Anna University Coimbatore 2010 B.E Electrical and Electronics Engineering Electrical machines I - Question Paper

ANNA UNIVERSITY COIMBATORE
B.E./B.TECH. DEGREE EXAMINATIONS: MAY /JUNE 2010
REGULATIONS: 2008
FOURTH SEMESTER: ELECTRICAL & ELECTRONICS ENGG>
080280003-ELECTRICAL MACHINES I
TIME : three Hours Max.Marks : 100
PART-A
(20 × two = 40 MARKS)
ans ALL ques.
1. What is meant by fringing effect?
2. Define reluctance.
3. State Lenz's legal regulations.
4. What is stacking factor?
5. Write down the emf formula of a transformer?
6. Draw the no load phasor diagram of transformer?
7. Write the condition for maximum efficiency of transformer?
8. List the applications of auto transformer?
9. Write down the formula for force in magnetic field system?
10. What is an electro mechanical system?
11. Describe multiply excited magnetic field system?
12. Define co energy?
13. Describe rotating magnetic field.
14. What is the significance of magnetic leakage in rotating machine?
15. Write down the basic expression for mmf space wave of an single coil rectangular in nature?
16. State the assumptions for peak value of the mmf wave?
17. State Fleming's left hand rule.
18. What is the necessity for starters in a dc motor?
19. What is the function of commutator?
20. State 1 method for compensation of armature reaction?



PART – B
(5 × 12 = 60 MARKS)
ans ANY 5 ques.
21. a) In a rectangular electro magnetic relay, the exciting coil has 1200 turns.
Cross sectional area of the core is A = six cm × six cm. neglect the reluctance of the magnetic circuit and fringing effects. With coil current kept constant at 2A,derive expression for force on armature as a function of air gap of length x. obtain the work done by the magnetic field when x reduces from one cm to 0.5 cm by integrating the force.
b) Compare statically induced emf and dynamically induced emf?
22. a) explain the origin of hysteresis and eddy current losses in electrical machines.
b) A straight conductor of two m length carries a current of 20A. 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.
23. What are the different losses in a transformer? Derive the equivalent circuit of single phase transformer from its phasor diagram.
24. A five KVA 200/1000 V, 50Hz,single phase transformer gave the subsequent test results:
O.C test(LV side): 2000 V,1.2A,90W
S.C test(HV side): 50 V, five A, 110 W
(1) compute the parameters of the equivalent circuit referred to the LV side.
(2) compute the output secondary voltage when delivering three KW at 0.8 pf lagging, the input primary voltage being 200 V. obtain the percentage regulation also.
25. a) Derive the expression for field energy produced in a doubly excited magnetic field system?
b) The magnetic flux density on the surface of an iron face is 1.6 T which is a typical saturation level value for ferromagnetic material. obtain the force density on the iron face.
26. With necessary diagram explain about mmf waves of distributed AC winding in detail with equation?
27. a) The lap wound armature of a four – pole generator armature has 51 slots. every slot
contains 20 conductors. What will be the emf generated in machine when driven
at 1500 r.p.m. If useful flux per pole is 0.01Wb?
b) A 230 V shunt motor has an armature resistance of 0.3 Ohm and takes an armature
current of 40 A on a certain load. By how much must the main flux be decreased to
raise speed by 50% and 25% if the developed speed is constant? Neglect saturation and armature reaction.
28. a) Derive the torque formula of a dc machine?
b) A 223 V compound generator is supplying a load of 100 A at 220 V. The resistance of its
armature, shunt and series winding are 0.1?,50? and 0.06? repectively. obtain the induced emf and thearmature current when the machine is connected in
1) Short shunt 2)Long shunt

Earning:   Approval pending.