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Council for Technical Education and Vocational Training{{CTEVT}}
Office of the Controller of Examinations
Sanothimi, Bhaktapur
Regular/ Back Exam-2079 Chaitra/ 2080 Baisakh
Full Marks: 80
Pass Marks: 32
Program: Diploma in Electrical & Electronics / Electrical Engg.
Year/Part: II/I (2014)
Subject: Electrical Machine-I
Time: 3 hrs.
Candidates are required to give their answers in their own words as far as practicable.The figures in the margin indicate full marks.
Attempt any FIVE questions.
1. a) Explain the operation of an ideal transformer on load with phasor diagram and equivalent circuit. [8]
b) A 2000/400v, 50Hz Single phase transformer drains 2A at a power factor of 0.2 lagging, when it has no-load. Calculate the primary current and p.f. when the secondary current is 200A at a p.f. of 0.8 lagging. Assume the voltage drop in the winding to be neglected. [8]
2. a) Explain briefly the open circuit and short circuit tests of a transformer. (8)
b) Obtain the equivalent circuit of 220/440V, 50Hz, 1-ϕ transformers of following data:
S.C. Test: 85 W, 9 A, 18 V ............H.V. Side
O.C. test: 70 W, 0.6A, 220 V ............L.V. Side
Calculate the secondary Voltage when delivery 7kw at 0.85 p.f. lagging, primary voltage being 200V.
3. a) Why is the rating of a transformer given is KVA? What are the advantages and disadvantages of three units of 1-ϕ transformer used as 3-ϕ transformer? [8]
b) Name different types of connections of 3- transformer and describe any one.
4. a) Why dc motor (series) never be started on No-load? Derive the Torque equation of DC motor. (4+4)
b) Describe the speed control of DC motor. [8]
5. a) Describe Armature reaction in brief. Derive the condition for the maximum efficiency of de generator. (8)
b) A short shunt cumulative compound de generator supplies 7.5KW at 230V. The shunt field, series field and armature reaction are 100, 0.3 and 0.4 Ω respectively. Calculate the induced emf and the load resistance. [8]
6. Write short notes on: [4x4=16]
i) Testing of Transformers[
ii) Back emf
iii) Auto transformer
iv) Three phase transformer connection
Good Luck!
