GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER- IV (New) EXAMINATION - WINTER 2019
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Subject Code: 2140906 Date: 12/12/2019Subject Name: AC Machines
Time: 10:30 AM TO 01:00 PM Total Marks: 70
Instructions:
- Attempt all questions.
- Make suitable assumptions wherever necessary.
- Figures to the right indicate full marks.
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MARKS
Q.1 (a) Explain working principle of Induction motor. 03
(b) Explain production of rotating magnetic field in poly-phase 04 induction motor.
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(c) Why single phase induction motors are not self-started? Explain 07 double field revolving theory for single phase induction motor.
Q.2 (a) What is Synchronous condenser? 03
(b) Explain with reason why synchronous motor is not self-starting. 04 Enumerate different methods of starting the synchronous motor.
(c) Explain the working principle of Induction Generator. Also discuss 07 the applications of Induction generator.
OR
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(c) Draw and explain speed — torque characteristic of an Induction 07 motor.
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 07 motor. Also describe the method to determine losses, efficiency and slip at full load condition using circle diagram.
OR
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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 07 kW, 50 Hz, 16-pole induction motor are 0.02 Q and 0.15 Q respectively. Full load torque is obtained at 360 rpm. Calculate: (1) 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.
Q.4 (a) Derive the emf equation of an alternator. 03
(b) Define the pitch factor and distribution factor for synchronous 04 generator.
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(c) A 3-phase water wheel generator is rated at 100 MVA, 0.9 power 07 factor, 11 kV, star-connected, 50 Hz, 120 rpm. Determine (1) 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)
OR
Q.4 (a) Explain various conditions for synchronization of the alternator. 03
(b) Explain the effect of variation of excitation on synchronous 04 generator.
(c) Explain determination of voltage regulation by ZPF method. 07
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Q.5 (a) Write a note on double cage induction motor. 03
(b) Derive the equation of electromagnetic torque for a three phase 04 induction motor with usual notations from first principles.
(c) Explain concept of linear induction motor. Also give relative 07 advantage and disadvantage.
OR
Q.5 (a) How direct axis and quadrature axis reactance can be measured for 03 salient pole machine?
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(b) Write a note on Schrage motor. 04
(c) Explain V-curves in brief. Also give its importance for synchronous 07 motors.
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