ELECTRICAL MACHINES — II Question paper EEE Fifth semester

Fourth Semester
Electrical and Electronics Engineering
Time : Three hours Maximum : 100 marks
Answer ALL questions.
PART A — (10 ? 2 = 20 marks)
1. Why is the field system of an alternator made as a rotor?
2. Compare salient pole rotor and cylindrical pole rotor.
3. What is synchronous reactance?
4. What is hunting?
5. What is the function of slip ring in 3-phase induction motor?
6. How do you reverse the direction of rotation of a 3-phase induction motor?
7. What is the function of capacitor in a single phase induction motor?
8. What is the use of shading coil in the shaded pole motor?
9. Why is the hysteresis motor free from mechanical and magnetic vibrations?
10. What type of motor is used in computer drives and wet grinders?
PART B — (5 ? 16 = 80 marks)
11. (i) State the requirements for paralleling alternators. (8)
(ii) A 2-pole, 50 Hz, 3-phase, turbo-alternator is excited to generate the bus-bar voltage of 11 kV on no load. The machine is star-connected and the short circuit current for this excitation is 1000 A.
Calculate the synchronizing power per degree of mechanical displacement of the rotor and the corresponding synchronizing torque. (8)
12. (a) (i) Explain the operating principle of three phase alternator. (8)
(ii) Explain the constructional details of a three phase alternator which is used for slow speed operation. (8)
(b) (i) What is the effect of armature reaction at different power factors on synchronous machine? (8)
(ii) A 3300 V, 3 phase star-connected alternator has a full load current of 100 A. On short circuit a field current of 5 amperes was necessary to produce full-load current. The e.m.f. on open circuit for the same excitation was 900 volts. The armature resistance was /phase. Determine the full-load voltage regulation for
(1) 0.8 p.f. lagging
(2) 0.8 p.f. leading. (8)
13. (a) (i) Develop the equivalent circuit for 3-phase induction motor. (8)
(ii) Explain the different speed control methods of squirrel cage induction motor. (8)
(b) (i) Describe the principle of operation of synchronous induction motor.
(ii) Compute the value of maximum torque in N-m for a 3-phase 440 V star connected 50 Hz, 4 pole induction motor for which and . Also determine the slip corresponding to it. (8)
14. (a) (i) Explain the operation of a single phase induction motor using double revolving field theory. (8)
(ii) Describe the constructional features and operating characteristics of single phase shaded pole motor. (8)
(b) (i) Explain the construction and principle of working of stepper motor.
(ii) Explain the principle of operation and applications of reluctance motor. (8)
15. (a) (i) Explain the principle of operation and constructional details of linear induction motor. (10)
(ii) List some applications of linear induction motor. (6)
(b) Write a brief note on :
(i) Hysteresis motor. (8)
(ii) A.C. series motor. (8)