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Reg. No. : ..................................... Name : ..........................................
Third Semester B.Tech. Degree Examination, November 2009 (2003 Scheme) Branch : Electrical and Electronics 08.306 : ELECTRICAL MACHINES – 1 (E) Time : 3 Hours
Max. Marks : 100 PART – A
Answer all questions. 1. Explain with neat sketches the difference between Lap and Wave Winding. 2. Explain the need of equilizer connection in lap winding. 3. What are the various losses in a DC machine ? Explain briefly. 4. The shunt field resistance is high, while the series field resistance is low. Explain why ? 5. How does a DC motor automatically adjust its input to match the mechanical load on the motor ? 6. What are the different methods of braking of DC motors ? Explain any one method. 7. Explain briefly the Retardation test on D.C. machine. 8. Derive the emf equation of transformer. 9. List the conditions to be fulfilled for paralleling two single phase transformers. 10. Explain the need of tap-changing in transformers. Distinguish between ON-load and OFF-load tap changing. P.T.O.
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PART – B Answer any one full question from each Module. MODULE – I 11. a) Describe with relevant diagrams, the different methods of excitation of D.C. machines. b) A 6-pole long shunt compound generator supplies 100A at a terminal voltage of 500 volt and has armature, series field and shunt field resistances of 0.02 : , 0.04 : and 80 : respectively. Calculate the generated emf. Take drop per brush as 0.5 V. Neglect armature reaction. 20 12. a) What is meant by armature reaction ? Explain the effects of armature reaction. b) What is meant by commutation in DC machines ? Explain the process. c) A 4-pole DC generator has 492 conductors (armature) and an armature current of 144A. The brushes have been given a lead of 10 degree mechanical. Find the demagnetizing AT/pole and cross magnetizing AT/pole if the machine is 1) lap connected 2) wave connected. 20 MODULE – II 13. a) Draw the circuit and explain a starter suitable for a DC shunt motor. b) A 4-pole 240V, wave connected shunt motor gives 11.19 KW when running at 1000 rpm and drawing an armature and field currents of 50A and 1A respectively. It has 540 conductors. Its armature resistance is 0.1 ohm. Assuming a drop of 1v/brush, find total torque, useful torque, Flux/pole, rotational losses and efficiency. 20 14. a) Explain the various methods of controlling the speed of DC motor. Comment on their relative advantages. b) With a circuit diagram, explain Hopkinson’s test.
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MODULE – III 15. a) Draw and explain the phasor diagram of a single phase transformer, when inductive load is connected to it.
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b) A 40 KVA transformer has a core loss of 450 watts and full load copper loss of 850 watts. If the power factor of the load is 0.8, calculate 1) Full load efficiency 2) Maximum efficiency 3) The load at which maximum efficiency occurs. 12 16. a) Derive the equivalent circuit of a single phase transformer.
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b) What is open-delta connection ? Enumerate its uses.
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c) What are the advantages of a single three phase transformer comparing with same rating of three single phase transformers ?
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