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Transformer
Sobuj Kumar Ray
EEN 257, Machine-I
Transformer
DEFINATION
Transformer is a static piece of apparatus by
means of which power in one circuit is
transformed into electric power of the same
frequency in another circuit.
Working Principle
It consists of two inductive coils which are
electrically separated but magnetically
linked. The two coils possess high mutual
inductance. If one coil is connected to a
source of alternating voltage, an alternating
flux is set up in the laminated core, most of
which is linked with the other coil in which
it produces mutually induced e.m.f
according to Faraday’s laws. If the second
coil ckt is closed a current flows in it and
electric energy is transferred from the first
coil to second coil. The first coil, in which
electric energy is fed from the ac supply
mains, is called the primary winding and
the other from which energy is drawn out,
is called secondary winding.
Transformer
Transformer
In brief, a transformer is a device that
a) TX transfers electric power from one ckt to another.
b) It does so without a change of frequency.
c) It accomplishes this by electromagnetic induction.
d) Where the two electric ckt are in mutual inductive influence of each other.
Components of a Transformer
•
•
•
•
•
•
•
•
Winding
Iron core
Transformer tank
Yoke
Conservator
Transformer Oil
Bushing
Oil level indicator
• Breather
• Radiator tubes for cooling
• Tap changer
• Buchholz relay
7
Transformer Construction
The simple element of a transformer consists of two
coils having mutual inductance and a laminated
steel core.
Necessary part are:
1. Some suitable container for assemble core and
winding
2. A suitable medium for insulating the core and its
winding from container
3. Suitable bushing for insulating and bringing out
the terminals of winding from the tank.
Transformer Construction
Transformer Construction
The eddy current loss is minimize by laminating
the core, the lamination being insulated from
each other by alight coat of core- plate vanish
or by an oxide layer on the surface.
The thickness 0.35mm for 50Hz frequency
0.50 for 25Hz frequency
Transformer Construction
Transformer Construction
Transformer Construction
Constructionally, the TX are of two general types. The two type are known as :
a) Core-type Transformer
b) Shell-type Transformer
• Core-type Transformer-Winding surrounds a considerable part of Core.
• Shell-type Transformer-Core Surrounds a considerable part of Winding.
Core-type &
Shell-type Transformer Construction
Shape of core
Joint of core
Bushing
• The bushing provides
insulation of the terminals
of winding. These bushings
are made of well china
clay. These are housed on
the upper side of tank
through gasket. Service
line and terminals of the
winding
is
connected
through bushing.
• High side bushings are
large
and
low
side
bushings are small
Low voltage bushing
High voltage bushing
17
Conservator
• It is transformer oil storage
tank and if the main tank oil
reduces the conservator will
feed the oil to main tank.
• And if the oil get expansion
that oil will go to
conservator
Fig: Conservator
18
Breather
• A transformer breather is an
accessory of an oil filled type
transformer which is attached
into the oil conservator tank.
• It protect moisture entering
the conservator tank.
• In the refrigeration breather
system, an air dryer is fitted
to the conservator vessel.
Fig: Breather
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Assembling of Transformer
Elementary Theory of an Ideal
Transformer
 An ideal Transformer is one which has no magnetic leakage and hence
which has no cupper and core losses.
Elementary Theory of an Ideal
Transformer
Elementary Theory of an Ideal
Transformer
E.M.F Equation of a Transformer
Let
• N1= No of turns in
primary
• N2= No of turns
secondary
• Φm=maximum flux in
core
Voltage transformer ratio
Transformer on No-load
At no-load condition the
primary current has to
supply
1. Iron loss in core i.e
hysteresis loss and
eddy current loss
2. Small amount of
copper loss in primary
Transformer on load
When the secondary is
loaded the magnitude &
phase of I2 with respect to
v2 is determined by the
characteristic of the load.
1. Current I2 is phase with
v2 if load is non
inductive
2. It lags if load is inductive
3. It leads if load is
capacitive
Vector diagram for a loaded
transformer
The total primary current
is the vector sum of I0
and
Transformer with winding resistance
but mo magnetic leakage
𝑉2 = 𝐸2 − 𝐼2 𝑅2
𝐸1 = 𝑉1 − 𝐼1 𝑅1
Equivalent circuit
Magnetic leakage
The flux linked with
primary does not link
the secondary but part
of it complete its
magnetic circuit by
passing through air
rather the core. This
flux is known as leakage
flux. This flux is directly
proportional to the e
primary ampere turn.
Leakage flux equivalent circuit
A
transformer
with
magnetic
leakage is
equivalent to an ideal
transformer
with
inductive
coils
connected in both
primary and secondary
circuits.
Equivalent circuit of transformer with
resistance and leakage reactance
𝑍1 =
𝑅12 + 𝑋12
𝑋2 =
𝑅22 + 𝑋22
Vector diagram with resistance and
leakage reactance
𝑉1 = 𝐸1 + 𝐼1 𝑅1 + 𝑗𝐼1 𝑋1
= 𝐸1 + 𝐼1 𝑍1
𝐸2 = 𝑉2 + 𝐼2 𝑅2 + 𝑗𝐼2 𝑋2
= 𝑉2 + 𝐼2 𝑍2
Equivalent circuit with resistance and
leakage reactance
𝑍01 =
2
2
𝑅01
+ 𝑋01
𝑍02 =
2
2
𝑅02
+ 𝑋02
Equivalent circuit
𝐸1
𝑋0 =
𝐼0
𝐸1
𝑅0 =
𝐼𝑤
Equivalent circuit
Transformer test
The purpose of this is to
determine core loss.
High voltage winding left
open and other is
connected to its supply
of normal voltage and
frequency.
No load current is small(2 to
10% of rated current) Cu
loss is negligible small in
primary and nil in
secondary.
If W is the wattmeter reading
𝑊 = 𝑉1 𝐼0 cos 𝛷0
𝑊
cos 𝛷0 =
𝑉1 𝐼0
𝐼µ = 𝐼0 sin 𝛷0 𝐼𝑤 = 𝐼0 cos 𝛷0
𝑋0 =
𝑉1
𝐼µ
and 𝑅0 =
𝑉1
𝐼𝑤
Short-circuit or impedance test
Purpose of this test is to
determine copper loss
at full load
A low voltage (usually 5 to
10% of primary voltage)
is applied to the
primary
and
is
cautiously till full-load
current are flowing
both in primary and
secondary.
Short circuit test
If Vsc is the voltage required to
circulate rated load current,
then 𝑍01 =
𝑉𝑠𝑐
𝐼1
𝑊 = 𝐼12 𝑅01
𝑤
𝑅01 =
𝐼0
2
𝑋01 =
2
𝑍01
+ 𝑅01
Why transformer rating KVA?
1. Cu loss of transformer depends on
current
2. Iron or core loss depend on voltage
Total loss depend on volt-ampere(VA)
and not on phase angle between V
& I i.e it is independent of load
power factor. That is Why
transformer rating KVA not kw.
Losses in transformer
In static transformer ,
there are no friction or
windage losses.
1. Core or iron loss
2. Cupper loss
Core loss two types
Hysteresis & Eddy current
loss
Efficiency & condition of maximum
efficiency
𝑂𝑢𝑡𝑝𝑢𝑡
𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝐼𝑛𝑝𝑢𝑡
Condition for maximum
efficiency 𝑊𝑖 = 𝐼12 𝑅02
Cu loss = Iron loss
Auto-transformer
It is a transformer with
one winding only, part
of this being common
to both primary and
secondary. The primary
and secondary are not
electrically
isolated
from each other.
Uses of transformer
1. To give small boost to a
distribution cable to correct the
voltage drop.
2. As auto- starter transformer to
give upto 50 to 60% of full voltage
to an induction during starting.
Parallel operation of single-phase
transformer
For supplying a load in
excess of the rating , we
have to connect more
transformer in parallel.
Condition of parallel operation
1. Primary winding of the transformer should be suitable for
the supply system voltage & frequency
2. The transformer should be properly connected with regard
polarity.
3. The voltage rating of both primaries and secondary's
should be identical.
4. The percentage impedance should be equal in magnitude
and have same X/R ratio in order to avoid circulating
currents and operation at different power factor.
5. With transformer having different kVA ratings, the
equivalent impedance should be inversely proportional to
the individual kVA rating.