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Sunday, 1 November 2015

Magnetic Circuits - Multiple Choice Questions

Magnetic Circuits - Multiple Choice Questions Points : magnetic circuits multiple choice questions and answers, electrical objective type question answers, mcqs 1. M.M.F. in a magnetic circuit correspond to in an electric circuit.
(i) voltage drop
(ii) potential difference
(iii) electric Intensity
(iv) e.m.f

2. Magnitude of AT necessary for air gap is much more than that necessary for iron part of a magnetic circuit as ……
(i) air is a gas
(ii) air has the lowest relative permeability
(iii) air is a conductor of magnetic flux
(iv) none of the above

3. The SI unit of reluctance be
(i) AT/Wb
(ii) AT/m
(iii) AT
(iv) N/Wb

4. Reluctance of a magnetic circuit is …… relative permeability of material comprise circuit.
(i) directly proportional to
(ii) inversely proportional to
(iii) independent of
(iv) none of the above

5. Permeance of a magnetic circuit is …….. area of x-section of circuit.
(i) inversely proportional to
(ii) directly proportional to
(iii) independent of
(iv) none of above

6. Value of Leakage co-efficient for electric al machines is frequently concerning …..
(i) 0.5 to 1
(ii) 4 to 10
(iii) above 10
(iv) 1.15 to 1.25

7. Magnetic circuit has a m.m.f. of 400 AT and a reluctance of 2 x 105 AT/Wb. The magnetic fIux in magnetic circuit is
(i) 3 x 10-5Wb
(ii) 2x 10-3Wb
(iii) 1.5 x 10-2 Wb
(iv) 2.5 x 10-4 Wb

8. Reluctance of a magnetic circuit depends upon ……
(i) current in the coil
(ii) no of turns of coil
(iii) flux density in the circuit
(iv) none of above

9. A 2cm long coil has 10 turns and carry a current of 750 mA. The magnetising force of coil is
(i) 225 AT/m
(ii) 675 AT/m
(iii) 450 AT/m
(iv) 375 AT/m

10. Whatever may be the flux density in …… material will by no means saturate.
(i) soft iron
(ii) cobalt steel
(iii) air
(iv) silicon steel

11. Hysteresis loop for materials have elevated receptivity is
(i) wide
(ii) narrow
(iii) straight line passing through origin
(iv) data insufficient

12. Out of following materials, area of hysteresis Loop will be slightest for ……
(i) wrought iron
(ii) hard steel
(iii) silicon steel
(iv) soft iron

13. In electro-mechanical change devices (e.g. motors and generators), a small air gap is left among the rotor and stator in order to …..
(i) whole magnetic path
(ii) decrease the unwillingness of magnetic path
(iii) permit mechanical clearance
(iv) increase flux density in air gap

14. Hysteresis is the occurrence of ...... in a magnetic circuit.
(i) lagging of B behind H
(ii) lagging of H behind B
(iii) setting up constant flux
(iv) none of the above

15. At low frequencies, material use for transformer cores is
(i) copper
(ii) silicon iron
(iii) soft iron
(iv) none of above

16. B.H curve of will not be a straight line.
(i) air
(ii) copper
(iii) wood
(iv) soft iron

17. Magnetic material use for ….. should contain a large hysteresis loop.
(i) transformers
(ii) d.c. generators
(iii) ac. motors
(iv) permanent magnets

18. While transformer primary is fed as of ac….. its core heats up due to ……
(i) permeability of core
(ii) reluctance of core
(iii) ferromagnetism
(iv) hysteresis loss

19. The B-H curve for ….. will be a straight line temporary through origin.
(i) air
(ii) soft iron
(iii) hardened steel
(iv) silicon steel

20. B-H curve is use to locate the m.m.f. of ……. in a magnetic circuit.
(i) air gap
(ii) iron part
(iii) both air gap and iron part
(iv) none of the above

21. Air-cored coil carries stable current. If air-core is replace through a ferromagnetic material, flux density in core will
(i) remain same
(ii) increase
(iii) decrease
(iv) none of above

22. Reluctance of a magnetic circuit varies as ……
(i) length x area
(ii) length + area
(iii) area + length
(iv) (length)2 + area

23. What is magnetic field strength in a material whose relative permeability is 1 while flux density is 0.005 T?
(i) 250 AT/m
(ii) 452 AT/m
(iii) 3980 AT/m
(iv) 1715 AT/m

24. Unit of relative permeability is
(i) AT/m
(ii) N/Wb
(iii) AT/m2
(iv) a number

25. The hyteresis cycle for material of a transformer is
(i) tall and narrow
(ii) tall and wide
(ii) short and narrow
(iv) short and wide

26. Saturation flux density for the majority magnetic materials is concerning …….
(i) 0.5Wb/m2
(ii) 10 Wb/m2
(iii) 2 Wb/m2
(iv) 1 Wb/m2

27. Slope of B.H curve of a material gives its
(i) absolute permeability
(ii) relative permeability
(iii) retentivity
(iv) none of above

28. Hysteresis loss in a ferromagnetic material is
(i) directly proportional to area of loop
(ii) inversely proportional to area of loop
(iii) directly proportional to square of area of loop
(iv) none of above

29. If a magnetic material is placed in a coil through which irregular current (50 Hz frequency) flows, then …… hysteresis loops will be formed each second
(i) 50
(ii) 23
(iii) 100
(iv) 150

30. At high frequencies, transformer cores is
(i) silicon iron
(ii) soft iron
(iii) ferrite
(iv) none of above

31. An electromagnet uses
(i) soft iron core
(ii) steel core
(iii) nickel core
(iv) copper core

32. B-H curve of a ferromagnetic material is
(i) Linear
(ii) non-Linear
(iii) circle
(iv) none of above

33. SI unit of μ0
(i) AT/m
(ii) Wb/m
(iii) Wb/AT-m
(iv) none of above

34. In practical magnetic circuits, air-gap is reserved
(i) very small
(ii) large
(iii) very large
(iv) none of above

35. Materials used for core of a go relay should have …… hysteresis loop.
(i) large
(ii) very large
(iii) narrow
(iv) none of the above

36. Ohm’s law for magnetic circuit is
(i) reluctance = flux/m.m.f.
(ii) m.m.f. = reluctance / flux
(iii) m.m.f. = flux/ reluctance
(iv) m.m.f. = flux x reluctance

37. Hysteresis loop of a material have low retentivity is
(i) wide
(ii) narrow
(iii) very wide
(iv) none of above

38. Ferromagnetic material exhibit hysteresis as
(i) they are taken to saturation only
(ii) they are not taken even to saturation
(iii) they are heated to very high temperature
(iv) none of above

39. Curie temperature of iron is
(i) 200°C
(ii) 430°C
(iii) 770°C
(iv) 550°C

40. Hysteresis loss can be compact by
(i) increasing m.m.f. of the circuit
(ii) using material of narrow hysteresis loop
(iii) laminating magnetic circuit
(iv) none of the above

41 Hysteresis loop of core material of a transformer must have
(i) a Large area
(ii) a small area
(iii) an infinite area
(iv) none of above

42. A permanent magnet must have
(i) low remanence
(ii) high remanence
(iii) zero remanence
(iv) none of above

43. Relative permeability of air is is
(i) 0
(ii) 1
(iii) infinite
(iv) none of above

44. Value of relative permeability of ferromagnetic materials ….. with enhance in temperature
(i) increases
(ii) decreases
(iii) remains the same
(iv) none of above

45. Magnetic shields are prepared as of materials having
(i) low permeability
(ii) high permeability
(iii) zero permeability
(iv) none of above

46. A permanent magnet must have
(i) high coercivity
(ii) low coercivity
(iii) zero coercivit
(iv) none of above

47. Magnetic materials that have elevated retentivity have a
(i) narrow hysteresis loop
(ii) circular hysteresis loop
(iii) nearly square hysteresis loop
(iv) none of above

48. In an electromagnet, flux density at saturation should be
(i) high
(ii) by
(iii) zero
(iv) none of above

49. Minimum relative permeability of a material can be
(i) 1
(ii) slightly less than 1
(iii) 0005
(iv) none of above

50. Owing to fringing at the air gaps in a magnetic circuit, efficient area of the air gaps is
(i) increased
(ii) decreased
(iii) remains same
(iv) none of above

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