Toroidal transformers are built around a ring-shaped core, which,depending on operating frequency, is made from a long strip ofsiliconsteel orpermalloy wound into a coil, powdered iron, orferrite.

Air cores

A physical core is not an absolute requisite and a functioningtransformer can be produced simply by placing the windings near eachother, an arrangement termed an "air-core" transformer. The air whichcomprises the magnetic circuit is essentially lossless, and so an air-coretransformer eliminates loss due to hysteresis in the core material.

2) Windings

Theprimary winding is the winding of the transformer whichis connected to the source of power. It may be either thehigh- or the low voltage winding, depending upon theapplication of the transformer.

The secondary winding is the winding of the transformer whichdelivers power to the load. It may be either the high- or the

low-voltage winding, depending upon the application of thetransformer

The conducting material used for the windings depends upon theapplication, but in all cases the individual turns must be electricallyinsulated from each other to ensure that the current travelsthroughout every turn.

For small power and signal transformers, in which currents arelow and the potential difference between adjacent turns is small,the coils are often wound from enamelled magnet wire, such asFormvar wire. Larger power transformers operating at highvoltages may be wound with copper rectangular strip conductorsinsulated by oil-impregnated paper and blocks ofpressboard.

Certain transformers have the windings protected by epoxy resin.

3) Cooling

A certain amount of the electrical energy delivered to a

transformer is transformed into heat energy because of theresistance of its windings and the hysteresis and eddy currents
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in the iron core. Means must be provided for removing this heatenergy from the transformer and dissipating it into thesurrounding air. If this were not done in a satisfactory manner,the transformer would operate at an excessively high

temperature, which would destroy or harm the insulation of thetransformer.

Some large transformers employ electric-operated fans or pumps forforced-air or forced-oil cooling orheat exchanger-based water-cooling.

Other transformers use coolants for maintaining the temperature.

There are different types of coolants that are used. Some of the

examples of coolants are silicone based oils, fluorinatedhydrocarbons, polychlorinated biphenyls etc.

Operation

Wire wound on an iron core have self-induction. When analternating voltage is applied to such turns, a current flowsthrough them that generates a countervoltage or EMF thatopposes the applied voltage.

Since the primary and secondary windings are on the samecore, the magnetic flux generated by the magnetizing orexciting current flowing in the primary winding also cuts theturns of the secondary winding and generates an EMF in them.

This EMF will be, opposite in direction to that impressed on theprimary. If the secondary circuit is open, no current can flow init, but if it is closed, a certain current, proportional to the

impedance of the secondary circuit, will flow.

This current, because of the direction of the EMF generated inthe secondary, will be in such a direction that the magnetic fluxproduced in the core by it will oppose the flux due to theprimary winding.

It will therefore decrease the effective or resultant flux in thecore by a small amount which will decrease the counter-EMF of

the primary winding and permit more current to flow into theprimary winding.
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Effect of loading the secondary of transformer

If a load is connected to the secondary, current will flow in the secondarywinding, and electrical energy will be transferred from the primary circuitthrough the transformer to the load.

In an ideal transformer, the induced voltage in the secondary winding

(Vs) is in proportion to the primary voltage (Vp) and is given by the ratio ofthe number of turns in the secondary (Ns) to the number of turns in theprimary (Np) as follows:

By appropriate selection of the ratio of turns, a transformer thusenables an alternating current (AC) voltage to be "stepped up" by

making Ns greater than Np, or "stepped down" by making Ns lessthan Np.

The windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception.
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