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Name of the College : Kings College Of Engineering
Department : Electrical & Electronics Engineering
Subject : Electrical Machine Design
Website : kings.ac.in
Document Type : Question Bank

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Kings Electrical Machine Design Question Paper

Unit I

Fundamental Concepts :
Part – A : (2 Marks)
1. What are the main dimensions of a rotating machine?
2. Define gap contraction factor for slots.
3. Define total gap contraction factor.

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4. What is Carter’s Co-efficient? What is its usefulness in the design of dc machine?
5. What is the effect of salient poles on the air gap mmf?
6. Define Field form factor.
7. List the methods used for estimating the mmf for tapered teeth.
8. What is real and apparent flux density?
9. In which way the air gap length influence the design of machines?
10. What is magnetic leakage and leakage co-efficient?

11. What is fringing flux?
12. What are the differences between leakage flux and fringing flux?
13. What is magnetic circuit and what are it’s constituents.
14. Define gap contraction factor for ducts.
15. What is tooth top leakage flux?
16. What is Zig – Zag leakage flux?
17. What is skew leakage flux?
18. How will you minimize the leakage flux?
19. Define slot space factor.
20. Discuss the parameters governing the selection of conductor dimensions.

Part – B : (16 Marks)
1. a) Discuss in detail about the cooling methods adopted in transformers. (10)
b) What are the advantages of hydrogen as a better cooling medium for turbo alternators? (6)
2. a) Discuss about the various types of thermal ratings of the electrical machines (10)
b) Discuss about the various Insulating materials and their grades. (6)
3. a) Explain in detail about the MMF calculation for tapered Teeth. (10)
b) Discuss in detail about the real and apparent flux densities. (6)
4. a) Write about specific magnetic loading and specific electric loading. (8)
b) Explain about the cooling of Turbo alternators. (8)
5. a) What are the direct and indirect cooling methods used in electrical machines (8)
b) Derive an equation for the slot leakage reactance (8)
6. a) Discuss in detail about the unbalance magnetic pull. (8)
b) Explain about the air gap reluctances in different types of armature slots (8)

Unit II

DC Machines :
Part – A (2 Marks) :
1. Define specific magnetic loading.
2. Define specific electric loading.
3. What is output equation?
4. Write the expressions for output coefficients
5. List the factors that influence the separation of D and L of a dc machine.
6. What is square pole criterion?
7. List the various L/_ ratios used for separation of D and L in induction motor.
8. In a dc machine what are the limiting values of armature diameter?
9. What is un-balanced magnetic pull and how its ill-effects can be overcome?
10. What are the factors that affect the size of rotating machines?
11. What are the factors that decide the choice of specific magnetic loading?
12. .What are the factors that decide the choice of specific electric loading?
13. What is magnetization curve?
14. What are the factors that modify the reluctance of air gap
15. What are the problems encountered in estimating the mmf for teeth?
16. What factor decides the number of turns in a winding?
17. How the area of cross-section of a conductor is estimated?
18. What are the ranges of specific magnetic loading and specific electric loading in dc machine?
19. What are the factors to be considered for the selection of number of poles in a dc machine?
20. What are the parameters that are affected by the number of poles?
21. List the advantages and disadvantages of large number of poles.
22. Why square pole is preferred?
23. State the difference between the armature winding of dc machine and the stator winding of ac machine.
24. Define winding pitch.
25. What is back pitch?
26. What is front pitch?
27. Define commutator pitch.
28. What is equalizer connection?
29. What are the factors to be considered for estimating the length of air gap in dc machines?
30. What are the effects of armature reaction?
31. How the polarities of inter pole are decided.
32. What is the effect of interpole on main pole?
33. What is the fundamental requirement of a good insulating material?
34. What is the importance of temperature as a factor in the life of insulating materials?
35. Why mineral insulating oils are the most widely used liquid insulation.
36. Why large size machines have large rating time constant?
37. Why ac armature winding is always made short-pitched.
38. Why equalizer connections are necessary for the armature winding of a dc machine with lap winding.
39. Why the voltage wave form of an ac armature winding contains harmonics. ?
40. Why the conductor eddy current loss increases if embedded deeper in the slot.

Part – B :(16 Marks)
1. a) A 4 pole, 25 HP, 500 V, 600 rpm series motor has an efficiency of 82%. The pole faces are square and the ratio of pole arc to pole pitch is 0.67. Take Bav = 0.58 Wb/m2 and ac = 17000 amp.cond/m. Obtain the main dimensions of the core. (12) b) Enumerate the procedure for shunt field design (4)
2. a) A 4 pole, 400 V, 960 rpm, shunt motor has an armature of 0.3 m in diameter and 0.2 m in length. The commutator diameter is 0.22 m. Give full details of a suitable winding including the number of slots, number of commutator segments and number of conductors in each slot for an average flux density of approximately 0.55wb/m2Wb / m2 in the air gap. (12)
b) Find the main dimension and number of poles of a 37 kW, 230V, 1400 rpm, shunt motor so that a square pole face is obtained. The average gap density is 0.5 wb/m2 and the ampere conductors / meter are 22000. The ratio of pole arc to pole pitch is 0.7 and the full load efficiency is 90% (4)
3. i) Derive the out put equation of a DC machine. (10)
ii) Distinguish between lap winding and wave winding. (6)
4. i) Determine the total commutator losses for a 1000kw, 500V, 800rpm, 10 poles generator. Given that commutator diameter is 1.0m, current density at brush contact =75×10-3 A/mm2 brush pressure = 14.7kv/ m2, co efficiency of friction = 0.28, brush contact drop= 2.2V. (10)
ii) Discuss the significance of specific loadings in the design of DC machines. (6)
5. i) Design a suitable commutator for a 350kw, 600rpm, 440V, 6poles DC generator having an armature diameter of 0.75m. The number of slots is 288. Assume suitable values where it’s necessary? (10)
ii) Discuss the choice of poles and speed in DC machine. (6)
6. i) List out the procedure involved in design of shunt field winding and series field winding? (10)
ii) Find the main dimension and number of poles of a 37 kW, 230V, 1400 rpm, shunt generator so that a square pole face is obtained. The average gap density is 0.7 wb/m2 and the ampere conductors / meter are 25000. The ratio of pole arc to pole pitch is 0.7 and the full load efficiency is 90 % (6)
7. Determine the shunt field winding of a 6-pole, 440V, dc generator allowing a drop of 15 % in the regulator. The following design date are available, mmf per pole = 7200 AT; mean length of turn = 1.2 m; winding depth = 3.5 cm; watts per sq.cm. of cooling surface = 650. Calculate the inner, outer and end surfaces of the cylindrical field coil for cooling. Take diameter of the insulated wire to be 0.4 mm greater than the bare wire. Assume 2 micro – ohm / cm as the resistivity of copper at the working temperature. (16)
8. Determine the diameter and length of armature core for a 55kW, 110V, 1000 rpm, 4 pole, shunt generator, assuming specific electric and magnetic loadings of 26000 amp.cond. / m and 0.5 Wb / m2 respectively. The pole arc should be about 70% of pole pitch and length of core about 1.1 times the pole arc. Allow 10 ampere for the field current and assume a voltage drop of 4 volts for the armature circuit. Specify the winding to be used and also determine suitable values for the number of armature conductors and slots. (16)