Magnetic Circuit

11. Laminated cores, in electrical machines, are used to reduce

  1. copper loss
  2. eddy current loss
  3. hysteresis loss
  4. all of the above

Correct answer: (B)
eddy current loss

12. Permanent magnets are normally made of

  1. alnico alloys
  2. aluminium
  3. cast iron
  4. wrought iron

Correct answer: (A)
alnico alloys

13. Permeability in a magnetic circuit corresponds to _________ in an electric circuit.

  1. resistance
  2. resistivity
  3. conductivity
  4. conductance

Correct answer: (C)
conductivity

14. Point out the wrong statement. Magnetic leakage is undesirable in electric machines because it

  1. lowers their power efficiency
  2. increases their cost of manufacture
  3. leads to their increased weight
  4. produces fringing

Correct answer: (A)
lowers their power efficiency

15. Reciprocal of reluctance is

  1. reluctivity
  2. permeance
  3. permeability
  4. susceptibility

Correct answer: (B)
permeance

16. Relative permeability of vacuum is

  1. 1
  2. 1 H/m
  3. 1/4JI
  4. 4n x 10-‘ H/m

Correct answer: (A)
1

17. Silicon steel is used in electrical machines because it has

  1. low coercivity
  2. low retentivity
  3. low hysteresis loss
  4. high coercivity

Correct answer: (C)
low hysteresis loss

18. The initial rate of rise of current through a coil of inductance 10 H when suddenly connected to a D.C. supply of 200 V is _________ Vs

  1. 50
  2. 20
  3. 0.05
  4. 500

Correct answer: (B)
20

19. The property of a material which opposes the creation of magnetic flux in it is known as

  1. reluctivity
  2. magnetomotive force
  3. permeance
  4. reluctance

Correct answer: (D)
reluctance

20. The rate of rise of current through an inductive coil is maximum

  1. at 63.2% of its maximum steady value
  2. at the start of the current flow
  3. after one time constant
  4. near the final maximum value of current

Correct answer: (B)
at the start of the current flow

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