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Reg. No. : .................................... Name : .........................................
Third Semester B.Tech. Degree Examination, June 2009 Branch : Electrical (2003 Scheme) 03-306: ELECTRICAL MACHINES – I (E) Time : 3 Hours
Max. Marks : 100 PART – A
Answer all questions. 1. What are the conditions in which a self excited d.c generator fails to build up voltage ? 2. With the help of simple sketches, explain the difference between lap-winding and wave-winding. 3. A 4-pole dc shunt generator has 24 slots with 10 conductors per slot and is rotated at 1000 r.p.m. The flux per pole is 0.03 Wb. Calculate the generated emf if the armature is lap-wound. 4. Derive the expression for the back emf in a dc motor. 5. Explain the various methods of speed control of dc series motor. 6. Compare the characteristics of dc shunt and series motor. 7. Distinguish between core type and shell type transformers. 8. Explain one method of cooling of power transformer. 9. Explain the use of tertiary winding in transformer. 10. Describe the principle of Auto-transformer. Show the distribution of current in the winding section of a step up auto-transformer. (10×4=40 Marks) P.T.O .
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3112 PART – B Answer one question from each Module. MODULE – I
11. a) Explain with neat sketches the main parts of a dc machine. Describe their functions.
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b) A 110 V shunt generator has a full load current of 100 A, shunt field resistance of 55 ohm and constant losses of 500 W. If full load efficiency is 88 percent, find armature resistance. Assuming voltage to be constant at 110 V, calculate efficiency at ½ full load and at 50 percent overload. 10 OR 12. a) Describe with schematic diagrams the various methods of excitation of dc machine. b) Explain the process of commutation in dc machine.
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c) Two shunt generators A and B operate in parallel and their load characteristics may be taken as straight lines. The voltage of A falls from 240 V at no load to 220 V at 200 A, while that of B falls from 245 V at no load to 220 V at 150 A. Determine the current which each machine supplies to a common load of 300 A and the bus-bar voltage at this load. 10 MODULE – II 13. a) Compare the different characteristics of dc series motor and dc shunt motor.
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b) A 500 V, 50 hp, 1000 rpm dc shunt motor has on full load an efficiency of 90 percent. The armature circuit resistance is 0.24 ohm and there is total voltage drop of 2 V at the brushes. The field current is 1.8 A. Determine i) full load line current ii) full load shaft torque in N-m and iii) total resistance in motor starter to limit the starting current to 1.5 times the full load current. 12
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14. a) Draw a neat schematic diagram of a 3-point starter and explain its working.
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b) The following results were obtained during Hopkinson’s test on two similar 230 V machines. Armature current 37 A and 30 A, field currents 0.85 A and 0.8 A. Calculate the efficiencies of machines if each has an armature resistance of 0.33 ohm. 10 MODULE – III 15. a) Draw the vector diagram of a transformer when it supplies inductive load. b) Describe the various losses in a transformer.
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c) Obtain the equivalent circuit of a 200/400 V, 50 Hz, single phase transformer from the following test data. OC test : 200 V, 0.7 A, 70 W - (carried out on L.V side) SC test : 15 V,
10 A, 85 W (carried out on H.V side)
The transformer is rated as 4 KVA. OR 16. a) Determine the saving of copper in an auto-transformer in comparison with an ordinary transformer.
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b) Write short notes on vector group of power transformers.
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c) What are the factors that determine the choice between star and delta connections ? ______________
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