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Reg.
APJ ABDUL KALAM TECHNOLOGICAL UNIVERSITY
THIRD SEMESTER B.TECH DEGREE EXAMINATION, JANUARY 2017
Course Code: EE 205
Course Name: DC MACHINES AND TRANSFORMERS
Max. Marks: 100 Duration: 3 Hours
PART A
(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 dc 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 lagging p.f
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. A 4 kVA, 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 B
(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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11. A short shunt compound generator has armature, series field shunt field resistances
of 0.06Ω, 0.03Ω and 110Ω respectively. It supplies 100 lamps rated at 250V, 40W.
Find the generated e.mf. Assume that contact drop per brush 1V.
PART C
(Answer any 2 questions, each question carries 10 marks)
12. Why is a starter necessary for a motor? Give the diagram and explain the working of a
three- point starter for a shunt motor including the features of no volt release' and
'overload release'.
13. A 200V series motor has a total resistance of 0.5Ω. It runs at 800 rpm taking an input
current of 10A. Find the series resistance required to reduce the speed to 600 rpm, the
input current being kept constant.
14. A transformer on no load has a core loss of 50W, draws a current of 2A and has
an induced emf of 230V (rms). Determine the no load power factor, core loss current
and magnetizing current. Also calculate the no load circuit parameters of the
transformer. Neglect winding resistances leakage flux.
PART D
(Answer any 2 questions, each question carries 10 marks)
15. Explain sumpner's method of testing transformers. What are its advantages over OC
and SC tests?
16. Explain the operation of an autotransformer. How saving of copper is achieved in an
autotransformer as compared to an ordinary two winding transformer. What are the
advantages of autotransformer over conventional two winding transformer?
17. The efficiency of 100kVA, 110V/220V, 50Hz single phase transformer is 98.5% at
half full load and 0.8 pf lead and 98.8% at full load upf, find Iron loss Full load
copper loss maximum efficiency at upf.