Download GTU (Gujarat Technological University) BE/BTech (Bachelor of Engineering / Bachelor of Technology) 2019 Winter 4th Sem New 2140906 Ac Machines Previous Question Paper
Seat No.: ________ Enrolment No.___________
GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER ? IV (New) EXAMINATION ? WINTER 2019
Subject Code: 2140906 Date: 12/12/2019
Subject Name: AC 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) Explain working principle of Induction motor. 03
(b) Explain production of rotating magnetic field in poly-phase
induction motor.
04
(c) Why single phase induction motors are not self-started? Explain
double field revolving theory for single phase induction motor.
07
Q.2 (a) What is Synchronous condenser? 03
(b) Explain with reason why synchronous motor is not self-starting.
Enumerate different methods of starting the synchronous motor.
04
(c) Explain the working principle of Induction Generator. Also discuss
the applications of Induction generator.
07
OR
(c) Draw and explain speed ? torque characteristic of an Induction
motor.
07
Q.3 (a) Why starter needed for poly phase induction motor? 03
(b) Write a note: Shaded pole motor and Universal motor. 04
(c) Explain the procedure to construct the circle diagram of induction
motor. Also describe the method to determine losses, efficiency and
slip at full load condition using circle diagram.
07
OR
Q.3 (a) Discuss working of Repulsion motor. 03
(b) Write a note on: (i) Crawling (ii) Cogging of induction motor. 04
(c) Rotor resistance and standstill reactance per phase of a 3-phase, 746
kW, 50 Hz, 16-pole induction motor are 0.02 ? and 0.15 ?
respectively. Full load torque is obtained at 360 rpm. Calculate: (i)
the ratio of maximum to full-load torque (ii) The speed at maximum
torque (iii)What should be the external resistance required at start in
rotor circuit to obtain maximum torque at start.
07
Q.4 (a) Derive the emf equation of an alternator. 03
(b) Define the pitch factor and distribution factor for synchronous
generator.
04
(c) A 3-phase water wheel generator is rated at 100 MVA, 0.9 power
factor, 11 kV, star-connected, 50 Hz, 120 rpm. Determine
(i) The number of poles
(ii) The kW rating
(iii) The current rating of the alternator
(iv) The input at rated kW if the efficiency is 97% (ignore
field losses)
(v) Input torque (prime-mover torque)
07
OR
Q.4 (a) Explain various conditions for synchronization of the alternator. 03
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1
Seat No.: ________ Enrolment No.___________
GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER ? IV (New) EXAMINATION ? WINTER 2019
Subject Code: 2140906 Date: 12/12/2019
Subject Name: AC 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) Explain working principle of Induction motor. 03
(b) Explain production of rotating magnetic field in poly-phase
induction motor.
04
(c) Why single phase induction motors are not self-started? Explain
double field revolving theory for single phase induction motor.
07
Q.2 (a) What is Synchronous condenser? 03
(b) Explain with reason why synchronous motor is not self-starting.
Enumerate different methods of starting the synchronous motor.
04
(c) Explain the working principle of Induction Generator. Also discuss
the applications of Induction generator.
07
OR
(c) Draw and explain speed ? torque characteristic of an Induction
motor.
07
Q.3 (a) Why starter needed for poly phase induction motor? 03
(b) Write a note: Shaded pole motor and Universal motor. 04
(c) Explain the procedure to construct the circle diagram of induction
motor. Also describe the method to determine losses, efficiency and
slip at full load condition using circle diagram.
07
OR
Q.3 (a) Discuss working of Repulsion motor. 03
(b) Write a note on: (i) Crawling (ii) Cogging of induction motor. 04
(c) Rotor resistance and standstill reactance per phase of a 3-phase, 746
kW, 50 Hz, 16-pole induction motor are 0.02 ? and 0.15 ?
respectively. Full load torque is obtained at 360 rpm. Calculate: (i)
the ratio of maximum to full-load torque (ii) The speed at maximum
torque (iii)What should be the external resistance required at start in
rotor circuit to obtain maximum torque at start.
07
Q.4 (a) Derive the emf equation of an alternator. 03
(b) Define the pitch factor and distribution factor for synchronous
generator.
04
(c) A 3-phase water wheel generator is rated at 100 MVA, 0.9 power
factor, 11 kV, star-connected, 50 Hz, 120 rpm. Determine
(i) The number of poles
(ii) The kW rating
(iii) The current rating of the alternator
(iv) The input at rated kW if the efficiency is 97% (ignore
field losses)
(v) Input torque (prime-mover torque)
07
OR
Q.4 (a) Explain various conditions for synchronization of the alternator. 03
2
(b) Describe the effect of armature reaction in case of a synchronous
generator.
04
(c) Explain determination of voltage regulation by ZPF method. 07
Q.5 (a) Write a note on double cage induction motor. 03
(b) Derive the equation of electromagnetic torque for a three phase
induction motor with usual notations from first principles.
04
(c) Explain concept of linear induction motor. Also give relative
advantage and disadvantage.
07
OR
Q.5 (a) How direct axis and quadrature axis reactance can be measured for
salient pole machine?
03
(b) Write a note on Schrage motor. 04
(c) Explain V-curves in brief. Also give its importance for synchronous
motors.
07
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This post was last modified on 20 February 2020