Transformer

Dr. Mohammed Moshiul Hoque

Md. Obaidur Rahman & Md. Mynul Hasan
 Transformer

• The transformer is a
device for transferring
electrical energy from
one circuit to another
without a change in
frequency.
 History
• Principle of electromagnetic
induction (Michael Faraday &
Joseph Henry, 1831)
• Faraday was the first to publish
the results of his experiments
and thus receive credit for the
discovery.
• First type of transformer to see
wide use Faraday's experiment with
 induction coil (Rev. Nicholas induction between coils of wire
Callan of Maynooth College,
Ireland, 1836)
 Reasons for Use
• It is known that the efficiency of
electrical power transmission had
been improved by the use of
higher voltages.
• This is one of the main reasons
that AC has nearly entirely
replaced DC for power
transmission & distribution.
• Transformer is used for
transmission off A..C. over long
distances by stepping it up.
• The transformer accomplishes
this change in voltage without
the use of moving parts, &
therein lies its great advantage.
 Reasons for Use
• It reduces current for a given
power requirement, hence
reduces losses due to Joulle’s
heating along the resistance off
the transmission line.
• At the city A..C. is again stepped
down to 220V for the
consumption.
• Cost/kw is low & efficiency is high.
• No moving parts: maintenance is
simpler & cheaper, required
isolation for the extremely high
voltages obtained can more easily
be constructed.
 Construction
• Consists of two coils having mutual
inductance & a laminated steel core.
• Two coils are insulated from each other.
• Others: container for assembled core &
windings; brushing for insulating &
bringing out the terminals of winding
from the tank.
• It consists of a laminated soft iron core.
• On which two enameled copper wires
are wound
• The eddy current loss is minimized by
laminating the core, the laminations
being insulated from each other by a
light coat of core-plate varnish/by an
oxide layer on the surface.
• The first coil in which A.C. input is fed
with called primary winding
• Other coils across the other output
supply is taken and it is called secondary
winding.
 Construction

• Transformer oil
ASKARELS
PYROCLOR
 Types
According to
construction
1. Core-type:
windings
surround a
considerable
part of the
core
2. Shell-type:
core surrounds
a considerable
portion of the
windings
 Types
 Shell type : more prevalent than core type Shell-Type
for distribution transformer applications
 Core type: more economical, more
prevalent, than shell type for high voltage
power transformer applications at the
lower end of their voltage & power rating
ranges (</=230 kV or 75 MVA).
 At higher voltage & power ratings, shell
type transformers tend to be more
prevalent.
 Shell type tends to be preferred for extra
high voltage & higher MVA applications
because, though more labor intensive to Core-Type
manufacture, shell type transformers are
characterized as having inherently better
kVA-to-weight ratio, better short-circuit
strength characteristics & higher immunity
to transit damage.
 Types
According to
Function/Purpose
1. If a transformer
changes low
voltage A.C. into
high voltage A.C. it
is called STEP-UP
TRANSFORMER
2. If a transformer
changes high
voltage A.C. into
low voltage A.C. it
is called STEP-
DOWN 3-phase step-down transformer
TRANSFORMER
 Basic Principle
Principle: MUTUAL
INDUCTION
According to
which an e.m.f. is
induced in a coil
when current in
the neighboring
coil changes.
E= M dI/dt
 Basic Principle
• Recall (generator): a voltage is induced in a coil
 by the coil moving past a magnetic field
 by the field poles moving past a stationary coil.
 Both cases, the total flux is substantially constant
 but there is a change in the amount of flux that
links the coil.
• Is the same principle still hold for
transformer???
 Coils & magnetic circuit are stationary,
 but the amount of flux is continually changing.
 Basic Principle
• Alternating current (AC) changing with
time & AC changing the flux.
• The current through the coil will vary
in magnitude with time & therefore,
the flux produced by this current will
also vary in magnitude with time.
• Any change of flux due to a current
change in the 1st coil also causes a
change in flux linking the 2nd coil.
• Hence magnetic flux linked with the
secondary coil changes
• Since there is a change of flux linking
the 2nd coil, there will also be a voltage
induced inn that coil.
• This voltage may then be applied to a
load connected to the 2nd coil.
 EMF Equation
• Assumptions:
 Ideal tωo winding
transformer
– ωinding resistances are
negligible
– Fluxes confined to magnetic
core
– Core lose negligible
– Core has constant
permeability
• Voltage of primary =V1
• Current at Primary= I1
• MMF = N1Ie
• No. of turns in Primary = N1
• No. of turns in 2ndary = N2
• Core flux φ follows, I very
closely.
• I & φ sinusoidal
• φ =φmax sinωt
 EMF Equation
φ =φmax sinωt
  2 f Sin (/2-x) = cosx
d   
e1   N 1   N 1max cos t  N 1max sin  t  
dt  2 
E 1max  N 1max
  
e1  E 1max sin  t  
 2   =2f
E 1max N 1max
E 1RMS    2 fN 1max  4.443fN 1max
2 2
d   
e 2  N 2   N 2max cos t  N 2max sin  t  
dt  2 
E 2 max N 2max
E 2 RMS    2 fN 2max  4.443fN 2max
2 2
E1 N1 E E
  2 f max 1
 4.44 f
2

E2 N2 N N
1 2
max
Voltage Transformation Ratio (K)
E 4.44 fN 
2 2 max

E 1
4.44 fN  1 max

N
 2

N 1

K
This constant is known as voltage transformation ratio.
(i) If N2 > N1, K >1 (Step-up)
(ii) If N2<N1, K<1 (Step-down)
For ideal transformer,
Input VA = Output VA
V1 I1 = V2 I2
I2/I1 = V1/V2 = 1/K

 Currents are in the inverse ratio of the voltage transformation ratio

Electrical Equivalent Circuit
Electrical Equivalent Circuit
Electrical Equivalent Circuit
Electrical Equivalent Circuit
Electrical Equivalent Circuit
Electrical Equivalent Circuit
 Why Transformer Rating in kVA?
• Cu loss of a transformer depends on current
• Iron loss on voltage
• Total transformer loss depends on volt-ampere
(VA) and not on phase angle between voltage &
current
• It is independent of load power factor
• P = VI cos = VI x power factor [ = 0; cos = 1]
• That is why rating of transformers is in kVA & not
in kW.
 Efficiency
In general η
output
output output  losses
efficiency  output
input η 
output  Cu loss  iron loss
But transformer
output
being a highly efficient  
input
piece of equipment input - losses
•Has very small loss 
input
Impractical measure losses
 1- 
Better method input
 Condition for Maximum Efficiency
Cu loss, Wcu  I 12 R 01  I 22 R 02
Iron loss  hysteresis loss  eddy current loss
Wi  Wh  We
Primary input, P1  V1I 1cosφ 1 Differenti ating both sides w. r. to I 1
input  losses
η dη R 01 Wi
input  0 
V I cosφ 1  Wcu  Wi dI 1 V1cosφ 1 V1 I 12cosφ 1
η 1 1
V1I 1cosφ 1 dη
I 12 R 01 Wi
For maximum  , 0
 η  1  dI 1
V1I 1cosφ 1 V1 I 1cosφ 1
R 01 Wi
IR
 η  1 - 1 01 
Wi  
V1cosφ 1 V1I 1cosφ 1 V1cosφ V1 I 12cosφ 1
 I 12 R 01  Wi
 cu loss  iron loss
 All Day Efficiency
output
• Ordinary efficiency η
input
• But certain transformer performance can not be judged by
this measure.
• Lighting/distributions
 All-day (24 hours): primary
 But 2ndary is light load
 It means that whereas core loss occurs throughout the day,
the cu loss occurs only when the transformer is loaded
 Good practice: core loss very low.
 Cu loss less important (depends on load)
 The performance of such a transformer should be judged by
all-day efficiency (operational efficiency)
 Computed on the basis of energy consumed during a certain
time (a day-24 hours)
 All Day Efficiency

output in kWh
ηallday  (for 24 hours )
input in kWh
• Less than the commercial efficiency
Transformer in Real World