electrical

Three Phase Induction Motor MCQ Questions and Answers Page - 7

1.
 Induction motors, when directly switched on across their normal rated supply voltage, the torque developed at starting is about _____of their full load torque. A Half B 1.5 to 2.5 times C 5 times D 10 times Answer : B Explanation :
2.
 _____starter is used for starting a 3-phase induction motor that needs frequent starting and operating in forward and reverse directions. A Direct on line B Star delta C Auto transformer D Reduced voltage Answer : A Explanation :
3.
 With DOL start of an induction motor the heating during acceleration of inertia load is proportional to A Slip at maximum torque B Maximum torque C Stored kinetic energy D Electromagnetic power developed on the rotor Answer : D Explanation :
4.
 A 3- phases squirrel cage induction motor is started by means of a star/delta switch. What is the starting current of the motor? A 3-times the current with direct on line starting B 1/3 time the current with direct on line starting C 1/√3 time the current with direct on line starting D √3 time the current with direct on line starting Answer : B Explanation :
5.
 In auto transformer starting of 3- phase induction motor, if the tapping used is x then the starting line current and starting torque will be ___and ____times respectively with direct on line starting. A x, x B x, x2 C x2, x D x2, x Answer : C Explanation :
6.
 for the purpose of starting an induction motor. Y/  switch is an equivalent to an auto transformer of ratio. A 33.3% B 57.7% C 73.2% D 100% Answer : B Explanation :
7.
 The starting torque of a 3-phase slip ring induction motor can be increased by connecting an external resistance in the A Rotor circuit B Stator circuit C Supply circuit D Both (a) and (b) Answer : A Explanation :
8.
 Which of the following starting methods cannot be used for starting a 3-phase squirrel cage induction motor? A Star delta B Auto transformer C Rotor resistance D Direct on line Answer : C Explanation :
9.
 An induction motor can run at synchronous speed if it is operated A On no load B On full load C With emf injected in the rotor circuit D On voltage higher than the rated voltage Answer : C Explanation :
10.
 The speed regulation of a 3-phase induction motor at full load is about A 4% B 8% C 15% D 25% Answer : A Explanation :
11.
 The speed of an induction motor depends on A Number of stator poles B Stator supply frequency C Input voltage to stator D All of the above Answer : D Explanation :
12.
 Smooth speed control can be achieved by A Rotor resistance control only B Rotor slip power control only C Variation of supply frequency only D Both rotor slip power control and variation of supply frequency Answer : D Explanation :
13.
 The speed of a slip ring three phase induction motor can be controlled from the rotor side by A Changing the supply voltage B Changing the supply frequency C Rheostatic control D Changing the number Answer : C Explanation :
14.
 Synchronous speed of an induction motor can be increased by A Reducing mechanical friction B Increasing supply voltage C Increasing number of poles D Increasing supply frequency Answer : D Explanation :
15.
 In ac motor control V/f is kept constant to A Make maximum use of the magnetic circuit B Make minimum use of the magnetic circuit C Maximize the current drawn D Make the power constant Answer : A Explanation :
16.
 In rotor resistance control method ____with the increase in speed A Torque increases B Torque decreases C Slip increases D Losses increase Answer : B Explanation :
17.
 In rotor resistance control of induction motor, the harness of speed torque characteristic A Increases B Decreases C Remains same D None of these Answer : B Explanation :
18.
 In case of voltage injection method of speed control, the injected emf should be of A Supply frequency (f) B Slip frequency (sf) C (1-s)f D (2-s)f Answer : B Explanation :
19.
 Speeds higher than synchronous speed can be had by A Line voltage control B Rotor slip power control C Rotor resistance control D Frequency control Answer : B Explanation :
20.
 Slip changing method of speed control can be used in case of A Slip ring induction motors only B Squirrel cage induction motors only C Squirrel cage as well as slip ring induction motors D None of the above Answer : A Explanation :
21.
 During plugging of an induction motor A Phase sequence is reversed B A dc source is connected to stator C One phase is open circuited D Power is fed back to mains Answer : A Explanation :
22.
 In dynamic braking of 3-phase induction motors A The supply terminals of any two-stator phase are interchanged B Any two stator terminals are earthed C The stator terminals are switched over to a dc source from the ac supply D A dc voltage is injected in the rotor circuit Answer : C Explanation :
23.
 In a 3-phase induction motor, the regenerative braking occurs when A The load is lowered by a hoisting machine B The load is raised by a hoisting machine C The number of poles is reduced in a pole changing motor D The motor speed falls due to overload Answer : A Explanation :
24.
 The stator of a 2/4 pole changing cage motor is initially wound for 2-poles. The reconnection of the stator winding to 4- poles through a changeover switch, while the motor is running would result in A Constant torque drive B Constant hp drive C Plugging to standstill D Regenerative braking to half the original speed Answer : D Explanation :
25.
 If a squirrel cage induction motor runs slow, the probable cause could be A Low voltage or Over load B Shorted stator coils C One phase open D Any of these Answer : D Explanation :
26.
 If a squirrel cage induction motor run hot, the probable cause could be A Uneven air gap B Overload C Low supply frequency D Any of these Answer : D Explanation :
27.
 If an induction motor hums during starting up, the possible cause could be A Open circuit B Unequal phase resistance C Inter turn short circuit on rotor D Any of the above Answer : D Explanation :
28.
 Three phase induction motor should preferably be mounted on A Wooden structure B Solid concrete foundation C Rigid steel structure D Either (c) and (d) Answer : D Explanation :
29.
 The large sized induction motors are protected against overload by A High voltage fuses B Automatic cutouts C Thermal or magnetic relay D Rewireable fuses Answer : C Explanation :
30.
 The insulating materials employed for motor windings are classified according to A Motor kW output rating B Level of temperature rise C Controller size D Overload protection available Answer : B Explanation :
31.
 The factor (s) to be considered in selection of motor is / are A Voltage rating of the motor B kW output rating of the motor C speed and frame enclosure size and bearings and protection provided D all of the above Answer : D Explanation :
32.
 Armature short circuits can be detected and identified by ____tests(s) A growler B bar-to-bar C voltage drop D any of these Answer : D Explanation :
33.
 Induction generator deliver power at A leading power factor only B lagging power factor only C leading as well as lagging power factor D unity power factor Answer : A Explanation :
34.
 A 3- phase induction motor is fed from a balanced 3-phase supply on the rotor side short circuiting the stator terminals. The speed of the rotating magnetic field is A Zero B Equal to rotor speed C Equal to synchronous speed D Equal to slip speed Answer : D Explanation :
35.
 A dual voltage single phase ac motor can be operated on either A 230 or 400 V B 30 or 400 V C 115 or 230 V D 30 or 400V Answer : C Explanation :
36.
 The rotor slots in an induction motor are, usually not kept parallel to the shaft as it A Helps the rotor teeth to remain under the stator teeth B Helps in reducing the tendency of the rotor teeth to remain under the stator teeth C Improves pf D Improves efficiency Answer : B Explanation :
37.
 If a single phasing occurs when the motor is running. It should not be loaded beyond A 5 % of rated load B 20 % of rated load C 50% of rated load D 90% of rated load Answer : C Explanation :
38.
 The overheating of an induction motor may be due to A Overloading B Loss of ventilation C Low supply voltage D Any of the above Answer : D Explanation :
39.
 Imbalance in shaft of the induction motor occurs due to A Overheating of the winding B Air gap is not uniform C Slip rings D Rugged construction Answer : B Explanation :
40.
 The reversing of 3-phase A.C motor is achieved by A Star-delta starter B DOL starter C An auto-transformer D Interchanging any two of the supply lines Answer : D Explanation :
41.
 Which of the following is not determined by circle diagram? A Efficiency B Power factor C Frequency D Number of poles and frequency of supply Answer : C Explanation :
42.
 What is the advantage of the slip ring induction motor over the squirrel cage induction motor? A It has a higher power factor B It can be started with the help of rotor resistances C It is suitable for higher speeds D It can be started with the help of rotor resistance Answer : D Explanation :
43.
 What is the advantage of starting a slip-ring induction motor with the help of rotor resistance as compared to other methods? A The starter has to be designed for only a very low current B The starting current is reduced C The starting torque increases due to rotor resistance D The starter can be built directly into the motor Answer : C Explanation :
44.
 What is the advantage of the double squirrel-cage rotor as compared to the round bar cage rotor? A The slip of the motor is larger B The efficiency of the motor is higher C The starting current of the motor is lower D The power factor of the motor is higher Answer : C Explanation :
45.
 Synchronous speed is defined as A The speed of a synchronous motor B The natural speed of the rotor of an induction motor C The speed of the rotor of an induction motor D The speed of an induction motor at no load Answer : B Explanation :
46.
 The speed of a three-phase cage-rotor induction motor depends upon A Number of poles only B Input voltage C Frequency of supply only D Number of poles and frequency of supply Answer : D Explanation :
47.
 When a Scharge motor runs exactly at synchronous speed, then A Secondary voltage E2 comes into phase with primary back e.m.f E1 B The speed of the motor field relative to that of regulating winding becomes zero C Voltage injected from the compensation winding into secondary becomes equal and opposite to the secondary induced voltage D It behaves, in a sense, like a synchronous motor with compensating winding functioning as D.C exciter Answer : D Explanation :
48.
 Under average service conditions, motor bearing may be safely operated at temperature upto A 120°C B  105°C C 95°C D 75°C Answer : C Explanation :
49.
 Which of the following statements is correct regarding a 3-phase motor? A Its magnetic flux alternates at the supply frequency B It is cheap, robust and needs no skilled maintenance C Stator is externally connected rotor D Three-phase supply is usually connected to its rotor Answer : B Explanation :
50.
 The most important factor in motor maintenance is to keep A It clean and free from foreign materials B Checking its insulation C Regular check on line voltage D Regular check of its lubrication Answer : A Explanation :
51.
 The two important parts of a3-phase induction motor are A Rotor and armature B Rotor and stator C Slip rings and brushes D Stator and field Answer : B Explanation :
52.
 In a wound 3-phase induction motor, brushes should be connected to A External D.C excitation B Equalizing coils C Power supply D  External star-connected resistor Answer : D Explanation :
53.
 Phase advancers are used with induction motors to A Reduce noise B Reduce vibrations C Reduce copper loss D Improve power factor Answer : D Explanation :
54.
 In moist surroundings which frames of motor will preferred? A Open type B Totally enclosed fan cooled C Drip proof D Splash proof Answer : B Explanation :
55.
 The principle of operation of a three-phase induction motor closely resembles to that of which of the following? A Synchronous motor B D.C series motor C Transformer with short-circuit secondary D Transformer with open-circuited secondary Answer : C Explanation :