1. A dc machine is run at rated speed in forward direction and then in backward direction. It is observed that, speeds of the rotation are different, then it leads to the conclusion of
Correct : A. incorrect brush placement
2. For a dc machine, its commutator has a diameter of 50 c rotating at 1000 rpm. For a brush width of 1 cm, the time commutation taken by the machine will be
Correct : A. 0.382 ms
3. An induction motor can be said analogous to
Correct : A. transformer
4. A 3-phase induction motor with its rotor blocked behaves similar to a
Correct : A. transformer under short circuit of secondary terminals
5. No load current in induction motor is 10- 20% of full load current and the no load current of transformer is 2-6%.
Correct : A. true
6. no load estimated current for the machine?
Correct : A. 12 a
7. The no load current of the transformer is very less due to
Correct : A. mutual flux having low reluctance iron core
8. The no load current of the induction motor is high due to
Correct : A. long and high reluctance path between stator and rotor
9. At no load induction motor has possible power factor as
Correct : A. 0.2
10. Mechanically air gaps in induction motor are kept very low to avoid
Correct : D. all of the mentioned
11. The low no load power factor
Correct : A. reduces full load operating pf
12. An induction motor when started on load, it does not accelerate up to full speed but runs at 1/7th of the rated speed. The motor is said to be
Correct : C. crawling
13. The great advantage of the double squirrel-cage induction motor over single cage rotor is that its
Correct : D. starting current is lower
14. The rotor of 3-phase slip ring induction motor is fed from a 3-phase supply with its stator winding short circuited having rotor rotating clockwise at a speed of Nr, then the
Correct : A. speed of air gap field w.r.t. stator is ns-nr anticlockwise
15. (II) If a large 3–phase squirrel-cage induction motor with no load is started at full voltage, it will be damaged.
Correct : A. i is true, ii is false
16. The PWM control of DC motor varies
Correct : A. linearly with speed
17. Ward-Leonard system of system of speed control is not recommended for
Correct : A. constant speed operation
18. Mark the wrong option. Which of the following cause and effect behaviour in speed control is correct when field resistance is increased.
Correct : C. increase in emf
19. resistance will
Correct : A. decrease the speed of motor
20. by
Correct : A. armature voltage control
21. must be ensured that
Correct : A. it is used on separately excited machine
22. works well for speed
Correct : A. below, above
23. The torque limit of speed for a shunt motor
Correct : A. remains constant till base speed
24. A laboratory group was working with a set of 3-hp shunt DC motor. But there was a mistake that it was fused with a 0.3A fuse instead 3A fuse. Then it was started
Correct : B. it ran for 3s and fuse was blown
25. avoided by using
Correct : B. compensating winding
26. fast when under-loaded?
Correct : A. due to weak field
27. Concentrated winding differ from distributed winding with the concern of
Correct : A. identical magnetic axis
28. We can place closed windings in
Correct : A. ac commutator machine
29. Which of the following machines can be used to place open slot winding?
Correct : A. ac machine
30. The simplex lap winding has the range of winding pitch of
Correct : A. (-2,2)
31. A 200V dc machine has 4 poles and 40 coils, having simplex lap winding. The number of commutator segments which required in the given machine will be?
Correct : A. 40
32. Commutator segments in a DC shunt machine is equal to the number of
Correct : A. coil sides
33. We can employ dummy coils in a DC machine to
Correct : A. compensate reactance voltage
34. In AC machines we should prefer double layer winding over single layer windings because
Correct : A. it requires identical coils
35. For an electrical machine with C number of coils and P poles, the distance between the coils connected by an equalizer ring is?
Correct : C. 2c/p
36. With a P-pole DC machine with armature current Ia, the current per brush arm for a lap connected windings is?
Correct : C. 2ia/p
37. unused component in the machine, then we must
Correct : A. s/p/2 should not be an integer
38. While doing regular checks on the dc machine with the lap connected winding, it is reported to have ammeter fluctuations, this can be due to
Correct : D. all of the mentioned
39. with the remaining brushes, voltage and power that could be obtained from the machine are
Correct : A. e, p
40. Ward Leonard method is
Correct : C. combination of armature control method and field control method
41. Reducing the armature voltage will give us
Correct : B. constant torque speed control
42. b) DC generator
Correct : D. decrease field current
43. Reducing the field current will give us
Correct : B. constant torque speed control
44. Speed-power characteristic for Ward Leonard speed control method
Correct : A. will start from origin
45. Efficiency of Ward Leonard method is
Correct : C. higher than rheostatic control method and shunted field control method
46. Ward Leonard method is an ideal choice for motor which undergoes frequent starting, stopping, speed reversal.
Correct : A. true
47. Starting gear used in Ward Leonard method
Correct : D. is absent
48. To get the speed of DC motor below the normal speed without wastage of electrical energy we use
Correct : A. ward leonard control
49. can give uniform speed variation
Correct : A. in both directions
50. control method is
Correct : D. high cost, high maintenance and low efficiency
51. TOPIC 5.1 SPEED CONTROL OF THREE PHASE INDUCTION MOTOR
Correct : A. slip frequency
52. An 8-pole, 3-phase, 50 Hz induction motor is operating at a speed of 720 rpm. The frequency of the rotor current of the motor in Hz is
Correct : A. 2
53. Calculate the moment of inertia of the thin spherical shell having a mass of 73 kg and diameter of 36 cm.
Correct : A. 1.56 kgm2
54. Calculate the phase angle of the sinusoidal waveform z(t)=78sin(456πt+2π÷78).
Correct : A. π÷39
55. A 50 Hz, 4poles, a single phase induction motor is rotating in the clockwise direction at a speed of 1425 rpm. The slip of motor in the direction of rotation & opposite direction of the motor will be respectively.
Correct : C. 0.05, 1.95
56. The frame of an induction motor is made of
Correct : C. cast iron
57. The slope of the V-I curve is 5°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
Correct : D. .3443 Ω
58. In an induction motor, when the number of stator slots is equal to an integral number of rotor slots
Correct : D. the machine may fail to start
59. rotor?
Correct : D. 50 revolution per minute
60. Calculate the active power in a 487 H inductor.
Correct : D. 0 w
61. Calculate the active power in a 788 ω resistor with 178 A current flowing through it.
Correct : A. 24.96 mw
62. an external voltage source is in phase with the main voltage then speed will
Correct : A. increase
63. R.M.S value of the periodic square waveform of amplitude 72 V.
Correct : A. 72 v
64. frequency of the rotor current of the motor in Hz is 2.
Correct : D. 9.37
65. phase with the main voltage then speed will
Correct : B. decrease
66. The rotor injection method is a part of the slip changing technique.
Correct : A. true
67. synchronous speed changing technique.
Correct : A. true
68. relationship between voltage and current is a straight line.
Correct : A. .16 Ω
69. If induction motor air gap power is 1.8 KW and gross developed power is .1 KW, then
Correct : A. s
70. KW.
Correct : B. 4 sec
71. Calculate the total heat dissipated in a resistor of 44 Ω when 0 A current flows through it.
Correct : A. 0 w
72. The value of slip at which maximum torque occurs
Correct : A. r2÷x2
73. AC voltage controllers convert
Correct : C. fixed ac to variable ac
74. devices were used for voltage control applications.
Correct : C. tap changing transformer
75. applications.
Correct : B. electric heating
76. In the principle of phase control
Correct : B. control is achieved by adjusting the firing angle of the devices
77. consists of
Correct : B. one scr is anti parallel with one diode
78. The below given controller circuit is a
Correct : B. full wave controller
79. circuit
Correct : C. both the cycles can be controlled
80. Calculate the resonant frequency if the values of the capacitor and inductor are 4 F and 4 H.
Correct : A. .25 rad/sec
81. Which test is used to determine the conditions where electric motors are being used?
Correct : A. thermography
82. 1 Btu/h is equal to
Correct : C. .293
83. Full form of IDMT is
Correct : A. inverse definite minimum time
84. Full form of THD is
Correct : C. total harmonic distortion
85. Calculate the value of THD if the distortion factor is .8.
Correct : B. 75 %
86. Second lowest order harmonic present in 3-Φ fully controlled rectifier is
Correct : B. 7th
87. The flux and the internally generated voltage of a dc machine is a function of its magneto-motive force.
Correct : A. non-linear
88. Choose the best option which identifies about the below characteristics.
Correct : A. (1)series motor (2) cumulative compound motor (3) shunt motor
89. The speed torque of the differential compound dc motor is shown below. What conclusions can be made?
Correct : D. all of the mentioned
90. A student is given a differential compound motor and he has been asked to make it start. How will he try?
Correct : D. all of the mentioned
91. For a 100 hp 250 V, compound dc motor with compensating winding has a field current of 5 A to produce a voltage of 250 V at 1200 rpm. What will be the shunt field current of this machine at no load?
Correct : A. 5 a
92. Load torques can be classified into how many types?
Correct : B. two
93. What is the relationship between torque and speed in constant type loads?
Correct : A. torque is independent of speed
94. Which of the following is the plugging method of braking?
Correct : B. reversal of armature connections
95. Electrical braking of any variety becomes less effective as
Correct : B. speed decreases
96. Plugging is used in
Correct : A. small motors only
97. Braking time in the dynamic braking is the function of
Correct : D. all- system inertia, load torque and motor rating
98. In dynamic braking, when braking is applied system acts as
Correct : C. generator
99. What will happen if DC shunt motor is connected across AC supply?
Correct : D. burn due to heat produced in the field winding
100. What will happen, with the increase in speed of a DC motor?
Correct : A. back emf increase but line current falls.