Transformer vaishnav gaurav 130450112037 yadav shalu 130450112038 Gharia Isha 140453111002 Matariya Rahisha 140453111004 Pinjari sadik 140453111013

Introduction The transformer is a static device(i.e. the one which does not contain any rotating or moving parts) which is used to transfer electrical energy from one ac circuit to another ac circuit, with increase or decrease in voltage/current but without change in the frequency.

Introduction Input and output of the transformer are alternating quantity. The electrical energy is generated and transmitted at an extremely high voltages. The voltage level can be reduced and increased using transformer. To reduce voltage the transformer is used which is then called as the step down transformer. To increase voltage the transformer is used which is then called as the step up transformer. When the transformer changes the voltage level, it changes the current level also.

Types of transformer On the basis of the supply transformers are designed and they are of two types:- Single phase transformer with single phase supply. Three phase transformer with three phase supply. However the principle for both the types is same.

Principle of the transformer Single phase transformer consists of two highly inductive coils(windings)wound on an iron or steel core. The winding connected to the ac supply is called as the primary winding whereas the other one which is connected to the load is called as the secondary winding. The primary and secondary winding are isolated from each other as well as from the iron core. There is no physical connection between primary and secondary windings.

Elementary transformer Symbol of transformer

The ac primary current produces an alternating flux ø in the core 1 The ac primary current produces an alternating flux ø in the core 2 When the primary winding is connected to the single phase ac supply, an ac current starts flowing through it. Most of the changing flux linked with the secondary winding through the core. Operating Principle of a transformer The varying flux will induce voltage into the secondary winding according to the faraday’s laws of electromagnetic induction 3 4

Thus due to the primary current, there is an induced voltage in the secondary winding due to the mutual induction. Hence the emf induced in the secondary winding is called as the mutually induced emf. Can the transformer operate on DC? Ans. no

Construction of a transformer The most important part of transformer is winding and core. However for the large capacity transformers, some other parts such as Suitable tanks conservator Buchhloz relay bushings Breather explosion vent,etc Are also used alongwith the core and windings.

1. Laminated steel core The material used for the construction of the transformer core is silicon steel. It is used for its high permeability and low reluctance. Due to this the magnetic field produced in the core is very strong. The core is in the form of stacks of the laminated thin steel sheets which are electrically isolated from each other. The laminations are typically 0.35 to 0.5 mm thick. The various ways of the construction of the core are as follows:- Core construction using l-shaped laminations. Core construction using I-Shaped laminations. The arrangement is in such a manner that there is the continuous path for the magnetic flux, with a minimum air gap

2. cross-sections of the transformer limbs. The cross-section of the limb of the core of small transformer is rectangular and the windings wound are also rectangular. But as the size increases circular cross-section windings are preferred. The cross section of such type of the transformer is either square or stepped. With the increase in the number of steps, the cross-section of the windings will be more and more closer to the circular cross-section and less copper is used to wind these coils. But due to the stepped structure of the limb of the core the labour charges to construct the core increases.

Windings of the transformer If we are practically dealing with this arrangement then a part of the flux produced in the core will not be linked with the secondary winding. This is called as the leakage flux. In order to avoid this, the primary and secondary windings are mounted on the same limb of the core.

Types of windings Concentric cylindrical type of winding Sandwiched type of winding

Concentric cylindrical type of winding The cylindrical coils are placed in the concentric way around the same limb with the low voltage winding placed onside and high voltage windings are placed outside it with the proper insulation between the windings. Both the windings are insulated from the core as well.

Sandwiched type winding Here the high and low voltage windings are divided into number of small coils and then these small windings are interleaved The top and bottom windings are low voltage coils because they are close to the core

Transformer tank The whole assembly of large size transformer is placed in a sheet metal tank. Inside the tank the whole assembly is immersed in oil which act as the insulator as well as the coolant. the oil will take out the heat produced by the transformer windings and core and transfer it to the surface of the transformer tank.

Conservator In transformers, some empty space is always provided above the oil level because due to the change in the temperature the oil tends to contract or expand. When the oil temperature increases and decreases , it expands and the air will be expelled out from the conservator and it contracts and the air gets sucked inside the conservator. This process is called as the breathing of the transformer. Due to this process the air comes in contact with the oil and because of the moisture oil will lose the insulating power and it can be prevented by the conservator.

Conservator is a cylindrical shaped air tight metal drum placed on the transformer tank. It is connected by a pipe to the tank. The oil level in the conservator is such a way that, always some empty space is available above the oil. Due to the use of the conservator, the main tank will be always full with oil and the surface of oil in the tank will not be exposed directly to the air.

Breather The apparatus through which breathing of the transformer take place is known as “Breather”. The air goes in or out through the breather. To reduce the moisture content of this air, some drying agent such as silica gel or calcium chloride is used in the breather. The dust particles present in the air are also removed by the breather.

Explosion Vent The explosion vent consists of the diaphragm or aluminium foil. When the transformer becomes faulty, the oil gets decomposed and various gases are liberated If the gas pressure reaches a certain level then the diaphragm in the explosion vent will burst to release the pressure. This will save the main tank from getting damaged.

Transformer types Core type transformer Shell type transformer Berry type transformer

Core type transformer Shell type transformer

Core and Shell type transformer Core type transformer Shell type transformer The core has only one window. Windings encircle the core Cylindrical windings are used. It is easy to repair. Better cooling since more surface is exposed to the atmosphere. Less mechanical protections to the coil. The construction is preferred for low voltage transformers. The core has two window. Core encircles the windings. Sandwich windings are used. It is not so easy to repair. Cooling is not very effective. Better mechanical protection to the coils. The construction is preferred for high voltage transformers.

Berry type transformer The berry type of the transformer has a distributed magnetic circuit. The low voltage windings are placed inside and the high voltage windings are placed outside.

EMF Equation of transformer For deriving the emf equation of a transformer The primary winding is connected across the ac supply This forces an alternating current through the primary winding to produce an alternating flux(ø) in the core. The varying flux gets linked with the secondary and primary winding to induce the mutually induced and self induced emfs in the secondary and primary windings respectively.

Expressions for the induced voltages Step 1 Expression for the instantaneous flux(ø): The instantaneous flux changes in a sinusoidal manner with respect to time. Its frequency “f” is same as that of the ac voltage applied to the primary winding. Therefore, ø= øm sinωt (1) Where, øm = Maximum value of the instantaneous flux. ω = 2Πf where f is the frequency of the flux waveform

step 2 Obtain the expression for induced voltage: According to the faraday’s law of electromagnetic induction, then induced emf due to varying flux is given by e= - N volts (2) e= emf induced due to varying flux.

Step 3 Obtain the maximum value of “e” per turn The value of the induced emf per turn can be obtained by substituting N=1 in equation (2) e= - Substitute ø=øm sinωt therefore e= - øm ωcosωt (3) the max. value of induced voltage per turn is given by substituting cosωt=1 e= - øm ω2Πf volts

Step 4 Obtain the rms value of “e” per turn = =4.44 f øm (4)

Step 5 Obtain the expressions for induced voltages E1 and E2 Let E1 be the rms induced voltage in the primary winding with N1 and E2 be the rms induced voltage in the secondary winding with N2 turns. Therefore, RMS value of the induced voltage in primary is, E1= RMS value of “e” per turn * number of primary turns E1= 4.44 f øm*N1 volts (5) Similarly rms value of induced voltage in the secondary winding is E2= RMS value of “e” per turn * number of secondary turns E2= 4.44 f øm*N2 volts (6)

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