# Download GTU B.Tech 2020 Summer 6th Sem 2160912 Design Of Dc Machines And Transformer Question Paper

Download GTU (Gujarat Technological University Ahmedabad) B.Tech/BE (Bachelor of Technology/ Bachelor of Engineering) 2020 Summer 6th Sem 2160912 Design Of Dc Machines And Transformer Previous Question Paper

Seat No.: ________
Enrolment No.___________
GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER? VI EXAMINATION ? SUMMER 2020
Subject Code: 2160912 Date:02/11/2020
Subject Name: Design of DC Machines and Transformer
Time: 10:30 AM TO 01:30 PM Total Marks: 70
Instructions:
1. Attempt all questions.

2. Make suitable assumptions wherever necessary.

3. Figures to the right indicate full marks.

MARKS
Q.1
(a) Define the following words related to transformer design:
03
(I) Window space factor
(II) Staking factor
(III) Circumscribing Circle

04

(c) Determine the main dimensions of core and yoke for a 200 KVA,
07
50 Hz, 1-phase core type transformer. Use the following data:
Window space factor=0.32, Current density=3A/mm2, Maximum
flux density=1.1 Wb/m2, Voltage per turn=14 V, Stacking
factor=0.9.Net iron area=0.56d2, where d is the diameter of
circumscribing circle. The cruciform core is used with the distance
between adjacent limbs=1.6 times width of core lamination. The
width of the largest stamping is 0.85d.

Q.2
(a) Give the comparison between core and shell type transformers.
03

(b) Differentiate between Radial Forces and Axial Forces in
04
transformer windings.

(c) Derive the expression of leakage reactance of a 3-phase core type
07
distribution transformer.

OR

(c) Write a short note on the classification of insulating materials.
07

Q.3
(a) Explain the importance of stepped core in the transformer.
03

(b) Why tapping's are generally provided on HV winding?
04

(c) What is design optimization? Derive necessary condition for
07
designing a transformer with minimum cost.

OR

Q.3
(a) How to reduce the demagnetizing effects and cross-magnetizing
03
effect?

(b) Give the comparison between the power transformer and
04
distribution transformer.

(c) A 200 kVA, 6600/440volts, 50Hz, three phase core type
07
transformer has the following design data: Max. flux density : 1.3
wb/m2, EMF/Turn : 10 volts, Stacking factor : 0.9, Window space
factor : 0.3 , Current density: 2.5 A/mm2. Overall width and
overall height are the same. If three stepped core is used to
determine overall dimensions.

Q.4
(a) Explain the heating of electrical machines.
03

(b) Explain how the following factors influence the main dimensions
04
of a DC Machine.
1

(1) Peripheral speed (2) voltage between adjacent segments.

(c) Derive the output equation of a 3-phase core type transformer.
07

OR

Q.4
(a) Explain Staggering of Brushes in a D.C. machine.
03

(b) Why Circular Coils are preferred in Transformer?
04

(c) Explain various factors affecting the selection of airgap length in
07
the D.C. machine.
Q.5
(a) Explain the disadvantages of having higher specific electric
03

(b) Which type of material is preferred for core laminations in
04
transformer? What is the advantage of using a mitred joint in the
core construction?

(c) Calculate the diameter and length of the armature for a 7.5kW, 4
07
poles, 1000 r.p.m, 220 V shunt motor. Given: full load efficiency=
0.83; maximum gap flux density=0.9 Wb/m2: specific electric
0.7. Assume that the maximum efficiency occurs at full load and
field current is 2.5% of rated current. The pole face is square.

OR
Q.5
(a) Discuss the necessity of inter-pole in D. C. machine design.
03
(b) Explain the losses at the commutator surface.
04
(c) Design is required for a 50 kW, 4 poles, 600 r.p.m. D.C. shunt
07
generator, the full load terminal voltage being 220 V. If the
maximum gap density is 0.83 Wb/m2 and the armature ampere
conductors per meter are 30,000, calculate suitable dimensions of
armature core to give a square pole face.
Assume that the full load armature voltage drop is 3 per cent of
the rated terminal voltage and that the field current is 1 per cent of
rated full load current. The ratio of pole arc to pole pitch is 0.67.

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