Subject Code:2160912
GUJARAT TECHNOLOGICAL UNIVERSITY
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SEMESTER-VI(NEW) — EXAMINATION — SUMMER 2019
Subject Name:Design of DC Machines and Transformer
Time:10:30 AM TO 01:30 PM
Instructions:
- Attempt all questions.
- Make suitable assumptions wherever necessary.
- Figures to the right indicate full marks.
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Q.1 (a) Describe importance of mitered joints in transformer. (MARKS 03)
(b) Discuss the factors affecting the selection of specific magnetic loading in transformer. (MARKS 04)
(c) For a transformer show that e.m.f. per turn Et = kNQ where Q =kVA rating. Explain how service condition of transformer affect the value of K. (MARKS 07)
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Q.2 (a) What is window space factor? On what factors does it depend? (MARKS 03)
(b) Show the complete magnetic path in a sketch of 4 pole d.c. machine. (MARKS 04)
(c) How will the output and losses in a transformer vary with the linear dimensions? (MARKS 07)
OR
(c) Design a 250KVA, 2000/400 Volt, 50 HZ, 1 phase, core type oil immersed self cooled power transformer with following data: (MARKS 07)
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Induced emf per turn = 15 Volt
Max flux density Bm= 1.2 wb/m?
Current density= 2.75 A/mm?
Window space factor = 0.3
Height of the window Hw/ width of window Ww =3
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Assume 3 stepped core and Ai=0.6d%, a=0.9d
Determine the main dimensions of the-core and yoke
Q.3 (a) Why circular coils are always preferred over rectangular coils for winding a transformer? (MARKS 03)
(b) Define specific magnetic loading and specific electrical loading in d.c. machine. (MARKS 04)
(c) Give the important guidelines in selecting no. of poles in d.c. machine design. (MARKS 07)
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OR
Q.3 (a) State the relation between Wight of copper and iron for minimum volume of transformer. (MARKS 03)
(b) Explain the differences between Power and Distribution transformer. (MARKS 04)
(c) Give reasons for following: (MARKS 07)
- Why are tapping provided on HV winding ?
- Why cores of transformers are stepped?
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Date:10/05/2019
Total Marks: 70
Q.4 (a) Explain different methods of cooling of transformer (MARKS 03)
(b) Explain different methods of cooling of transformer (MARKS 04)
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(c) What are the types of windings commonly used in transformer and on what basis they are selected? (MARKS 07)
OR
Q.4 (a) Explain design of brushes in d.c. machine (MARKS 03)
(b) Explain No load current calculations for 3 phase transformers. (MARKS 04)
(c) A 600kVA, 6600/400V, 50 hz delta-star 3 ph. Core type transformer has the following data: width of h.v. and L.v. winding are 3 cm for both .; Height of H.V.winding and L.V. winding =40 cm. ; Length of mean turn =1.5 m ; h.v. winding turns =220 ; width of duct between hv and v winding =2 cm. Calculate the leakage reactance of the transformer referred h.v. side. (MARKS 07)
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Q.5 (a) The length of air gap is not uniform under the entire pole face, why it is done in d.c. machine? (MARKS 03)
(b) Explain the factors affecting for the choice of armature diameter and armature core length in d.c. machine. (MARKS 04)
(c) Find the suitable no of poles and the diameter of the core of a 400 KW 550 volt, 180 RPM, d.c. generator having 92% efficiency. Assume an average flux density in the air gap of about 0.6 wb/m? and ampere conductor per meter to be 35000 and ratio of pole arc/pole pitch =0.7. (MARKS 07)
OR
Q.5 (a) Discuss how the size of the core of a d.c. machine depends upon its(i) rating (i1) speed (MARKS 03)
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(b) Write note on selection of number of armature slots in D.C.machine. (MARKS 04)
(c) Find main dimensions of a 5 kW , 250 V , 4 pole, 1500 rpm D.C. shunt generator if the required data is : full load efficiency = 87%, pole arc/pole pitch = 0.66 ; avg. flux density = 0.42 Wb/m*; Ampere conductors / m =15000. Machine is designed with pole arc to-pole pitch ratio equal to 0.66. (MARKS 07)
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