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B.Tech III (Third) Semester Examination 2012-13 Course Code: EEE304 Paper ID: 0113320 DC Machines and Transformer Time: 3 Hours Max. Marks: 70 Note: Attempt six questions in all. Q. No. 1 is compulsory. 1. a) b) c) d) e) f) g) h) 2. Answer any five of the following (limit your answer to 50 words). (4x5=20) Show that the reaction of coupling magnetic field on the electrical or mechanical system is essential for the electromechanical energy conversion process. State and explain with suitable examples, singly excited and doubly excited magnetic field systems. Also mention their applications. What are the advantages of analyzing energy conversion devices by field energy concept? Give the materials and functions of the following parts of a DC machine: i) Field poles ii) Yoke iii) Commutator vi) Commutating poles Explain in brief how efficiency is obtained from Swinburne’s Test. Explain what would happen, if the dc motor is directly switched on to the supply without any starter. What is the difference between 3 single phase transformer bank and a 3-phase transformer unit? Discuss the conditions necessary for the successful parallel operation of 1-phase transformers. How can you check these conditions? Develop the relationship between energy and co-energy in a magnetic field system. Also give a diagram of flow of energy in an electro-mechanical energy conversion device via a coupling field (10) 3. Distinguish between self-excited and separately excited D.C. generators. How are self-excited D.C. generators classified? Give their circuit diagrams with voltage equations. (10) 4. A short-shunt compound D.C. generator delivers 100 A to a load at 250V. The generator has shunt field, series field and armature resistance of 130 Ω, 0.1 Ω and 0.1Ω respectively. Calculate the voltage generated in armature winding. Assume 1 V drop per brush. (10) 5. Draw and explain the speed-torque characteristics of a dc shunt, series and compound motors. Which characteristic is more suitable for traction purpose and why? (10) 6. Explain with necessary diagrams how two 3-phase transformers can be used to convert a 3- phase supply to a 2phase one. If the load is balanced on one side, show that it will be balanced on other side. (10) 7. Two transformers A and B are joined in parallel to the same load. Determine the current delivered by each transformer, given: open-circuit emf 6600 V for A and 6400 V for B. Equivalent leakage impedance in terms of the secondary = (0.3 + j 3) Ω for A and (0.2 + j 1) Ω for B. The load impedance is (8 + j 6) Ω. (10) 8. Derive an expression for saving in conductor material in an autotransformer over a two-winding transformer of equal rating. State the advantages and disadvantages of autotransformers over two-winding transformers. Also write its applications. (10)