1. EMT 113/4 ELECTRICAL ENGINEERING TECHNOLOGY
Lecturers :
Ms Sanna Taking
Ms Syarifah Norfaezah
Mr Amir Razif
Chap 1: Transformer

2. Assessments (a) Coursework: 50 % (i) 30 % Practical:
- 70 % from Lab Reports.
- 30% from Lab Test.
(ii) 20 % :
- 15 % from Written Test 1 & Test 2
- 5 % from Tutorials, Attendance &
Quizzes.
(b) Final Exam: 50 %
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3. Course Outlines Electrical machines Transformer DC machines
AC machines
Instrumentation
DC Bridges
AC Bridges
Sensors and Transducers
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4. List of Experiments Lab 1 – Lab Introduction
Lab 2 - Single Phase Transformer; voltage and current ratio
Lab 3 – DC Series Motor
Lab 4 – Three Phase AC Induction Motor
Lab 5 – d'Arsonval Galvanometer
Lab 6 – The Basic Voltmeter Design
Lab 7 – The Wheatstone Bridge
Lab 8 - Practical Test
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5. Text Books
Chapman S.J., “Electric Machinery Fundamentals”, Fourth Edition, 2005, McGraw Hill, Singapore.
Z.A. Yamayee & J.L. Bala, “ Electromechanical Energy Devices & Power Systems”, 1994, Wiley & Sons, USA
Bhas S. Guru & Huseyin R. Hiziroglu, “Electric Machinery and Transformers”, 2001, Oxford University Press.
A.K. Sawhney & P.Sawhney, “A Course in Electronic and Electrical Measurement and Instrumentation” Dhanpat Rai & Co. (P) Ltd., 2001.
C.S Rangan, G.R. Sarma & V.S. Mani, “Instrumentation Devices & System” Tata, McGraw-Hill Publishing Company Limited, 2004.
Chap 1: Transformer

6. EMT 113 ELECTRICAL ENGINEERING TECHNOLOGY
Chapter 1 : Transformer
Chap 1: Transformer
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7. Contents Introduction Ideal transformer Practical transformer
Transformer equivalent circuit
Transformer characteristics
Open loop test, close loop test
Introduction to 3 phase transformer
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8. Elements of a Power Transmission and Distribution System.
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9. Why need transformer ? Power efficiency over a long distance
Power at high voltage is necessary to decrease the lines losses.
Power at low voltage is necessary to be used at safe level in home appliances and most equipments.
Chap 1: Transformer

10. Introduction: What is Transformer ?
EMT 113 ELECTRICAL ENGINEERING TECHNOLOGY
Introduction: What is Transformer ?
Electrical device that closely related to electrical machines (device that can convert either mechanical energy to electrical energy or vice versa). It converts ac electrical energy at one voltage level to ac electrical energy at another voltage level.
Chapman S.J., “Electric Machinery Fundamentals”
Operates depending on the action of magnetic field.
Chap 1: Transformer
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11. Introduction
As a conclusion, transformer is a device that changes ac electric energy at one voltage level to ac electric energy at another voltage level through the action of magnetic field.
Similarly for motor and generator as illustrated below
Chap 1: Transformer

12. Transformer classifications
EMT 113 ELECTRICAL ENGINEERING TECHNOLOGY
Transformer classifications
Step-up transformers
connected between the generator and transmission line.
permit a practical design voltage for generators
an efficient transmission line voltage
Step-down transformers
connected between the transmission line and various electrical loads.
permit the transmitted power to be used at a safe utilization voltage.
Chap 1: Transformer
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13. Construction 1. The primary winding -the input winding, connected to
Secondary winding
Core
1. The primary winding -the input winding, connected to
an ac power source
2. The secondary winding is the output winding.
3. Consists of two or more coils of wire physically wrapped around the ferromagnetic core.
Chap 1: Transformer

14. Construction The core is formed of a stack of steel laminations.
The steel has a high magnetic permeability and provides a high-performance path for the flux, which is mutual to the primary and secondary windings.
The core is built up of thin laminations, which are electrically insulated from each other.
Two types of core construction are used:-
core type
shell-type.
Core type shell type
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15. Operation
AC voltage is applied to the primary winding, result an AC current.
The AC primary current i1 sets up a time-varying magnetic flux φ in the core. The flux links the secondary winding of the transformer.
max
 = max sin t………(1.1)
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16. Operation
Electromagnetic forces (emf’s) are induced in N1 and N2 due to a time rate of change of φM (mutual flux), as stated by the Faraday’s Law
…………………………(1.2)
Where,
e = instantaneous voltage induced by magnetic field (emf)
 = number of flux linkages between the magnetic field and the electric circuit.
= effective flux
The sign depends on Len’z law and the polarity of the circuit terminals.
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17. Operation
The voltage induced in the primary is nearly equal to the applied voltage, and the voltage at the secondary winding also differs by only a few percent from the voltage induced into that winding.
Thus, the primary-to-secondary voltage ratio is essentially equal to the ratio of the number of turn in the two windings.
………………………………(1.3)
According to Faraday’s law, the voltage induced is proportional to the number of the turn in the windings, thus
and ………………….……(1.4)
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18. Operation If the resistance is neglected, eqn (1.2) becomes.
… …..(1.5)
By neglecting the power losses,
Power in primary winding = Power in secondary winding
……………………………………………………...(1.6)
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19. Operation Substituting equation 1.1 into equation 1.4
…………......……..1.7
Solve for this equation …then the rms value of the induced voltage is given as
……………….1.8
f = frequency in hertz ; also known as the emf equation
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20. Operation Combine equations 1.3; 1.4;1.5 and 1.6,
…………………………… (1.9)
In term of phasor quantities (or rms value), these quantities are
… (1.8)
Where is the turn ratio.
If, a > 1  Step down transformer
a < 1  Step up transformer
a = 1  Isolation Transformer
Chap 1: Transformer

21. Operation
If a load is connected to the secondary terminals, a current i2 will flow in the secondary winding and electric power will be transferred to the load.
The direction of the current in the secondary winding is determined by Lenz’s law. The secondary current’s direction is such that the flux produced by this current opposes the changes in the original flux with respect to time and the flux varies sinusoidally.
Chap 1: Transformer

22. Ideal Transformer Characteristics of an ideal transformer
Windings with zero impedance
Lossless
Infinite permeability core
Therefore, the efficiency = 100%
Zero resistance result in zero voltage drops between the terminal voltages
and induced voltages
v1 = e1
v2 =e2
Sketch of ideal transformer Schematic symbol
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23. Dot convention
The dot convention appearing at one end of each winding tell the polarity of the voltage and current on the secondary side of the transformer.
If the primary voltage is positive at the dotted end of the winding with respect to the undotted end, then the secondary voltage will be positive at the dotted end also. Voltage polarities are the same with respect to the doted on each side of the core.
If the primary current of the transformer flow into the dotted end of the primary winding, the secondary current will flow out of the dotted end of the secondary winding.
Chap 1: Transformer

24. Transformer Characteristics
Transformer characteristics can be defined by:-
Efficiency
Voltage regulation.
Good transformers has high efficiency and low voltage regulation.
Through short circuit and open circuit test, parameter, power loss, efficiency and voltage regulation can be determined
Chap 1: Transformer

25. Transformer Characteristics : Efficiency
Efficiency of a transformer is defined as
= (Output Power /Input Power ) X 100%
In practice, the efficiency of a transformer is about 97% or better
For a non-ideal transformer, the output power is less than the input power because of losses.
2 types of losses – Copper losses (winding or I2R losses)
- Core losses (Hysteresis & eddy-current losses )
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26. Transformer Characteristics : Efficiency
Ideally,
For non-ideal transformer, losses are considered, therefore
Then,
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27. Transformer Characteristics : Voltage Regulation
Voltage regulation - a measure of the change in the terminal voltage of the transformer with respect to loading.
Defined as
V.R
In calculation of voltage regulation, the equivalent circuit can be referred to primary and secondary side.
Good practice to have a small voltage regulation as possible.
For an ideal transformer, V.R = 0 %
Chap 1: Transformer

28. Power in an Ideal Transformer
The power supplied to the transformer by the primary winding:
where
cos  is the power factor
1 = the angle between the primary voltage and the primary current
The power supplied by the transformer secondary winding:
Where
2 = the angle between the secondary voltage and the secondary current
For an ideal transformer, 1=2 ;same power factor, then
Pin = V1I1 cos 1
Pout = V2I2 cos 2
Pout = Pin
Chap 1: Transformer

29. Power in an Ideal Transformer
The reactive power (Q)
The apparent (complex) power (S)
Qout = Qin = V1I1 sin  = V2I2 sin  (VAR)
Sout = Sin = V1I1 = V2I2 (VA)
Chap 1: Transformer

30. Review
Unit transformer – Connected the output of a generator and used to step the voltage up to transmission levels (110kV)
Substation transformer – Connected at the other end of the transmission line which steps the voltage down from transmission level to distribution levels
(2.3 to 34.5 kV).
Distribution transformer – Takes the distribution voltage and steps it down to the final voltage
(110V, 208V,220V,etc)
Special-purpose transformers :
Potential transformer
Current transformer
Chap 1: Transformer

31. Exercises 1.1 Q1) A transformer has the following parameters;
N1= 1000, N2 = 10, I1=200A, V1 = 100kV
a) Determine I2 and V2
b) Which type of transformer is this?
Q2) A 250 kVA, V/400V, 50Hz single-phase transformer has 80 turns on the secondary. Calculate:
a) The values of the primary and secondary currents
b) The number of primary turns
c) The maximum value of flux, Фm.
Q3) How many turns must the primary and the secondary windings of a 220 V-110 V, Hz ideal transformer have if the core flux is not allowed to exceed 5 mWb?
Note: Assume the transformer is ideal for all cases
Answers will be given during class session
Chap 1: Transformer

32. Transformer applications
Voltage level adjustment (step-up and step-down transformers).
Voltage and current measurement.
Isolation for safety (isolation transformers)
Impedance matching (for maximum power transfer from the source to the load)
The resistance of the load, as seen from the primary-side of the transformer by the source, equal to the internal source resistance. In other words, the objective is to realize: Rin = Rs.
Chap 1: Transformer

33. Real, reactive and apparent power in transformer.
S = VI = P ± jQ
S = Apparent power, unit=VA.
P = Average power (also known as real power) , unit = Watt
Q = Reactive power, unit=VAR
Power factor also = ratio between real power and complex power
= P/S
Chap 1: Transformer

34. Impedance transformation through the Transformer
Impedance - the ratio of the phasor voltage across it to the phasor current flowing through it.
Figure (a) Definition of Impedance
Figure (b): Impedance scaling through a through transformer
From the eqn, It is possible to match the magnitude of a load impedance to a source impedance simply by picking proper turns ratio.
Chap 1: Transformer

35. Practical transformer
For the practical transformer,
The resistances and inductances on the primary and secondary windings
The leakage fluxes exist on both primary and secondary sides.
The core experiences eddy current and hysteresis losses
The permeability of the core material is finite resulting in a non-zero reluctance.
Two components of flux exist:
leakage flux - flux links only the primary or secondary winding.
mutual flux - links both primary and secondary windings
For a non-ideal/practical transformer, the output power is less than the input power because of losses.
2 types of losses – Copper losses (winding or I2R losses)
- Core losses (Hysteresis & eddy-current losses )
Chap 1: Transformer

36. THE EQUIVALENT CIRCUIT OF A TRANSFORMER
EXACT EQUIVALENT
APPROXIMATE EQUIVALENT
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37. Losses in transformer
Copper losses – The resistive heating losses in the primary and secondary windings
Eddy Current Losses - The resistive heating losses in the core of the transformer
Hysteresis losses - Associated with the re-arrangement of the magnetic domains in the core during each half cycle. They are complex, nonlinear function of the voltage applied to the transformer.
Leakage flux – the fluxes at primary and secondary which escape the core and pass through only one of the transformer windings.
These losses that occurred in real transformers are modeled in the
transformer model
- Exact Equivalent model
- Approximate model
Chap 1: Transformer

38. EXACT EQUIVALENT MODEL
Under load
No-load
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39. Exact Equivalent Model (Under Load)
Ideal Transformer
Symbol
Description a
Turns ratio E1 E2
Primary and secondary induced voltages V1 V2
Primary and secondary terminal voltages I1 I2
Primary and secondary currents I I0
No load current r1 x1
Primary winding resistance and reactance r2 x2
Secondary winding resistance and reactance Im Xm
Magnetizing current and reactance Ic Rc
Core loss current and resistance
Self inductance of the coil
Copper Losses
Core excitation effect
Chap 1: Transformer

40. Exact Equivalent Model (No-Load)
Power out = 0 (no load at secondary )
Power in = power out + power loss
Power loss = core loss + Cu loss
Cu = 0 (no load)
Power in = core loss
=Ic2Rc Watt
whereby
Chap 1: Transformer

41. The previous figures are accurate model of a transformer, but to analyze practical circuits containing transformer, it is necessary to refer to its primary side or to its secondary side because it is necessary to convert the entire circuit to an equivalent circuit at a single voltage level.
Chap 1: Transformer

42. Non-Ideal Transformer with LOAD and Exact Equivalent Model
Referred to the primary
Referred to the secondary
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43. The previous model more complex than necessary…………………..
APPROXIMATE
EQUIVALENT
MODEL
This model……
The excitation branch has a very small current compared to the load current of the transformer
Negligible voltage drop in R1 and X1
Excitation branch is moved to the front of the transformer
Primary and secondary impedance left in series with each other (impedances just added) creating the following…..
Chap 1: Transformer

44. Approximate Equivalent Circuits of a Transformer
I2/a
Referred to primary side
Req_1 = R1 + a2R2
jXeq_1 = X1 +a2X2
Chap 1: Transformer

45. Approximate Equivalent Circuits of a Transformer
Referred to secondary side
Req_2 = R1/a2 + R2
jXeq_2 = X1/a2 + X2
Excitation branch
Chap 1: Transformer

46. Exercise 1.2
An ideal, single phase 2400 V-240 V transformer. The primary is connected to a 2200 V source and the secondary is connected to an impedance of 2Ω 36.9°.
a) Find the secondary output current and voltage.
b) Find the primary input current.
c) Find the load impedance as seen from the primary side.
d) Find the input and output apparent powers.
e) Find the output power factor.
Chap 1: Transformer

47. Open circuit and short circuit test
Why need open circuit test and short circuit test ???
Experimentally determine the values of inductances and resistances in the transformer model.
Open circuit test – transformer’s secondary winding is open-circuited transformer’s primary winding is connected to a full-rated line voltage. LV HV
Note:
The open circuit test is conducted by applying rated voltage at rated frequency to one of the windings, with the other windings open circuited. The input power and current are measured. For reasons of safety and convenience, the measurements are made on the low-voltage (LV) side of the transformer.
Chap 1: Transformer

48. Open Circuit Test In the open circuit test,
The terminals of the high voltage (HV) side of the transformers are open circuited.
Full line voltage is applied at the low-voltage (LV) terminals
The input power, input voltage and input current are measured
Get the power factor of the input current and both magnitude and angle of the excitation impedance.
From these parameters, the values of RC and Xm is determined by comparing the following equation.
Chap 1: Transformer

49. Assignment #01 Based on the above equation, prove the following;
Chap 1: Transformer

50. Short Circuit Test In short circuit test
The secondary terminals of the transformer are short-circuited
The primary terminals are connected to a fairly low-voltage source.
The input voltage is adjusted until the current in the short-circuited windings is equal to the rated voltage.
The input power, voltage and current are again measured
Chap 1: Transformer

51. Short Circuit Test
The input voltage is so low – negligible current flows through the excitation branch.
If the excitation current is ignored, then all the voltage drop in the transformer can be attributed to the series elements in the circuit.
Approximate model with no excitation branch;
Referred to primary side
Approximate model with no excitation branch;
Referred to secondary side
Chap 1: Transformer

52. Short Circuit Test
The magnitude and the angle of the series impedance referred to the primary side is
From the equation, the values of Reqp and X eqp is determined by comparing the above equation
Note :
These same tests may also be performed on the secondary side of the transformer if it is convenient to do so.
Chap 1: Transformer

53. Short Circuit Test In short circuit test
The secondary terminals of the transformer are short-circuited
The primary terminals are connected to a fairly low-voltage source.
The input voltage is adjusted until the current in the short-circuited windings is equal to the rated voltage.
The input power, voltage and current are again measured
Chap 1: Transformer

54. Short Circuit Test
The input voltage is so low – negligible current flows through the excitation branch.
If the excitation current is ignored, then all the voltage drop in the transformer can be attributed to the series elements in the circuit.
Approximate model with no excitation branch;
Referred to primary side
Approximate model with no excitation branch;
Referred to secondary side
Chap 1: Transformer

55. Short Circuit Test
The magnitude and the angle of the series impedance referred to the primary side is
From the equation, the values of Reqp and X eqp is determined by comparing the above equation
Note :
These same tests may also be performed on the secondary side of the transformer if it is convenient to do so.
Chap 1: Transformer

56. Phasor Diagram What is phasor diagram?
A sketch of phasor voltages and currents in the transformer.
Why need it?
Easiest way to determine the effect of the impedances and the current phase angles on the transformer voltage regulation.
It is easy to determine the effect of the impedances and the current phase angles on the transformer voltage regulation by drawing the phasor diagram.
Vs is assumed to be at an angle of 0 degree, and all other voltages and currents are compared to that references.
A transformer phasor diagram is presented by applying Kirchhoff's Voltage law to the transformer equivalent circuit and an equation will be as follows.
Chap 1: Transformer

57. Phasor Diagram Lagging Power Factor Unity Power Factor
Leading Power Factor
Chap 1: Transformer

58. Exercise 1.2
An ideal, single phase 2400 V-240 V transformer. The primary is connected to a 2200 V source and the secondary is connected to an impedance of 2Ω 36.9°.
a) Find the secondary output current and voltage.
b) Find the primary input current.
c) Find the load impedance as seen from the primary side.
d) Find the input and output apparent powers.
e) Find the output power factor.
Chap 1: Transformer

59. Exercise 1.3
A transformer has the following impedances of a 20-kVA, 8000/240-V, 60Hz transformer is determined. The open circuit test and the short circuit tests are performed on the secondary side of the transformer, and the following data were taken:
a) Sketch the approximate circuit model of the transformer referred to:
i) primary voltage level
ii) secondary voltage level
b) Find the impedances of the approximate equivalent circuit referred to the primary side and secondary side.
c) Sketch the circuit for both cases.
Chap 1: Transformer

60. The transformer model referred to its primary voltage level
Solution
Q1.2a
The transformer model referred to its primary voltage level
The transformer model referred to its secondary voltage level
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61. Solution
Open circuit test
Short circuit test
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62. Solution
= 38.4Ω
= kΩ
= 192Ω
= 38.4kΩ
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63. Introduction to Three Phase Transformer
Chap 1: Transformer

64. Introduction to Three Phase Transformer
Almost all the major power generation and distribution systems in the world today are three-phase ac system.
Two ways of constructing transformer of three-phase circuit;
(i) Three single phase transformers are connected in three-phase bank.
Chap 1: Transformer

65. Introduction to Three Phase Transformer
(ii) Make a three-phased transformer consisting of three sets of windings wrapped on a common core.
The three-phased transformer on a common core is preferred because it is lighter, smaller, cheaper and slightly more efficient.
Chap 1: Transformer

66. Introduction to Three Phase Transformer
Advantages three phase transformer
Less material for the same three phase power and voltage ratings.
Smaller/lighter because all connection are made internally
Less cost to manufacture.
Less external wiring
It has slightly better efficiency
Disadvantages three phase transformer
Failure of one phase puts the entire transformer out of service.
Chap 1: Transformer

67. Introduction to Three Phase Transformer
The primary and secondary windings of the three phase transformer may be
independently connected in either a WYE (Y) or DELTA () connection
As a result, four types of three phase transformers are commonly use.
Chap 1: Transformer

68. Review
Faraday’s Law : “the e.m.f (electromotive force) induced between the ends of a loop or coil is proportional to the rate of change of magnetic flux enclosed by the coil; or the e.m.f induced between the ends of a bar conductor is proportional to the time rate at which magnetic flux is cut by the conductor."
Lenz’s Law: "A change in the magnetic flux passing through or linking with, a loop or coil causes e.m.f to be induced in a direction to oppose any change in circuit conditions, this opposition being produced magnetically when current flows in response to the induced e.m.f’’
A transformer is a device that changes ac electric energy at one voltage level to ac electric energy at another voltage level through the action of magnetic field.
Transformer construction; primary winding, secondary winding and core.
The powered inductor in a transformer is called the primary winding.
The un-powered inductor in a transformer is called the secondary winding.
For an ideal transformer; efficiency = 100% and V.R=0%
Power in an Ideal Transformer ;
(S = Apparent power, unit=VA)
(P = Average power (also known as real power) , unit = Watt)
(Q = Reactive power, unit=VAR)
Chap 1: Transformer

69. Review Transformer characteristics can be defined by:- Efficiency
Voltage regulation.
Good transformers has high efficiency and low voltage regulation.
Through short circuit and open circuit test, parameter, power loss, efficiency and voltage regulation can be determined
Phasor diagram : sketch of phasor voltages and currents in the transformer.
Power transmission and distribution System ; Unit transformer, substation transformer and distribution transformer.
Special purpose transformers :
Potential transformer - Used to measure a high ac voltage.
Current transformer (C.T) - used to measure a high ac current
Chap 1: Transformer