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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) &


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) &


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) &


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


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


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




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


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.




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.




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




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?




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.




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 commutator 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 mechanical 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

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