electrical machine design Mcqs
1. A current density of __________ is ilsed for large power transformers with forced circulation of oil or with water cooling coils?
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A. 1.5 to 2.5 A/mm2

B. 4.0 to 5.0 A/mm2

C. 5.4 to 6.2 A/mm2

D. 3.5 to 4.5 A/mm2

2. In D.C. machines the width of the duct is usually_____________?
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A. 6 mm

B. 4 mm

C. 8 mm

D. 10 mm
3. ____________are always double layer type?
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A. Open windings

B. Closed windings

C. Either of the above

D. None of the above

4. If an insulating material is operated beyond the maximum allowable temperature, its life is_____________?
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A. unaffected

B. drastically increased

C. none of the above

D. drastically reduced
5. In a D.C. machine the number of brush arms is _________ the number of poles?
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A. less than

B. none of the above

C. equal to

D. greater than

6. The design of electromagnets is based upon which of the following fundamental equations ?
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A. Force equation

B. Voltage equation

C. All of the above

D. Magnetic circuit equation

E. Heating equation

7. D.C. machines designed with a large value of air gap length have_____________?
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A. worst, ventilation

B. none of the above

C. poor ventilation

D. better ventilation
8. impose limitation on design____________?
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A. Power factor

B. Efficiency

C. All above

D. Saturation

E. Temperature rise

9. The action of electromagnetic machines can be related to which of the following basic principles __________?
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A. Induction

B. Interaction

C. Alignment

D. All of the above
10. Direct water cooling of rotor winding presents_____________?
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A. none of the above

B. lesser mechanical difficulties

C. no mechanical difficulties

D. greater mechanical difficulties
11. When the two coil sides forming a coil are spaced exactly one pole pitch apart they are said to be of_____________?
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A. full pitch

B. none of the above

C. either of the above

D. short pitch

12. The length cf air gap to be provided in __________ is primarily determined by power factor consideration?
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A. d.c. shunt motor

B. induction motor

C. d.c. series motor .

D. synchronous motor

13. Hot rolled sheets have ___________ value of permeability?
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A. low

B. zero

C. none of the above

D. high

14. The insulation between windings and grounded core and the insulation between the windings of the same phase is called__________________?
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A. minor insulation

B. either of the above

C. none of the above

D. major insulation
15. In _________ machines, the size of the shaft is decided by the critical speed which depends on the deflection of the shaft?
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A. medium

B. any of the above.

C. small

D. large
16. With direct water cooling it is possible to have ratings of about_____________?
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A. 300 MW

B. 400 MW

C. 600 MW

D. 200 MW

17. The input impedance of a JFET is _____________that of an ordinary transistor?
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A. less than

B. more than

C. none of the above

D. equal to

18. __________is extensively used for those portions of magnetic circuit which carry steady flux and need superior mechanical properties ?
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A. Grey cast-iron

B. Stainless steel

C. High carbon steel

D. Cast steel
19. A practical formula for determining the thickness of insulation between low voltage and high voltage windings is__________________?
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A. 1 + 0.2 kVmm

B. 5 + 0.9 kV mm

C. 2 + 0.5 kVmm

D. 4 + 0.7 kV mm

20. Which of the following in an application of D.C. motors?
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A. Battery driven vehicles

B. All of the above

C. Traction

D. Automatic control

E. Drives for process industry

21. D.C. windings are____________?
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A. none of the above

B. never 2-layer type

C. always 2-layer type

D. sometimes 2-layer type

22. By which of the following methods the temperature rise of windings and other parts may be determined ?
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A. Embedded temperature detector method

B. Resistance method

C. Thermometer method

D. Any of the above
23. Commercial available medium size machines have a speed range of __________ ?
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A. 600 to 1000 r.p.m

B. 2000 to 2500 r.p.m

C. 1000 to 1500 r.p.m

D. 200 to 400 r.p.m

24. The surge phenomenon is particularly important in case of_______________?
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A. high voltage transformers

B. medium voltage transformers

C. any of the above

D. low voltage transformers

25. The voltage control in electric supply networks in required on account of which of the following reasons ?
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A. All of the above

B. Control of active and reactive power

C. Adjustment of voltage at consumers premises within statutory limits

D. Adjustment of short period daily and seasonal voltage variations in accordance with variations of load

26. The laminations of the armature of a D.C. machine are usually ___________ thick?
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A. 0.2 to 0.3 mm

B. 0.3 to 0.4 mm

C. 0.1 to 0.2 mm

D. 0.4 to 0.5 mm
27. Tractive magnets are operated from____________?
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A. either a.c. or d.c. supply

B. d.c. supply

C. none of the above</strong>

D. a.c. supply

28. Machines cooled by direct cooling method may be called_____________?
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A. any of the above

B. supercharged

C. inner cooled

D. conductor cooled

29. In a D.C. machine the current per brush arm should not be more than_____________?
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A. 300 A

B. 200 A

C. 400 A

D. 100 A

30. The cold rolled grain oriented steel has ___________ permeability in the direction of the grain orientation?
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A. nil

B. maximum

C. minimum

D. none of the above

31. Electrical machines having a power output up to about 750 W may be called__________machines?
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A. any of the above

B. large size

C. medium size

D. small size
32. The value of exciting or magnetizing current depends upon which of the following factors ?
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A. The number of turns in the exciting winding

B. All of the above

C. The way in which the winding is distributed

D. Total m.m.f. required

33. Yokes with rectangular cross-section are used for_______________?
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A. any of the above

B. large capacity transformers

C. medium capacity transformers

D. small capacity transformers
34. In D.C. machines, ir order to prevent excessive distortion of field form by the armature reaction, the field mmf must be made____________?
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A. less in comparison with the armature mmf

B. large in comparison with the armature mmf

C. none of the above

D. equal to that of armature mmf

35. Which of the following methods is most accurate ?
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A. Method of average losses

B. Equivalent power method

C. Equivalent torque method

D. Equivalent current method
36. Electrical machines having power outputs ranging from a few kW up to approximately 250 kW may be classified as_____________?
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A. any of the above

B. medium size machines

C. large size machines

D. small size machines

37. __________ has a low relative permeability and is used principally in field frames when cost is of primary importance and extra weight is not objectionable?
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A. Soft steel

B. Aluminium

C. Cast iron

D. Cast steel

38. Disc windings are primarily used in_____________-?
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A. medium capacity transformers

B. short capacity transformers

C. high capacity transformers

D. any of the above

39. Transformers with a capacity of up to __________ have a cooling radiator system with natural cooling?
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A. 5 MVA

B. 2 MVA

C. 7.5 MVA

D. 10 MVA
40. In D.C. machines the usual limit of slot pitch is________________?
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A. between 5 to 10 mm

B. between 15 to 20 mm

C. between 10 to 15 mm

D. between 25 to 35 mm
41. On which of the following variables heat convected depends ?
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A. Gravitational constant

B. Thermal resistivity, density, specific heat

C. All of the above

D. Temperature difference between heated surface and coolant

E. Power density

42. The noise produced by a ___________ cooled machine is less as the rotor moves in a medium of smaller density?
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A. hydrogen

B. air

C. none of the above

D. either A. or B.

43. The flash point of transformer oil should be higher than_______________?
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A. 104C

B. 60C

C. 80C

D. 40C

44. Materials exhibiting zero value of resistivity are known as ____________ ?
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A. conductors

B. superconductors

C. insulators

D. semiconductors

45. The distance between the starts of two consecutive coils measured in terms of coil sides is called_____________?
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A. winding pitch

B. back pitch

C. commutator pitch

D. front pitch

46. For 275 kV transformers, using cold rolled grain oriented steel, which of the following values of flux density may be used?
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A. 1.6 Wb/m2

B. None of the above

C. 1.0 Wb/m2

D. 1.1 Wb/m2

E. 1.3 Wb/m2

47. The slot leakage can be calculated by making which of the following assumptions ?
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A. All of the above

B. The leakage path is straight across the slot and around the iron at the bottom

C. The permanence of air paths is only considered. The reluctance of iron paths is assumed as zero

D. The current in the slot conductors is uniformly distributed over their cross-section

48. The heat dissipating capability of transformers of ratings higher than 30 kVA in increased by providing which of the following ?
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A. Corrugations

B. Fins

C. All of the above

D. Tubes

E. Radiator tanks

49. Which of the following methods is used for air cooling of turbo-alternators ?
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A. All of the above

B. One sided axial ventilation

C. Two sided axial ventilation

D. Multiple inlet system

50. Which of the following problem arises in D.C. motors which are fed from thyristor bridge circuits ?
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A. Increased I R losses

B. Increased core losses

C. Poor commutation

D. All of the above

E. Change in motor parameters