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Electrical Machines I 4th ND11 EE2251

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B.E./B.Tech. DEGREE EXAMINATION, NOVEMBER/DECEMBER 2011.
Fourth Semester
Electrical and Electronics Engineering
EE 2251 - ELECTRICAL MACHINES - I
(Regulation 2008)

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PART A — (10 × 2 = 20 marks)

1. Define statically and dynamically induced EMF.
2. What is Hysteresis loss and how can this loss be minimized?
3. Why is transformer rated in KVA?
4. Compare two winding transformer and auto transformer.
5. What are the advantages of analyzing energy conversion devises by field energy concept?
6. Draw the general block diagram of electromechanical energy conversion device.
7. What is back EMF in a D.C. motor?
8. Define winding factor.
9. What is armature reaction in DC machines?
10. Explain why Swinburne’s test cannot be performed on DC series motor.

PART B — (5 × 16 = 80 marks)

11. (a) Compare electric and magnetic circuit by their similarities and dissimilarities. (16)
Or
(b) A ring composed of three sections. The cross section area is 0.001 m2 for each section. The mean arc length are Ia = 0.3 m, Ib,= 0.2 m, Ic = 0.1 m. an air gap length of 0.1 mm is cut in the ring. r ยต for sections a, b and c are 5000, 1000 and 10000 respectively. Flux in the air gap is 7.5 × 10–4 Wb. Find (i) mmf (ii) exciting current if the coil has 100 turns (iii) reluctance of the sections. (16)

12. (a) (i) Explain clearly the causes of voltage drop in a power transformer on load and develop the equivalent circuit for a single phase transformer. (10)
(ii) Derive an expression for saving of copper when an auto transformer is used. (6)
Or
(b) A 3-phase step down transformer is connected to 6.6 KV mains and takes 10 Amps. Calculate the secondary line voltage and line current for the (i) (ii) Y Y (iii) Y and (iv) Y connections. The ratio of turns per phase is 12 and neglect no load losses. (16)

13. (a) Obtain an expression for the mechanical force of field origin in a typical attracted armature relay. (16)
Or
(b) Find an expression for the magnetic force developed in a doubly excited magnetic systems. (16)

14. (a) Derive an expression for emf generated in
(i) Synchronous machine (8)
(ii) D.C machine. (8)
Or
(b) A 3F, 50 Hz, star connected alternator with two layer winding is running at 600 rpm. It has 12 turns/ coil, 4 slots/pole/phase and a coil pitch of 10 slots. If the flux per pole is 0.035 Wb sinusoidally
distributed, Find the phase and line emf induced. Assume that the total turns/phase are series connected. (16)

15. (a) Explain the different methods of excitation and characteristics of a DC motors with suitable diagrams. (16)
Or
(b) Explain the Ward-Leonard system of controlling the speed of a DC shunt motor with help of neat diagram. (16)