PHYSICS PROJECTCLASS XII A

By : NIKHIL SHARMAXII-A

CERTIFICATE

This is to certify that NIKHIL SHARMA of Class XII-A has completed her project under my supervision and guidance and to my full satisfaction. I also certify that information contained in this project is true and authentic to best of my knowledge.

ACKNOWLEDGEMENT

I would like to thank my Physics Teacher Ms. sheetal for her interest and valuable guidance without which the work could not have been completed. I would also like to express my deep appreciation to my colleagues for providing me with their valuable suggestions and comments for improvement of project.

Most importantly I would like to thank the board for providing me with such a nice opportunity that helped me to get a better insight and a practical knowledge about the subject.NIKHIL SHARMA

Transformer

transformer,  device that transfers electric energy from one alternating-current circuit to one or

more other circuits, either increasing (stepping up) or reducing (stepping down) the voltage.

Transformers are employed for widely varying purposes; e.g., to reduce the voltage of

conventional power circuits to operate low-voltage devices, such as doorbells and toy electric

trains, and to raise the voltage from electric generators so that electric power can be transmitted

over long distances.

Transformers change voltage through electromagnetic induction; i.e., as the magnetic lines of

force (flux lines) build up and collapse with the changes in current passing through the primary

coil, current is induced in another coil, called the secondary. The secondary voltage is calculated

by multiplying the primary voltage by the ratio of the number of turns in the secondary coil to the

number of turns in the primary coil, a quantity called the turns ratio.

Discovery of induction phenomenon

The principle behind the operation of a transformer, electromagnetic induction, was discovered independently by Michael Faraday and Joseph Henry in 1831. However, Faraday was the first to publish the results of his experiments and thus receive credit for the discovery.[86] The relationship between emf and magnetic flux is an equation now known as Faraday's law of induction:

.where is the magnitude of the emf in volts and ΦB is the magnetic flux through the circuit in webers.[87]

Faraday performed the first experiments on induction between coils of wire, including winding a pair of coils around an iron ring, thus creating the first toroidal closed-core transformer.[88] However he only applied individual pulses of current to his transformer, and never discovered the relation between the turns ratio and emf in the windings.

Basic principles

The ideal transformer

Consider the ideal, lossless, perfectly-coupled transformer shown in the circuit diagram at right having primary and secondary windings with NP and NS turns, respectively.

The ideal transformer induces secondary voltage ES =VS as a proportion of the primary voltage VP = EP and respective winding turns as given by the equation

,

where,- VP/VS = EP/ES = a is the voltage ratio and NP/NS = a is the winding turns ratio, the value of these ratios being respectively higher and lower than unity for step-down and step-up transformers.- VP designates source impressed voltage,- VS designates output voltage, and,- EP & ES designate respective emf induced voltages.

Any load impedance connected to the ideal transformer's secondary winding causes current to flow without losses from primary to secondary circuits, the resulting input and output apparent power therefore being equal as given by the equation

.Combining the two equations yields the following ideal transformer identity

.This formula is a reasonable approximation for the typical commercial transformer, with voltage ratio and winding turns ratio both being inversely proportional to the corresponding current ratio.

The load impedance is defined in terms of secondary circuit voltage and current as follows.

The apparent impedance of this secondary circuit load referred to the primary winding circuit is governed by a squared turns ratio multiplication factor relationship derived as follows

.Induction law

The transformer is based on two principles: first, that an electric current can produce a magnetic field and second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction).

The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that:

where Vs = Es is the instantaneous voltage, Ns is the number of turns in the secondary coil, and dΦ/dt is the derivative of the magnetic flux Φ through one turn of the coil. If the turns of the coil are oriented perpendicularly to the magnetic field lines, the flux is the product of the magnetic flux density B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer, the instantaneous voltage across the primary winding equals

Taking the ratio of the above two equations gives the same voltage ratio and turns ratio relationship shown above, that is,

.The changing magnetic field induces an emf across each winding. The primary emf, acting as it does in opposition to the primary voltage, is sometimes termed the counter emf. This is in accordance with Lenz's law, which states that induction of emf always opposes development of any such change in magnetic field.

The real transformer

Real transformer deviations from ideal

The ideal model neglects the following basic linear aspects in real transformers:Core losses collectively called magnetizing current losses consisting of:

• Hysteresis losses due to nonlinear application of the voltage applied in the transformer core

• Eddy current losses due to joule heating in core proportional to the square of the transformer's applied voltage.

Whereas the ideal windings have no impedance, the windings in a real transformer have finite non-zero impedances in the form of:

• Joule losses due to resistance in the primary and secondary windings

• Leakage flux that escapes from the core and passes through one winding only resulting in primary and secondary reactive impedance.

Energy losses

An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers, energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those rated for electricity distribution usually perform better than 98%.

Experimental transformers using superconducting windings achieve efficiencies of 99.85%.The increase in efficiency can save considerable energy, and hence money, in a large heavily loaded transformer; the trade-off is in the additional initial and running cost of the superconducting design.

Transformer losses arise from:Winding joule losses

Current flowing through winding conductors causes joule heating. As frequency increases, skin effect and proximity effect causes winding resistance and, hence, losses to increase.

Core lossesHysteresis lossesEach time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core.

Eddy current lossesFerromagnetic materials are also good conductors and a core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness.[31] Eddy current losses can be reduced by making the core of a stack of plates electrically insulated from each other, rather than a solid block; all transformers operating at low frequencies use laminated or similar cores.

Magnetostriction related transformer humMagnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction, the frictional energy of which produces an audible noise known as mains hum or transformer hum.[3][34] This transformer hum is especially objectionable in transformers supplied at power frequencies [i] and in high-frequency flyback transformers associated with PAL system CRTs.

Mechanical vibration and audible noise transmission

In addition to magnetostriction, the alternating magnetic field causes fluctuating forces between the primary and secondary windings. This energy incites vibration transmission in interconnected metalwork, thus amplifying audible transformer hum.[36]

Toroidal cores

Toroidal transformers are built around a ring-shaped core, which, depending on operating frequency, is made from a long strip of silicon steel or permalloy wound into a coil, powdered iron, or ferrite. A strip construction ensures that the grain boundaries are optimally aligned, improving the transformer's efficiency by reducing the core's reluctance. The closed ring shape eliminates air gaps inherent in the construction of an E-I core.[18] The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimizes the length of wire needed, and also provides screening to minimize the core's magnetic field from generating electromagnetic interference.

Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and greater choice of shapes

Windings

The conducting material used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn. For small power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from enamelled magnet wire, such as Formvar wire. Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard.

Classification parameters

Transformers can be classified in many ways, such as the following:

• Power capacity : From a fraction of a volt-ampere (VA) to over a thousand MVA.

• Duty of a transformer: Continuous, short-time, intermittent, periodic, varying.

• Frequency range: Power-frequency, audio-frequency, or radio-frequency.

• Voltage class: From a few volts to hundreds of kilovolts.

• Circuit application: Such as power supply, impedance matching, output voltage and current stabilizer or circuit isolation.

• Utilization: Pulse, power, distribution, rectifier, arc furnace, amplifier output, etc..

• Basic magnetic form: Core form, shell form.

• Constant-potential transformer descriptor: Step-up, step-down, isolation.

• General winding configuration: By EIC vector group - various possible two-winding combinations of the phase designations delta, wye or star, and zigzag or interconnected star; other - autotransformer, Scott-T, zigzag grounding transformer winding.

Types

A wide variety of transformer designs are used for different applications. Important common transformer types include:

• Autotransformer : Transformer in which part of the winding is common to both primary and secondary circuits.

• Capacitor voltage transformer : Transformer in which capacitor divider is used to reduce high voltage before application to the primary winding.

• Distribution transformer, power transformer : International standards make a distinction in terms of distribution transformers being used to distribute energy from transmission lines and networks for local consumption and power transformers being used to transfer electric energy between the generator and distribution primary circuits.

• Phase angle regulating transformer : A specialised transformer used to control the flow of real power on three-phase electricity transmission networks.

• Scott-T transformer : Transformer used for phase transformation from three-phase to two-phase and vice versa.

• Polyphase transformer : Any transformer with more than one phase.

• Grounding transformer: Transformer used for grounding three-phase circuits to create a neutral in a three wire system, using a wye-delta transformer, or more commonly, a zigzag grounding winding.

• Leakage transformer : Transformer that has loosely coupled windings.

• Resonant transformer : Transformer that uses resonance to generate a high secondary voltage.

• Audio transformer : Transformer used in audio equipment.

• Output transformer : Transformer used to match the output of a valve amplifier to its load.

• Instrument transformer : Potential or current transformer used to accurately and safely represent voltage, current or phase position of high voltage or high power circuits.

Applications

Transformers are used to increase voltage before transmitting electrical energy over long distances through wires. Wires have resistance which loses energy through joule heating at a rate corresponding to square of the current. By transforming power to a higher voltage transformers enable economical transmission of power and distribution. Consequently, transformers have shaped the electricity supply industry, permitting generation to be located remotely from points of demand. All but a tiny fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer.

Transformers are also used extensively in electronic products to step-down the supply voltage to a level suitable for the low voltage circuits they contain. The transformer also electrically isolates the end user from contact with the supply voltage.

Signal and audio transformers are used to couple stages of amplifiers and to match devices such as microphones and record players to the input of amplifiers. Audio transformers allowed telephone circuits to carry on a two-way conversation over a single pair of wires. A balun transformer converts a signal that

is referenced to ground to a signal that has balanced voltages to ground, such as between external cables and internal circuits.