51. Iron losses of a machine are
a. directly proportional to flux density
b. directly proportional to the square of flux density
c. inversely proportional to flux density
d. inversely proportional to the square of flux density

52. While considering hysteresis loss in a transformer under which of the following the loss will not increase
a. when flux density is increased by 10%
b. when thickness of lamination is caused by 10%
c. when frequency is increased by 10%
d. when both thickness and frequency are increased by 10%

53. The shaft of electric motors is generally made of
a. mild steel
b. cast iron
c. copper
d. aluminium alloy

54. The critical speed of the shaft for an electrmotor should be
a. away from the operating speed
b. exactly the same as operating speed
c. half the operating speed
d. double of the operating speed

55. The shaft of electric motors is generally supported in
a. magnetic bearings
b. bush bearings
c. ball or roller bearings
d. cast iron bearings

56. Ball and roller bearings are generally lubricated by
a. graphite
b. grease
c. water
d. air

57. In case of ball bearings the ball are made of
a. cast iron
b. mild steel
c. carbon chrome steel
d. none of the above

58. A ball bearing is secured to the shaft by means of
a. key
b. slot
c. pin
d. sleeve

59. In case of 110 MW generators, the shaft is supported on
a. ball bearings
b. roller bearings
c. needle bearings
d. bush bearings

60. Which of the following is classed as anti friction bearing?
a. Ball bearing
b. roller bearing
c. needle bearing
d. all of the above