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Electrical Engg by Ashish Sir

Magnetizing Inrush Current

MCQ

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1. Area of hysteresis loop represents: - (A) Copper loss (B) Eddy current loss (C) Dielectric loss (D) Hysteresis loss 2. The purpose of conservator in transformer: - (A) To cool the winding (B) To prevent moisture in the transformer (C) To prevent short circuit of primary and secondary winding (D) To take up contraction and expansion of oil 3. A 3-phase transformer has its primary connected in delta and secondary in star. Secondary to primary turns ratio per phase is 6. For a primary voltage of 200 V the secondary voltage would be (A) 2078 V (B) 693 V (C) 1200 V (D) 58 V 4. The iron loss per unit frequency in a ferromagnetic core, when plotted against frequency, is a (A) Straight line with positive slope (B) Straight line with negative slope (C) Parabola (D) Constant 5. Hysteresis losses are present in iron core coil when (A) The current in the coil is alternating (B) The current is unsymmetrical alternating only. (C) The current in the coil is d.c. only (D) The current in the coil is sinusoidal only. 6. Eddy current loss in ferromagnetic core is proportional to (A) Square of the frequency (B) Square root of the frequency (C) Frequency (D) Reciprocal of the frequency 7. The total flux at the end of a long permanent bar magnet is 100 ×10 -6 Wb. The end of this magnet is withdrawn through a 1000 turn coil in 1/20 seconds. The induced emf in the coil is (A) 20 V (B) 2.0 V (C) 0.2 V (D) 0.02 V 8. Silicon content in iron lamination is kept within 5% as it (A) Makes the material brittle (B) Reduces the Curie point (C) Increases hysteresis area (D) Increases cost 9. The high voltage and low voltage winding resistances of a distribution transformer of 100 kVA, 1100/220 Volts, 50 Hz are 0.1 Ω and 0.004 Ω respectively. The equivalent resistances referred to high voltage side and low voltage sides are respectively (A) 2.504 Ω and 0.2 Ω (B) 0.2 Ω and 0.008 Ω (C) 0.10016 Ω and 2.504 Ω

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(D) 0.008 Ω and 0.10016 Ω 10. An inductor with a ferromagnetic core is supplied from a sinusoidal voltage source with frequency f. The current drawn by the inductor will be (A) Sinusoidal with frequency f (B) Sinusoidal with frequency 2f (C) A saw tooth wave (D) Non- sinusoidal with frequency f 11. A 10 Ω resistive load is to be impedance matched by a transformer to a source with 6250 Ω of internal resistance. The ratio of primary to secondary turns of transformer should be (A) 25 (B) 10 (C) 15 (D) 20 12. The primary and secondary windings of a transformer are wound on the top of each other in order to reduce……… (A) Iron losses (B) Winding resistance (C) Copper losses (D) Leakage reactance 13. The no load primary current Io is about …… of full load primary current of a transformer. (A) Above 40% (B) 30-40% (C) 15-30 % (D) 3-5% 14. Leakage flux in a transformer occurs because (A) Transformer is not an efficient device (B) Applied voltage is sinusoidal (C) Air is not good magnetic insulator (D) Iron core has high permeability 15. Which of the following about the force between two charges is true? (A) Unlike charges repel each other (B) Like charges attract each other (C) No force exists between two unlike charges (D) A force of repulsion exists between two like charges 16. The electrons revolve around the nucleus with high velocity. Which type of force acts against the centrifugal force and keeps the electrons in their orbits? (A) Electrostatic force of attraction (B) Electromagnetic force of attraction (C) Gravitational force (D) Adhesive force 17. What is the difference between an atom and an ion? (A) Ions have always larger mass than the atoms of the same element. (B) Ions are neutral particles while atoms always carry a positive charge. (C) Ions are always charged particles while the atoms are neutral as a whole. (D) Ions can only exist in liquid solutions.

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18. Which particles act as a current carrier in a metallic conductor? (A) Only electrons (B) Only holes (C) Both electrons and holes (D) none of these 19. The resistance of the conductor, when its temperature is increased…. (A) Remains constant (B) Decreases (C) Varies (D) Increases 20. If the AC supply to transformer is replaced by DC (A) The primary winding will burn (B) The secondary winding will burn (C) The transformer has no effect (D) All options are correct 21. According to Lenz’s law, the induced emf sets up a current in which direction? (A) Same direction of cause producing it (B) Direction of gravity (C) Direction opposite of cause producing it (D) None of these 22. The material having low retentivity is suitable for making….. (A) Temporary magnets (B) Permanent magnets (C) Weak magnets (D) None of these 23. In which of the following voltage sources are the movements of conductors in a magnetic field used to produce voltage? (A) In a thermocouples (B) In a zinc copper element (C) In a DC generator (D) In a transformer 24. Reluctance offered by the magnetic circuit depends upon……. (A) Nature of magnetic material (B) Length of magnetic flux path (C) Cross-sectional area of the material (D) All options are correct 25. For a 100% efficient transformer, the primary winding has 1000 turns and the secondary 100 turns. If the power input to the above transformer is 1000 watts, the power output is…….. (A) 1000 watts (B) 100 Watts (C) 10 Watts (D) 10 kW 26. For the production of induced e.m.f field system of an electric machine (A) Must be on stator (B) May be on stator and rotor (C) Must be on rotor (D) None of these

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27. Which of the following transformers is smaller? (A) 1 kVA, 50 Hz (B) 1 kVA, 200 Hz (C) 1 kVA, 400 Hz (D) 1 kVA, 600 Hz 28. The short circuit test in a transformer is used to determine (A) Iron loss at any load (B) Copper loss at any load (C) Hysteresis loss (D) Eddy current 29. In relation to a transformer the ratio 20:1 indicates that (A) There are 20 turns on primary and one turns on secondary (B) Secondary voltage is 1/20 th of primary (C) Primary current is 20 times greater than secondary current (D) For every 20 turns on primary there is one turn on secondary 30. The main purpose of using core in a transformer is to…. (A) Decrease iron loss (B) Prevent eddy current loss (C) Eliminate magnetic hysteresis (D) Decrease reluctance of the common magnetic hysteresis 31. Centrifugal methods of reconditioning transformer oil are effective for removal of…… (A) Water (B) Dissolved gases (C) Solid impurities (D) All options are correct 32. While conducting Short circuit test on transformer the following side is short-circuited…. (A) HV side (B) LV side (C) Primary side (D) none of these 33. An electromagnets can be made by….. (A) Single touch method (B) Double touch method (C) Divided touch method (D) Passing current through solenoid 34. The iron core material used at high frequencies in radio circuits is…… (A) Ferrite (B) Material of low permeability (C) Material of high permeability (D) A material which reduces inductance as well as losses 35. Which is the only sure proof of testing the polarity of magnet? (A) Attraction and repulsion both (B) Repulsion (C) Attraction (D) None of these 36. The field at any point on the axis of a current carrying coil will be: - (A) Perpendicular to the axis

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(B) Parallel to the axis (C) At an angle of 45° with the axis (D) Zero 37. A straight conductor of circular cross-section carries a current. Which one of the following statements is true in this regard? (A) No force acts on the conductor at any point (B) An axial force acts on the conductor tending to increase its length (C) A radial force acts away from the axis tending to reduce its cross-section (D) A radial force acts away from the axis tending to increase its cross-section. 38. Which of the following is correct? (A) B= µoH+M (B) B = µoM+H (C) B= µo(H+M) (D) B = M/µo 39. Sludge formation in transformer oil is due to which one of the following? (A) Ingress of dust particles and moisture in the oil. (B) Appearance of small fragments of paper, varnish, cotton and other organic materials in the oil. (C) Chemical reaction of transformer oil with the insulating materials (D) Oxidation of transformer oil 40. A circular metallic disc is placed in a vertical magnetic field of constant induction in the downward direction. If the disc is rotated in a horizontal plane, the mmf induced will be……. (A) Zero (B) Constant and independent of disc size (C) Increasing radially in the outward direction (D) Decreasing radially in the outward direction 41. Consider the following statements: The force per unit length between two stationary parallel wires carrying (steady) currents. A. Is inversely proportional to the separation of wires. B. Is proportional to the magnitude of each current. C. Satisfies Newton’s third law Out of these: (A) A and B are correct (B) B and C are correct (C) A and C are correct (D) A, B and C are correct 42. According to Faraday’s law of electromagnetic induction an e.m.f in a conductor whenever it (A) Lies in a magnetic field (B) Lies perpendicular to the magnetic field. (C) Cuts the magnetic flux. (D) Moves parallel to the direction of magnetic field. 43. ‘In all cases of electromagnetic induction, an induced voltage will cause a current to flow in a closed circuit in such a direction that the magnetic field which is caused by that current will oppose the change that produces the current’ is the original statement of: - (A) Lenz’s law (B) Faraday’s law

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(C) Flemings' law of induction (D) Ampere’s law 44. In power transformer, the breather is provided in order to (A) Filter transformer oil (B) Prevent ingress of moisture with air (C) The cooling oil (D) Provide fresh air for increasing cooling effect 45. Which property of a material opposes the passage of magnetic flux through it? (A) Permeance (B) Capacitance (C) Inductance (D) Reluctance 46. Determine the intensity of magnetization (in A/m) of a magnet when the pole strength of the magnet is 30 A-m and the pole of the magnet is 2 sq.m. (A) 60 (B) 30 (C) 25 (D) 15 47. What will be the produced mmf (in Amp-turns) in a coil, if the coil has 60 turns and carries a current of 0.15 A? (A) 32 (B) 24 (C) 16 (D) 8 48. Determine the reluctance (in Amp-turns/Wb) of a coil, when the flux through the coil is 25 Wb and the value of produced mmf is 50 Amp-turns. (A) 2 (B) 4 (C) 6 (D) 8 49. Determine the magnetic field intensity (in Amp-turns/m) of 5 meter long coil when the coil has 100 turns and carries a current of 0.6A. (A) 15 (B) 12 (C) 10 (D) 8 50. Determine the self inductance (in mH) of a 3 m long air-cored solenoid, when the coil has 300 turns and the diameter of the coil 12 cm. (A) 0.41 (B) 0.35 (C) 0.32 (D) 0.24 51. Determine the current (in A) through a 60 cm long solenoid when the solenoid has 400 turns and the value of magnetic field at the center of the solenoid is 6mT. (A) 4.4 (B) 5.6 (C) 7.2 (D) 8.4 52. In a transformer the resistance between primary and secondary is: (A) Zero (B) 1 ohm (C) 1000 ohm (D) Infinite 53. Which one of the following is the correct expression of magnetic reluctance? (A) I/H (B) B/H (C) ɸ/Fm (D) Fm /ɸ 54. Magnetic flux through any closed surface is

(A) One (B) Infinite (C) Zero (D) Negative

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55. Determine the self inductance (in mH) of a 4m long air-cored solenoid. If the solenoid has a cross sectional area of 0.02 square meter and has 60 turns. (A) 0.064 (B) 0.036 (C) 0.045 (D) 0.023 56. Determine the value of magnetic field (in mT) at the center of a 20 cm long solenoid. If the solenoid has 200 turns and carrying current of 4 A. (A) 5 (B) 8 (C) 9 (D) 11 57. Determine the magnetic susceptibility of a material if the field strength of the material is 0.8 × 106 Am-1 and the

magnitude of magnetization is 0.97×105 Am-1

(A) 1.01 (B) 1.21 (C) 1.41 (D) 1.61 58. What will be the magnitude of the induced emf (in V) in a coil of area of 100 square centimeters with 200 turns? If the coil is removed from a magnetic field of 10 T acting at right angles to the coil in 1 second. (A) 10 (B) 30 (C) 60 (D) 20 59. Tapings of a transformer are provided (A) At the phase end of LV side (B) At the phase end of HV side (C) At the neutral side end of HV side (D) At the middle of HV side 60. The core of the transformer is made of

(A) Copper (B) Aluminium (C) Air (D) Laminated sheath 61. Which of the following statement is TRUE about the magnetic reluctivity? (A) Reluctivity is the reciprocal of reluctance. (B) Reluctivity is the reciprocal of susceptibility. (C) Reluctivity is the reciprocal of permeance. (D) Reluctivity is the reciprocal of permeability.

62. The ratio of total flux linkage to used flux is known as….. (A) Flux gain factor (B) Flux leakage factor (C) Field gain factor (D) Field leakage factor 63. A transformer has a percentage resistance of 2% and percentage reactance of 4%. What are the regulations at

0.8 pf lag and 0.8 pf lead?

(A) 4% and -0.8% (B) 3.2% and -1.6% (C) 1.6% and -3.2% (D) 4.8% and -0.6% 64. If copper loss of the transformer at 7/8th full load is 4900 W. then its full load copper loss would be: (A) 5600 W (B) 6400W (C) 373 W (D) 429W 65. A transformer is working at its full load and its efficiency is also maximum at which iron loss is 1000 Watts. Then its copper loss at half of the full load will be: - (A) 250 Watt (B) 300 Watt

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(C) 400 Watt (D) 500 Watt 66. A 230/110 V transformer is supplied with 50 Hz frequency. If the supply voltage remains same and frequency is

now 400 Hz then what happened with iron losses?

(A) Decreases (B) Increases (C) Remains the same (D) None 67. Midway between two equal and similar charges, a third equal and similar charge is placed. Then this third charge will…. (A) Remain in stable equilibrium (B) Be in unstable equilibrium (C) Not be in equilibrium (D) Will move out of the field to influence the two charges 68. During S.C. test, the iron loss of a transformer is negligible because…… (A) The entire input is just sufficient to meet copper losses only (B) Voltage applied across the HV (high voltage) side is a small fraction of the rated voltage and so its flux. (C) Iron core becomes fully saturated. (D) Supply frequency is held constant 69. The maximum efficiency of a 500 kVA, 3300/500 V, 50 Hz single phase transformer is 97 % and occurs at 75% of full load at unity p.f. Find the iron losses. (A) 0.116 p.u. (B) .0116 p.u. (C) 1.16 p.u. (D) 1.06 p.u. 70. A transformer has maximum efficiency at full load, when iron losses are 800 Watts, copper losses at half load

will be

(A) 1600 W (B) 800 W (C) 400 W (D) 200 W 71. A 2 kVA transformer has iron loss of 150 W and full load copper loss of 250 W. The maximum efficiency of the transformer will occur when the total loss is: - (A) 500 W (B) 400 W (C) 300 W (D) 275 W 72. In a 1-phase transformer, the copper loss at full load is 600 Watts. At half of the full load the copper loss will be (A) 150 Watt (B) 75 Watt (C) 600 Watt (D) 300 Watt 73. The iron loss in a 100 kVA transformer is 1kW an full load copper losses are 2kW. The maximum efficiency occurs at a load of (A) 100 kVA (B) 70.7 kVA (C) 141.4 kVA (D) 50 kVA 74. The voltage regulation of a transformer having copper loss 1% of output and percentage reactance drop of 5% and power factor 0.9 lagging is…… (A) 3.08% (B) 3% (C) -3.08% (D) 3.8%

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75. A transformer has a core loss of 64 W and copper loss of 144 W. When it is carrying 20 % overload current. The load at which this transformer will operate at the maximum efficiency….. (A)) 80% (B) 66% (C) 120% (D) 44% 76. The ordinary efficiency of a given transformer is maximum when…..

(A) It runs at half full-load (B) It runs at full-load (C) Its copper loss equals iron loss (D) It runs overload 77. What is the value of the magnetic vector potential due to an infinitesimally small current element, evaluated at infinite distance from it? (A) Infinity (B) Unity (C) Zero (D) Any number between zero and infinity depending on the strength of the current element 78. What is required to remove static electricity from machinery? (A) Grounding the frame work has to be done. (B) Humidify the surroundings has to be done. (C) Constructing insulated cabins has to be done. (D) Insulating the machinery has to be done. 79. The all day efficiency of a distribution transformer will be high with low…….. (A) Copper losses (B) Iron losses (C) Operating temperature (D) Copper as well as iron losses 80. A 500 kVA transformer has constant loss of 500 W and copper losses at full load are 2000 W. then at what load, is the efficiency maximum? (A) 250 kVA (B) 500 kVA (C) 1000 kVA (D) 125 kVA 81. The all day efficiency of a transformer depends primarily on…..

(A) Its copper losses (B) The amount of load (C) The duration of load (D) Both the amount and duration of load 82. Stepped cores are used in transformer in order to reduce (A) Volume of iron (B) Volume of copper (C) Iron loss (D) Reluctance of the core 83. In a transformer zero voltage regulation is achieved at a load power factor which is (A) Zero (B) Leading (C) Lagging (D) Unity 84. A consideration of the power losses in electrical machines is essential for which of the following reasons

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(A) Operating cost (B) Temperature rise (C) Voltage drops (D) All options are correct 85. A 40 kVA transformer has a core loss of 450 W and total loss of 800 W. Find the copper loss for maximum efficiency. (A) 350 W (B) 800 W (C) 450 W (D) None of these 86. The heat generated in the transformer is dissipated mainly by (A) Conduction (B) Convection (C) Radiation (D) All options are correct 87. The overall power factor of an On-load transformer (A) Depends on the power factor of the load (B) Is always lagging (C) Is always unity (D) Is always leading 88. In an auto transformer a part of energy transfer is through:

(A) Convection process (B) Conduction process (C) Induction process (D) Both B and C 89. Determine the eddy current loss ( in W) in a material having eddy current coefficient of 1, thickness of 0.04 m and volume of 2 cubic meter. Which is kept in a magnetic field having a maximum flux density of 4 T and supply by a frequency of 50 Hz? (A) 140 (B) 128 (C) 108 (D) 100 90. The working principle of transformer depends on (A) Coulombs law (B) Faraday law (C) Amperes law (D) Newton's law 91. If the area of the cross section is reduced to half of its original value. Then what will happen to the flux density?

(A) Doubled (B) Halved (C) Remains same (D) One- fourth 92. A conductor of length of 0.6 m is situated at the right angle to a uniform magnetic field of flux density 4 Wb/m2. The conductor is moving with a velocity of 50 m/s. calculate the e.m.f induced (in V) in the conductor? (A) 100 (B) 115 (C) 118 (D) 120 93. A flux of 2 mWb passes through strip of length and breadth 10 cm and 20 cm respectively. It is placed at an angle of 30 degree to the direction of uniform magnetic field. What is the magnetic flux density (in T)? (A) 0.1 (B) 0.115 (C) 0.9 (D) 1 94. A 600 turn coil has a resistance of 7 ohms and an inductance of 0.8H. If 1/5th of the turns are removed then

determine the new inductance (in H)?

(A) 0.512 (B) 1.25 (C) 2 (D) 2.5 95. A magnetic circuit of reluctance 300 AT/Wb is wounded by wire of 400 turns. Determine the average e.m.f induced in Volt in the coil, If a current of 2 A is flowing in the coil is reversed in 15 seconds?

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(A) 14.24 (B) 16.28 (C) 17.96 (D) 19.62 96. Which of the following is not a part of transformer? (A) Commutator (B) Conservator tank (C) Radiator (D) tap changer 97. The current in the circuit shown below is 4 A. Calculate the magnetic field intensity (in AT/m) in the core, if the winding has 50 turns?

(A) 520 (B) 524 (C) 526 (D) 528 98. If K is the phase to phase voltage ratio, then the line to line voltage ratio in a 3 phase, star-delta transformer is:

(A) K (B) K/√3

(C) √3K (D)√3/K 99. A delta-star transformer has phase to phase voltage transformation ratio of a:1. The line to line voltage ratio of star-delta is given by

(A) a/1 (B) √3/√𝑎

(C) √3𝑎/1 (D) √3/a 100. In a transformer the core loss is 100 W at 40 Hz and 72 watt at 30 Hz, then the eddy current and hysteresis

losses at 50 Hz respectively are: -

(A) 20 watt and 100 watt (B) 100 watt and 32 watt (C) 32 watt and 100 watt (D) 25 Watt and 105 watt 101. The core flux in transformer depends mainly on: - (A) Supply voltage (B) Supply voltage, frequency and load (C) Supply voltage and load (D) Supply voltage and frequency 102. The type of oil used in transformer is: - (A) Olive (B) Coconut (C) Mineral (D) Palm 103. The transformer used for AC welding sets is (A) Booster type

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(B) Step up transformer (C) Step down transformer (D) None of these 104. In case of core type transformer which type of winding is done near the core? (A Low voltage (B) High voltage (C) Primary winding (D) Secondary winding 105. Input of a transformer is square wave, then the output will be

(A) Pulsed wave (B) Square wave (C) Triangular wave (D) Sine wave 106. The most essential condition for parallel operation of two 1-phase transformers is that they should have same

(A) KVA rating

(B) Percentage impedance

(C) Polarity

(D) Voltage ratio

107. The ratio of primary/secondary voltages is 2:1. The saving in terms of weight of copper required if an

autotransformer is used instead of a two winding transformer will be:

(A) 95% (B) 66.7% (C) 50% (D) 33.3% 108. For a two winding power transformer with the effects of the no-load current being neglected, the voltage regulation can be zero at load, when the load power factor is (A) Lagging only (B) Leading only (C) Either lagging or leading, depending upon power rating of the transformer (D) Unity 109. A 50 kVA transformer has a core loss of 500 W and full-load core loss of 900 W. The load at which the

efficiency is maximum is

(A) 27.45 kVA (B) 37.75 kVA (C) 45.5 kVA (D) 47.5 kVA 110. A 200 kVA, 1-phase transformer is in circuit continuously for 8 hours in a day, the load is 160 kW at 0.8 p.f.. For 6 hours, the load is 80 kW at unity p.f. and for the period remaining out of 24 Hours, it runs on no-load. If the full-load copper losses are 3.02 kW. The total copper losses in 24 hours are (A) 35.62 kW (B) 24.16 kW (C) 11.46 kW (D) 38.40 kW 111. Tapping are normally provided on the high voltage winding of a transformer only because (A) It has larger number of turns which allows smoother variation of voltage (B) It has to handle low currents (C) It is easily accessible physically (D) It has larger number of turns, has to handle low currents and is also easily accessible physically. 111. When a transformer is first energized, the transient current during first few cycles, is

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(A) Less than full load current (B) Equal to full load current (C) Equal to no-load current (D) Much higher than full load current 112. For power transformer of larger ratings, the tapping are located in the middle portion of the windings to (A) Increase the breakdown strength of the winding insulation (B) Enable better cooling of the windings (C) Enable better distribution of inter-turn voltage (D) Reduce the mechanical forces affecting the windings during short-circuit. 113. A 10 kVA, 2200/220 V transformer gave the following test results. Open circuit test, high voltage side open: V=220 V, I= 1.5 A, W= 150 W. Short circuit test. Low voltage side short-circuited: V= 115 V, I= rated, W= 200 W. The half full load efficiency of the transformer operating at unity power factor is (A) 95% (B) 95.5% (C) 96% (D) 96.5% 114. A 4kVA, 400/200 V single phase transformer has resistance of 0.02 p.u. and reactance of 0.06 p.u. The resistance and reactance referred to high voltage side are: - (A) 0.2Ω and 0.6Ω (B) 0.8Ω and 2.4Ω (C) 0.08Ω and 0.24Ω (D) 1Ω and 3Ω 115. A single phase 50 kVA, 1000/100 V two winding transformer is connected as an autotransformer as shown in

figure: -

The kVA rating of the autotransformer is…………..

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