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GUJARAT TECHNOLOGICAL UNIVERSITY

ELECTRONICS (10) / ELECTRONICS & COMMUNICATION (11)

/INSTRUMENTATION & CONTROL (17) ELECTRICAL MACHINES

SUBJECT CODE: 2131005

B.E. 3RD

SEMESTER

Type of course: Engineering Science (ELECTRICAL)/(INTERDISCIPLINARY)

Prerequisite: Fundamentals of Electrical Engineering

Rationale: NA

Teaching and Examination Scheme:

Teaching Scheme Credits Examination Marks Total

Marks L T P C Theory Marks Practical Marks

ESE

(E)

PA (M) PA (V) PA

(I) PA ALA ESE OEP

3 0 2 4 70 20 10 20 10 20 150

Contents:

Sr.

No.

Subject Content Hours Module

Weightage

1. Transformers:

Single Phase Transformer: Working principle, Construction, types, EMF

equation, Transformer on no load and on load, vector diagram, exact and

approximate equivalent circuit, O.C & S.C.test on transformer, regulation of

transformer, losses &efficiency, condition for maximum efficiency, Allday

efficiency, Efficiency curve, Sumpner’s test, Auto transformer, Saving of

conductor material, Parallel operation, Conditions, Parallel with equal and unequal

voltage ratio.

3 Phase transformers: Construction, connections, Scott connection, V-V

Connection, Instrument transformers, Current transformers and potential

transformers.

8 18

2. Induction Machines:

3 Phase induction motor: Construction, types, rotating magnetic field, principle

of operation, slip, frequency of rotor current, rotor emf, rotor current, expression

for torque, conditions for maximum torque, torque slip characteristics, starting

torque in squirrel cage and slip ring motors, effect of change in supply voltage on

torque, slip and speed , relation between full load torque and maximum torque,

Power stages in induction motor, vector diagram and equivalent circuit, circle

diagram, construction and calculation, speed control of 3 phase motor, starting

methods for 3 phase induction motor.

Single phase motor:

10 22

Double revolving field theory, starting methods, no load and block rotor test,

equivalent circuit, types of single phase motor

3. Synchronous Machine: Alternator,Basic principle, construction, pitch factor,

distribution factor, emf equation, alternator on load, voltage regulation,

synchronous impedance method, mmf method, ZPF method, parallel operation,

synchronization of alternator.

Synchronous motor:

Basic principle, methods of starting, application.

10 20

4. DC Machines:

DC Generator: Construction features, emf equation of dc generator, methods of

excitation, losses condition for maximum efficiency, armature reaction, interpoles

and compensating winding, commutation, methods of improving commutation,

characteristics of separately excited and self excited dc generator.

DC Motor: Working principle, voltage equation, condition for maximum power,

characteristics, operating characteristics of dc motor, torque developed, starting ,3

point and 4 point starter, speed control methods, swinburn’s and break test of dc

shunt motor.

10 20

5 Basic of Power transmission and distribution

Operation of different power plants using block diagram-different terminologies

like load factor, diversity factor, plant utilization factors etc.

Classification of transmission lines, transmission line parameters, ABCD

constants, Voltage regulation, Ferranti effect, efficiency of transmission line.

3-phase 3-wire and 3-phase 4-wire distribution system, feeders, distributors, main

lines, comparison of various distribution systems, load power factor improvement

techniques.

10 20

Total 48Hrs 100

Reference Books:

1. Text of Electrical Technology;Vol -2; B. L. Theraja, and A. K. Theraja; S. Chand Publication

2. Electrical machines BY AshfaqHussain; Dhanpatrai and Co.

3. Principles of Electrical power systems by J. B. Gupta

4. Generalised theory of rotating machines By P S Bhimra

Course Outcome:

After learning the course the students should be able to :

- Understand working principle, performance, control and applications of AC, DC Machines and

Transformer.

- Carry out basic experiments on AC, DC machine and Transformer.

- Identify, formulate and solve AC, DC machine and Transformer related problems.

- Understand the working and performance analysis of Transmission and Distribution of power.

List of Experiments:

1. To operate two single phase transformers of different KVA ratings in parallel and plot the variation

of currents shared by each transformer versus load current.

2. To perform Open Circuit and Short circuit Test on a transformer and find its efficient and

regulation.

3. Speed control of DC Shunt Motor using a) Armature control and b) field control methods

4. To obtain Magnetizing Characteristics, Internal & External Characteristic of Self Excited DC Shunt

Generator. Also obtain the critical filed resistance of the machine from magnetizing Characteristics.

5. To conduct direct load test on a D.C. compound generator with a) Shunt field alone b) Cumulative

and differential compounding for short and long shunt connections.

6. To obtain Speed-Torque characteristics of DC Series Motor

7. To obtain Speed-Torque characteristics of DC Shunt Motor.

8. To study different starters of D. C. motor.

9. To study different starters of three phase induction motor

10. To perform No load and Block rotor test on induction motor and plot equivalent circuit

11. To Study the effect of Inserting resistance on rotor of Slip ring induction motor.

12. To draw the V curves for synchronous machine

13. To find the voltage regulation of synchronous machine

14. To study capacitor start and capacitor run induction motor

15. Find Voltage regulation and efficiency of Medium transmission line

16. Find Voltage regulation and efficiency of Long transmission line

17. To study various power factor improvement methods

18. To study the block diagram of various power plants

Design based Problems (DP)/Open Ended Problem:

1. To find the applications of various single phase/ three AC and DC motors

2. Design capacitor bank to improve the load power factor

3. Design electrical power distribution system to achieve priscribed voltage regulation

Major Equipments:

Transformer, Single phase and three phase induction motor, synchronous machine,

List of Open Source Software/learning website:

Web-based tools for design:

- http://www.fairchildsemi.com/support/design-tools/power-supply-webdesigner/

- http://www.ti.com/lsds/ti/analog/webench/overview.page

Circuit Lab:

- https://www.circuitlab.com/editor/

Open source Math Tools:

- http://maxima.sourceforge.net/

- http://www.sagemath.org/

- http://www.scilab.org/

- http://www.gnu.org/software/octave/

Learning website

- http://www.electrical-engineering-portal.com/

- http://nptel.iitm.ac.in/courses.php

ACTIVE LEARNING ASSIGNMENTS: Preparation of power-point slides, which include videos,

animations, pictures, graphics for better understanding theory and practical work – The faculty will

allocate chapters/ parts of chapters to groups of students so that the entire syllabus to be covered. The

power-point slides should be put up on the web-site of the College/ Institute, along with the names of

the students of the group, the name of the faculty, Department and College on the first slide. The best

three works should submit to GTU.

1

Seat No.: ________ Enrolment No.___________

GUJARAT TECHNOLOGICAL UNIVERSITY BE - SEMESTER–III (NEW) - EXAMINATION – SUMMER 2017

Subject Code: 2131005 Date: 09/06/2017 Subject Name: Electrical Machines Time: 10:30 AM to 01:00 PM Total Marks: 70 Instructions:

1. Attempt all questions. 2. Make suitable assumptions wherever necessary. 3. Figures to the right indicate full marks.

MARKS

Q.1 (a) State whether the following statements are true or false 5

1 In an ideal transformer iron losses are zero.

2 The induced e.m.f. in a DC machine is directly proportional to speed.

3 A 3-phase induction motor has no starting torque.

4 The field winding of an alternator is on rotor side.

5 A capacitor starts capacitor run 1-phase induction motor uses two

capacitors.

Q.1 (b) Choose an appropriate option from the following 9

1 In DC generator commutator act as

(a) Rectifier (b) Inverter (c) Current booster (d) Transformer

2 The main purpose of performing short circuit test on transformer is to

measure its

(a) Cu loss (b) Total loss (c) Core loss (d) insulation resistance

3 The number of parallel path in wave winding are ______

(a) 2 (b) 4 (c) 6 (d) No. of pole

4 Friction and windage losses in transformer is ______

(a) 0 % (b) 20% (c) 90% (d) 50%

5 For a step up transformer number of turn in primary winding is _______

than secondary winding.

(a) less (b) more (c) Equal (d) none of the above

6 The method of putting an alternator in parallel with bus bars is called __

(a) Synchronizing (b) Short circuit (c) slip test (d) none of the above

7 A transformer ______________

(a) Converts AC to DC (b) Steps up or down DC Voltages & Current

(c) Converts DC to AC (d) Steps up or down AC Voltages & Current

8 Core is laminated to reduce ________

(a) Cu loss (b) Efficiency (c) Voltage regulation (d) Eddy current loss

9 Which DC motor is more suitable in electric traction system

(a) Shunt (b) Separately excited (c) Compound (d) Series

Q.2 (a) Derive EMF equation of DC generator 03

(b) Explain hysteresis loss and eddy current loss in DC machine. 04

(c) Explain different parts of DC machine with neat diagram. 07

OR

(c) Discuss the principle of operation and construction of 1-phase

transformer 07

Q.3 (a) Develop the condition for maximum efficiency of transformer. 03

(b) A 6-pole lap wound DC generator has 600 conductors on its armature. 04

2

The flux per pole is 0.02 Weber. Calculate (i) the speed at which the

generator must be run to generate 300 V. (ii) What would be the speed if

the generator were wave wound?

(c) Write technical note on open circuit test and short circuit test of 1-phase

transformer. 07

OR

Q.3 (a) Why cylindrical rotor alternators have small diameter and large length of

core? 03

(b) Explain working of capacitor-start capacitor-run 1-phase induction

motor 04

(c) What is the necessity of starter in DC motor? Explain 3-point starter

with neat diagram.. 07

Q.4 (a) “An induction motor may be considered to be a transformer with a

rotating short circuited secondary.” Justify this statement. 03

(b) Draw and explain power stages of an induction motor. 04

(c) Discuss starting of 3-phase induction motors? Explain any one method

of starting in detail. 07

OR

Q.4 (a) What do you mean by synchronous speed of induction motor? Why can

not 3-phase induction motor run at synchronous speed? 03

(b) Derive equation of starting torque of 3-phase induction motor. 04

(c) What is slip? Explain torque-slip characteristic of induction motor. 07

Q.5 (a) What is pitch factor? It is always less than or more than 1? 03

(b) Discuss the main constructional features of cylindrical rotor and salient

pole alternator. 04

(c) What is power factor? How can we improve power factor? 07

OR

Q.5 (a) Draw block diagram of Thermal Power Station 03

(b) Briefly explain Ferranti effect. 04

(c) What is voltage regulation? Explain any one method to find voltage

regulation of alternator. 07

*************

1


GUJARAT TECHNOLOGICAL UNIVERSITY BE - SEMESTER–III(New) • EXAMINATION – WINTER 2016

Subject Code:2131005 Date:09/01/2017

Subject Name:Electrical Machines

Time: 10:30 AM to 01:00 PM Total Marks: 70 Instructions:

1. Attempt all questions.

2. Make suitable assumptions wherever necessary.

3. Figures to the right indicate full marks.

Q.1 Choose the correct answer for the following multiple choice questions. 14

1 With increase in load of a transformer, core flux will …

(a) increase. (b) decrease.

(c) remain constant. (d) depends on type of load.

2 What is the current drawn by an actual transformer, when primary is

supplied rated voltage and secondary is kept open.

(a) No-load current. (b) Full-load current.

(c) Zero current. (d) Magnetizing current.

3 The maximum e.m.f. will induce in the rotor of a 3-phase induction motor,

when it …

(a) is blocked. (b) runs at synchronous speed.

(c) runs at no-load. (d) runs at full-load.

4 What is the purpose of starting winding in 1-phase induction motor?

(a) To limit temperature rise of the machine.

(b) To reduce losses.

(c) To produce rotating magnetic field in conjunction with main winding.

(d) Both (a) and (b).

5 When 3-phase a.c. supply of 50 Hz is given to stator of a 4-pole, 3-phase

induction motor, in standstill condition the rotor frequency will be ...

(a) 25 Hz. (b) 50 Hz.

(c) 0 Hz. (d) 12.5 Hz.

6 The current from the stator of an alternator is taken out to the external load

circuit through …

(a) carbon brushes. (b) solid connections.

(c) slip rings. (d) commutator segments.

7 Which type of alternator is used in hydro-electric power stations?

(a) Salient pole alternator. (b) Non-salient pole alternator.

(c) Turbo generator. (d) Steam turbine alternator.

8 While starting a synchronous motor its field winding should be ….

(a) kept open. (b) connected to a d.c. source.

(c) short-circuited. (d) connected to a a.c. source.

9 In a d.c. generator, the effect of armature reaction on the main flux is to …

(a) reduce it. (b) distort it.

(c) both, reduce and distort it. (d) reverse it.

10 In d.c. generators, the polarity of interpoles is ...

(a) same as that of the main pole behind.

(b) same as that of the main pole ahead.

(c) opposite to that of the main pole ahead.

(d) None of the above.

2

11 What is the function of brush and commutator in a d.c. motor.

(a) To convert a.c. into d.c.

(b) To reduce demagnetizing effect of armature reaction.

(c) To produce unidirectional current in the armature.

(d) To produce unidirectional torque.

12 The economisers are used to heat …

(a) air. (b) feed water.

(c) steam. (d) None of the above.

13 The skin effect in stranded conductor compared to solid conductor is ...

(a) less. (b) more.

(c) equal. (d) None of the above.

14 The main consideration in the design of a feeder is the ...

(a) current carrying capacity. (b) voltage drop.

(c) power handling capacity. (d) None of the above.

Q.2 (a) Draw a complete phasor diagram for a step-down transformer when the

load power factor is lagging. 03

(b) Draw the torque-speed characteristics of 3-phase induction motor and

explain the effect of change in rotor resistance. 04

(c) Explain the back-to-back test with necessary diagram for separation of

losses in two identical 1-phase transformers. 07

OR

(c) A 746 kW, 3-phase 50 Hz, 16-pole induction motor has a rotor impedance

of (0.02 + j0.15) Ω at standstill. Full-load torque is obtained at 360 RPM.

Calculate (i) the ratio of maximum to full-load torque, (ii) the speed for

maximum torque and (iii) the rotor resistance to be added to get maximum

starting torque.

07

Q.3 (a) Explain power stages in 3-phase induction motor. 03

(b) Derive e.m.f. equation of an alternator. 04

(c) Describe, with neat sketches, the constructional details of salient pole and

non-salient pole type of rotors in an alternator. 07

OR

Q.3 (a) State the advantages of stationary armature in an alternator. 03

(b) Explain diversity factor and plant utilization factor. 04

(c) Explain the synchronous impedance method of voltage regulation in an

alternator with neat diagrams. 07

Q.4 (a) Derive e.m.f. equation of a d.c. generator. 03

(b) Explain commutator action in d.c. generator with necessary diagrams. 04

(c) A d.c. shunt generator supplies 96 A at a terminal voltage of 200 V. The

armature and shunt field resistances are 0.1 Ω and 50 Ω respectively. The

iron and frictional losses are 2500 W. Find (i) generated e.m.f., (ii) copper

losses and (iii) commercial efficiency.

07

OR

Q.4 (a) Explain critical resistance for a d.c. series generator. 03

(b) Explain the characteristics of d.c. shunt motor. 04

(c) What is the necessity of d.c. motor starter? Explain three-point starter. 07

Q.5 (a) Define power factor. What is the need of improving power factor? 03

(b) What is Ferranti effect? How is it effect the voltage of transmission line? 04

(c) Derive the equitation of starting torque and running torque of 3-phase

induction motor. 07

OR

Q.5 (a) Why 1-phase induction motor is not self-starting? 03

(b) Explain the types of d.c. distributors. 04

(c) A 3-phase induction motor has a 4-pole star-connected stator winding. The

motor runs on a 50 Hz supply with 200 V between lines. The rotor 07

3

resistance and standstill reactance per phase are 0.1 Ω and 0.9 Ω

respectively. The ratio of rotor to stator turns is 0.67. Calculate (i) torque at

4 % slip, (ii) maximum torque and (iii) speed at maximum torque.

*************

1



BE - SEMESTER–III(New) EXAMINATION – SUMMER 2016


Subject Name:Electrical Machines Time:10:30 AM to 01:00 PM Total Marks: 70 Instructions:


2. Make suitable assumptions wherever necessary. 3. Figures to the right indicate full marks.

MARKS

Q.1 Answer the following Questions 14

1 Which winding of a step up transformer has more number of turns

(a) low voltage winding (b) high voltage winding

(c) tertiary winding (d) either (a) or (b)

2 Which side is short-circuited while performing short-circuit test on a

transformer

(a) L.V. side (b) H.V. side (c) star point (d) primary side

3 Just at start of a three-phase induction motor, the slip is

(a) zero (b) infinite (c) one (d) between zero and one

4 The speed regulation of a synchronous motor is

(a) 100 % (b) 50 % (c) 1 % (d) 0 %

5 Starter is connected on rotor side in

(a) synchronous motor (b) squirrel cage induction motor

(c) slip ring induction motor (d) none of the above

6 The output voltage of a simple DC generator is

(a) pure DC (b) pulsating DC

(c) AC sinusoidal (d) AC square wave

7 The frequency of rotor current in induction motor is given by relation

(a) fr = pn/120 (b) fr = 120f/p (c) fr = sf (d) fr = f/s

8 The function of pole shoe in DC machine is

(a) to spread out the flux in air gap (b) to reduce reluctance

(c) to support field coils (d) all of the above

9 A line which connects distributor to substation is called

(a) distributor (b) service main (c) feeder (d) none of these

10 Define load factor.

11 Define power factor.

12 What is Ferranti effect ?

13 What is armature reaction ?

14 What is synchronous condenser ?

Q.2 (a) Explain working principle of single phase transformer. 03

(b) State conditions for parallel operation of two transformer. 04

(c) Explain V–V connection for 3-phase transformer. 07

OR

(c) Explain open circuit test on single phase transformer with diagram. 07

Q.3 (a) A 440V, 3-phase, 50Hz, 6-pole induction motor has 0.04 Ω rotor

resistance/phase. Its maximum torque occurs at 860 r.p.m. Find the

developed torque as a percentage of maximum torque at starting.

03

2

(b) Explain different starting methods for single phase induction motor. 04

(c) Explain rotating magnetic field produced in 3-phase induction motor

with neat diagram. 07

OR

Q.3 (a) Derive EMF equation of single phase transformer. 03

(b) Enlist different starters used for 3-phase induction motor. Explain any

one in detail. 04

(c) Draw and explain torque-slip characteristic of induction motor. 07

Q.4 (a) Write function of super heater, economizer and air-pre heater. 03

(b) What is diversity factor and plant utilization factor ? 04

(c) Explain 3-phase 3-wire and 3-phase 4-wire distribution system. 07 OR

Q.4 (a) Explain classification of transmission line. 03

(b) What is pitch factor and distribution factor in alternator ? 04

(c) Explain synchronous impedance method for finding voltage regulation

of alternator. 07

Q.5 (a) Explain different types of DC generator. 03

(b) A long shunt compound generator delivers a load current of 20A at

250V. It has armature, series field and shunt field resistances are 0.15

Ω, 0.05 Ω and 250 Ω respectively. Find generated voltage. Neglect

brush voltage drop.

04

(c) Define commutation and explain methods of improving it. 07

OR

Q.5 (a) Explain working principle of DC motor. 03

(b) Explain different starting methods for synchronous motor. 04

(c) Explain speed control methods for DC shunt motor. 07

*************

1


GUJARAT TECHNOLOGICAL UNIVERSITY BE - SEMESTER–III (New) EXAMINATION – WINTER 2015


Subject Name: Electrical Machines

Time: 2:30pm to 5:00pm Total Marks: 70 Instructions:




MARKS

Q.1 Answer the following : 14

1 A transformer having 1000 primary turns is connected to a 250-V a.c.

supply. For a secondary voltage of 400 V, the number of secondary

turns should be

(a) 1600 (b) 250 (c) 400 (d) 1250

2 Before removing the ammeter from a current transformer, its secondary

must be short circuited in order to avoid

(a) excessive heating of the core (b) high secondary e.m.f.

(c) increase in iron losses (d) all of the above.

3 The principle of operation of a 3-phase. Induction

motor is most similar to that of a

(a) synchronous motor

(b) repulsion-start induction motor

(c) transformer with a shorted secondary

(d) capacitor-start, induction-run motor.

4 One of the characteristics of a single- phase

motor is that it

(a) is self-starting (b) is not self-starting

(c) requires only one winding (d) can rotate in one direction only.

5 The frequency of voltage generated by an alternator

having 4-poles and rotating at 1800 r.p.m.

is .......hertz.

(a) 60 (b) 7200 (c) 120 (d) 450

6 In a d.c. generator, the generated e.m.f. is directly proportional to the

(a) field current (b) pole flux

(c) number of armature parallel paths (d) number of dummy coils

7 Overall efficiency of Thermal Power Plant is

(a) 45 (b) 25 (c) 55 (d) 85

8 What do you understand by step up and step down transformers ?

9 What is meant by single Phasing?

10 Why is a single Phase Induction Motor not self starting?

11 State the advantages of Synchronous Motors.

12 What are the remedies of Armature Reaction ?

13 What is load factor?

14 What is Ferranti effect?

2

Q.2 (a) (i) A 25-kVA transformer has 500 turns on the primary and 50 turns on

the secondary winding. The primary is connected to 3000-V, 50-Hz

supply. Find the full-load primary and secondary currents, the secondary

e.m.f. and the maximum flux in the core. Neglect leakage drops and no-

load primary current.

03

(b) (ii) Explain how 3-phase supply can be converted into 2-phase supply

using scott connection?

04

(c) Derive EMF equation of single phase transformer. Which parameters

are responsible for variation in output voltage?

07

OR

(c) What is the constructional difference between core type and shell type

transformer? What are the merits and demerits of each? 07

Q.3 (a) What is basic principle and construction of synchronous motor? 03

(b) Write short note on O.C test and S.C. test on transformer. 04

(c) What is autotransformer? Derive its equation for saving in copper. 07

OR

Q.3 (a) A 3-φ induction motor is wound for 4 poles and is supplied from 50-Hz

system. Calculate (i) the synchronous speed (ii) the rotor speed, when

slip is 4% and (iii) rotor frequency when rotor runs at 600 rpm.

03

(b) Explain cogging and crawling in a 3 phase induction motor with their

remedies.

04

(c) What is slip in induction motor? Explain torque-slip curve of induction

motor.

07

Q.4 (a) Discuss the conditions to be satisfied before a 3- phase alternator is

synchronized with infinite bus.

03

(b) What is pitch factor and distribution factor in alternator ? 04

(c) Explain ZPF method for finding regulation of alternator. 07 OR

Q.4 (a) An 8-pole d.c. shunt generator with 778 wave-connected armature

conductors and running at 500 r.p.m. supplies a load of 12.5 Ω

resistance at terminal voltage of 50 V. The armature resistance is 0.24Ω

and the field resistance is 250 Ω. Find the armature current, the induced

e.m.f. and the flux per pole.

03

(b) Explain armature reaction of the d.c. machine. 04

(c) Derive the e.m.f. equation of D.C. generator. 07

Q.5 (a) A d.c. motor takes an armature current of 110 A at 480 V. The armature

circuit resistance is 0.2 Ω. The machine has 6-poles and the armature is

lap-connected with 864 conductors. The flux per pole is 0.05 Wb.

Calculate (i), the speed and (ii) the gross torque developed by the

armature.

03

(b) Explain Swinburn’s test of dc shunt motor. 04

(c) Write short note on Three point starter. 07

OR

Q.5 (a) What is diversity factor and plant utilization factor ? 03

(b) Explain three phase four wire and three phase three wire system. 04

(c) What do you mean by power factor? How can we improve power

factor ? 07

*************

1


GUJARAT TECHNOLOGICAL UNIVERSITY BE - SEMESTER– III (NEW) EXAMINATION – SUMMER 2015

Subject Code: 2131005 Date: 04/06/2015

Subject Name: ELECTRICAL MACHINES

Time: 02.30pm-05.00pm Total Marks: 70 Instructions:




Q.1 (a) Explain working of single phase transformer under no load condition with the

help of vector diagram.

07

(b) What is the constructional difference between core type and shell type

transformer? What are the merits and demerits of each?

07

Q.2 (a) How can you perform open circuit test on transformer? Which parameter you

can obtain from this test?

07

(b) Why are we connecting two transformers in parallel? What are the different

conditions for parallel operation of transformer?

07

OR

(b) What is autotransformer? Derive its equation for saving in copper.

07

Q.3 (a) Explain Sumpner’s test performed on transformer.

07

(b) How can we convert three phase to two phase in transformer?

07

OR

Q.3 (a) Explain torque-slip curve of induction motor.

07

(b) What is difference between squirrel cage and slip ring induction motor?

07

Q.4 (a) What do you mean by distribution factor and pitch factor in synchronous

machine?

07

(b) Explain ZPF method to find out regulation of alternator.

07

OR

Q.4 (a) What is the basic principle and construction of synchronous motor?

07

(b) Derive the e.m.f equitation of D.C.Generator.

07

Q.5 (a) Explain different type of D.C.Generator with diagram.

07

(b) Explain different method of speed control of D.C Motor.

07

OR

Q.5 (a) What do you mean by power factor? How can we improve power factor?

07

(b) What is Ferranti effect? How is it effect voltage of transmission line?

07

1


GUJARAT TECHNOLOGICAL UNIVERSITY BE - SEMESTER–III • EXAMINATION – WINTER • 2014

Subject Code: 2131005 Date: 30-12-2014


Time: 02.30 pm - 05.00 pm Total Marks: 70 Instructions:




Q.1 (a) Why induction motor is self started? Explain with rotating magnetic field. 07

(b) What is construction difference between slip-ring and squirrel cage induction motor?

What are the merits of each?

07

Q.2 (a) Derive EMF equitation of single phase transformer. Which parameters are

responsible for variation in output voltage?

07

(b) Explain construction difference between core type and shell type transformer. 07

OR

(b) Explain scott connection in transformer. 07

Q.3 (a) What are the different conditions for parallel operation of two transformers? 07

(b) Explain open circuit and short circuit test on single phase transformer. 07

OR

Q.3 (a) Explain working of transformer under NO load condition with the help of vector

diagram.

07

(b) Explain equivalent circuit of single phase transformer. 07

Q.4 (a) What is slip in induction motor? Explain torque/slip characteristics of induction

motor.

07

(b) Derive equitation of starting torque and running torque of induction motor. 07

OR

Q.4 (a) What pitch factor and distribution factor in alternator? 07

(b) Explain ZPF method for finding regulation in alternator. 07

Q.5 (a) What is need of starter in DC machine? Explain construction and working of three

point starter.

07

(b) Explain different type of DC generator according to its field winding. 07

OR

Q.5 (a) What is load factor, diversity factor and plant utilization factor? 07

(b) Explain three phase four wire and three phase three wire system. Explain merits of

each.

07

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1


GUJARAT TECHNOLOGICAL UNIVERSITY BE - SEMESTER–III (OLD) - EXAMINATION – SUMMER 2017

Subject Code: 131701 Date: 07/06/2017 Subject Name: Electrical Machines Time: 10:30 AM to 01:00 PM Total Marks: 70 Instructions:


Q.1 (a) Explain the losses of transformer. How these losses are reduce? 07

(b) Explain different parts of DC machine with neat and clean diagram. 07

Q.2 (a) Explain the Swinburne’s test of a d.c. machine for finding losses with

necessary diagram.

07

(b) Explain working principle of transformer in detail and also derive E.M.F.

equation of transformer

07

OR

(b) A 4 pole d.c. shunt generator with a shunt field resistance of 100 ohm and an

armature resistance of 1 ohm has 378 wave connected conductors in its

armature. The flux per pole is 0.02 Wb. If a load resistance of 10 ohm is

connected across the armature terminals and the generator is driven at 1000

r.p.m. calculate the power absorbed by the load.

07

Q.3 (a) What is voltage regulation? How it can be determined by using Zero power

factor method in synchronous machine?

07

(b) What is cogging and crawling? Compare slip ring induction motor with

squirrel cage induction motor

07

OR

Q.3 (a) Why single phase induction motor is not self start? How it is made self start? 07

(b) What is hunting? Why damper winding use to reduce hunting? What are the

applications of synchronous motor

07

Q.4 (a) What is armature reaction? What are the methods to improve armature

reaction?

07

(b) Explain torque-slip characteristic of three phase induction motor. 07 OR

Q.4 (a) Explain the double revolving field theory for a single-phase induction motor. 07

(b) Explain construction and working principle of synchronous generator. 07

Q.5 (a) Explain 3-point starter use to start DC motor. 07

(b) A 25 KVA transformer has 500 turns on the primary & 50 turns on the

secondary winding. The primary is connected to 3000 V, 50 Hz supply. Find

the full load primary & secondary currents, the secondary e.m.f. & the

maximum flux in the core. Neglect leakage drops & no load primary current

07

OR

Q.5 (a) Explain the equivalent circuit of 1-phase transformer. 07

(b) Explain working principle of d.c. generator & also derive e.m.f. equation of

d.c. generator.

07

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1


GUJARAT TECHNOLOGICAL UNIVERSITY BE - SEMESTER–III(OLD) • EXAMINATION – WINTER 2016



Time: 10:30 AM to 01:00 PM Total Marks: 70 Instructions:




Q.1 (a) What is an IDEAL transformer? How it differs from an actual transformer? 07

(b) Explain different losses in transformer and derive condition for maximum

efficiency of transformer.

07

Q.2 (a) Explain ward Leonard method for speed control of DC shunt motor. 07

(b) On what basis DC generators are classified? Classify DC generators. 07

OR

(b) Explain synchronous impedance method for finding regulation of synchronous

alternator.

07

Q.3 (a) Explain No-load and Blocked rotor test on three phase induction motor. 07

(b) Explain power stages of induction motor. 07

OR

Q.3 (a) Explain different parts of DC machine. 07

(b) Explain Parallel operation of two three phase transformers. 07

Q.4 (a) A 30KVA, 2400/120-V, 50Hz transformer has high voltage winding resistance

of 0.1 ohm and a leakage reactance of 0.22 ohm. The low voltage winding

resistance is 0.035 ohm and the leakage reactance is 0.012ohm. Find the

equivalent winding resistance, reactance and impedance referred to the (1) high

voltage side (2) low voltage side.

07

(b) Explain SCOTT connection. 07

OR

Q.4 (a) For an induction motor establish relation between rotor input, rotor copper loss

and rotor output.

07

(b) Explain speed torque characteristics of three phase induction motor. 07

Q.5 (a) A short shunt compound wound DC generator delivers a load current of 28 A at

250 V. If the resistances of shunt field, series field and armature winding is 125

Ω, 0.25 Ω and 0.4 Ω respectively. Find the generated EMF. Consider 1 V/brush

for brush contact drop.

07

(b) Draw and Explain different characteristics of self-excited compound generator. 07

OR

Q.5 (a) Explain parallel operation of two alternators. 07

(b) Draw and explain vector diagram of loaded alternator with different power

factors.

07

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1



BE - SEMESTER–III EXAMINATION – SUMMER 2016



Time:10:30 AM to 01:00 PM Total Marks: 70

Instructions: 1. Attempt all questions.



Q.1 (a) Derive an equation for induce EMF of transformer. Also explain transformation

ratio.

07

(b) Explain parallel operation of two single phase transformers with necessary

conditions.

07

Q.2 (a) List different methods for speed control of DC shunt motor. Explain any one in

detail.

07

(b) Explain armature Reaction. 07

OR

(b) Explain synchronous impedance method for finding regulation of synchronous

alternator.

07

Q.3 (a) Explain No-load and Blocked rotor test on three phase induction motor. 07

(b) Explain Equivalent circuit of three phase induction motor. 07

OR

Q.3 (a) Explain Swinburne’s method for testing DC machine. 07

(b) Derive condition for maximum efficiency of transformer. 07

Q.4 (a) A 50KVA, 4400/220-V, 50Hz transformer has high voltage winding resistance

of 3.45 ohm and a leakage reactance of 5.2 ohm. The low voltage winding

resistance is 0.009 ohm and the leakage reactance is 0.015ohm. Find the

equivalent winding resistance, reactance and impedance referred to the (1) high

voltage side (2) low voltage side.

07

(b) Explain SCOTT connection. 07 OR

Q.4 (a) Explain power stages of induction motor. 07

(b) Derive an equation for torque under running condition for an induction motor. 07

Q.5 (a) A long shunt compound wound DC generator delivers a load current of 18 A at

250 V. If the resistances of shunt field, series field and armature field is 125 Ω,

0.2 Ω and 0.3 Ω respectively. Find the generated EMF. Consider 1 V/brush for

brush contact drop.

07

(b) Draw and Explain different characteristics of self excited shunt generator. 07

OR

Q.5 (a) Explain parallel operation of two alternators. 07

(b) Explain Chording factor and winding factor. 07

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1


GUJARAT TECHNOLOGICAL UNIVERSITY BE - SEMESTER–V EXAMINATION – WINTER 2015

Subject Code: 150802 Date:05/12/2015


Time: 10:30am to 1:00pm Total Marks: 70 Instructions:




Q.1 (a) Explain with the help of connection and phasor diagram how to obtain 2-phase

supply from 3-phase supply mains using scott connection.

07

(b) With net circuit diagram explain swinburne’s test and write the equations of

efficiency of D.C. motor and D.C. generator also give advantages and

disadvantages of swinburne’s test.

07

Q.2 (a) Explain construction and working of switched reluctance motor. Give applications

of it. Compare variable reluctance motor and switched reluctance motor.

07

(b) The hopkinson’s test on two shunt machines gave the following results for full-

load: line voltage = 250 V, current taken from supply system excluding field

currents = 50 A, motor armature current = 380 A, generator field current is 5 A

and motor field current is 4.2 A. calculate efficiency of machine working as a

generator. Armature resistance of each machine is 0.2 Ω.

07

OR

(b) What is commutation? Explain different methods to improve commutation. Also

explain function of equalizing connection.

07

Q.3 (a) Enlist speed control methods of 3-phase induction motor. Explain rotor rheostat

control with net diagram.

07

(b) Why single phase induction motor is not self-started? Explain double field

revolving theory.

07

OR

Q.3 (a) What is the need of starter in 3-phase induction motor? Explain star-delta starting

of 3-phase induction motor.

07

(b) With net diagram explain working of single value capacitor start and run motor.

Also give advantages of capacitor start and run single phase induction motor.

07

Q.4 (a) Draw the no-load and short circuit diagram for a 14.92 kW, 400 V, 50 Hz, 3-

phase star-connected induction motor from the following data(line values):

No load test: 400 V, 9 A, cosΦ = 0.2

Short Circuit test: 200 V, 50 A, cosΦ = 0.4

Rotor Cu loss at standstill is half the total Cu loss.

From the diagram find (a) the line current and power factor at full load, and (b)

the maximum output power.

10

(b) Draw and explain equivalent circuit of single phase induction motor without core

loss.

04

OR

2

Q.4 (a) A 20 h.p., 400 V, 950 r.p.m, 3-phase, 50 Hz, 6-pole cage motor with 400 V

applied takes 6 times full-load current at standstill and develops 1.8 times full-load

running torque. The full-load current is 30 A. Determine,

(a) What voltage must be applied to produce full-load torque at starting?

(b) What current will this voltage will produce?

(c) If voltage is obtained by an auto transformer, what will be line current?

(d) If starting current is limited to full load current by an auto transformer, what

will be the starting torque as a percentage of full load torque?

Ignore the magnetizing current and stator impedance drops.

07

(b) Explain construction working of shaded pole single phase induction motor. 07

Q.5 (a) Explain “V curve” and “invert V curve” of synchronous motor. 07

(b) Explain construction and 1-phase ON & 2-phase ON mode of operation of

variable reluctance stepper motor.

07

OR

Q.5 (a) Give comparison between synchronous motor and induction motor 07

(b) Explain construction and working of permanent magnet stepping motor with net

diagram.

07

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1


GUJARAT TECHNOLOGICAL UNIVERSITY BE - SEMESTER–V • EXAMINATION – SUMMER • 2015

Subject Code: 150802 Date:07/05/2015


Time: 02.30pm-05.00pm Total Marks: 70 Instructions:




Q.1 (a) Describe Hopkinson’s test for computing the efficiency and its advantages and

disadvantages.

07

(b) What is armature reaction? Describe the phenomena of armature reaction. 07

Q.2 (a) Draw the developed diagram of progressive lap winding for 4 poles and 12 slots

armature with two coil sides per slot. 07

(b) What is mean by tertiary winding? Write applications of tertiary winding. 07

OR

(b) What are the conditions of parallel operation of three phase transformers? 07

Q.3 (a) Explain the Scott connection of transformers with diagram and mark the

terminals and turn-ratio.

07

(b) Explain crawling and cogging phenomena for three phase induction motor. 07

OR

Q.3 (a) Describe in detail any two methods of controlling speed of an Induction motor. 07

(b) Draw the circle diagram of a 20 hp, 400 V, 50 Hz, 3-Ø star-connected induction

motor from the following test data(line values)

No Load test =400 V 9 A p.f.=0.2

SC test =200 V 50 A p.f.=0.4

The stator and rotor copper losses are divided equally in the SC test. From the

circle diagram find (1) line current and power factor at full load (2) maximum

power output.

07

Q.4 (a) Explain power developed by a synchronous motor with neat diagram. 07

(b) Compare Synchronous motors and Induction motors. 07 OR

Q.4 (a) Obtain V-curves with diagram for synchronous motors. What is its

significance? 07

(b) Explain in detail capacitor start and capacitor run single phase induction motor. 07

Q.5 (a) Explain double field revolving theory for single phase induction motor. 07

(b) Discuss the construction, operation and speed control for the Universal motor. 07

OR

Q.5 (a) Discuss the construction and working of Permanent Magnet DC (PMDC)

motor. Also state its advantages and disadvantages. 07

(b) Explain the operation of single stack variable reluctance stepper motor. Also

discuss about the micro stepping. 07

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1


GUJARAT TECHNOLOGICAL UNIVERSITY BE - SEMESTER–III • EXAMINATION – WINTER • 2014

Subject Code: 131701 Date: 30-12-2014 Subject Name: Electrical Machines Time: 02.30 pm - 05.00 pm Total Marks: 70 Instructions:


Q.1 (a) Discuss open delta connections of transformers with necessary circuit and vector

diagrams.

07

(b) What do mean of an ideal transformer and derive emf equation of a single phase transformer. Also define Transformation Ratio.

07

Q.2 (a) Derive the condition for Maximum torque for induction motor and Explain Torque - Slip characteristics.

07

(b) Obtain the equivalent circuit of a 200/400 –V, 50 Hz, 1 Phase Transformer from the following test data O.C.Test : 200 V,0.7 A,70 W - on L.V. side S.C. Test : 15 V, 10 A, 85 W - on H.V. side Calculate the secondary voltage when delivering 5 KW at 0.8 p.f. lagging , the primary voltage being 200V.

07

OR (b) Write and explain the conditions of parallel operation of 3-phase transformer. 07 Q.3 (a) Explain the phenomena of armature reaction of a DC machine. State its Remedies. 07 (b) What is commutation? Give remedies for commutation. 07 OR

Q.3 (a) Draw and explain the internal and external characteristics of d.c. shunt generators. 07 (b) Explain different parts of DC machine with neat and clean diagram. 07 Q.4 (a) Explain different speed control methods for 3 phase induction motor. 07 (b) Explain the procedure to construct the circle diagram for induction

motor & how various quantities are measured from circle diagram. 07

OR Q.4 (a) Explain different methods for speed control of series motors. 07

(b) Write different starters used for 3 phase induction motor and explain any one of them. 07 Q.5 (a) Define and state the expressions for (i) Pitch factor. (ii) Distribution factor for

alternator. 07

(b) What is voltage regulation? How it can be determined by using Zero power factor method in synchronous machine?

07

OR Q.5 (a) State the different methods of finding voltage regulation in alternator. Explain any one

of them in detail. 07

(b) State and explain condition of synchronizing alternator with infinite bus bar. 07

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1


GUJARAT TECHNOLOGICAL UNIVERSITY BE – SEMESTER - V • EXAMINATION – SUMMER-2014



Time: 10:30 am to 01:00 pm Total Marks: 70 Instructions:




Q.1 (a) Explain swinburne's test in detail. 07

(b) What is armature reaction? Explain in detail the phenomenon of armature

reaction. 07

Q.2 (a) Differentiate Lap and Wave winding. 07

(b) Explain scott connection. 07

OR

(b) Why transformers need to be connected in parallel? What are the conditions to

be satisfied for parallel operation of transformers?

07

Q.3 (a) Explain blocked rotor and no load test on three phase induction motor. 07

(b) What are the various methods for induction motor speed control? Explain any

two methods in detail 07

OR

Q.3 (a) Why single phase induction motors are not self starting? Explain double field

revolving theory.

07

(b) Explain universal motor and shaded pole induction motor. 07

Q.4 (a) Explain the procedure to draw circle diagram. 07

(b) With neat diagram describe operation of permanent magnet brushless DC

motor.

07

OR

Q.4 (a) What is the principle of stepper motors? Explain its construction. 07

(b) Explain the terms: (1) Coil Span (2) Pole pitch (3) Dummy coil 07

Q.5 (a) How V-curves for synchronous motors are obtained? What is its significance? 07

(b) Explain power developed by synchronous motor. 07

OR

Q.5 (a) Draw the equivalent circuit of induction motor and explain in detail. 07

(b) Describe the construction of three phase transformer. 07

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1


GUJARAT TECHNOLOGICAL UNIVERSITY BE - SEMESTER–V • EXAMINATION – WINTER 2013



Time: 10:30 pm to 01:00 pm Total Marks: 70 Instructions:




Q.1 (a) Explain the effect of variable excitation on the behavior of synchronous motor

under constant load condition.

07

(b)) Why Synchronous motors are not self starting? Explain. Also explain V and

Inverted V curves of synchronous motor.

07

Q.2 (a) Describe the merits and demerits of various connections of three phase

transformer.

07

(b) With the help of phasor diagram explain how two phase supply can be obtained

from three phase supply using scott connection.

07

OR

(b) A 440 V d.c. shunt motor takes a no load current of 2.5 A (From Swinburne’s

Test). The resistance of the shunt field and the armature are 550 Ω and 1.2 Ω

respectively. The Full load line current is 32 A. Find the full load output and

the efficiency of the motor.

07

Q.3 (a) Draw the circle diagram for a 3.73 KW, 200 V, 50 Hz, 4-pole. 3-phase star-

connected induction motor from the following data:

No-load: Line Voltage 200 V, Line Current 5 A, Total Input 350 W

Blocked rotor: Line Voltage 100 V, Line Current 26 A, Total Input 1700 W

Estimate from the circle diagram for full load condition, the line current, power

factor and also the maximum torque in terms of the full – load torque. The

rotor cu loss at standstill is half the total cu loss.

10

(b) Explain the construction, operation and equivalent circuit of double cage

induction motor.

04

OR

Q.3 (a) What are the conditions of parallel operation of three phase transformers? 07

(b) A Three phase step down transformer with per phase turns ratio is 47.6 : 1

connected in delta/star and is supplying a load of 400 KW, 0.8 p.f. lagging at

400 V. sketch the connection diagram and show in it different line voltages.

Also calculate primary line current.

07

Q.4 (a) Using double-revolving field theory, explain why a single phase induction

motor is not self starting?

07

(b) Write short note on shaded pole induction motor. 07 OR

Q.4 (a) Give Comparison between Synchronous motor and induction motor. 07

(b) With the help of phasor diagrams, discuss the behavior of synchronous motor

with the constant field excitation and variable load.

07

Q.5 (a) A DC Generator is delivering the rated current with a brush shift in the

direction of rotation. Neatly sketch and explain the distribution of flux density

in the air gap due to (1) Main field MMF only (2) Armature MMF only and (3)

Both main field and armature MMF.

07

2

(b) Describe Hopkinson’s test in detail with its advantages and disadvantages. 07

OR

Q.5 (a) Discuss the construction, working, performance, advantages and disadvantages

of PMDC (Permanent magnet DC) motor.

07

(b) Explain the construction and working of variable reluctance steeper motors. 07

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1/2


GUJARAT TECHNOLOGICAL UNIVERSITY BE - SEMESTER–III • EXAMINATION – SUMMER 2013

Subject Code: 131701 Date: 29-05-2013 Subject Name: Electrical Machines Time: 02.30 pm - 05.00 pm Total Marks: 70 Instructions:


Q.1 (a) Differentiate between self-excited and separately excited d.c. machines.

Draw the load characteristics of shunt, series and compound generators. 07

(b) Explain three point starter for D.C.Shunt motor. 07

Q.2 (a) Derive the E.M.F. equation of single phase transformer and explain effect of turns ratio on output voltage

07

(b) Explain scott connection for transformer with diagram. 07 OR (b) An ideal 25 KVA transformer has 500 turns on the primary winding and

40 turns on the secondary winding. The primary is connected to 3000 V,50 Hz supply. Calculate (1) primary and secondary currents on full load (2) secondary e.m.f. (3) maximum core flux

07

Q.3 (a) Differentiate between single excited and multiple excited systems. Derive

the expression for magnetic field energy stored in a singly excited system. 07

(b) Define energy and co-energy. Derive the expression for force developed for current excited and voltage controlled systems

07

OR Q.3 (a) Discuss power angle characteristic of an alternator. Also discuss its

operation at constant load with variable excitation. 07

(b) Discuss the conditions to be satisfied before a 3-phase alternator is synchronized with infinite bus. Two 3-phase alternators operate in parallel. The rating of one machine is 50 MW and that of the other is 100 MW. Both alternators are fitted with governors having a droop of 4 %. How will the machines share a common load of 100 MW?

07

Q.4 (a) State the type of three phase induction motor. Explain how rotor

rotates when three phase induction motor is connected across three phase supply & Define Slip.

07

(b) A 3-phase ,50 Hz,500V Induction motor with 6 poles gives an output of 20 Kw at 950 rpm with a power factor of 0.8 The mechanical losses are equal to 1 Kw. Calculate for this load(i)slip (ii)rotor copper loss (iii)input if the stator losses are 1500 W(iv)line current

07

OR Q.4 (a) With reference to induction motor, attempt the following

(1) Explain “cogging” and “crawling” in a 3-phase induction motor with their remedies. (2) Why single-phase induction motor is not self-starting? Explain any one method to make it self-starting.

07

(b)

A 4-pole, lap wound D.C shunt generator has a useful flux per pole of 0.07 wb. The armature winding consists of 220 turns each of 0.004 ohm resistance.Calculate the terminal voltage when running at 900 r.p.m if the armature current is 50 amp.

07

2/2

Q.5 (a) Explain the difference between cylindrical and salient pole rotors used

in large alternator . Define (1) pitch factor (2) Distribution factor (3) form factor.

07

(b) Define Voltage regulation of alternator. State various methods to find voltage regulation and Explain any one method in detail.

07

OR Q.5 (a) Discuss power angle characteristic of an alternator. Also discuss its

operation at constant load with variable excitation. 07

(b) Explain synchronization of alternators. Which conditions must be satisfied for proper synchronization of 3-phase alternators?

07

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