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Reg. No. : ..................................... Name : ..........................................
Third Semester B.Tech. Degree Examination, November 2009 (2008 Scheme) Branch : ELECTRICAL AND ELECTRONICS 08.306 : Electrical Machines-I (E) Time : 3 Hours
Max. Marks : 100 PART – A
Answer all questions. 1. Derive the expressions for the back emf in a d.c. motor. 2. Describe the factors on which the core loss of a transformer depend. 3. Derive the e.m.f. equation of a transformer. 4. Define all-dry efficiency. 5. Derive the e.m.f. equation of a D.C. generetor. 6. Enumerate the losses in a D.C. Machines. 7. Draw the performance characteristics of a compound motor. 8. Briefly explain why interpoles are provided in a D.C. machine. 9. Explain the open circuit test on single phase transformer and the parameters obbined from it. 10. Explain with neat sketches, the load test on DC series motor.
(10Ă—4=40 Marks) P.T.O.
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PART – B Answer one question from each Module. Module – I 11. a) Explain with neat sketches the main ports of a d.c. machine. Describe their functions.
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b) A 500V, wave wound, 750 rpm shunt generator supplies a load of 105A. The armature has 720 conductors and 120 commutator segments. The shunt field resistance is (100 ohm). Find the demagnetizing AT/pole if the brushes are advanced through three commutator segments at this load. Also calculate the extra shunt field turns required to neuitralize this demagnetization. 10 OR 12. a) What is meant by armature reaction ? Explain the effect of armature reaction.
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b) Two shunt generators operating in parallel to deliver a total current of 250 A. One of the generator is rated at 50 kw and the other 100 kW. The voltage rating of both machines are 500V and have regulation of 6 percent (smeller one) and 4 percent. Assume linear characteristics and determine : i) The current delivered by each machine and ii) The terminal voltage. 12 Module – II 13. a) Discuss the different characteristics of D.C. series motor and d.c. shunt motor.
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b) A 4 pole, 240V, wave connected shunt motor gives 9 kW when running at 1000 rpm and drawing armature and field current of 40A and 1A respectively. It has 500 conductors. Its resistance is 0.1 ohm. Assuming a drop of 1V per brush find : i) total torque ii) useful torque iii) useful flux per pole iv) rotational losses and v) efficiency. 12 OR
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14. a) Explain different methods of speed control of d.c. series motor.
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b) The following results were obtained during Hopkinson’s test on two similar 230V machines. Armature current 37A and 30 A, field current 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.
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b) A 10 kVA 200/400V, 50 Hz, single phase transformer gave the following test results. Open circuit test (HT winding open) : 200V, 1.3A, 120W Short circuit test (LT winding shorted) : 22V, 30A, 200W. Calculate : i) The magnetising current and the current corresponding to core loss at normal voltage and frequency. ii) The parameter of equivalent circuit referred to low voltage side ?
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OR 16. a) Derive the condition for maximum efficiency of a transformer.
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b) A 500 kVA, 3 phase, 50 Hz transformer has a voltage ratio of line voltages 33000/11000 and is delta / star connected. The resistance per phase are 35 ohms on high voltage side and 0.876 ohm on low voltage side. The iron loss is 3050 watts. Calculate the efficiency of this transformer at half full load, 0.8 power factor. 12 (3×20=60 Marks) —————————