CONSTRUCTION OF TRANSFORMER

A transformer is the simplest of all electrical devices and this is because of the simplicity and ease in building a transformer. A transformer is indeed very simple in construction, yet the amount of engineering involved in maintaining its elements of construction is vast.

The transformer is made up of the following parts:

  1. Two coils (in case of single phase transformers)

  2. Laminated core

  3. Container for assembled core and windings

  4. Insulating medium

  5. Bushings

These are the main parts of a single phase transformer. In case of a three phase transformer, there are six windings or coils in total and rest the construction is the same as mentioned above.

LAMINATED CORE –

The laminated core is made up of sheet steel laminations which are insulated from each other with minimum air gap between them. The steel which is used to make the core has high silicon content to produce high permeability and low hysteresis loss.

Why the core is laminated?

  1. To provide a continuous magnetic path.

  2. To reduce eddy current losses.

Varnishing or deposition of oxide layer on the surface of steel is done in order to insulate the laminations from each other.

Construction wise on the basis of cores, transformers can be classified as:-

  1. Core type

  2. Shell type

CORE TYPE TRANSFORMERS – 

In core type, there are two vertical legs or limbs over which the coils are wound. The coils used are cylindrical which may be circular, oval or rectangular in shape.

2 D VIEW OF CORE TYPE TRANSFORMER

For large transformers, circular cylindrical type of coils is preferred.

For small transformers, rectangular cylindrical type of coils is preferred.

The coils are arranged concentrically. The low voltage winding is placed prior to the high voltage winding. That means, the low voltage winding is first to be wound on the core and then high voltage winding is wound around the low voltage winding.

3 D VIEW OF CORE TYPE TRANSFORMER

In order to insulate the cylindrical windings from the core and from each other, insulating cylinders made of fuller board are used.

The laminations used are of various types like L, E and U type. Laminations are stacked together just like a pile and their joints are staggered in order to reduce the air gap between them. Larger air gaps increases the reluctance of the core, therefore, minimum air gaps are preferred.

Because of the laminations and insulations, effective core area is reduced by 10%.

SHELL TYPE TRANSFORMERS –

In shell type transformers, the windings are wound on the central limb of the core. Other two limbs are left as it is. The purpose of these two limbs is to provide low reluctance path.

2 D VIEW OF SHELL TYPE TRANSFORMER

3 D VIEW OF SHELL TYPE TRANSFORMER

The low voltage and high voltage windings are kept one over the other just like a sandwich. Therefore, these type of windings are also called disc or sandwich winding.

Another question that holds importance is – 

Why the cross section of the core is preferred to be circular?

This is because circle has minimum perimeter than any other shape for a given area. Therefore,

  1. Minimum boundary means less length of mean turns.

  2. Less resistance of the winding.

  3. I²R loss is reduced.

  4. Volume of the conductor material reduced and therefore cost is also reduced.

But circular core requires large number of laminations of different sizes to be stacked together. The solution to this problem is the stepped core arrangement, also known as the cruciform core.

 

SQUARE AND STEPPED CORE CROSS-SECTIONS

This type of core requires the size of the laminations according to the step sizes. As the number of steps increases, the number of sizes of laminations also increases.

CONTAINER AND THE INSULATING MEDIUM –

Now this whole assembly of core and windings is kept in a container, which is called tank. The tank is made air tight and filled with insulating oil. Along with insulation this oil also serves the purpose of cooling the coils of the transformer.

What should be the properties of transformer oil?

  1. Good transformer oil should be free from any type of reagents that can react with the components of transformer.

  2. The oil should be free from moisture.

Because, presence of moisture lowers the dielectric strength of oil and it is not desirable as the oil is used for insulation purpose.

The transformer oil or the insulating medium is that part of the transformer which is taken care, the most. The life of a transformer is dependent on these insulating mediums and therefore they are selected on the basis of their high quality and their ability to preserve that quality even after prolonged use.

Long-time use of oil also causes Sledging. Sledging is the decomposition of oil leading to heavy and dark coloured deposits which can clog the ducts of transformer.

Thus, transformer oil is regularly checked and changed, if required to do so.

BUSHINGS –

Transformer bushings are made of porcelain for small transformers and large transformers are equipped with oil filled condenser bushings. The bushings are used for bringing out the terminals of the windings.

TRANSFORMER

A transformer is a static device which is used to transfer electrical energy from one circuit to another by using the principle of mutual induction. It is generally used to

CIRCUIT SYMBOL OF TRANSFORMER

transform voltage from one level to another. It means transformer can be used to raise or lower down the voltage level by corresponding decrease or increase in the current.

Transformers use the principle of mutual induction. The two circuits are electrically isolated but are magnetically linked to each other by a mutual flux. The transfer of energy takes place without any change in the frequency of the system.

CONSTRUCTION –

Consider the circuit given below:

The rectangular part represents the core, which is a material of high magnetic permeability. The core is laminated in order to reduce eddy current losses. Two coils are wound on the either side of core having turns N₁ and N₂. The vertical portions of the core are called limbs and bottom portions are called yokes.

The two coils are insulated from each other and the steel core. The whole setup of core and windings is kept in a container which is further insulated from the setup by means of a suitable medium (generally called transformer oil). Transformer oil also serves the purpose of cooling.

Electrical transformer is one of the best examples of those devices which are simplest in their construction, yet serve the most important function in our electrical generation and transmission system.

OPERATION –

When an alternating current is passed through coil 1(primary coil), it produces a magnetic flux. Since, the current is alternating in nature a changing magnetic flux is produced. This changing magnetic flux induces a voltage across the ends of the coil 2 (secondary coil), which is governed by the relation, $e=-\frac{d\phi }{dt}$ .

Simply stating, changing current leads to changing magnetic field which in turn leads to the voltage induced in the secondary circuit. Thus, energy is transferred from one circuit to the other.

IDEAL TRANSFORMER –

To study about transformers, we generally consider the ideal case. The ideal case of a transformer should have the following properties:

1. The magnetic permeability of the core should be infinite. I.e. no magneto motive force (mmf) is needed to setup the flux.
2. Leakage flux should be zero. i.e. the flux is confined within the core and entirely links both the windings.
3. Resistances of windings should be zero.
4. There are no losses due to resistance, eddy currents and hysteresis.

In short, an ideal transformer has no losses and stores no energy. The phasor diagram for an ideal transformer is as follows:

However, there is no such transformer that exists and is ideal in nature. The ideal case is just to be used for study purpose. It is that no transformer has 100% efficiency, but there are transformers which have efficiency of 99.75% (very close to ideal one).

EMF EQUATION OF TRANSFORMER –

When an alternating voltage is applied to the primary of a transformer, a varying magnetic flux is setup. Let this varying flux be represented by

                      \[\phi ={{\phi }_{m}}\sin \omega t={{\phi }_{m}}\sin 2\pi ft\]

Where ${{\phi }_{m}}$ = maximum flux

               f= frequency of variation of flux

By faraday’s law of electromagnetic induction, the induced emf in winding of N turns is given by,

                \[E=-N\frac{d\phi }{dt}=-N\frac{d}{dt}({{\phi }_{m}}sin\omega t)\]

              \[=-N\omega {{\phi }_{m}}\cos \omega t=N\omega {{\phi }_{m}}\sin (\omega t-\frac{\pi }{2})\]

Thus, maximum value of induced emf, ${{E}_{m}}=N\omega {{\phi }_{m}}$

Rms value of induced emf, ${{E}_{R}}=\frac{{{E}_{m}}}{\sqrt{2}}=\frac{N\omega {{\phi }_{m}}}{\sqrt{2}}=\frac{2\pi fN{{\phi }_{m}}}{\sqrt{2}}$

                                               \[{{E}_{R}}=4.44fN{{\phi }_{m}}\]

TRANSFORMATION RATIO (K) –

Transformation ratio is given by ratio of secondary voltage to the primary voltage. To obtain this ratio:

The induced emf’s in primary and secondary windings (rms values) is given by:

                                    \[{{E}_{1}}=4.44f{{N}_{1}}{{\phi }_{m}}\] and

                                        \[{{E}_{2}}=4.44f{{N}_{2}}{{\phi }_{m}}\]

Thus transformation ratio is, $\frac{{{E}_{2}}}{{{E}_{1}}}=\frac{4.44f{{N}_{2}}{{\phi }_{m}}}{4.44f{{N}_{1}}{{\phi }_{m}}}$

\[\Rightarrow \frac{{{E}_{2}}}{{{E}_{1}}}=\frac{{{N}_{2}}}{{{N}_{1}}}=\frac{{{V}_{2}}}{{{V}_{1}}}=K\]

This is for a two winding transformer. If the transformer has three windings (primary, secondary and tertiary), then the ratio of emf’s is given by E₁:E₂:E₃:: N1:N2:N3.