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MCQ of Electrical Machines page-3

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41. The relationship between no-load generated e.m.f. in the armature and field excitation of a d.c. generator is known as

(a) Internal characteristic of generator.
(b) External characteristic of generator.
Magnetic characteristic or open circuit characteristic of generator.
(d)Total characteristic of generator.

42. The critical resistance of the d.c. generator is the resistance of

(a) armature.
(b) load.
field.
(d) brushes.

43. When a bank of two single-phase transformers in an open delta arrangement is used, each of them supplies

(a) 33.3% of its output rating.
86.6% of its output rating.
(c) 48.6% of its output rating.
(d) 100% of its output rating.

44. Two single phase transformers with proper connections can be used to achieve a three phase output from three phase input.

true.
b)false.

45. A thicker wire is used in the d.c. series field winding than d.c. shunt field winding in d.c. machines

(a) To prevent mechanical vibrations.
(b) To produce large flux.
Because it carries the load current which is much higher than shunt field current for the same rating of d.c machines.
(d) To provide strength.

46. Transformer core is laminated

(a) Because it is difficult to fabricate solid core.
(b) Because laminated core provides high flux density.
To avoids eddy current and hysteresis losses.
(d) To increase the main flux.

47. The mechanical power developed by the d.c. motor is maximum when

(a) Back e.m.f. is equal to applied voltage.
(b) Back e.m.f is equal to zero.
Back e.m.f. is equal to half the applied voltage.
(d) None of the above.

48. Which of the motor is used to drive the constant speed line shafting, lathes, blowers and fans ?

D.C. shunt motor.
(b) D.C. series motor.
(c) Cumulative compound motor.
(d) None of the above.

49. In series-parallel control method when two d.c. series motors are connected in series, the speed of the set is

(a) Half of the speed of the motors when connected in parallel.
One-fourth of the speed of motors when connected in parallel.
(c) Same as in parallel.
(d) Rated speed of any one of the motors.

50. The torque produced by series combination of two d.c. series motors is

(a) Equal to the torque when they are connected in parallel.
(b) Half of the torque when they are connected in parallel .
Four times of the torque when they are connected in parallel.
(d) Twice of the torque when they are connected in parallel.

51. The use of the starter in d.c. motors is necessary because

(a) They are not self-starting.
Initially there is no back e.m.f. and armature current is very high which is to be avoided by using high resistance starter.
(c) To overcome back e.m.f..
(d) None of the above.

52. The speed of the d.c. motor can be varied

(a) By varying field current only.
(b) By varying field current or armature resistance.
(c) By varying supply voltage only.
(b) or (c).

53. The electrical energy can be converted into chemical energy.

true.
(b)false.

54. The mercury are rectifier can be used as an inverter.

true.
(b) false.

55.The hum in the transformer is mainly attributed to

(a) Laminations of the transformer.
Magnetostriction.
(c) Oil of the transformer.
(d) Walls of the tanks.

56. To obtain the sinusoidal voltage the poles should have the shape such that the length of the air gap at any point is

(a) Proportional to 1/sinθ, where θ is the angle measured in electrical degrees between the point in the question and centre of the pole.
Proportional to 1/cos⁡θ, where θ is same as defined in (a).
(c) Proportional to sin⁡θ.
(d) Proportional to cos⁡θ.

57. The power transformer is a

(a) Constant current device.
Constant main flux device.
(c) Constant voltage device.
(d) Constant power device.

58. An induction motor is

(a) Self-starting with zero torque.
(b) Self-starting with high torque.
Self-stating with small torque than rated torque.
(d) None of the above.

59.As the load is increased the speed of d.c. shunt motor will

(a) Increase proportionately.
(b) Remains almost constant.
(c) Increase slightly.
Reduce slightly.

60. Two transformers operating in parallel will share the load depending upon their

(a) ratings.
(b) leakage reactance.
(c) efficiency.
per unit impedance.

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