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Lesson plan
(www.meri.edu.in)
Name if the faculty : Mr. Manoj Bansal
Discipline : Electrical & Electronics Engineering
Semester : 3rd
Subject : Electrical Machine- I
Lesson Plan Duration : 15 weeks (From August, 2018 to November 2018)
Work Load (Lecture/ Practical) per week (in hours): Lecture-04, Practical-02
Week Theory Practical
Lecture
day
Topic(Including
assignment/test)
Practical
Day
Topic
1st
1st TRANSFORMERS: Principle
1st
Conversion of 3 Phase to
six phase using 3 single
phase transformers 2nd
Construction of core
3rd
Winding & tank operation
4th
Testing of single phase
transformer
2nd
1st Equivalent circuit, phasor
diagram, parameters
determination
2nd
To study three phase
rectifiers & supply
configuration . In 3 phase
2nd
P.U. representation of parameters
3rd
Regulation, losses & efficiency
4th
Separation of iron losses
3rd
1st Parallel operation of single phase
transformers
3rd
To perform Sumpner's
Back to back test on 1-
phase transformers 2
nd Auto-transformer: Principle,
construction
3rd
Comparison with two winding
transformers
4th
Application.
4th
1st Various types of connection of
three phase transformer
4th
Parallel operation of two 1-
phase transformers
2nd
Comparative features of three
phase transformer
3rd
Zig-Zag connection
4th
Parallel operation of single phase
5th
1st Three phase transformers
5th
To convert three phase to
2-phase By Scott-
connection 2nd
Auto-transformer: Principle
3rd
Construction
4th
Comparison with two winding
transformers
6th
1st Application
6th
To perform load test on
DC shunt generator
2nd
Nature of magnetizing current
3rd
Plotting of magnetising current
from B-H curve
4th
Inrush current, harmonics
7th
1st
Sessional -1
examination+Activity 7
th
Sessional -1
examination+Activity
2nd
3rd
4th
8th
1st Effect of construction on input
current
8th
Speed control of DC shunt
motor
2nd
Connection of three phase
transformer
3rd
Phase-Conversion
4th
Three to two phase
9th
1st Three to six phase
9th
Swinburne’s test of DC
shunt motor
2nd
Three to twelve phase conversions
3rd
Introduction to three winding, tap-
changing
4th
Phase-shifting transformers
10th
1st D.C. MACHINES: Elementary
DC machine
10th
Hopkinson’s test of DC
shunt M/Cs
2nd
Principle & construction of D.C.
generator
3rd
Simplex lap
4th
Wave windings
11th
1st E.M.F. equation
11th
Ward Leonard method of
speed control
2nd
Armature reaction, compensating
winding
3rd
Commutation, methods of
excitation
4th
Load characteristics, parallel
operation
12th
1st Principle of DC Motors
12th
Revision
2nd
Torque and output power
equations
3rd
Revision
4th
Revision
13th
1st
Sessional -II
Examination+Activity 13
th
Sessional -II
Examination+Activity
2nd
3rd
4th
14th 1
st Load characteristics 14
th Revision
2nd
Starting, speed control
3rd
Braking, testing, efficiency &
applications
4th
Revision
15th
1st
Pre-University Exam 15th
Pre-University Exam 2
nd
3rd
4th
MANAGEMENT EDUCATION AND RESEARCH INSTITUTE COLLEGE OF ENGINEERING AND TECHNOLOGY
(DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING)
Session - 2018-19 Semester: 3rd Name of the Faculty: Er. Manoj Bansal Subject & Code: Electrical Machines - I (EE-207-F) COURSE OBJECTIVE- This subject emphasizes on basics of electrical machines with their basic operating principles and construction. The most important area of electrical engineering i.e. transformer and D.C. Machine will be explained. After the completion of the course student will be able to have in depth knowledge of all said components which will be utilized in later part of engineering education. METHODOLOGY: The pedagogy will be lectures, presentations, Tutorials, assignments and class work.
EVALUATION- Besides the semester end – examination, the students will be continuously assessed during the course on the following basis
i. Mid Term Examinations : 20 Marks ii. Attendance : 10 Marks
iii. Class performance +Assignments : 10 Marks iv. Regularity & Discipline : 10 Marks
v. End Semester Examination : 100 Marks Total : 150 Marks
SYLLABUS AS PER MDU
EE-207-F ELECTRICAL MACHINES - I
L T P Class Work marks : 50
3 1 0 Theory marks : 100
Total marks : 150
Duration of Exam : 3 hr
NOTE: For setting up the question paper, Question No. 1 will be set up from all the four sections which will
be compulsory and of short answer type. Two questions will be set from each of the four sections. The
students have to attempt first common question, which is compulsory, and one question from each of the four
sections. Thus students will have to attempt 5 questions out of 9 questions.
SECTION -A
TRANSFORMERS: Principle, construction of core, winding & tank, operation, testing of single phase transformer,
equivalent circuit, phasor diagram, parameters determination, P.U. representation of parameters, regulation, losses &
efficiency, separation of iron losses. Parallel operation of single phase transformers. Auto-transformer: Principle,
construction, comparison with two winding transformers, application.
SECTION -B
Various types of connection of three phase transformer, their comparative features, Zig-Zag connection.
Parallel operation of single phase & three phase transformers.Auto-transformer: Principle, construction, comparison
with two winding transformers, application.
Nature of magnetizing current, plotting of magnetising current from B-H curve, Inrush current, harmonics, effect of
construction on input current, connection of three phase transformer.
Phase-Conversion: Three to two phase, three to six phase and three to twelve phase conversions.
Introduction to three winding, tap-changing & phase-shifting transformers.
SECTION-C
D.C. MACHINES: Elementary DC machine, principle & construction of D.C. generator, simplex lap and wave
windings, E.M.F. equation, armature reaction, compensating winding, commutation, methods of excitation, load
characteristics, parallel operation.
SECTION-D
Principle of DC Motors, torque and output power equations, load characteristics, starting, speed control, braking,
testing, efficiency & applications.
Detailed Course Contents
References
Contact Hours
SECTION 1: TRANSFORMERS:
Principle, construction of core, winding & tank, operation, testing of single phase transformer, equivalent circuit, phasor diagram, parameters determination, P.U. representation of parameters, regulation, losses & efficiency, separation of iron losses. Parallel operation of single phase transformers. Auto-transformer: Principle, construction, comparison with two winding transformers, application.
Electric Machines: I.J.Nagrath and D.P.Kothari, TMH, New Delhi. (Chapter No. 3)
&
Fundamental of Electrical & Electronics Engineering: S.K.Sahdev: TMH (Chapter No. 13)
12
SECTION 2: Various types of connection of three phase transformer, their comparative features, Zig-Zag connection. Parallel operation of single phase & three phase transformers. Auto-transformer: Principle, construction, comparison with two winding transformers, application. Nature of magnetizing current, plotting of magnetizing current from B-H curve, Inrush current, harmonics, Effect of construction on input current, connection of three phase transformer. Phase-Conversion: Three to two phase, three to six phase and three to twelve phase conversions. Introduction to three winding, tap-changing & phase-shifting transformers.
Electric Machines: J.B.Gupta: TMH, (Part-3; Chapter No. 1,2) &
Fundamental of Electrical & Electronics Engineering: S.K.Sahdev: TMH (Chapter No. 13)
10
07
SECTION 3:
D.C. MACHINES: Elementary DC machine, principle & construction of D.C. generator, simplex lap and wave windings, E.M.F. equation, armature reaction, compensating winding, commutation, methods of excitation, load characteristics, parallel operation.
Electric Machines: I.J.Nagrath and D.P.Kothari; (Chapter No. 7) &
Fundamental of Electrical & Electronics Engineering: S.K.Sahdev: TMH (Chapter No. 12)
08
SECTION 4:
Principle of DC Motors, torque and output power equations, load characteristics, starting, speed control, braking, testing, efficiency & applications.
Electric Machines: I.J.Nagrath and D.P.Kothari, (Chapter No. 7) &
Fundamental of Electrical & Electronics Engineering: S.K.Sahdev: TMH (Chapter No. 12)
08
Total Lectures:45
TEXT BOOKS:
1. Electric Machines: I.J.Nagrath and D.P.Kothari, TMH, New Delhi. 2. Electric Machines: J.B. Gupta, Kataria Publications. 3. Electrical Machines – (Vol – II) By B L Theraja , S Chand
REF. BOOKS:
1. Electric Machinery, Fitzgerald & Kingsley, MGH. 2. Theory of alternating current machinery, A.S. Langsdorf, TMH. 3. Electrical Machines, P.S.Bhimbra, Khanna Publishers Delhi 4. Fundamental of Electrical & Electronics Engineering, S.K.Sahdev: Dhanpat Rai Publication
MANAGEMENT EDUCATION AND RESEARCH INSTITUTE (MERI)
COLLEGE OF ENGINEERING AND TECHNOLOGY
(DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING)
Session - 2018-19 Semester: 3rd
Name of the Faculty: Er. Manoj Bansal
Subject & Code: Electrical Machines - I (EE-207-F)
Focal Points
1. Group discussion will be organized to remove hesitation of local
students.
2. Special seminar will be conduct by expert faculties of concern field.
3. Extra class will be given to poor students.
4. Special attention on practically visible site will be given.
5. Students’ presentations/seminars related to the particular subject, will
be organized unit-wise and a schedule has been prepared for this
purpose
6. Visit to nearby 132 sampla sub station will be organized time to time
to learn the thing practically
7. Special attention on numerical problems so that students will prepare
themselves for company interviews.
8. We will take special class for understanding the behavior of AC &
DC current like how to measure, how to work, numbers of precautions
we should follow when we work with this.
MANAGEMENT EDUCATION AND RESEARCH INSTITUTE (MERI) COLLEGE OF ENGINEERING AND TECHNOLOGY
(DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING)
Session - 2018-19 Semester: 3rd Name of the Faculty: Er. Manoj Bansal Subject & Code: Electrical Machines - I (EE-207-F)
Assignment Chart
S.No Assignments D O A D O S D OD
1 Assignment no. 1 18 August,2018 20 August,2018 21 August,2018
2 Assignment no. 2 21 August,2018 27 August,2018 28 August,2018
3 Assignment no. 3 4 Sep,2018 10 Sep.,2018 11 Sep,2018
4 Assignment no. 4 3 oct.2018 8 oct.,2018 9 oct.,2018
5 Assignment no. 5 24 oct.2018 29 oct.2018 31 oct.2018
6 Assignment no.6 31 oct2018 5 nov.2018 9 nov.2018
7 Assignment no. 7 9 nov.2018 14 nov.2018 16 nov.2018
MANAGEMENT EDUCATION AND RESEARCH INSTITUTE (MERI) COLLEGE OF ENGINEERING AND TECHNOLOGY
(DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING)
Session - 2018-19 Semester: 3rd Name of the Faculty: Er. Manoj Bansal Subject & Code: Electrical Machines - I (EE-207-F)
ASSIGNMENT-1
1. What is a transformer? What is its necessity in the power system?
2. Explain the working principle of a transformer.
3. State why silicon steel is selected for the core of a transformer and why the core of a
transformer is laminated?
4. Give the constructional details of a core type transformer.
5. Derive an expression for the e.m.f induced in a transformer wiring.
6. What is an ideal transformer? What are the necessary conditions for an ideal
transformer? Draw & explain the phasor diagram for an ideal transformer.
7. Can a transformer work on D.C supply? Justify your answer.
8. Describe the working of a loaded transformer neglecting winding resistance and
leakage flux.
9. Draw and explain the phasor diagrams of a loaded transformer neglecting voltage
drop in windings and ampere-turns balance.
10. A 25KVA transformer has 500 turns on primary and 40 turns on the secondary
winding. The primary is connected to 3000V, 50 Hz mains. Calculate:
a) Primary & Secondary currents
b) Secondary emf
c) Maximum flux in the core
Neglect magnetic leakage, resistance of winding and primary no load current in
relation to the full load current.
MANAGEMENT EDUCATION AND RESEARCH INSTITUTE (MERI)
COLLEGE OF ENGINEERING AND TECHNOLOGY
(DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING)
Session - 2018-19 Semester: 3rd Name of the Faculty: Er. Manoj Bansal Subject & Code: Electrical Machines - I (EE-207-F) L (3) T (1) P (0)
ASSIGNMENT-2
1. What are the various losses in a transformer? Where do they occur and how do they
vary with load?
2. What do you mean by voltage regulation in a transformer?
3. Define efficiency in a transformer. What is the condition for maximum efficiency?
4. What information can be obtained from open circuit test of a transformer? How can
you get this information?
5. Draw and explain the equivalent circuit of a transformer.
6. What is an auto transformer? Explain its construction and working principle.
7. Distinguish between an auto transformer with two winding transformer.
8. Describe the parallel operation of single phase transformer.
9. Explain the Per Unit (P.U) system in case of transformers.
10. What is an actual transformer? Draw and explain the phasor diagram of an actual
transformer by using R, L, and C load.
MANAGEMENT EDUCATION AND RESEARCH INSTITUTE (MERI) COLLEGE OF ENGINEERING AND TECHNOLOGY
(DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING)
Session - 2018-19 Semester: 3rd Name of the Faculty: Er. Manoj Bansal Subject & Code: Electrical Machines - I (EE-207-F)
ASSIGNMENT-3
1. Explain the constructional details of a three phase transformer with diagrams.
2. Describe the different types of connections of a three phase transformer.
3. What is parallel operation of transformer? What is its necessity and what are the conditions
for satisfactory operation of transformer in parallel?
4. Explain the working principle of an Auto Transformer & compare it with a two winding
transformer.
5. Describe the concept of tap changing in transformers.
6. Explain three phase to two phase conversion of transformer.
7. What are the effects of harmonics components in magnetizing currents?
8. Find the turn-ratio (primary to secondary) of a 11,000/415 V, delta/star connected three
phase transformer.
9. Can transformer work on D.C supply? Justify your answer.
10. Explain open circuit and short circuit test in a single phase transformer.
MANAGEMENT EDUCATION AND RESEARCH INSTITUTE (MERI) COLLEGE OF ENGINEERING AND TECHNOLOGY
(DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING)
Session - 2018-19 Semester: 3rd Name of the Faculty: Er. Manoj Bansal Subject & Code: Electrical Machines - I (EE-207-F)
ASSIGNMENT-4
1. Name the various parts of a d.c machine and give the function of each part.
2. Explain the working action of a d.c generator. Describe briefly its important
parts.
3. Explain how commutator works in a d.c machine to generate d.c voltage.
4. Derive an e.m.f equation of a d.c machine.
5. Explain how can you distinguish a lap and wave winding. How can you
recognize the winding of a d.c machine by counting its brushes?
6. What are the different types of excitation employed for d.c generators?
7. What do you mean by armature reaction of a d.c machine? Describe different
methods for minimizing armature reaction.
8. Sketch the load characteristics of:
a) DC shunt generator
b) DC series generator
Give reason for the particular shape in each case.
9. Write a short note on external characteristics of series shunt and compound
wound d.c generator.
10. What are the various energy losses in a d.c machine
MANAGEMENT EDUCATION AND RESEARCH INSTITUTE (MERI) COLLEGE OF ENGINEERING AND TECHNOLOGY
(DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING)
Session - 2018-19 Semester: 3rd Name of the Faculty: Er. Manoj Bansal Subject & Code: Electrical Machines - I (EE-207-F)
ASSIGNMENT-5
1. Explain the operating principle of a d.c motor.
2. Explain the concept of back e.m.f.
3. On what factors do the torque developed by a d.c motor depends?
4. Mention various types of d.c motors and their uses.
5. With the help of speed-armature current characteristics, show that a shunt
motor runs at almost constant speed irrespective of the load.
6. Sketch the speed-torque curve of a d.c series motor. What are the
applications of d.c series motor?
7. Describe the speed control methods of d.c shunt motor.
8. Why is a starter necessary for a d.c motor?
9. Describe briefly the methods of speed control of d.c series motor.
10. “A d.c motor should not be started without load”, why?
MANAGEMENT EDUCATION AND RESEARCH INSTITUTE (MERI) COLLEGE OF ENGINEERING AND TECHNOLOGY
(DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING)
Session - 2018-19 Semester: 3rd Name of the Faculty: Er. Manoj Bansal Subject & Code: Electrical Machines - I (EE-207-F)
ASSIGNMENT-6
1. Explain the short circuit test of a single phase transformer. Why do we
perform this test?
2. Write a short note on all day efficiency of transformer.
3. A single phase, 50Hz transformer has 30 primary and 350 secondary turns.
The net cross sectional area of core is 250 cm2. If the primary winding is
connected to a 230V, 50Hz supply, calculate:
a) Maximum flux density in the core
b) Voltage induced in secondary winding
Neglect losses, what is the primary current when the secondary current is
100 amperes.
4. The armature of a 12 pole d.c shunt generator has 50 slots and is wave
wound with12 conductors per slot. The generator is running at a speed of
625 r.p.m and supplies a resistive load of 15Ω at a terminal voltage of 300V.
The armature resistance is 0.5Ω and field resistance is 60Ω. Draw the circuit
diagram and find the armature current, the generated e.m.f, and the flux per
pole.
5. What are the causes of sparking at brushes and necessity of inerpoles?
6. What are the causes of failure to build up voltage in a generator?
7. What is the efficiency of a d.c generator? What is the condition for maximum
efficiency?
8. What are the various losses in d.c generator?
9. Describe the method for speed control of separately excited d.c motor.
10. Draw the speed-armature current characteristics of d.c shunt motor.
MANAGEMENT EDUCATION AND RESEARCH INSTITUTE (MERI) COLLEGE OF ENGINEERING AND TECHNOLOGY
(DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING)
Session - 2018-19 Semester: 3rd Name of the Faculty: Er. Manoj Bansal Subject & Code: Electrical Machines - I (EE-207-F)
ASSIGNMENT-7
1. Explain the operating principle of a d.c motor.
2. Explain the concept of back e.m.f.
3. Write a short note on all day efficiency of transformer
4. Describe the speed control methods of d.c shunt motor.
5. Why is a starter necessary for a d.c motor?
6. Derive an e.m.f equation of a d.c machine.
7. Explain how can you distinguish a lap and wave winding. How can you
recognize the winding of a d.c machine by counting its brushes?
8. Describe the parallel operation of single phase transformer.
9. Explain the Per Unit (P.U) system in case of transformers
10. Draw the speed-armature current characteristics of d.c shunt motor.
MERI COLLEGE OF ENGG. AND TECHNOLOGY
QUESTIONS BANK
DEPARTMENT OF EEE
SUBJECT:-EM-1 Code:- EE-205-F
Q.1. Develop the phasor diagram of single phase transformer under lagging power
Factor load.
Q.2. The maximum efficiency of a 500 KVA, 3300/500 V, 50 Hz single phase
Transformer is 97% and occurs at ¾ full load, unity power factor. If the
Impedance is 10%, calculate the regulation at full load and 0.8 power
Factor lagging.
Q.3. Discuss the relative merits and demerits of a auto-transformer. Distinguish
Between potential divider and autotransformer.
Q.4. A 400/100 V, 5 KVA, 1-ø two winding transformer is to be used as an auto
Transformer to supply 400 V from 500 V source. When tested as a two winding
Transformer at rated load and 0.8 p.f. (lag), its efficiency was found to be 0.95.
Determine its KVA rating as an autotransformer. Also calculate the transformed
KVA and conducted KVA.
Q.5. Explain the principle of single phase ideal transformer. Derive an expression for
Induced emf in a transformer and also draw its no-load phasor diagram.
Q.6. A 230/460 V transformer has a primary winding resistance of 0.2 Ω and a
Reactance of 0.5 Ω and the corresponding values for the secondary winding are
0.75 Ω and 1.8 Ω respt. Find the secondary terminal voltage when suppling
(i) 10 A at 0.8 p.f. lagging.
(ii) 10 A at 0.8 p.f. leading.
Q.7. The following test results were odtained on a 20 KVA, 2200/220 V, 50 Hz 1-ø
Transformer :
O.C. test (L.V.side): 220V, 1.1A, 125W
S.C. test (H.V.side): 52.7V, 8.4A, 287W
The transformer is loaded at unity p.f. on secondary side with a voltage of 220V.
Determine the maximum efficiency and the load at which it occur.
Q.8. Explain the working principle and construction of an auto transformer. Draw the phasor
diagram under no load condition.
Q.9. State Faradays law of electro-magnetic induction and explain how it is applied for working
of a d.c. motor.
Q.10.Discuss various types of losses in magnetic circuits.
Q.11.Define the voltage regulation of a transformer. Deduce the expression of voltage regulation.
Q.12.What is an electromechanical energy conversion device? Explain the working of a
generator with the help of a power flow diagram.
Q.13.Explain how torque is produced in a rotating electrical machine. Whatn do yau understand
by torque angle.
Q.14.Derive the expression for generated emf in a d.c. generator.Dfine all symbols with their
units.
Q15.Discuss the effect of armature reaction in a d.c. generator.
Q.16.What is meant by back emf ? Is the back emf greater or lessr than the applied voltage ? By
what amount the two voltage differ ?
Q.17.Discuss the flux control method for the speed of a shunt motor.
Q.18.Develop suitable equations for D.C. shunt motor for speed-current, torque-current and
speed-torque characteristics and draw characteristics.
Q.19.A 250 V, D.C. shunt motor has an armature resistance of 0.5 Ω and a fixed resistance of
250 Ω. When driving a constant torque load at 600 rpm, the motor draws 21 A of current.
What will be speed of the motor if an additional 250 Ω resistance is inserted in the field
circuit ?
Q.20.Develop the circuit model of a D.C. machine and explain ihe generating mode.
Q.21.A 6-pole, lap wound d.c. motor takes 340 A when the speed is 400 rpm. The flux per pole
is 0.05 Wb and the armature has 864 turns. Neglecting machine losses, calculate the brake
horse power of the motor.
Q.22.In a 50 KW, 230 V on no load and 250 V on full load over compound D.C. generator (long
shunt), the flux per pole required to produce 230 V on no load at 1050 rpm is 0.06 Wb. The
resistance of armature and series field are 0.04 Ω and 0.01 Ω respectively and the shunt
field resistance is 100 Ω. Calculate the value of the flux per pole at full load speed 1000
rpm. Neglect brush drop.
Q.23.A d.c. series motor runs at 500 rpm drawing 40 A from 600 V supply. Determine the value
of the external resistance to be added in series with the armature for the motor to run at 450
rpm. The load torque varies as the square of the speed. Assume liner magnetisation and
take armature resistance as 0.3 Ω and series field resistance 0.2 Ω.
Q.24.Draw the torque-speed characteristics of polyphase induction motor and clearly indicate the
effect of change in resistance.
Q.25.Explain the terms slip, slip frequency, wound rotor and cage rotor.
Q26.Discuss the point of similarities between a transformer and induction motor. Why an
induction motor on no load operates at a very low power factor.
Q27.A 25 H.P. 400 V, 50 Hz 4-pole star connected induction motor has the following
impedances per phase in ohms referred to the stator side:
R = 0.6410 Ω, Rr = 0.332 Ω, Xs = 1.106 Ω
X = 0.464 Ω, and Xm = 26.3 Ω.
The rotational losses are 14 KW (constant) and core losses are assumed negligible. If the
slip is 2.2% at rated voltage and frequency, find speed, stator current, power factor, output
and input power and efficiency of the motor.
Q.28.Explain the two field revolving theory for a single phase induction motor. Draw its
equivalent circuit diagram.
Q.29.A 230 V, 380 W, 50 Hz, 4-pole single phase induction motor gave the following test
results:
No-load test: 230 V, 84 W, 2.8 A
Blocked rotor test: 110 V, 460 W, 6.2 A
The stator winding resistance is 4.6 Ω and during blocked rotor test, the auxiliary winding
is open. Determine the equivalent circuit parameters.
Q.30.A 3-ø induction motor with r2/x2 = 0.5 has a starting torque of 25.0 Nm. For negligible
stator impedance and no load current, determine starting torque in case rotor circuit
resistance per phase is (i) doubled, (ii) halved.
Q.31.Why starting is required for induction motor? Compare stator resistance and
Autotransformer starting methods. Develop suitable equations.
Q.32.Draw the phasor diagram of a three phase induction motor loading with lagging p.f. load.
Q.33.From the equivalent circuit of a polyphase induction motor, obtain the following relation:
I2st/I2 = √ [S2 + S2mt/S
2 (1+S2mt)]
Q.34.For a salient pole synchronous motor working at lagging p.f., show that
Tan δ = Ia (Xq cos θ – γa sin θ)
_____________________
Vt – Ia (Xq sin θ + γa cos θ)
Symbols have suitable meanings.
Q.35.A 433 V, 3 phase Y- connected synchronous motor has a Xs = 5 ohm/phase. For a power
output of 15 KW, find its minimum armature current, excitation voltage and power angle.
Ra is negligible.
Q.36.Calculate the r.m.s. value of the induced emf per phase of a 10-pole, 3-phase, 50 Hz
alternator with 2 slot per pole per phase and 4 conductors per slot in two layers. The coil
span is 150. The flux per pole has a fundamental component of 0.12 Wb and a 20% third
harmonics component.
Q.37.Write short notes on the following:
(a) Starting of Synchronous motor.
(b) V – curves of Synchronous motor.
UNIT –I D.C. MACHINES
Principles of d.c. machines D.C. machines are the electro mechanical energy converters which
work from a d.c. source and generate mechanical power or convert mechanical power into a d.c.
power.
Construction of d.c. machines A D.C. machine consists mainly of two part the stationary part
called stator and the rotating part called rotor. The stator consists of main poles used to produce
magnetic flux ,commutating poles or interpoles in between the main poles to avoid sparking at the
commutator but in the case of small machines sometimes the interpoles are avoided and finally the
frame or yoke which forms the supporting structure of the machine. The rotor consist of an armature
a cylindrical metallic body or core with slots in it to place armature windings or bars,a commutator
and brush gears The magnetic flux path in a motor or generator is show below and it is called the
magnetic structure of generator or motor. The major parts can be identified as, 1. Frame 2. Yoke 3.
Poles Institute of Technology Madras 4. Armature 5. Commutator and brush gear 6. Commutating
poles 7. Compensating winding 8. Other mechanical parts
Frame Frame is the stationary part of a machine on which the main poles and commutator poles are
bolted and it forms the supporting structure by connecting the frame to the bed plate. The ring shaped
body portion of the frame which makes the magnetic path for the magnetic fluxes from the main
poles and interpoles is called Yoke.
Why we use cast steel instead of cast iron for the construction of Yoke?
In early days Yoke was made up of cast iron but now it is replaced by cast steel.This is because cast
iron is saturated by a flux density of 0.8 Wb/sq.m where as saturation with cast iron steel is about 1.5
Wb/sq.m.So for the same magnetic flux density the cross section area needed for cast steel is less
than cast iron hence the weight of the machine too.If we use cast iron there may be chances of blow
holes in it while casting.so now rolled steels are developed and these have consistent magnetic and
mechanical properties.
End Shields or Bearings If the armature diameter does not exceed 35 to 45 cm then in addition to poles end shields or frame
head with bearing are attached to the frame.If the armature diameter is greater than 1m pedestral
type bearings are mounted on the machine bed plate outside the frame.These bearings could be ball
or roller type but generally plain pedestral bearings are employed.If the diameter of the armature is
large a brush holder
yoke is generally fixed to the frame.
Main poles: Solid poles of fabricated steel with separate/integral pole shoes are fastened to the frame
by means of bolts. Pole shoes are generally laminated. Sometimes pole body and pole shoe are
formed from the same laminations. The pole shoes are shaped so as to have a slightly increased air
gap at the tips. Inter-poles are small additional poles located in between the main poles. These can be
solid, or laminated just as the main poles. These are also fastened to the yoke by bolts. Sometimes the
yoke may be slotted to receive these poles. The inter poles could be of tapered section or of uniform
cross section. These are also called as commutating poles or com poles. The width of the tip of the
com pole can be about a rotor slot pitch.
Armature The armature is where the moving conductors are located. The armature is constructed by
stacking laminated sheets of silicon steel. Thickness of these lamination is kept low to reduce eddy
current losses. As the laminations carry alternating flux the choice of suitable material, insulation
coating on the laminations, stacking it etc are to be done more carefully. The core is divided into
packets to facilitate ventilation. The winding cannot be placed on the surface of the rotor due to the
mechanical forces coming on the same. Open parallel sided equally spaced slots are normally
punched in the rotor laminations. These slots house the armature winding. Large sized machines
employ a spider on which the laminations are stacked in segments. End plates are suitably shaped so
as to serve as ’Winding supporters’. Armature construction process must ensure provision of
sufficient axial and radial ducts to facilitate easy removal of heat from the armature winding. Field
windings: In the case of wound field machines (as against permanent magnet excited machines) the
field winding takes the form of a concentric coil wound around the main poles. These carry the
excitation current and produce the main field in the machine. Thus the poles are created
electromagnetically. Two types of windings are generally employed. In shunt winding large number
of turns of small section copper conductor isof Technology Madras used. The resistance of such
winding would be an order of magnitude larger than the armature winding resistance. In the case of
series winding a few turns of heavy cross section conductor is used. The resistance of such windings
is low and is comparable to armature resistance. Some machines may have both the windings on the
poles. The total ampere turns required to establish the necessary flux under the poles is calculated
from the magnetic circuit calculations. The total mmf required is divided equally between north and
south poles as the poles are produced in pairs. The mmf required to be shared between shunt and
series windings are apportioned as per the design requirements. As these work on the same magnetic
system they are in the form of concentric coils. Mmf ’per pole’ is normally used in these calculations.
Armature winding As mentioned earlier, if the armature coils are wound on the surface of the
armature, such construction becomes mechanically weak. The conductors may fly away when the
armature starts rotating. Hence the armature windings are in general pre-formed, taped and lowered
into the open slots on the armature. In the case of small machines, they can be hand wound. The coils
are prevented from flying out due to the centrifugal forces by means of bands of steel wire on the
surface of the rotor in small groves cut into it. In the case of large machines slot wedges are
additionally used to restrain the coils from flying away. The end portion of the windings are taped at
the free end and bound to the winding carrier ring of the armature at the commutator end. The
armature must be dynamically balanced to reduce the centrifugal forces at the operating speeds.
Compensating winding One may find a bar winding housed in the slots on the pole shoes. This is
mostly found in d.c. machines of very large rating. Such winding is called compensating winding. In
smaller machines, they may be absent.
Commutator: Commutator is the key element which made the d.c. machine of the present day
possible. It consists of copper segments tightly fastened together with mica/micanite insulating
separators on an insulated base. The whole commutator forms a rigid and solid assembly of insulated
copper strips and can rotate at high speeds. Each com- mutator segment is provided with a ’riser’
where the ends of the armature coils get connected. The surface of the commutator is machined and
surface is made concentric with the shaft and the current collecting brushes rest on the same. Under-
cutting the mica insulators that are between these commutator segments has to be done periodi- cally
to avoid fouling of the surface of the commutator by mica when the commutator gets worn out. Some
details of the construction of the commutator are seen in Fig. 8.
Brush and brush holders: Brushes rest on the surface of the commutator. Normally electro-graphite
is used as brush material. The actual composition of the brush depends on the peripheral speed of the
commutator and the working voltage. The hardness of the graphite brush is selected to be lower than
that of the commutator. When the brush wears out the graphite works as a solid lubricant reducing
frictional coefficient. More number of relatively smaller width brushes are preferred in place of large
broad brushes. The brush holders provide slots for the brushes to be placed. The connection Brush
holder with a Brush and Positioning of the brush on the commutator from the brush is taken out by
means of flexible pigtail. The brushes are kept pressed on the commutator with the help of springs.
This is to ensure proper contact between the brushes and the commutator even under high speeds of
operation. Jumping of brushes must be avoided to ensure arc free current collection and to keep the
brushcontact drop low. Other mechanical parts End covers, fan and shaft bearings form other
important me- chanical parts. End covers are completely solid or have opening for ventilation. They
support the bearings which are on the shaft. Proper machining is to be ensured for easy assembly.
Fans can be external or internal. In most machines the fan is on the non-commutator end sucking the
air from the commutator end and throwing the same out. Adequate quantity of hot air removal has to
be ensured.
Bearings Small machines employ ball bearings at both ends. For larger machines roller bearings are
used especially at the driving end. The bearings are mounted press-fit on the shaft. They are housed
inside the end shield in such a manner that it is not necessary to remove the bearings from the shaft
for dismantling. Generator E.M.F Equation Let Φ = flux/pole in weber Z = total number of armture conductors = No.of slots x No.of
conductors/slot P = No.of generator poles A = No.of parallel paths in armature N = armature rotation
in revolutions per minute (r.p.m) E = e.m.f induced in any parallel path in armature Generated e.m.f
Eg = e.m.f generated in any one of the parallel paths i.e E. Average e.m.f geneated /conductor =
dΦ/dt volt (n=1) Now, flux cut/conductor in one revolution dΦ = ΦP Wb No.of revolutions/second
= N/60 Time for one revolution, dt = 60/N second Hence, according to Faraday's Laws of
Electroagnetic Induction, E.M.F generated/conductor is For a simplex wave-wound generator
No.of parallel paths = 2 No.of conductors (in series) in one path = Z/2 E.M.F. generated/path is For
a simplex lap-wound generator No.of parallel paths = P No.of conductors (in series) in one path =
Z/P E.M.F.generated/path In general generated e.m.f where A = 2 - for simplex wave-winding A = P
- for simplex lap-winding METHODS OF EXCITATION: Various methods of excitation of the field windings are shown in Fig.
Figure shows Field-circuit connections of dc machines: (a) separate excitation, (b) series, (c) shunt,
(d) compound.
Consider first dc generators.
Separately-excited generators.
Self-excited generators: series generators, shunt generators, compound generators.
o With self-excited generators, residual magnetism must be present in the machine iron to get the
self-excitation process started.
o N.B.: long- and short-shunt, cumulatively and differentially compound.
Typical steady-state volt-ampere characteristics are shown in Fig.7.5, constant-speed operation
being assumed.
The relation between the steady-state generated emf Ea and the armature terminal voltage Va is
Va=Ea−IaRa (7.10)
Figure Volt-ampere characteristics of dc generators. Any of the methods of excitation used for
generators can also be used for motors.
Typical steady-state dc-motor speed-torque characteristics are shown in Fig.7.6, in which it is
assumed that the motor terminals are supplied from a constant-voltage source.
In a motor the relation between the emf Ea generated in the armature and and the armature terminal
voltage Va is