Introduction of Transformer:

A transformer is an a.c static electrical device as its coil is not movable. It transfers the electrical energy from one circuit to another circuit, keeping the frequency on both the primary and secondary sides. In it, the two circuits are electrically isolated but magnetically linked, having the common flux. The energy transfer usually takes place with a change of voltage (which is not necessary in all cases). A transformer is based on the principle of “Faraday’s law of electromagnetic induction”.

Functions of the transformer:

• It can raise or lower the voltage and current in A.C. circuits.
•  It can increase or decrease the value of capacitance, inductance, or resistance in
• An a.c circuit thus it can thus act as an impedance transferring device.
• It can isolate two circuits electrically.
•  It can be used to prevent DC passing from one circuit to another.

Application transformer:

Power supply, industries, substations,hydro-power, etc.

Construction of a transformer

A transformer consisting of an iron core is shown in Figure 1.2. Two separate coils are provided on two separate limbs of the core. The horizontal member of the core is known as the yoke. The coils are made by winding enamel-insulated copper wire.

A transformer is a very simple device consisting of two windings (primary and secondary) having mutual inductance and a laminated steel core. The winding coils are well insulated from the core.

Steel core:
The magnetic current is set up in the steel core. Due to the high permeability of steel, the magnitude of the exciting current necessary to set up the required flux in the core is very small, and consequently, almost 100% of the magnetic flux (generated by the primary) links with the secondary. The core is built of thin sheets of silicon steel alloy containing 4-5% silicon. Such steels possess very high permeability, and hence magnetic flux linkage is nearly 100%. A steel core is either circular or rectangular in shape and is laminated. The core has different shapes as L, I, U, and E.

Lamination of steel core:
When the transformer is excited with an a.c voltage, alternating flux is set up in the steel core. If the core were made of a solid block of steel, eddy currents would be induced, thereby causing a loss of power. Since the eddy current loss is proportional to the thickness of the core, in order to reduce such loss, the thickness has to be reduced. Hence, the core is made by stacking thin laminations (about 0.35 to 0.50 mm thick), well-insulated from each other by a light coat of core-plate varnish (or by an oxide layer on the surface) with a minimum air gap.

Bobbins and winding of the transformer
A bobbin is a spindle or cylinder, with or without flanges, on which wire, yarn, thread, or film is wound. In the case of the transformer, the bobbin is a permanent container for the wire forming the shape of the coil. The bobbin supports the windings, aligns the cores, channels the winding, and provides the termination and connection method. Each bobbin is designed for use with a specific core shape. So, it is very important to design the best bobbin. There are two windings in the transformer, which are the primary and secondary windings. In primary winding, an a.c source is connected, or input is given, whereas in secondary winding load is connected, or from where the output is taken.

Types of transformer

Based on use, the transformer is of two types, which are as follows:

1. Step-up transformer: The transformer that raises the voltage level is called a step-up transformer. or When the secondary winding has more number of turns than the primary winding, then the transformer is said to be a Step-up transformer. Here, the induced EMF is greater than the input signal.

2. Step-down transformer: The transformer that lowers the voltage level is called a step-down transformer. or When the secondary winding has a lesser number of turns than the primary winding, then the transformer is said to be a step-down transformer. Here, the induced EMF is less than the input signal

Based on construction, the transformer is of two types, which are as follows:

1. Shell-type transformer
2. Core-type transformer

Shell-type transformer

A shell-type transformer is a type of transformer in which the magnetic core surrounds the windings. In this transformer, the primary and secondary windings are placed on the central limb of the core, while the outer limbs provide the return path for magnetic flux. This design offers better mechanical strength, reduced leakage flux, and high efficiency. The construction shell-type transformer core is made of thin laminated silicon steel sheets to reduce eddy current losses.

Main construction features:

• The core has three limbs: one central limb and two outer limbs.
• Both primary and secondary windings are wound on the central limb.
• Windings are usually arranged in sandwich form (alternating layers of primary and secondary).
• The outer limbs carry the return magnetic flux.

Working principle of a shell-type transformer
A shell-type transformer works on the principle of mutual induction, the same as a core-type transformer, but with a different magnetic and winding arrangement.

Principle:
When an AC supply is applied to the primary winding, it produces an alternating magnetic flux in the iron core. This flux links both the primary and secondary windings, inducing an emf in the secondary winding according to Faraday’s law of electromagnetic induction.

Core-type transformer:

A core-type transformer is a transformer in which the windings surround the magnetic core. In this type, the primary and secondary windings are placed on different limbs of the core. The core-type transformer is widely used due to its simple construction, good cooling, and easy maintenance. The construction of the core-type transformer core is built using thin laminated silicon steel sheets to reduce eddy current losses.

Main construction features:

• The core has two limbs and two yokes.
• The primary winding is placed on one limb and the secondary winding on the other limb.
• Windings are usually cylindrical in shape.
• Laminations are insulated from each other to minimize losses.
• The windings are placed concentrically to reduce leakage flux.

Working Principle of Core-Transfermer
The core-type transformer works on the principle of mutual induction. When an alternating current (AC) is applied to the primary winding, it produces an alternating magnetic flux in the laminated iron core. This magnetic flux flows through the core and links both the primary and secondary windings. According to Faraday’s law of electromagnetic induction, this changing magnetic flux induces an electromotive force (EMF) in the secondary winding. As a result, electrical energy is transferred from the primary to the secondary winding without any direct electrical connection, and the voltage level changes depending on the turns ratio of the windings.

Working principle of a transformer:

An alternating voltage is applied to the primary winding (N1) of the transformer then the coil draws the alternating current. Since this winding links with soft iron core, this alternating current through this winding produces the alternating flux (ⱷ) in the core. As the flux is alternating in nature and links with the secondary winding also, induces an emf in the secondary winding. The induced emf in the secondary winding enables it to deliver current to an external load connected across it. Hence, the energy is transformed from the primary winding to the secondary winding by means of Faraday’s law of electromagnetic induction. Therefore, the main principle of a transformer is mutual induction.
For the primary side,

So, the transformation ratio is defined as the ratio of secondary voltage to primary voltage or secondary no of turns to primary no of turns.

Special cases:

•  If k >1, i.e, N2>N1, then the transformer is called a step-up transformer.
•  If k<1, i.e, N2<N1, then the transformer is called a step-down transformer.
•  If k=1, i.e, N2=N1, then the transformer is called an isolation transformer.

EMF equation of the Transformer

Let, “V” be the a.c voltage applied at the primary side, “I” be the current that flows through primary winding of transformer. Let us consider that the coil used is purely inductive i.e“I” lags the applied voltage “V” by 900. The current “I” in the primary coil produces the time-varying flu,x, which is in phase with the current “I”. When time-varying flux produced by the primary winding reaches the secondary coil via the iron core, the time-varying flux cuts the stationary secondary coil, so from Faraday’s law of electromagnetic induction, emf “E2” is induced in the secondary coil. The coil on which the supply voltage is applied is called the primary winding, and the second coil on which the emf “E2” is induced is called the secondary coil.

Transformation ratio:

The transformation ratio is defined as the ratio of secondary voltage to primary voltage or secondary no of turns to primary no of turns.

Where k= = transformation ratio

Transformer on NO LOAD:

If the AC source is supplied to the primary side, keeping the secondary side of the transformer open and then the primary will draw the current known as no-load current. It
is denoted by I₀

In the case of a real transformer, the winding consists of some resistance as it is not purely inductive. Hence, the primary current I₀ will lag the applied voltage V₁ by some angle ∅_,0, which will be less than 90⁰ as shown in the figure below:

The no-load current can be resolved into two components as follows:

• I_m = component of I0 which lags V₁ by 90⁰ = I₀sin∅₀ = magnetizing component
• I_w = component of I0 in phase with V₁ = I₀cos∅₀ = magnetizing component of I0
• The power consumed by the transformer at no-load is given by:
• W₀ = V₁I₀cos∅₀ watts

Where cos∅₀ is known as the no-load power factor of the transformer. From the phasor diagram,

I0 = √(𝐼𝑤² + 𝐼𝑚² ). No load equivalent circuit of the transformer is shown in Figure 

Transformer on LOAD:

Current “I₂” will flow while connecting the load on secondary side after supply is given through primary side. Now, the secondary mmf N₂I₂ will set up its own magnetic flux ∅₂ whose direction is opposite to main ∅. The output of the transformer at no-load is zero (i.e. V₂I₂ = 0), but the input power is V₁I₀ which will be power loss within the transformer. When the transformer is loaded, the output of the transformer is V₂I₂ (greater than zero). Therefore, some additional currents “I₂” will flow in the primary winding to increase the power in the primary circuit so that there is power balance between primary circuit and secondary circuit. This additional current, “I₂”, in the primary winding will set up its own magnetic flux ∅₂’ whose direction will be opposite to the direction of ∅₂. The additional power in the primary winding should be equal to the power in the secondary winding.

The reluctance for both cases is the same, i.e.,∅₂=∅’₂ so they cancel each other. Therefore, the net magnetic flux in the core is always constant at any load and is equal To ∅. The equivalent circuit of the transformer on load is shown in the figure.

Losses and efficiency of the transformer:

The output of a transformer is always less than the input because there are some power losses within the transformer while it transfers power from one circuit to another circuit. There are mainly two types of power losses in the transformer.
i. Copper loss
ii. Iron loss

Compiled by Er. Basant Kumar Yadav