Electrical Machines -II

Transcription

1 Objective Type Questions: 1. Basically induction machine was invented by (a) Thomas Alva Edison (b) Fleming (c) Nikola Tesla (d) Michel Faraday Electrical Machines -II 2. What will be the amplitude and frequency of the rotating magnetic field in induction motors? (a) Supply frequency and 1.5 Φ m (b) more than supply frequency and Φ m (c) less than supply frequency and Φ m (d) 50 Hz and Φ m 3. The torque produced in an induction motor depends on (a) Speed & slip (b) stator & rotor field (c) stator voltage & current (d) all of the above 4. Skewing of rotor bars in squirrel cage induction motor will be effective in (a) Preventing cogging (b) increasing starting torque (c) preventing crawling (d) producing rotating magnetic field 5. The torque produced by a squirrel cage induction motor depends on irrespective of the supply frequency (a) Mechanical load (b) supply voltage (c) rotor resistance (d) slip speed 6. A three phase induction motor is running at a synchronous speed of 1500 rpm, the rotor runs at a slip of 4%, calculate the following (a) speed of rotating magnetic field (b) rotor speed (c) speed of rotor magnetic field 7. The power factor of squirrel cage induction motor will be generally (a) poor at low/no loads (b) high at full load (c) poor at no load and high at full load (d) high at no load and low at full load Page 1 of 26

2 8. The magnetizing current drawn by the induction motor is that of the transformer of same power rating. (a) same as (b) greater than (c) less than (d) not comparable to 9. The effect of higher air gap length in an induction motor is to (a) reduce the leakage reactance (b) increased overload capacity (c) reduced noise (d) all of these 10. A three phase induction motor running at a speed of 1450 rpm, is supplied from 60 Hz supply? What will be the frequency of the rotor current? (a) 60 Hz (b) Hz (c) 50 Hz (d) 30 Hz 11. A 6 pole, 50 Hz, three phase induction motor running at 960 rpm on certain load has the rotor copper loss of 5 kw. What will be the rotor input? 125 kw 12. In a three phase squirrel cage induction motor supply voltage is reduced to half from the rated value. What will be the starting torque at the half voltage compared to the rated voltage? (a) same (b) half (c) one fourth (d) double 13. The slip of an induction motor can also be expressed as the ratio of (a) rotor cu loss & rotor input (b) stator cu loss & stator in put (c) stator input & rotor input (d) stator cu loss & rotor cu loss 14. The synchronous speed of a linear induction motor does not depend upon (a) length of the pole pitch (b) supply frequency (c) number of poles (d) all of these Page 2 of 26

3 15. The magnitude of torque developed by a three phase induction motor depends upon (a) supply voltage, stator current, stator impedance (b) number of poles, speed, frequency (c) Synchronous speed, rotor speed, frequency (d) rotor current, rotor emf, rotor pf 16. In case of centrifugal type of loads connected to induction motors the power consumption is proportional to (a) speed (b) square of speed (c) cube of the speed 17. Which of the following decides the thermal loading of the motor? (a) Ambient temperature (b) Temperature of the winding (c) Load cycle (d) Aging of the motor 18. The speed of induction motors can be controlled by (a) Changing number of poles (b) changing the frequency (c) Using multiple windings (d) all of these 19. After rewinding of an A C Motor which of the following parameters are to be measured (a) no load current (b) air gap length (c) winding resistance (d) all of these 20. Stray losses in an induction motor are generally proportional to (a) stator current (b) rotor current (c) square of rotor current (d) square of stator current 21. In an ac motor if the supply frequency is increased by 5% the change in synchronous speed is (a) + 5% (b) 5% (c) +10 % (d) 10 % Page 3 of 26

4 22. Which of the following starter is used for starting of a three phase squirrel cage induction motor? (a) 3 point starter (b) 4 point starter (c) star delta starter (d) resistance starter 23. Which of these material is used in construction of stator of the induction motor (a) Silicon steel (b) ) carbon steel (c) iron (d) copper 24. Double cage rotors are employed in squirrel cage induction motors to (a) Increase rotor size (b) decrease rotor resistance (c) decrease starting current (d) increase starting torque 25. In induction motor the torque is produced as a result of interaction between (a) stator and rotor (b) stator speed and rotor speed (c) currents and air gap flux (d) voltage and current 26. The frame or yoke of an induction motor is usually made of (a) Silicon steel (b) cast steel (c) aluminium (d) carbon steel 27. In an induction motor the no load slip is generally (a) less than 1% (b) greater than 1% (c) more than 5% (d) more than 10% 28. In slip ring induction motors the slip rings are made of (a) copper (b) phosphor bronze (c) carbon (d) aluminium Page 4 of 26

5 29. If Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be (a) Ns (b) sns (c) (1-s)Ns (d)(1 + s) Ns 30. The crawling in the induction motor is caused by (a) Low voltage (b) improper design (c) harmonics in the motor (d) over loads 31. Number of slip rings on the rotor of induction motor are (a) two (b) three (c) four (d) zero 32. During the development of equivalent circuit of a double cage induction motor the two cages can be considered to be connected (a) in series (b) in parallel (c) in series parallel (d) in star / delta 33. Rheostatic method of speed control of induction motor is used in (a) squirrel cage motor (b) double cage motor (c) slip ring motor (d) all of these 34. In circle diagram of an induction motor, the diameter of the circle represents (a) torque (b) rotor current (c) slip (d) supply voltage 35. In which type of induction motor the speed can be controlled from rotor side (a) squirrel cage (b) double cage (c) wound rotor (d) all of these Page 5 of 26

6 36. Rotor resistance speed control can be used for which of these motors (a) squirrel cage (b) double cage (c) slip ring (d) all of these 37. What happens if the phase sequence of the supply voltage is changed for the induction motor (a) motor will not run (b) nothing will happen (c) motor runs in reverse direction (d) motor gets damaged 38. In a squirrel cage induction motor for certain combination of stator and rotor slots, motor runs at 1/7 th of the rated speed, the phenomenon is called (a) hunting (b) cogging (c) humming (d) crawling 39. Wound rotor induction motors are employed for the application where (a) high starting torque is required (b) speed control is required (c) frequent starting stopping and reversing is required (d) all of these 40. Direct on line starting of induction motors is avoided because it (a) takes high starting current (b) produces very high starting torque (c) runs at very speed 41. What happens to the operating power factor of the induction motor as the load is increased (a) decreases (b) remains constant (c) increases up to full load and then decreases 42. Size of the high speed induction motor is compared to low speed induction motor (a) larger (b) not related (c) smaller (d) same Page 6 of 26

7 43. In a three phase induction the number of poles in the rotor winding are (a) Zero (b) less than stator poles (c) equal to stator poles (d) more than stator poles 44. If the slip of an induction is negative it indicates that (a) magnetic field is rotating in opposite direction to rotor (b) rotor speed is less than synchronous speed (c) rotor speed is greater than synchronous speed (d) rotor speed is equal synchronous speed 45. The fuse in one phase blows off in a three phase induction motor running at full load then (a) the motor continues running without any problem (b) the motor continues running at lower speed and can get damaged (c) the motor stops 46. Wound motor induction motors are employed where (a) high starting torque is required (b) speed control is required (c) frequent on- off operation is required (d) all of these 47. OC and SC test is conducted on a three phase induction motor, which of the following can not be determined from the test results (a) Copper loss (b) iron loss (c) friction & windage loss (d) transformation ratio 48. A three phase 415 volt, 50 HZ, 4 pole induction motor operates with a 4% slip. The synchronous speed and the actual rotor speed are and respectively. 49. Power dissipation in the term R r /s of the equivalent circuit represents (a) Input power (b) air gap power (c) copper loss (d) mechanical power output 50. Power dissipation in the term R r of the equivalent circuit represents (a) Mechanical loss (b) air gap power (c) copper loss (d) mechanical power output Page 7 of 26

8 51. Power dissipation in the term R r (1-s)/s of the equivalent circuit represents (a) Mechanical loss (b) air gap power (c) copper loss (d) mechanical power output 52. In the approximate equivalent circuit of an induction motor the shunt elements X m and R m are neglected because (a) values are negligibly small compared to series elements (b) same as series elements (c) values are very much larger compared to series elements (d) equal to zero 53. During no load test on a three phase induction motor the entire power drawn by the motor is consumed as (a) copper loss (b) core loss (c) friction & windage loss (d) mechanical loss 54. The equivalent circuit parameters obtained form OC and SC test are referred to (a) stator only (b) rotor only (c) both stator and rotor 55. In medium rated three phase induction motors which type of winding is employed (a) mush winding (b) single layer lap winding (c) double layer lap winding (d) concentric winding 56. A 4 pole, 3- phase induction motor runs at 1500 rpm, at what speed a single phase motor with same number of pole runs (a) less than 1500 rpm (b) 1500 rpm (c) more than 1500 rpm 57. In case of squirrel cage induction motor the rotor bars are made of (a) steel or aluminium (b) copper or aluminium (c) steel or carbon steel (d) all of these Page 8 of 26

9 58. The rotor winding of a wound rotor induction motor is connected in (a) series (b) star (c) delta (d) parallel 59. If there is an excessive rise in phase current of the three phase induction motor, the possible reason is (a) mechanical load greater than the kw rating (b) the rotor is blocked (c) winding insulation failure (d) all of these 60. In star delta starting of an induction motor the applied voltage per phase is (a) 50 % of line voltage (b) % of line voltage (c) 57.7 % of line voltage (d) 100 % 61. The induction motor suitable for driving high inertia loads is (a) slip ring (b) squirrel cage (c) double cage 62. The reason for having poor power factor in induction motors is due to (a) stator leakage reactance (b) rotor leakage reactance (c) magnetizing current (d) all of these 63. In a double cage induction motor the inner cage has (a) low reactance and low resistance (b) high resistance and high reactance (c) high reactance and low resistance (d) low resistance and high reactance 64. In an induction motor if R 2 is the rotor resistance, X 2 is the rotor reactance and s is the slip then the condition at which the maximum torque occurs at running condition is (a) R 2 = X 2 (b) R 2 = sx 2 (c) sr 2 = X 2 (d) R 2 = s/x 2 Page 9 of 26

10 65. Less maintenance is required for which type of motor (a) Squirrel cage induction motor (b) slip ring induction motor (c) both a & b 66. Under normal running conditions the rotor current in an induction motor is than the stator current (a) very much smaller (b) very much larger (c) equal 67. Under what condition the induction can be run at synchronous speed (a) run on no load (b) supplied with the voltage greater than rated voltage (c) run on a supply with higher frequency (d) when emf is injected to the rotor circuit 68. The type of coupling between stator and rotor windings of an induction motor is (a) conductive (b) inductive (c) direct (d) electrical 69. What is the frequency of induced rotor current for a 3 phase, 50 Hz, 4 pole induction on full load (a) 50 Hz (b) 2 Hz (c) 25 Hz (d)100 Hz 70. A balanced 3 phase 50Hz, voltage is supplied to a 4 pole induction motor. The full load slip is found to be The speed of rotor mmf relative to the rotor is (a) 1500 rpm (b) 1450 rpm (c) 50 rpm (d) 25 rpm 71. The frequency of rotor current of a 3 phase, 4 pole, 50 Hz, induction motor at full load speed is about (a) 50 Hz (b) 5 Hz (c) 2 Hz (d) 20 Hz Page 10 of 26

11 72. In a three phase induction motor the mechanical power developed in terms of slip and air gap power can be given by (a) (s -1)P g (b) (1- s)p g (c) (s-1)/p g (d) P g / (s-1) 73. In a three phase induction motor the maximum torque developed is (a) directly proportional to rotor resistance (b) inversely proportional to rotor resistance (c) proportional to square of rotor resistance (d) does not depend on rotor resistance 74. The motor used in water pumps are (a) induction motor (b) synchronous motor (c) d c motor (d) universal motor 75. The synchronous speed of a three phase 6 pole induction motor is 1200 rpm. If the number of poles is reduced to 4 with supply frequency remaining same, what will be the rotor speed with a slip of 5%? (a) 1200 rpm (b) 1600 rpm (c) 1750 rpm (d) 1710 rpm 76. In three phase induction motors the magnetizing current will be minimum for which type of slot combination (a) open rotor and stator slots (b) semi closed stator and rotor slots (c) open stator and semi closed rotor slots (d) semi closed stator and open rotor slots 77. Generally and type of ventilating ducts are employed for normal cooling of induction motors. 78. Fins or ribs are provided on the yoke of the induction motors for providing (a) strength to yoke (b) increased surface area (c) look to the yoke (d) protection to yoke Page 11 of 26

12 79. Unsuitable combination of stator and rotor slots in case of induction motor will lead to (a) Hooks and cusps in T-S characteristics (b) Running at lower speed (c) Running at higher speed (d) a and b 80. Which winding will have the higher resistance in three phase squirrel cage induction motor (a) stator winding (b) Rotor winding (c) Field winding (d) All of these 81. Which type of windings are generally employed in three phase induction motors (a) Concentric winding (b) Mush winding (c) Fractional slot and integral slot winding (d) All of these 82. Silicon content in the laminations of the three phase induction motor will be than that in a transformer. (a) less (b) more (c) equal 83. In three phase induction motors the magnetizing current will be generally in the range of of rated current (a) 1 5% (b) % (c) 0 1% (d) 30 50% 84. In case of induction motors the rotor voltage and the frequency are dependent on (a) stator voltage (b) synchronous speed (c) slip (d) power developed 85. Which of the following method of speed control can be used for speed control of wound rotor induction motors (a) varying stator line voltage (b) varying resistance of the rotor circuit (c) injecting and varying external voltage in rotor circuit (d) all of these Page 12 of 26

13 86. List the different methods of starting of three phase induction motors 87. Half coils are generally employed in (a) Stator of squirrel cage induction motors (b) Stator of wound rotor induction motors (c) Rotor of squirrel cage induction motors (d) Rotor of wound rotor induction motors 88. For a given application an induction motor is preferred compared to a d c motor because it (a) has better speed characteristics (b) simple and strong construction (c) has higher starting torque 89. Under blocked rotor test condition the of a three phase induction motor the slip is (a) 1.0 (b) 0.1 (c) 0.0 (d) The full load slip of a three phase induction motor is generally in the range of (a) 2 5% (b) 5 10% (c) 10 20% (d) 20 30% 91. The speed torque characteristics of a three phase induction motor is almost similar to that of (a) D C shunt motor (b) D C series motor (c) Cumulative compound motor (d) Differential compound motor 92. The voltage induced in the rotor of a three phase induction motor will be maximum when it is (a) running at full load (b) running at no load (c) at stand still (d) blocked 93. If the slip of a three phase induction motor is made to increase then the p.f of the motor (a) increases (b) decreases (c) remains same Page 13 of 26

14 94. For the three phase induction motors to have higher efficiency the slip should be (a) large (b) small (c) very large (d) as small as possible 95. If a three phase induction motor is running at a slip of s then the efficiency of this motor expressed in terms of slip is approximately equal to (a) 1+S (b) 1-S (c) S-1 (d) 1/s 96. In the transformer analogy the three phase induction motor running on no load is equivalent to the transformer secondary (a) short circuited (b) open circuited (c) supplying light load (d) None of these 97. The number of stator windings used in a four speed squirrel cage three phase induction motor is (a) one (b) two (c) three (d) four 98. Under blocked rotor test condition most of the power supplied to the motor is utilized as (a) copper loss (b) core loss (c) friction & windage loss (d) mechanical loss 99. In three phase induction motors when the stator is STAR connected which harmonics do not have the effect (a) Even (b) Odd (c) Triple n (d) All of the above 100. The shaft of an induction motor is made of (a) Stainless steel (b) High speed steel (c) Cast iron (d) Carbon steel Page 14 of 26

15 101. If the air gap of an induction motor is increased (a) The magnetizing current will increase (b) The power factor will decrease (c) Speed of the motor will increase (d) Friction and windage loss will increase 102. A double cage induction motor has (a) Two rotors moving rotating in opposite direction (b) Two parallel windings in rotor (c) Two parallel windings in stator (d) Two parallel windings in both rotor and stator 103. Stepless speed control is possible in (a) Rotor emf injection method (b) Pole changing method (c) Cascade operation (d) v/f control method 104. For which induction motor speed can be controlled from rotor side (a) Slip ring induction motor (b) Squirrel cage induction motor (c) Both of the above (d) None of the above 105. A n induction motor is used for a water pump. It is switched on to a supply which is 30% less than its rated voltage. The pump starts running, however what may happen eventually if it is left to run for long time? (a) Stalls immediately (b) Stalls after sometime (c) Subsequently gets damaged (d) Will continue to run without any problem 106. What is plugging in case of three phase induction motor (a) Connecting the motor directly on line (b) Starting the motor on over load (c) Interchanging two phases of supply for quick stopping (d) Locking of rotor 107. The current drawn by a three phase induction motor during plugging will be (a) Normal full load rated current (b) less than the current under blocked rotor condition (c) equal to the current under blocked rotor condition (d) more than the current under blocked rotor condition Page 15 of 26

16 108. Under which condition an induction motor can be run at synchronous speed (a) Run on no load (b) Run on higher voltages than the rated voltage (c) Run in reverse direction (d) Run on emf injected to rotor circuit 109. In cascade control of two induction motors how many speeds can be obtained? (a) 2 (b) 4 (c) 6 (d) Consider the following cases. In which a squirrel cage induction motor is preferred than the slip ring induction motor (a) External emf has to be injected to the rotor (b) Cost is the major consideration (c) Wide range of speed control is required (d) Higher starting torque is required 111. The MMF caused by which type of excitation generates backward revolving flux pattern (a) zero sequence (b) positive sequence (c) negative sequence 112. In induction motors 10 % reduction in supply voltage will change the torque by (a) 10% (b) 38 % (c) 19 % (d) None of these 113. Turbo alternators are run by (a) Francis turbine (b) Kaplan turbine (c) Steam turbine (d) All of these 114. Salient pole alternators are run by (a) Francis turbine (b) Kaplan turbine (c) Pelton wheel turbine (d) All of these Page 16 of 26

17 115. What type of rotor is suitable for turbo alternators which are designed to run at very high speeds (a) salient pole type (b) Non salient pole type (c) Round rotor type (d) Both b and c 116. The frequency of the emf generated by an alternator with 12 poles and running at 1000 rpm is (a) 50 Hz (b) 100 Hz (c) 25 Hz (d) 60 Hz 117. The number of electrical degrees passed through per revolution in an alternator with 6 poles is (a) 360 (b) 720 (c) 960 (d) An alternator develops a voltage of 240 volts per phase while running at 1500 rpm. If the speed is reduced to 1000 rpm what will be the induced emf per phase (a) 240 volts (b) 200 volts (c) 160 volts (d) 120 volts 119. Fleming's left hand rule can be applied to an alternator to determine (a) Direction of force on the rotor (b) Direction of field in the air gap (c) Direction of induced emf (d) Direction of current in rotor 120. In an electrical generator the direction of flux around the current carrying conductor can be found by (a) Flemings right hand rule (b) Lenz s law (c) Flemings left hand rule (d) Corkscrew rule 121. Fractional pitch windings are used in synchronous generators to (a) To reduce the harmonics in MMF produced (b) To reduce the harmonics in the emf induced (c) Both (a) and (b) (d) Non of these Page 17 of 26

18 122. A three phase 10 pole alternator has 2 slots per pole per phase. The coil span is The distribution factor to eliminate 5 th harmonic for this machine is (a) (b) (c) (d) When an alternator is running no load condition the total power supplied by the prime mover is utilized to supply the (a) Iron losses in the machine (b) Copper losses in the machine (c) Total no load losses (d) All of these 124. The ratio of no load speed to full load speed of a 3 phase, 500 kva, 400 volts, 1500 rpm synchronous generator is (a) 1.1 (b) 1.5 (c) 1.0 (d) Very high 125. When the armature of an alternator is loaded the voltage drop in terminal voltage occurs due to (a) armature resistance (b) armature leakage reactance (c) armature impedance (d) armature impedance and armature reaction 126. The magnitude of various voltage drops that occur in a loaded alternator depends upon (a) load current (b) power factor of the load (c) voltage rating of the alternator 127. When an alternator is working at lagging power factor, the magnitude of generated voltage per phase, as compared to that at unity power factor must be (a) less than the terminal voltage (b) same as the terminal voltage (c) more than the terminal voltage (d) 3 times the terminal voltage Page 18 of 26

19 128. When an alternator is working at leading power factor, the magnitude of generated voltage per phase, as compared to that at unity power factor must be (a) less than the terminal voltage (b) same as the terminal voltage (c) more than the terminal voltage (d) 3 times the terminal voltage 129. If the power input to an alternator is kept constant and the excitation is varied then the (a) active component of power output varies (b) reactive component of the output power varies (c) power factor of the load varies (d) active and reactive power output remains constant 130. When an alternator is under excited the alternator works at (a) unity power factor (b) leading power factor (c) lagging power factor (d) zero power factor 131. When an alternator is over excited the alternator works at (a) unity power factor (b) leading power factor (c) lagging power factor (d) zero power factor 132. The purpose of providing damper winding in the pole face of alternators is to (a) eliminate hunting (b) mechanically balance the rotor (c) produce higher power output (d) both (a) and (b) 133. The purpose of providing damper winding in the pole face of a synchronous motor is to (a) eliminate hunting (b) mechanically balance the rotor (c) produce self starting torque (d) all of these 134. In a synchronous machine if the field winding is placed in the stator and the armature winding is placed in the rotor than under steady running condition, the air gap field (a) rotates at synchronous speed w.r.t rotor (b) rotates at synchronous speed w.r.t stator (c) remains stationary w.r.t rotor (d) remains stationary w.r.t stator Page 19 of 26

20 135. In a synchronous generator, for a particular air gap flux distribution and number of turns, which coil will produce maximum emf (a) short pitched coil (b) long pitched coil (c) short chorded coil (d) full pitched coil 136. In a synchronous machine the space angle between the rotating magnetic field and the rotor poles rotating at synchronous speed is called (a) power factor angle (b) power angle (c) load angle (d) mechanical angle 137. In a synchronous machine the relation between the short circuit ratio and synchronous reactance X s can be expressed as (a) SCR = X s (b) SCR = (X s ) 2 (c) SCR = 1/X s (d) SCR = (1/X s ) If a synchronous machine is having lower value of short circuit ratio then the machine will have (a) Higher cost (b) Higher over load capacity (c) Higher stability limit (d) Lower stability limit 139. In a synchronous machine connected to the bus bars, if the axis of field flux coincides with the axis of armature flux then the (a) the machine is working as a synchronous generator (b) the machine is working as a synchronous motor (c) the machine is floating on the bus bars 140. Relation between electrical and mechanical degrees of rotation in synchronous machines is (a) θ m = 2 θ e /p (b) θ m = θ e /p (c) θ m = θ e (d) θ m = θ e /2p 141. Brushless excitation system in brushless alternator eliminates (a) commutator (b) collector rings (c) brushes (d) all of these Page 20 of 26

21 142. Which one of these methods is not used for finding the regulation of alternators (a) EMF method (b) MMF method (c) UPF method (d) ZPF method 143. Which one of these methods is best suited for finding the regulation of alternators (a) EMF method (b) MMF method (c) ASA method (d) ZPF method 144. Which one of these method for finding the regulation of alternator is called optimistic method (a) EMF method (b) MMF method (c) ASA method (d) ZPF method 145. Which one of these method for finding the regulation of alternator is called pessimistic method (a) EMF method (b) MMF method (c) ASA method (d) ZPF method 146. The following methods are suited for alternator for finding the regulation. (a) EMF method (b) MMF method (c) ASA method (d) ZPF method 147. When the excitation of a salient pole synchronous motor running under no load rated speed on infinite bus bars fails (a) continues to run as induction motor (b) continues to run as synchronous motor (c) motor stops (d) non of these 148. The torque that is developed by the synchronous machine to maintain the machine in synchronism (a) braking torque (b) pullout torque (c) synchronizing torque (d) full load torque Page 21 of 26

22 149. In a synchronous motor if the induced emf in armature is equal to the supply voltage then the excitation is called (a) over excitation (b) under excitation (c) 100 % excitation (d) None of these 150. Which one of the following motors is not a self starting motor (a) DC motor (b) Squirrel cage induction motor (c) Slip ring induction motor (d) Synchronous motor 151. Which on of the following motor can be made to work as a capacitor (a) DC motor (b) Asynchronous motor (c) Single phase induction motor (d) Over excited Synchronous motor 152. A synchronous motor is made to work at zero power factor lagging with respect to the excitation voltage, then the nature of armature reaction mmf will be (a) cross magnetizing (b) demagnetizing (c) magnetizing 153. A synchronous motor is made to work at zero power factor leading with respect to the excitation voltage, then the nature of armature reaction mmf will be (a) cross magnetizing (b) demagnetizing (c) magnetizing 154. In case of synchronous motors the torque angle is the angle between (a) armature current and induced emf (b) induced emf and supply voltage (c) rotating magnetic field and rotor poles (d) magnetizing current supply voltage 155. A three phase, 6 pole, 60 Hz synchronous motor is running with a load angle of 15 degree electrical, then the actual physical load angle will be (a) 5 0 (b) 10 0 (c) 2 0 (d) Page 22 of 26

23 156. In a synchronous motor keeping the excitation constant, if the mechanical load on the motor is doubled what happens to the torque angle (a) Approximately becomes half (b) Approximately gets doubled (c) Approximately remains same (d) None of these 157. V curve for a synchronous motor represents the relation between (a) power factor and field current (b) armature current and field current (c) power factor and armature current (d) power and power factor 158. Inverted V curve for a synchronous motor represents the relation between (a) power factor and field current (b) armature current and field current (c) power factor and armature current (d) power and power factor 159. In a three phase synchronous motor negative phase sequence exists under the condition (a) motor is over loaded (b) motor is over excited (c) operating under unbalanced voltage (d) non of these 160. In a three phase synchronous motor what happens to the direction of rotation if the direction of excitation is reversed (a) gets reversed (b) remains same (c) motor will not run (d) field winding gets damaged 161. In synchronous motor the synchronizing power will come in to action when (a) the speed is equal to synchronous speed (b) the speed is more than synchronous speed (c) the speed is less than synchronous speed (d) the motor is tending to loose synchronism 162. For a synchronous motor running on infinite bus bar and operating at a leading power factor (a) excitation voltage is equal to the supply voltage (b) excitation voltage is less than the supply voltage (c) excitation voltage is more than the supply voltage (d) excitation voltage is independent of supply voltage Page 23 of 26

24 163. In a synchronous motor at any instant if the rotor speed becomes more than that of rotating magnetic field (a) harmonics are developed in stator circuit (b) negative sequence currents will flow in the stator circuit (c) motor starts working as induction generator using damper winding (d) the motor stops 164. In the V curves shown in the figure below identify the region of lagging power factor (a) left half (b) right half (c) full region 165. the V curves shown in the figure above identify the region of leading power factor (a) left half (b) right half (c) full region 166. In construction the synchronous motors are of the type (a) salient pole type (b) nonsalient pole type (c) induction motor type (d) d c motor type 167. In a synchronous motor the maximum magnitude of the torque angle that can be obtained is (a) 45 degree electrical (b) 45 degree mechanical (c) 90 degree electrical (d) 90 degree mechanical 168. Brushless excitation system is better suited for (a) medium power rated system (b) low power rated system (c) High power rated system (d) All of these 169. Which one the following is not the advantage of short pitched coil (a) Saving of cu in overhang portion (b) Reduction in harmonics (c) Sinusoidal emf waveform (d) Reduction in cost Page 24 of 26

25 170. Which of these power is always generated in synchronous machines (a) active power (b) reactive power (c) reluctance power (d) apparent power 171. Synchronous speed of n th order harmonic is ---- times with respect to the synchronous speed (a) n (b) 1/ n (c) n 2 (d) 1/ n An alternator coupled to which of the following prime mover will have the highest speed rotating magnetic field (a) Steam turbine (b) Diesel Engine (c) Francis turbine (d) Kaplan turbine 173. In which type of coil the harmonic component of the generated emf will be more (a) Full pitched coil (b) Fractional pitched coil (c) Short pitched coil (d) All of the above 174. Which harmonic can be totally eliminated by using the fractional pitch of 4/5 in an alternator (a) Third (b) Fifth (c) Seventh (d) Ninth 175. Three phase alternators are usually star connected to (a) Reduce size of the conductor (b) Save the amount of copper (c) Reduce friction and windage loss (d) To get maximum terminal voltage 176. In turboalternators how much portion of the rotor is wound (a) 1/3 rd (b) 2/3 rd (c) Half (d) Full Page 25 of 26

26 177. In alternators the cross magnetizing effect of armature reaction can be minimized by (a) Using inter poles (b) Shifting brush position (c) Improving filed strength (d) Reducing speed 178. Which of the following has the significant influence on the operating power factor of the alternators (a) Nature of load connected (b) Speed of the alternator (c) Nature of Excitation (d) Power input to the prime mover 179. In which of the following cases the rise in terminal voltage of the alternator will be maximum with excitation held constant (a) Leading power factor load is suddenly thrown off (b) Lagging power factor load is suddenly thrown off (c) Unity power factor load is suddenly thrown off (d) None of the above 180. The speed of the prime movers coupled to alternators is maintained constant by (a) Governors (b) Brakes (c) Alternators (d) Exciters Page 26 of 26

27 1. (c) Nikola Tesla 2. (a ) Supply frequency and 1.5 Φ m 3. (d) all of the above 4. (c) preventing crawling 5. (c) rotor resistance 6. a rpm b rpm, c rpm 7. (c) poor at no load and high at full load 8. (b) greater than 9. (d) all of these 10. (b) Hz kw 12. (c) one fourth 13. (a) rotor cu loss & rotor input 14. (c) number of poles 15. (d) rotor current, rotor emf, rotor pf 16. (c) cube of the speed 17. (c) Load cycle 18. (d) all of these 19. (d) all of these 20. (c) square of rotor current 21. (a) + 5% 22. (c) star delta starter 23. (a) Silicon steel 24. (d) increase starting torque 25. (c) currents and air gap flux 26. (b) cast steel 27. (a) less than 1% 28. (b) phosphor bronze 29. (c) (1-s)Ns 30. (c) harmonics in the motor 31. (d) zero 32. (b) in parallel 33. (c) slip ring motor 34. (b) rotor current 35. (c) wound rotor 36. (c) slip ring 37. (c) motor runs in reverse direction 38. (d) crawling 39. (d) all of these 40. (a) takes high starting current 41. (c) increases up to full load and then decreases 42. (c) smaller 43. (c) equal to stator poles 44. (c) rotor speed is greater than synchronous speed 45. (b) the motor continues running at lower speed and can get damaged 46. (d) all of these Page 1 of 4

28 47. (d) transformation ratio rpm & 1440 rpm 49. (b) air gap power 50. (c) copper loss 51. (d) mechanical power output 52. (c) values are very much larger compared to series elements 53. (b) core loss 54. (a) stator only 55. (c) double layer lap winding 56. (b) 1500 rpm 57. (b) copper or aluminium 58. (b) star 59. (d) all of these 60. (c) 57.7 % of line voltage 61. (a) slip ring 62. (d) all of these 63. (c) high reactance and low resistance 64. (b) R 2 = sx (a) Squirrel cage induction motor 66. (b) very much larger 67. (d) when emf is injected to the rotor circuit 68. (b) inductive 69. (b) 2 Hz 70. (c) 50 rpm 71. (c) 2 Hz 72. (b) (1- s)p g 73. (d) does not depend on rotor resistance 74. (a) induction motor 75. (d) 1710 rpm 76. (b) semi closed stator and rotor slots 77. radial and axial 78. (b) increased surface area 79. a and b 80. (a) stator winding 81. (a) All of these 82. (a) Less 83. (b) % 84. (c ) slip 85. (d) all of these 86. three phase, single phase, squirrel cage, slip ring etc 87. (d) rotor of wound rotor induction motor 88. (b) simple and strong construction 89. (a) (a) 2 5% 91. (a) D C shunt motor 92. (d) blocked 93. (b) decreases Page 2 of 4

29 94. (d) as small as possible 95. (b) 1-s 96. (b) open circuited 97. (b) two 98. (a) copper loss 99. (c ) Triple n 100. (d) Carbon steel 101. (a) magnetizing current will increase & (b) power factor will decrease 102. (b) Two parallel windings in rotor 103. (b) pole changing method 104. (a) slip ring induction motor 105. (c) subsequently gets damaged 106. (c) interchanging two phases of supply for quick stopping 107. (d) more than the current under blocked rotor condition 108. (d) run on emf injected to rotor circuit 109. (b) (c) cost is the major consideration 111. (c ) negative sequence 112. (c ) 19 % 113. (c) Steam turbine 114. (d) All of these 115. (d) Both b and c 116. (b) 100 Hz 117. (d) (c ) 160 volts 119. (c) direction of induced emf 120. (d) cork screw rule 121. (c) Both (a) and (b) 122. (a) (c ) Total no load losses 124. (c ) (d) armature impedance and armature reaction 126. (a) load current 127. (c ) more than the terminal voltage 128. (a) less than the terminal voltage 129. (b) reactive component of the output power varies 130. (c ) lagging power factor 131. (b) leading power factor 132. (d) both (a) and (b) 133. (d) all of these 134. (d) remains stationary w.r.t stator 135. (d) full pitched coil 136. (c ) load angle 137. (c ) SCR = 1/X s 138. (d) Lower stability limit 139. (c) the machine is floating on the bus bars 140. (a) θ m = 2 θ e /p Page 3 of 4

30 141. (d) all of these 142. (c ) UPF method 143. (c ) ASA method 144. (b) MMF method 145. (a) EMF Method 146. salient pole 147. (a) continues to run as induction motor 148. (c) synchronizing torque 149. (c) 100 % excitation 150. (d) Synchronous motor 151. (d) Over excited Synchronous motor 152. (b) demagnetizing 153. (c ) magnetizing 154. (c) rotating magnetic field and rotor poles 155. (a) (b) Approximately gets doubled 157. (b) armature current and field current 158. (a) power factor and field current 159. (c) operating under unbalanced voltage 160. (b) remains same 161. (d) the motor is tending to loose synchronism 162. (c ) excitation voltage is more than the supply voltage 163. (c) motor starts working as induction generator using damper winding 164. (a) left half 165. (b) right half 166. (a) salient pole type 167. (c ) 90 0 electrical 168. (a) medium power rated system 169. (d) reduction in cost 170. (c ) reluctance power 171. (b) 1/n 172. (a) steam turbine 173. (a) Full pitched coil 174. (b) Fifth 175. (b) save amount of copper 176. (a) 1/3 rd 177. (b) Shifting brush position 178. (c ) Nature of excitation 179. (b) lagging power factor load is suddenly thrown off 180. (d) alternators Page 4 of 4