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

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

**(b)**State factor to be considered for selection of specific loading.

**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**

loading in DC Machine Design.

**(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

loading=30,000 ampere conductors per meter; field form factor =

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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This post was last modified on 04 March 2021