Download GTU (Gujarat Technological University) BE/BTech (Bachelor of Engineering / Bachelor of Technology) 2019 Winter 3rd Sem New 3132407 Electrical Machines And Applications Previous Question Paper
Seat No.: ________ Enrolment No.___________
GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER ? III (New) EXAMINATION ? WINTER 2019
Subject Code: 3132407 Date: 3/12/2019
Subject Name: Electrical Machines and Applications
Time: 02:30 PM TO 05:00 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) Derive expression for the r.m.s. values of the induced
voltages in the two windings of a single phase transformer
connected to a sinusoidal supply.
03
(b) Derive the condition for maximum efficiency for a single
phase transformer.
04
(c) The connected instrument readings obtained from open
and short circuit tests on 10 kVA, 450/120 V, 50 Hz
transformer are:
O.C. test: V1 = 120 V; I1 = 4.2 A, W1 = 80 W; (test
conducted on l.v. side)
S.C. test: V1 = 9.65 V; I1 = 22.2 V; W1 = 120 W; (l.v. side
short circuited)
Compute the equivalent circuit constants and draw it.
07
Q.2 (a) Compare cage and wound three phase induction motor
with reference to construction, performance and
application.
03
(b) A 4-pole, 3-phase induction motor operates from a supply
whose frequency is 50 Hz. Calculate:
(i) The speed at which the magnetic field of the stator
is rotating
(ii) The speed of the rotor when the slip is 0.04
(iii)The frequency of the rotor currents when the slip
of 0.03
(iv) The frequency of the rotor currents at standstill
04
(c) Why starters are necessary for starting induction motors?
Name different starting methods for 3-phase induction
motors and explain any one briefly.
07
OR
(c) Draw a typical torque slip characteristic and derive the
condition for maximum torque.
07
Q.3 (a) State the principle of operation of dc generator and derive
the expression for the emf generated.
03
(b) What do you mean by demagnetizing effect of armature
reaction in dc machine?
04
(c) A 4-pole, 240 V, wave connected shunt motor gives 1119
kW when running at 1000 rpm and drawing armature and
field currents of 50 A and 1.0 A respectively. It has 540
conductors. Its resistance is 0.1 ohm. Assuming a drop of
1 volt per brush, find (i) total torque (ii) useful torque (iii)
useful flux/pole (iv) rotational losses and (v) efficiency.
07
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Seat No.: ________ Enrolment No.___________
GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER ? III (New) EXAMINATION ? WINTER 2019
Subject Code: 3132407 Date: 3/12/2019
Subject Name: Electrical Machines and Applications
Time: 02:30 PM TO 05:00 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) Derive expression for the r.m.s. values of the induced
voltages in the two windings of a single phase transformer
connected to a sinusoidal supply.
03
(b) Derive the condition for maximum efficiency for a single
phase transformer.
04
(c) The connected instrument readings obtained from open
and short circuit tests on 10 kVA, 450/120 V, 50 Hz
transformer are:
O.C. test: V1 = 120 V; I1 = 4.2 A, W1 = 80 W; (test
conducted on l.v. side)
S.C. test: V1 = 9.65 V; I1 = 22.2 V; W1 = 120 W; (l.v. side
short circuited)
Compute the equivalent circuit constants and draw it.
07
Q.2 (a) Compare cage and wound three phase induction motor
with reference to construction, performance and
application.
03
(b) A 4-pole, 3-phase induction motor operates from a supply
whose frequency is 50 Hz. Calculate:
(i) The speed at which the magnetic field of the stator
is rotating
(ii) The speed of the rotor when the slip is 0.04
(iii)The frequency of the rotor currents when the slip
of 0.03
(iv) The frequency of the rotor currents at standstill
04
(c) Why starters are necessary for starting induction motors?
Name different starting methods for 3-phase induction
motors and explain any one briefly.
07
OR
(c) Draw a typical torque slip characteristic and derive the
condition for maximum torque.
07
Q.3 (a) State the principle of operation of dc generator and derive
the expression for the emf generated.
03
(b) What do you mean by demagnetizing effect of armature
reaction in dc machine?
04
(c) A 4-pole, 240 V, wave connected shunt motor gives 1119
kW when running at 1000 rpm and drawing armature and
field currents of 50 A and 1.0 A respectively. It has 540
conductors. Its resistance is 0.1 ohm. Assuming a drop of
1 volt per brush, find (i) total torque (ii) useful torque (iii)
useful flux/pole (iv) rotational losses and (v) efficiency.
07
2
OR
Q.3 (a) Draw the power flow diagram of a d.c. motor. 03
(b) What is the function of interpole in dc machine? 04
(c) A shunt generator delivers 195 A at terminal p.d. of 250 V.
the armature resistance and shunt field resistance are 0.02
ohm and 50 ohm respectively. The iron and friction losses
equal 950 W. Find
(i) Emf generated
(ii) Cu losses
(iii) Output of the prime mover
(iv) Commercial, mechanical and electrical
efficiencies.
07
Q.4 (a) What is an auto-transformer? State its merits and demerits
over the two winding transformer.
03
(b) Explain the principle of operation of a 3-phase
synchronous motor.
04
(c) Explain synchronous impedance method of determining
the voltage regulation of an alternator. Comment on the
merits and limitations of this method.
07
OR
Q.4 (a) Explain how Scott connections are used to obtain two-
phase supply form 3-phase supply mains.
03
(b) State the necessary conditions for parallel operation of
alternators.
04
(c) Why is synchronous motor not self-starting? What
methods are generally used to start the synchronous
motors? Explain any one.
07
Q.5 (a) What type of motor would you use in the following
applications: (i) washing machine (ii) sewing machine (iii)
dishwasher (iv) portable electric drill (v) food mixer and
(vi) ceiling fan
03
(b) Describe working and application of reluctance motor. 04
(c) What are the main differences in the behavior of variable
reluctance type stepper motor and permanent magnet type
stepper motor?
07
OR
Q.5 (a) State some important application of brushless dc motor. 03
(b) Explain the operating principle of linear induction motor. 04
(c) What is an amplidyne? Draw its connection diagram and
explain its working.
07
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This post was last modified on 20 February 2020