Transformers:

A transformer is a static AC machine which transfers electrical energy from one circuit to
another circuit with the same frequency, but with different voltages and currents
It is static device because it has no rotating parts
It is the most important components of any power systems
It changes level of voltages from one value to the other at constant frequency
It is used either for rising or lowering the voltage of an AC supply with a decrease or increase
in current
It has a efficiency of 99%
The winding which is connected to the supply is known as primary winding
The winding which is connected to the supply is known as secondary winding
The winding which is connected to higher voltage circuit is called the High voltage (HV)
winding
The winding which is connected to lower voltage circuit is called the Low voltage (LV)
winding
Application of Transformer:
The following are some of the most common uses for transformer:
1. Increasing or reducing the voltage level in an AC circuit to ensure the correct operation
of the circuit’s various electrical components.
2. It stops DC from flowing from one circuit to another.
3. It separates two separate electric circuits.
4. Before transmission and distribution can take place, the voltage level at the electric
power plant must be increased.

Working principle of transformer:

The transformer works on the principle of mutual induction states that when two coils are
inductively coupled and if current in one coil is changed uniformly then an emf gets induced
in the other coil. This emf can drive a current, when a closed path is provided to it.
It consists of two inductive coils which are electrically separated but linked through a
common magnetic circuit.
The two coils have high mutual inductance.
The basic transformer is shown in fig.
One of the two coils is connected to a source of alternating voltage.
This coil in which electrical energy is fed with the help of source is primary winding (P).
The electrical energy transformed to this winding is drawn out to the load is called the
secondary winding (S).
The primary winding has N1 number of turns while the secondary winding has N 2 number of
turns.
When primary winding is excited by an alternating voltages, it circulates the alternating
current.
This current produces an alternating flux (Φ ¿ which completes its path through common
magnetic core.
Thus an alternating flux links with the secondary winding.
The flux is alternating according to faraday’s law of an electromagnetic induction mutually
induced emf gets developed in the secondary winding.
If now load is connected to the secondary winding, this emf drives a current through it.
There is no electrical contact between the two windings, an electrical energy is transferred
from primary to the secondary.
The frequency of the mutually induced emf is same as that of the alternating source which is
supplying energy to the primary winding.
Types of Transformers Based on Classification
A transformer is a static device used to transfer electrical energy from one circuit to another
circuit by mutual induction. The transformers are classified depending upon the service
requirement for different purposes like at power generating plants, transmitting the generated
power to the load centers, and distributing the transmitted power to the various consumers
depending upon the voltage requirement.
 Classification of Transformers:
The various types of transformers that are classified based upon,
1. Number of Phases:
o Single-phase transformer
o Poly-phase transformer
 2-two phase transformer (Scott connection)
 3-phase transformer
2. Based on Construction:
o Core type
o Shell type
o Berry type
3. Based on Function:
o Power transformers
 Step-up transformers (k > 1)
 Step-down transformers (k < 1)
o Distribution transformers always step-down (K < 1)
o Instrument transformers
 Potential transformer (PT)
 Current transformer (CT)
o Welding transformer
4. Number of windings:
o Single-winding transformer (autotransformer)
o Multi-winding transformer
5. Type of cooling:
o Air-cooled transformer
 Natural air cooled transformer
 Forced air-cooled transformer
 Induced air-cooled transformer
o Oil cooled transformer
 Oil-filled natural or self-cooled transformer
 Oil-filled forced air-cooled transformer
 Oil-filled forced water-cooled transformer
6. Based on Location:
o Indoor transformer
o Outdoor transformer
7. Frequency:
o Low-frequency (50 Hz or power frequency and audio frequency).
o High-frequency transformer, pulse transformer, radiofrequency, etc.
Give reasons for using laminations in transformer core (or) use of laminations:
In all types of transformers the core is made of steel having silicon content.
High silicon content reduces the hysteresis loss
Core is laminated to reduce the eddy currents losses.
Laminations are insulated from each other by light coat of varnish
Laminations are easy to handle than a big pieces
Laminations are small thickness.
Define transformation ratio of a transformer.
Transformation ratio:
1. The transformation ratio is defined as the ration of the number of turns in the secondary coil to
the number of turns in the primary coil of the transformer.
2. It is defined as the ratio of output voltage to the input voltage of the transformer
The number of turns in the secondary coil is N2 and the number of turns in the primary coil is N1
The transformation ratio is K

N 2 E2
K= =
N 1 E1
1. if N2 > N1 i.e., K>1, we get the E2>E1 then the transformer is called step-up transformer.
2. if N2 < N1 i.e., K<1, we get the E2<E1 then the transformer is called step-down transformer.
3. 1. if N2 = N1 i.e., K = 1, we get the E2 = E1 then the transformer is called isolation transformer or
1:1 transformer.
For an ideal transformer
V 2 E2
K= =
V 1 E1

Types of Losses in a Transformer

There are different kinds of losses that will be occurred in the transformer such as iron,
copper, hysteresis, eddy, stray & dielectric. The copper loss mainly occurs due to the
resistance in the transformer winding whereas hysteresis losses will be occurred due to the
magnetization change within the core.
The following power losses may occur in a practical transformer −
 Iron Loss or Core Loss
 Copper Loss or I2R Loss
 Stray Loss
 Dielectric Loss
In a transformer, these power losses appear in the form of heat and cause two major problems
 Increases the temperature of the transformer.
 Reduces the efficiency of the transformer.

Iron Losses in a Transformer

Iron losses mainly occur through the alternating flux within the transformer’s core.
Once this loss occurs within the core then it is called core loss.
This kind of loss mainly depends on the material’s magnetic properties within the core of the
transformer.
The core in the transformer can be made with iron, so these are called iron losses.
This type of loss can be categorized into two types like hysteresis as well as eddy current.
Hysteresis Loss
This kind of loss mainly occurs when the alternating current is applied to the core of the
transformer then the magnetic field will be reversed. This loss mainly depends on the core
material used in the transformer. To reduce this loss, the high-grade core material can be
used. CRGO- Cold rolled grain oriented Si steel can be used commonly like the core of the
transformer so that Hysteresis loss can be reduced.
Copper Loss or I2R Loss
Power loss in a transformer that occurs in both the primary and secondary windings due to
their Ohmic resistance is called copper loss or I2R loss.
Stray Loss
In practical transformer, a fraction of the total flux follows a path through air and this flux is
called leakage flux. This leakage flux produces eddy currents in the conducting or metallic
parts like tank of the transformer. These eddy currents cause power loss, which is known
as stray loss.
Dielectric Loss
The power loss occurs in insulating materials like oil, solid insulation of the transformer, etc.
is known as dielectric loss. The dielectric loss is significant only in transformers working on
high voltages.
Although, in practice, the stray loss and dielectric loss are very small, constant and may be
neglected.
Efficiency of Transformer
The efficiency of a transformer is also known as commercial efficiency. It is represented
by the letter ‘η’. The efficiency of a Transformer is described as the ratio of output (in W or
kW) to input (in W or kW).
the efficiency of transformer may be expressed as follows:
Efficiency(η ) = Output Power / Input Power.
The above equation can be used for an ideal transformer in which there are no transformer
losses and all input energy is transferred to the output. As a result, the following equation is
mostly used if transformer wastes are taken into account and the efficiency of the
transformer is evaluated across the practical states.
Efficiency = ((Power O/P) / (Power O/P + Losses)) × 100%
or
Efficiency = (Power i/p – Losses) / Power i/p × 100 = 1− (Losses/ i/p Power) × 100
Voltage Regulation?
Transformer’s voltage regulation is the ratio of the difference between transformer no load
and full load output voltage to its full load output voltage expressed as a percentage (%).
In other words, transformer voltage regulation is the measure of supplying constant output
voltage with different load currents.
E No Load−V Full Load
Voltage Regulation=
V Full Load
E2−V 2
% Voltage Regulation= × 100
E2
Voltage regulation for primary winding of the transformer
E1−V 1
% Voltage Regulation= × 100
E1
Where:
 E1 = No load primary terminal voltage
 V1 = Full load primary terminal voltage
 E2 = No load secondary terminal voltage
 V2 = Full load secondary terminal voltage

Auto transformer:
An auto transformer is an electrical transformer having only one winding.
The winding has at least three terminals which is explained in the construction details below.
Some of the advantages of auto-transformer are that,

 they are smaller in size,

 cheap in cost,
 low leakage reactance,
 increased kVA rating,
 low exciting current etc.

Construction of Auto Transformer

An auto transformer consists of a single copper wire, which is common in both primary as well as
secondary circuit.
The copper wire is wound a laminated silicon steel core, with at least three tappings taken out.
Secondary and primary circuit share the same neutral point of the winding.
The construction is well explained in the diagram.
Variable turns ratio at secondary can be obtained by the tappings of the winding (as shown in the
figure), or by providing a smooth sliding brush over the winding. Primary terminals are fixed.
Thus, in an auto transformer, you may say, primary and secondary windings are connected
magnetically as well as electrically.

Working Of Auto Transformer

Auto transformer has only one winding which is shared by both primary and secondary circuit, where
number of turns shared by secondary are variable. EMF induced in the winding is proportional to the
number of turns.
Therefore, the secondary voltage can be varied by just varying secondary number of turns.
As winding is common in both circuits, most of the energy is transferred by means of electrical
conduction and a small part is transferred through induction.
Advantages of Auto transformer
 Less costly
 Better regulation
 Low losses as compared to ordinary two winding transformer of the same
rating.
Disadvantages of Auto transformer:
 any undesirable condition at primary will affect the equipment at secondary (as
windings are not electrically isolated),
 due to low impedance of auto transformer, secondary short circuit currents are very
high,
 harmonics generated in the connected equipment will be passed to the supply.
 The secondary winding is not insulated from the primary winding.
Applications of Auto transformer
 It is used as a starter to give up to 50 to 60% of full voltage to the stator of a squirrel
cage induction motor during starting.
 It is used to give a small boost to a distribution cable, to correct the voltage drop.
 It is also used as a voltage regulator
 Used in power transmission and distribution system and also in the audio system and
railways.
APPLICATIONS OF TRANSFORMER:
Impedance Matching Transformers
Special Transformers called Impedance Matching Transformers can be used to match
impedance.
Impedance-matching transformers are used to match the impedance of a source and
that of its load, for most efficient transfer of energy.
Impedance matching transformers can be used in any AC circuit and any application in which
maximum power transfer is desired.
They are frequently used in audio equipment, microphones, amplifiers, data networks and
systems, telephone grids, phone systems, and airplane communication systems.
Audio transformers using sheet steel cores, such as those used in vacuum tube amplifier
circuits to match the high impedance of the tube to the low impedance of the speaker, have a
bandwidth of 20Hz to 20kHz.
RF transformers made using ferrite or even air cores can have bandwidths of 1MHz-30MHz.
Isolation transformer:
An isolation transformer is also similar to other transformers, specially designed for
providing electrical isolation between two circuits (primary and secondary) without
changing the secondary parameters (voltage, current, and frequency levels).
Basically, in step-up transformers, the secondary turns are greater than the primary
turns, and vice-versa in the case of step-down transformers.
S.No Winding Turns Voltage Current

Step-up Transformer N2 > N1 V2 > V1 I2 <I1

Step-down
N2 < N1 V2 < V1 I2 >I1
Transformer

Isolation
N2 = N1 V2 = V1 I2 = I1
Transformer

Hence, it is also called a 1:1 ratio transformer in terms of voltage, current, and turns
ratio.
An isolation transformer provides isolation physically and electrically between two
circuits. It isolates and protects the electronic circuits and the persons against
electrical shock from the main lines.
Example:
Stabilizers.
Applications of Isolation Transformer:
1. An isolation transformer is used as a Pulse transformer in power electronics devices..
2. Isolation transformer in ultra-isolation mode built with special features eliminates
common mode signal. Here, the transverse mode noise gets reduced and also common
mode noise gets attenuated with the use of high-isolating materials and specialized
shielding techniques. Ultra mode isolation transformer is used for very sensitive
computer peripherals and CNC machines etc.
3. Isolation transformers with electrostatic shields are used for power supplies for
sensitive instruments in medical and laboratory.
4. It is used to reduce or eliminate voltage spikes in supply lines that arise due to sudden
changes in supply voltage or lighting.
5. Isolation transformer also eliminates the grounding issue on the secondary side and
hence reduces the interference of noise into the load.
6. It’s used as a power adapter in computer laptops and cell phones.
7. It is used in the UPS unit.
Potential transformer (PT):
A potential transformer (P.T.) is an instrument transformer which is used for the protection
and measurement purposes in the power systems.
A potential transformer is mainly used to measure high alternating voltage in a power system.
Potential transformers are also known as voltage step-down transformers or voltage
transformers or instrument transformer, in which the voltage of a circuit is reduced to a lower
voltage for measurement.
The electromagnetic device used for the transformation of the higher voltage of the circuit to
the lower voltage is called a potential transformer. The output of a low voltage circuit can be
measured through voltmeters or wattmeters. These are capable of increasing or decreasing the
voltage levels of a circuit, without a change in its frequency and windings.
Current transformer (CT):
A current transformer (C.T.) is an instrument transformer which is used for the protection
and measurement purposes in a power system.
The C.T. is primarily used to measure high alternating currents in a power system.
This is also known as a series transformer
These are used to reduce high voltage currents to low voltage currents.
They are also used to step down the current at a specific ratio to insulate the instrument from
the high voltage lines.
Current transformer
Also known as CTs, current transformers are devices that measure alternating current. They
are widely used to measure high magnitude currents.
A current transformer essentially lowers (steps down) a high current to a lower, safer level
that you can manage properly.
It steps down the current to be measured so that you can measure it with an average range
ammeter.
Functions of a current transformer include:
 Converting large primary currents into small 1A/5A current
 Providing the current for the coil of measuring device and protective relaying
 It separates primary voltage from secondary voltage.
Characteristics of a current transformer include:
 The resistance of the instrument’s current coil with which the CT’s secondary
winding is connected is small. The CT transformer operates under a state close to the
short circuit under normal condition
 The primary winding is installed in series in the current.
Potential Transformers
Potential transformers, also known as voltage transformers, measure an aspect of the power
supply.
The potential transformer measures voltage.
Functions of the potential (voltage) transformers include:
 It measures and reduces high voltage values into lesser values
  Voltage transformers proportionally convert the high voltage into a standard
secondary voltage of 100V or lower for easier utilization of protective and measuring
instruments/devices
 To isolate the high voltage from electricians using the PT.
Difference between Current Transformer and Potential Transformer
Both current and potential transformers are types of instrument transformer used for
measurement and protection in an electrical power system.
Basis of
Current Transformer Potential Transformer
Difference
Transform the current from high Transform the voltage from high
Definition
value to the low value. value to the low value.
CT is used as the abbreviated name PT (or sometimes VT) is used to
Abbreviated name
to represent a current transformer. denote a potential transformer.

Symbol

CT reduces the large current to a PT reduces a high voltage to a

Function
safer and measurable level. safer and measurable level.
Current transformer is a voltage Potential transformer is a voltage
Step up/down step-up and current step-down step-down and current step-up
transformer. transformer.
Core material Usually built up with lamination of It is made up of with high quality
silicon steel. steel operating at low flux
densities
Primary Winding It carries the current which is to be It carries the voltage which is to
measured be measured.
Secondary It is connected to the current It is connected to the meter or
Winding winding of the instrument. instrument.
Connection Connected in series with the Connected in parallel with the
instrument instrument.
Primary Circuit Has a small number of turns Has a large number of turns
Secondary Circuit Has a large number of turns and Has a small number of turns and
cannot be open circuit. can be open circuit.
Range 5A or 1A 110v
Transformation High Low
Ratio
Burden Does not depends on secondary Depends on the secondary
burden burden
Input Constant current Constant Voltage
Full line current The primary winding consists the The primary winding consists the
full line current. full line voltage.
Types Two types ( Wound and Closed Two types (Electromagnetic and
Core ) Capacitor voltage)
Impedance Low High
Applications Measuring current and power, Measurement, power source,
monitoring the power grid operating protective relay,
 operation, for operating protective
relay,

Specification of a transformer:
Electrical Specifications:
1. KVA rating:
Kilo volt ampere of transformer. This indicates the size of transformer.
2. Phase: 3 For three phase, 1 for single phase.
3. Voltage Class: (line to line voltage):
this indicates the voltage level of transformer.
4. Frequency: This indicates the supply of transformer.
Mechanical specifications:
1. Connection: this indicates the interconnection of the winding.
2. Thermal specifications: this gives the maximum operating temperature of the
transformer.