Transformer

A transformer is a static machine.

The word ‘transformer’ comes from the word ‘transform’.
Transformer is not an energy conversion device, but is a device that changes AC electrical power at one
voltage level into AC electrical power at another voltage level through the action of magnetic field, without a
change in frequency.
It can be either to step-up or step down.

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Working principle of transformer

The basic principle behind working of a transformer is the phenomenon of mutual induction between
two windings linked by common magnetic flux. The figure shows the simplest form of a transformer.
Basically, a transformer consists of two inductive coils; primary winding and secondary winding. The coils
are electrically separated but magnetically linked to each other. When, primary winding is connected to a
source of alternating voltage, alternating magnetic flux is produced around the winding. The core provides
magnetic path for the flux, to get linked with the secondary winding. Most of the flux gets linked with the
secondary winding which is called as 'useful flux' or main 'flux', and the flux which does not get linked with
secondary winding is called as 'leakage flux'. As the flux produced is alternating (the direction of it is
continuously changing), EMF gets induced in the secondary winding according to Faraday's law of
electromagnetic induction. This emf is called 'mutually induced emf', and the frequency of mutually induced
emf is same as that of supplied emf. If the secondary winding is closed circuit, then mutually induced
current flows through it, and hence the electrical energy is transferred from one circuit (primary) to another
circuit (secondary).

Overall, a transformer carries out the following operations:

1. Transfer of electrical energy from one circuit to another

2. Transfer of electrical power through electromagnetic induction

3. Electric power transfer without any change in frequency

4. Two circuits are linked with mutual induction

Basic construction of transformer

Basically, a transformer consists of two inductive windings and a laminated steel core. The coils are
insulated
from each other as well as from the steel core. A transformer may also consist of a container for winding
and
core assembly (called as tank), suitable bushings to take the terminals, oil conservator to provide oil in the
transformer tank for cooling purposes etc. The figure illustrates the basic construction of a transformer.
In all types of transformers, core is constructed by assembling (stacking) laminated sheets of steel, with
minimum air-gap between them (to achieve continuous magnetic path). The steel used is having high
silicon
content and sometimes heat treated, to provide high permeability and low hysteresis loss. Laminated
sheets of steel are used to reduce eddy current loss. The sheets are cut in the shape as E,I and L. To avoid
high reluctance at joints, laminations are stacked by alternating the sides of joint. That is, if joints of first
sheet assembly are at front face, the joints of following assemble are kept at back face.
The Ideal Transformer

Properties of Ideal Transformer

1. Primary and secondary windings have no resistance.

2. All the flux produced by the primary links the secondary winding i,e., there is no leakage flux.
3. Permeability of the core is infinitely large. In other words, to establish flux in the core vanishingly
small (or zero) current is required.
4. Core loss comprising of eddy current and hysteresis losses are neglected.

Equivalent circuit of an ideal transformer

The equivalent circuit of a transformer can be drawn (i) showing both the sides along with parameters, (ii)
referred to the primary side and (iii) referred to the secondary side.

In whichever way the equivalent circuit is drawn, it must represent the operation of the transformer correctly
both under no load and load condition. The figure shows the equivalent circuits of the transformer.
Practical transformer

Characteristics of Practical Transformer

1. The core material will have finite permeability.

2. There will be eddy current and hysteresis losses taking place in the core.
3. There will be leakage fluxes, and finite winding resistances.

In a practical transformer, core material will have (i) finite value of permeability, (ii) winding resistances, (iii)
leakage fluxes and (iv) core loss.

A transformer core is subjected to an alternating time varying field causing eddy current and hysteresis
losses to occur inside the core of the transformer. The sum of these two losses is known as core loss of the
transformer.

Eddy current loss is essentially I2R loss occurring inside the core. The current is caused by the induced
voltage in any conceivable closed path due to time varying field. Obviously to reduce eddy current loss in a
material we have to use very thin plates instead of using solid block of material which will ensure very less
number of available eddy paths. Eddy current loss per unit volume of the material directly depends upon
the square of the frequency, flux density and thickness of the plate. Also, it is inversely proportional to the
resistivity of the material. The core of the material is constructed using thin plates called lamination. Each
plate is given a varnish coating for providing necessary insulation between the plates.

Transformer Types

Transformers are used in various fields like power generation grid, distribution sector, transmission and
electric energy consumption. There are various types of transformers which are classified based on the
following factors:

 Working voltage range

  The medium used in the core

 Winding arrangement

 Installation location

Based on Voltage Levels

Commonly used transformer types, depending on the voltage, are classified as follows:

 Step-up Transformer: They are used between the power generator and the power grid. The
secondary output voltage is higher than the input voltage.

 Step-down Transformer: These transformers are used to convert high-voltage primary supply to
low-voltage secondary output.

Based on the Medium of Core Used

In a transformer, we will find different types of cores that are used.

 Air Core Transformer: The flux linkage between primary and secondary winding is through the air.
The coil or windings wound on the non-magnetic strip.

 Iron Core Transformer: Windings are wound on multiple iron plates stacked together, which
provides a perfect linkage path to generate flux.

Based on the Winding Arrangement

 Autotransformer: It will have only one winding wound over a laminated core. The primary and
secondary share the same coil. Auto means “self” in the Greek language.

Based on Install Location

 Power Transformer: It is used at power generation stations, as they are suitable for high voltage
application

 Distribution Transformer: It is mostly used at distribution lanes for domestic purposes. They are
designed for carrying low voltages. It is very easy to install and characterized by low magnetic
losses.

 Measurement Transformers: They are mainly used for measuring voltage, current and power.

 Protection Transformers: They are used for component protection purposes. In circuits, some
components must be protected from voltage fluctuation, etc. Protection transformers ensure
component protection.

Parts of a Single-phase Transformer

The major parts of a single-phase transformer consist of

1. Core

The core acts as a support to the winding in the transformer. It also provides a low reluctance path to the
flow of magnetic flux. The winding is wound on the core, as shown in the picture. It is made up of a
laminated soft iron core in order to reduce the losses in a transformer. The factors, such as operating
voltage, current, power, etc., decide core composition. The core diameter is directly proportional to copper
losses and inversely proportional to iron losses.

2. Windings

Windings are the set of copper wires wound over the transformer core. Copper wires are used due to the
following:

 The high conductivity of copper minimises the loss in a transformer because when the conductivity
increases, resistance to current flow decreases.

 The high ductility of copper is the property of metals that allows it to be made into very thin wires.

There are mainly two types of windings: primary windings and secondary windings.

 Primary winding: The set of turns of windings to which the supply current is fed.

 Secondary winding: The set of turns of winding from which output is taken.

The primary and secondary windings are insulated from each other using insulation coating agents.

3. Insulation Agents

Insulation is necessary for transformers to separate windings from each other and to avoid short circuits.
This facilitates mutual induction. Insulation agents have an influence on the durability and stability of a
transformer.

The following are used as insulation mediums in a transformer:

 Insulating oil

 Insulating tape

 Insulating paper

 Wood-based lamination

EMF Equation of Transformer

N1 – Number of turns in the primary

N2 – Number of turns in the secondary

Φm – Maximum flux in the weber (Wb)

T – Time period. It is the time taken for 1 cycle

The flux formed is a sinusoidal wave. It rises to a maximum value of Φm and decreases to a negative
maximum of Φm. So, flux reaches a maximum in one-quarter of a cycle. The time taken is equal to T/4.

Average rate of change of flux = Φm/(T/4) = 4fΦm

Where, f = frequency

T = 1/f

Induced EMF per turn = Rate of change of flux per turn

Form factor = RMS value / average value

RMS value = 1.11 (4fΦm) = 4.44 fΦm [form factor of a sine wave is 1.11]

RMS value of EMF induced in winding = RMS value of EMF per turn x No. of turns

Primary Winding

RMS value of induced EMF = E1 = 4.44 fΦm * N1

Secondary Winding

RMS value of induced EMF = E2 = 4.44 fΦm * N2

This is the EMF equation of the transformer

For an ideal transformer at no load condition,

E1 = Supply voltage on the primary winding

E2 = Terminal voltage (theoretical or calculated) on the secondary winding

Voltage Transformation Ratio

K is called the voltage transformation ratio, which is a constant.

Case 1:If N2 > N1, K>1, it is called a step-up transformer.

Case 2: If N2< N1, K<1, it is called a step-down transformer.

Efficiency of Transformer

Just like any other electrical machine, efficiency of a transformer can be defined as the output power
divided by the input power. That is efficiency = output / input

Transformers are the most highly efficient electrical devices. Most of the transformers have full load
efficiency between 95% to 98.5% . As a transformer being highly efficient, output and input are having
nearly same value, and hence it is impractical to measure the efficiency of transformer by using output /
input. A better method to find efficiency of a transformer is using, efficiency = (input - losses) / input = 1 -
(losses / input).

Efficiency of a transformer will be maximum when copper loss and iron losses are equal.
That is Copper loss = Iron loss.