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Reg. No. Name:
APJ ABDUL KALAM TECHNOLOGICAL UNIVERSITY
THIRD SEMESTER B.TECH DEGREE EXAMINATION, JANUARY 2017

Course Code: EE 205
Course Name: DC MACHINES AND TRANSFORMERS (EE)

Max. Marks: 100 Duration: 3 Hours
PARTA

(Answer all questions, each question carries 5 marks)

1. Explain the working of a DC motor. What is the significance of back e.m.f in DC
motors?

2. What is armature reaction in ‏عل‎ machines? How it affects the main flux distribution and
how can armature reaction be reduced?

3. Draw the phasor diagram of a practical transformer on load at (a) lagging 0.1 (0) leading
p.f (€) u.p.f.

Also draw the phasor diagram of an ideal transformer at no load.

4. Define the terms critical resistance and critical speed and bring out their roles in the
process of self excitation in dc machines. What are the conditions for voltage build up
in a DC shunt generator?

5. A4kVA, 400/200 V single phase transformer has resistance of 0.02 p.u and reactance
of 0.06 p.u. What is the value of its resistance and reactance referred to h.v side?

6. What are the necessary and desirable conditions for parallel operation of transformers?

7. Explain the principle of operation of a transformer. Derive the emf equation of a single
phase transformer.

8. What are the different connections of three phase transformers?

PART ந
(Answer any 2 questions, each question carries 10 marks)
9. Give the constructional features and working principle of a DC generator. Draw the
cross-sectional view of a 4 pole DC generator and label all the parts. Explain the
function of each part.

10. Explain the different methods of excitation of DC generators with suitable diagrams.

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