Anna University Chennai 2005 B.E Electrical and Electronics Engineering Third Semester EE 232 — ELECTRICAL MACHINES – I - Question Paper

B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2003.
Third Semester
Electrical and Electronics Engineering
EE 232 — ELECTRICAL MACHINES – I
Time : 3 hours Maximum : 100 marks
ans ALL ques..
PART A — (10 ? two = 20 marks)
1. What is meant by leakage flux?
2. State Fleming's left hand rule.
3. Why transformers are rated in kVA instead of kW?
4. Why is the efficiency of transformers more than that of other rotating
machines?
5. Mark the co energy region in the i ? ? graph.
6. What is a multiply excited magnetic field system?
7. Write the relation ranging from electrical and mechanical degree.
8. describe distribution factor.
9. What is the function of yoke in a D.C. machine?
10. provide 2 reasons for failure of build up of emf in d.c. shunt generator.
PART B — (5 ? 16 = 80 marks)
11. (i) discuss the different methods of commutation. (8)
D 035
2 D 035
(ii) With neat sketch, discuss the function of three point starter. (8)
12. (a) (i) What is a magnetization curve and why is it used in magnetic
circuit calculations? (8)
(ii) An iron ring of circular cross–section 10 cm2 and mean
circumference 30 cm has an air–gap of two mm. If the ring is wound
with 500 turns, obtain the exciting current to establish a flux of
0.4 mWb in the air gap. The relative permeability of iron may be
presumed to be 2500. (8)
Or
(b) (i) discuss about a.c. operation of magnetic circuits. (8)
(ii) A straight conductor of 1.5 m length carries a current of 40 A. It is
lying at right angles to a uniform magnetic flux density of 0.8 T.
obtain : (1) the force developed on the conductor (2) the power
needed to drive the conductor at a uniform speed of 25 m/s and
(3) the emf induced in the conductor. (8)
13. (a) (i) What is meant by equivalent circuit of a transformer? What is its
use? (8)
(ii) A 600 kVA, single phase transformer when working at u.p.f. has an
efficiency of 92% at full load and also at half load. Determine its
efficiency when it operates at unity p.f. and 60% of full load. (8)
Or
(b) (i) discuss the back to back method of testing of 2 identical single
phase transformers. (8)
(ii) State and discuss the necessary conditions for satisfactory parallel
operation of 2 3 phase transformers. (8)
14. (a) (i) discuss about energy stored in magnetic system. (8)
(ii) provide a brief note on flow of energy in electromechanical devices. (8)
Or
(b) (i) Derive expression for coenergy in a multiply–excited magnetic field
system. (8)
3 D 035
(ii) The magnetic flux density on the surface of an iron face is 1.8 T
which is a typical saturation level value for ferromagnetic material.
obtain the force density on the iron face. (8)
15. (a) (i) discuss the constructional features of elementary synchronous
machines. (8)
(ii) A 3–phase 440 kVA, 50 Hz, star connected synchronous generator
running at 400 rpm is designed to develop 3600 V ranging from
terminals. The armature consists of 180 slots, every slot having one
coil side with 8 conductors. Determine the peak value of the
fundamental mmf in AT/pole when the machine is delivering full
load current. (8)
Or
(b) (i) discuss about rotating MMF waves in A.C. machines. (8)
(ii) Derive an expression for induced emf in a synchronous machine. (8)
———————

Earning:   Approval pending.