EET 208
ELECTRICAL
POWER
TECHNOLOGY
SINGLE- PHASE
TRANSFORMER
Salsabila binti Ahmad
OUTLINES
1
WORKING PRINCIPLE
2
BASIC STRUCTURE
3
MAGNETIC CORE
4
5
PRIMARY & SECONDARY
WINDING
EMF EQUATION OF A
TRANSFORMER
OUTLINES
6
TRANSFORMER RATIO
7
IDEAL TRANSFORMER
&EQUIVALENT CIRCUIT OF A
TRANSFOMER
8
OPEN & SHORT CIRCUIT TEST
9
VOLTAGE REGULATION
OUTLINES
LOSSES & EFFICIENCY OF A
10
TRANSFORMER
11
AUTO TRANSFORMER
12
PARALLEL OPERATION OF
SINGLE PHASE TRANSFORMER
WORKING PRINCIPLE
• Transformer is an
electromagnetic device
• It works on the Faraday’s Law of
electromagnetic induction.
• It is an AC static machine that
functions to transfer electrical
energy from one electric
circuit(primary) to another
(secondary) through magnetic
domain without changing the
frequency.
5
Transformer wiring
Connected to
any load
Connected to
an AC voltage
source
The coil at the source is
called the primary
winding
[1] https://upload.wikimedia.org/wikipedia/commons/6/64/Transformer3d_col3.svg
The coil at the load is
called the secondary
winding
6
• It consists of two inductive coils which are
electrically separated but magnetically
linked.
• The two coils possess high mutual
inductance.
• The first coil, in which electric energy is fed
from the ac supply is called the primary
winding.
• The other winding where energy is drawn is
called the secondary winding.
• The rate of change of flux, dø/dt results in
an induced voltage in the secondary winding.
If the secondary winding is connected to a
load, the induced voltage will drive a
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secondary current.
• The secondary current and voltage may or
may not be the same as in the primary
depending on their types, but the power
is the same (if only the transformer is
lossless).
• The flux in core will always be constant in
no load and loaded conditions.
• Transformers are bidirectional device
(both winding can be used as primary
when connected to an AC source).
8
BASIC STRUCTURE
A) Core type
B) Shell type
[4] http://www.electronics-tutorials.ws/transformer/transformer-construction.html
9
CORE TYPE
• Half of the primary winding and
half of the secondary winding are
placed one over the other
concentrically on each leg.
• This is done in order to increase
magnetic coupling allowing
practically all of the magnetic
lines of force go through both the
primary and secondary windings
at the same time.
• However, with this type of
transformer construction, a small
percentage of the magnetic lines
of force flow outside of the core,
and this is called “leakage flux”.
10
SHELL TYPE
• This type overcomes leakage flux as both the
primary and secondary windings are wound
on the same centre leg or limb which has
twice the cross-sectional area of the two
outer limbs.
• The advantage is; the magnetic flux has two
closed magnetic paths to flow around
external to the coils on both left and right
hand sides before returning back to the
central coils.
• This means that the magnetic flux circulating
around the outer limbs of this type of
transformer construction is equal to Φ/2.
• As the magnetic flux has a closed path
around the coils, this has the advantage of
decreasing core losses and increasing overall
efficiency.
http://www.electronics-tutorials.ws/transformer/transformer-construction.html
11
CORE TYPE VS SHELL TYPE
Core
Low magnetic coupling
> High magnetic flux
leakage
High core losses
Low efficiency
Shell
High magnetic coupling
> Decreased flux
leakage
Decrease core losses
Increased overall
efficiency
12
MAGNETIC CORE
Three Main Types of Steel Cores
Solid Iron/ Steel
•Was popular for strong
magnetic fields
•But produces large eddy
currents and heats
Laminated Silicon
•Allows efficient magnetic
Alloy/ Silicon Steel propagation
•Reduced eddy currents and
heat dissipations
Amorphous Steel
•Extremely thin strips
•Eddy current greatly reduced
at mid to high frequencies.
13
LAMINATED MAGNETIC
CORE




Circulating currents, called “eddy currents”, cause
heating and energy losses within the core decreasing
the transformers efficiency.
One way to reduce these unwanted power losses is to
construct the transformer core from thin steel
laminations.
In all types of transformer construction, the central
iron core is constructed from of a highly permeable
material made from thin silicon steel laminations
assembled together to provide the required magnetic
path with the minimum of losses.
The high resistivity of the steel sheet itself reduces
the eddy current losses by making the laminations
very thin.
[5] http://www.electronics-tutorials.ws/transformer/transformer-construction.html
14




In both the shell and core type, the individual
laminations are formed into strips of thin steel
resembling the letters “E’s”, “L’s”, “U’s” and “I’s” as
shown .
These lamination stampings are connected together to
form the required core shape.
These individual laminations are tightly butted together
during the transformers construction to reduce the
reluctance of the air gap at the joints producing a highly
saturated magnetic flux density.
E-I core laminated transformer construction is mostly
used in isolation transformers, step-up and step-down
transformers as well as auto transformers.
15
PRIMARY & SECONDARY WINDING
• A varying current in the transformer's primary
winding creates a varying magnetic flux in the
transformer core and a varying magnetic field
impinging on the secondary winding.
• This varying magnetic field at the secondary
winding induces a varying EMF or voltage in the
secondary winding due to electromagnetic
induction.
• This variables are achieve while varying the
number of turns in the coil.
16
Step-up
Np < Ns
Vp < Vs
Step-down
Np > Ns
Vp > Vs
17
Can a transformer has the same
number of winding at the primary and
the secondary, which means that the
Vs and Vp are the same?
• Yes. It is called the isolation transformer.
Isolation
Np= Ns
Vp = Vs
18
Isolation Transformer
• An isolation transformer is a
transformer used to transfer
electrical power from a source of
alternating current (AC) power to
some equipment or device while
isolating the powered device from
the power source, usually for safety
reasons.
Back to main menu
[2] https://en.wikipedia.org/wiki/Isolation_transformer
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EMF EQUATION OF A TRANSFORMER
d
e N
dt
From Faraday’s Law of
Electromagnetism
Where
E = induced Voltage in Volts (V)
N = No of turns
 = flux in Weber (Wb)
t = time in second (s)
20
The rms value of induced e.m.f in
primary and secondary winding
are:
E1  4.44 fN1max
E2  4.44 fN2max
Note:
for an ideal transformer
V1= E1
V2 = E2
21
Flux density,
Flux density,
B

Ac
B  H
Back to main menu
22
TRANSFORMER RATIO
Transformation ratio =
Vp
Vs

Np
Ns
I p N p  Is Ns
Turn ratio, a 
Derived
from
emf
equation
Np
Ns
23
IDEAL TRANSFORMER & EQUIVALENT
CIRCUIT OF A TRANSFORMER
IDEAL TRANSFORMER
• An ideal transformer is a lossless transformer.
• Any amount of energy comes in will produce the
equal.
Does not exist
in the real
world
• In the real world, there is no ideal transformer,
but there is a practical transformer with losses.
24
Losses in Transformer
1.Losses in the core
•Eddy current loss, Pe
•Hysteresis loss, PA
2.Losses in the coil
•Copper loss, Pcu
Hysteresis Loss, PA
Eddy Current Loss, Pe
Copper Loss, Pcu
25
Practical Transformers
Chapter 4 Single-Phase Transformer
26
Ideal Transformer
Practical
Transformer
Permeability, μ → infinite (∞)
Permeability, μ → finite
Bsat → 0
Bsat → finite
No hysteresis loop
Finite area of hysteresis loop
Resistivity of core material → ∞
ː eddy current loss →0
Resistivity of core material →
finite
Coil winding resistance → 0
Coil winding resistance → finite
*** The ability to allow magnetic
flux to flow is called
permeability.
ie CRGO = 1.2T
CRNGO= 1T
Ferrites= 0.3T
Will be explained in Losses and efficiency of
transformer
27
EQUIVALENT CIRCUIT
 An ideal transformer is a lossless device
with an input winding and an output
winding.
 The figure shows an accurate model of a
transformer which is not very useful.
Equivalent Circuit 1
28
 To analyze practical circuits containing
transformers, it is normally necessary to
convert the entire circuit to an equivalent
circuit at a single voltage level.
 It may be required to calculate the total
internal impedance of an electrical power
transformer viewing from primary side or
secondary side as per requirement.
 Impedance extends the concept of resistance
to AC circuits, and possesses both magnitude
and phase, unlike resistance, which has only
magnitude.
Equivalent Circuit 2
29
Transformer model referred to its primary voltage
Where a = Np/Ns
30
Transformer model referred to its secondary
voltage
31
OPEN & SHORT CIRCUIT TEST
•These two tests are performed on a
transformer to determine the approximate
value of inductances and resistances
•The power required for these Open
Circuit test and Short Circuit test on
transformer is equal to the power loss
occurring in the transformer.
32
 The losses that occur in real transformers have
to be included in accurate model of transformer
behavior. The major items to be considered in
the construction of a such model are:
1) Copper losses (I2R) = the resistive heating losses
in the primary and secondary windings of the
transformer.
2) Eddy current losses = resistive heating losses in
the core of the transformer.
3) Hysteresis losses = associated with the
rearrangement of the magnetic domains in the
core (they are complex).
4) Leakage flux = the fluxes which escape the core
pass through only one of the transformer
windings (rare).
33
OPEN CIRCUIT TEST
• The main purpose of this test is to
determine the N.L losses.
• One winding of the transformer(usually high voltage winding) is kept
open, and the other is connected to a
supply of normal voltage and
frequency
• A wattmeter W, voltmeter V, and
ammeter A are connected in the lowvoltage winding
35
LV

HV
Since the high voltage side is open, the input
current IOC is equal to the excitation current
through the shunt excitation branch. Because
this current is very small, about 2-6% of rated
current, the voltage drop across the low voltage
winding and the winding copper losses are
neglected.
36
• The measurement in the open-circuit test is normally done
on the low voltage side of the transformer, since lower
voltages are easier to work with. It is possible to determine
the power factor of the input current and
both the
magnitude and the angle of excitation impedance.
The conductance of the core loss resistor is given by
1
GC 
RC
The susceptance of the magnetizing inductor is given by
BM
1

XM
The total excitation admittance is
YE  GC  jBM
37
The magnitude of excitation admittance can be found from
the open-circuit test voltage and current.
YE
I OC

VOC
The angle of excitation admittance can be found from the
knowledge of the circuit power factor. The power factor is
given by
POC
PF  cos  
VOC I OC
the power factor angle is given by
POC
  cos
??
VOC I OC
I OC
1
Y



cos
PF
The admittance is
E
VOC
1
Open circuit Test Parameters
PF  cos
Gc 
Poc
  cos
Voc I oc
1
YE 
I oc
Voc
Where
YE = excitation admittance
Gc= core resistance
BM= magnetizing reactance
Yoc 
BM 
Poc
(Voc ) 2
Yoc  Gc
2
2
Poc  Piron
I oc
1
1
    Gc  jBM 
 j
Voc
Rc
XM
39
SHORT CIRCUIT TEST
• Method to determine
– Equivalent impedance (Z), leakage
reactance and resistance referred to
the winding placed.
– Copper loss, Pcu at full load (used to
calculate efficiency, μ of transformer)
– The total voltage drop
41
HV


LV
In the short circuit test, the low voltage side is short
circuited, and the high voltage is connected to a variable
low voltage source.
Measurement of power, current and voltage are made at
the high voltage side. The applied voltage is adjusted
until rated short circuit current flow in the winding. The
voltage generally much smaller (3%-5%) then the rated
primary voltage.
42
• Since the input voltage is so slow during the short-circuit test,
negligible currents flows through the excitation branch
(excitation current is ignored), all the voltage drop in the
transformer can be attributed to the series elements in the
circuit.
The magnitude of the series impedances referred to the primary
side of the transformer is
Z SE
VSC

I SC
The power factor of the current is given by
PSC
PF  cos  
VSC I SC
the current angle is given by
  cos
1
PSC
VSC I SC
43
Therefore, the series impedance is
Z SE
VSC 0
VSC


 
I SC    
I SC
series impedance is equal to

Z SE  Req  jX eq


Z SE  RP  a 2 RS  j X P  a 2 X S

***Note
The open-circuit test is usually performed on the low-voltage side
( RC & XM usually found referred to LV side) of the transformer,
and the short-circuit test is usually performed on the high-voltage
side ( Req & Xeq usually found referred to HV side).
All the elements must referred to the same side (either High or
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Low) to create the final equivalent circuit.
45
VOLTAGE REGULATION
The
voltage
regulation
of
a
transformer is defined as the change
in the magnitude of the secondary
voltage as the current changes from
full load to no load with the primary
voltage held fixed.
This variation in VS from N.L to F.L is
called the Voltage Regulation.
VR 
VS ,nl  VS , fl
VS , fl
100%
LOSSES & EFFICIENCY OF A
TRANSFORMER
Losses in Transformer
• Core loss (iron loss) remains constant
for all load since the variation of 1-3%
(from no load to full load) is negligible
• two types of losses occur in the cores
are “eddy current losses” and
“hysteresis losses”.
• The other loss is at the coil is named
“copper loss”.
• Copper Loss is due to the ohmic
resistance of the transformer
windings.
Eddy current loss
where
Ke – co-efficient of eddy current (depends on magnetic material)
Bm – maximum value of flux density in Wb/m2
t – thickness of lamination in meters
f – frequency of reversal of magnetic field in Hz
48
Hysteresis loss
where
Kh – hysteresis constant
Bm – maximum value of flux density in wb/m2
f – frequency of reversal of magnetic field in Hz
Copper loss
49
Can losses in transformer
be eliminate?
• No. Can only be reduced.
• Need to reduce the all of the 3 losses
– Hysteresis loss
– Eddy current loss
– Copper loss
50
Reducing Hysteresis Loss
• Caused by the friction of the molecules against the
flow of the magnetic lines.
• This friction causes heat (loss) to be developed.
• This loss is expressed using B-H curve.
• To reduce this loss, soft core materials is used.
Narrow curve
Hysteresis reduce
B-H curve
51
Β(ø/A2)
Slope =
permeability, μ
= B/H
H(NI/lm)
μ
Bsat
∞
∞
I≈0
Hysteresis loss= 0
52
Reducing Eddy Current Loss
Eddy currents are loops of electrical current induced within conductors by a
changing magnetic field.
Eddy current
Flux
53
•Eddy current losses within a transformer core can
not be eliminated completely.
•they can be greatly reduced and controlled by
reducing the thickness of the steel core.
•This can be done through laminations
•By doing this the resistance is increased and the
current is greatly reduce.
[3] http://nptel.ac.in/courses/108105053/pdf/L-22(TB)(ET)%20((EE)NPTEL).pdf
54
Reducing Copper Loss
• Transformer Copper Losses are mainly
due to the electrical resistance of the
primary and secondary windings
• conductors of large diameters are used
in order to reduce the resistance per
unit length of the conducting windings
of the electrical device
• improve the winding technique
• Use of materials with higher electrical
conductivities, such as copper).
Chapter 4 Single-Phase Transformer
55
The efficiency of a transformer is defined as
the ratio of the power output (Pout) to the power
input (Pin).
 
 
Poutput
100%
Pinput
Poutput
100%
Poutput  Ploss
  1
 
losses
100%
Pinput
Pinput  Ploss
100%
Pinput
56
AUTO TRANSFORMER
Autotransformer having
N1 turns on the primary
and N2 turns on the
secondary
Autotransformer under
load. The currents flow in
opposite directions in the
upper and lower windings
57
• An autotransformer (sometimes
called autostep down/ autostep up
transformer) is an electrical
transformer with only one winding
• On some occasions, it is desirable to
change voltage levels by only a small
amount i.e to increase from
– 110 to 120V
– 13.2 to 13.8kV
58
• Since to wind a transformer with 2 full
windings, each rated about the same
voltage is wasteful and very expensive.
• Therefore, a special-purpose
transformer, the autotransformer, is
used.
59
• The winding has at least three taps
where electrical connections are
made.
60
• Since part of the winding does
"double duty", autotransformers
have the advantages of often being
smaller, lighter, and cheaper
• The main disadvantage is, doesn’t
provide electrical isolation.
61
•In an autotransformer, portions of the same
winding act as both the primary and
secondary sides of the transformer. In
contrast, with conventional transformer with
separate windings.
Conventional step-up transformer vs. step up autotransformer
62
Standard 15
kVA, 600
Transformer reconnected as
V/120 V
an autotransformer to give a
transformer
ratio of 600 V/480 V.
Transformer reconnected to
give a ratio of 600 V/720 V.
63
PARALLEL OPERATION OF SINGLE
PHASE TRANSFORMER
OBJECTIVES
• to increase power
capability of transformer
• ease of maintenance
64
CONDITIONS TO SATISFY:
• Primary windings of the transformers should
suitable for the supply system voltage and
frequency.
•The transformer should be properly connected
with regard to polarity.
•The voltage rating of both primary and
secondary should be identical. In other words,
the transformer should have the same
transformation ratio.
•The percentage impedance should be equal.
65
The current carries by each transformer
The VA carries by each transformer
66
END
OF
CHAPTER
4
TQ
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