Electrical Drive MCQ

61. The condition of regenerative braking can be achieved by

(a) speed higher than no-load speed of overhauling load.
(b) increasing the excitation while supply voltage remains constant.
(c) increasing the armature current.
(d) either (a) or (b).

62. In a dc shunt motor, regenerative braking is limited by

(a) saturation of flux at high speed.
(b) saturation of flux at low speed.
(c) armature current at high speed.
(d) either (a) or (b).

63. For regenerative braking with dc series motors, its field windings are excited.

(a) separately
(b) series
(c) shunt
(d) either (b) or (c)

64. The most economical method of electric braking is

(a) plugging
(b) dynamic braking with separate excitation.
(c) dynamic braking with self excitation.
(d) regenerative braking.

65. The plugging provides……..braking torque in comparison to rheostatic and regenerative braking systems.

(a) negligible
(b) small
(c) highest

66. Rheostatic braking may be applied to an induction motor provided

(a) it is a squirrel cage type.
(b) it is a wound type.
(c) separate dc source for field excitation is available.
(d) variable external resistance is available.

67. The motor enclosure used for collieries, chemical plants is

(a) flame proof type.
(b) splash proof type.
(c) totally enclosed type.
(d) pipe ventilated type.

68. The motor enclosure used in woodworking industry is

(a) protected type.
(b) totally enclosed fan cooled type.
(c) flame-proof type.
(d) splash-proof type.

69. The motor enclosure used for industrial purposes is

(a) protected type.
(b) drip proof type.
(c) totally enclosed type.
(d) open type.

70. Rotor of a motor is usually supported on

(a) bush hearings.
(b) thrust bearing.
(c) ball or roller bearings.
(d) any of the above.

71. Ball-bearings are

(a) used up to 75 kW motors.
(b) of long life and low friction loss.
(c) costlier and noisy particularly at high motor speed.
(d) all of the above.

72. Sleeve bearings

(a) are normally of bronze.
(b) have self-lubricating properties.
(c) are used where noise is to be avoided.
(d) all of the above.

73. Hot bearings of a dc motor may be caused by

(a) poor ventilation.
(b) incorrect voltage.
(c) loose coupling.
(d) lack of or dirty lubricant.

74. Excessive motor vibration is caused by

(a) worn bearings.
(b) open armature coil.
(c) bent shaft.
(d) excessive brush tension.

75. Excessive sparking at the brushes may be caused due to

(a) loose coupling.
(b) worn bearings.
(c) dirt on commutator.
(d) open armature coil.

76. Intermittent sparking at the brushes of a dc motor may be caused due to

(a) intermittent load.
(b) open armature coil.
(c) loose coupling.
(d) incorrect voltage.

77. The least expensive drive is

(a) belt drive.
(b) rope drive.
(c) chain drive.
(d) gear drive.

78. For very high speed ratio the indispensable drive is

(a) direct drive.
(b) rope drive.
(c) chain drive.
(d) any of the above.

79. In rotating electrical machines, the insulation temperature limit for class B type is

(a) 105°C
(b) 130°C
(c) 150°C
(d) 180°C

80. For a particular motor, the cooling time constant is usually

(a) smaller than the heating time constant.
(b) greater than the heating time constant.
(c) equal to the heating time constant.