## electrical

### Three Phase Induction Motor MCQ Questions and Answers Page - 3

1.
 A three phase 6 pole, 50 Hz, induction motor is running at 5% slip. What is the speed of the motor? A 850 rpm B 900 rpm C 950 rpm D 1000rpm Answer : C Explanation :
2.
 At the instant of starting, the per unit slip of the 3-phase induction motor is A 0.05 B 0.1 C 1.0 D 0.5 Answer : C Explanation :
3.
 The mmf produced by the current of a 3-phase induction motor A Rotates ate the speed of rotor in the air gap B Is standstill with respect to stator mmf C Rotates at slip speed with respect to stator mmf D Rotates at synchronous speed with respect to rotor Answer : B Explanation :
4.
 The mmf produced by the rotor currents of a 3-phase induction motor A Rotates at the speed of rotor in the air gap B Is at standstill with respect to stator mmf C Rotates at slip speed with respect to stator mmf D Rotates at synchronous speed with respect rotor Answer : B Explanation :
5.
 A 3- phase induction motor is operating at slip s. If its two supply leads are interchanged then its slp at that instant will be A 2-s B 2+s C 1+s D A-s Answer : A Explanation :
6.
 The frequency of rotor currents at stand still is equal to A Zero B 2f C f D sf Answer : C Explanation :
7.
 In a slip ring induction motor, the frequency of rotor currents can be measured with a A galvanometer B wattmeter C dc moving coil milli voltmeter D none of the above Answer : C Explanation :
8.
 The direction of rotor current produced in an induction motor can be determined by A Len’s law B Induction law C Fleming’s right hand rule D Fleming’s left-hand rule Answer : C Explanation :
9.
 In case of an induction motor the leakage, flux is more in comparison to that of a transformer. This is due to A Revolving rotor B Air gap between rotor and stator C Higher flux densities in induction motor D None of the above Answer : B Explanation :
10.
 In an induction motor the phase reactance is _____in comparison to  phase resistance A Quite high B Very small C Slightly high D Almost some Answer : A Explanation :
11.
 In a 3- phase induction motor reactance under running condition is less than as standstill value. This is due to reduction in A Rotor inductance B Stator magnetic flux C Frequency of rotor emf D Mutual flux linking the stator and rotor Answer : C Explanation :
12.
 In an induction motor under running condition, the rotor reactance per phase is _______its standstill phase reactance A S times B equal to C 1/s times D (1-s) times Answer : A Explanation :
13.
 The torque developed by a 3-phase induction motor is approximately proportional to A √s B S2 C S D 1/s Answer : C Explanation :
14.
 Insertion of resistance in the rotor circuit of an induction motor to develop a given torque causes ____its rotor current A Almost no change B Increase C Decease D None of these Answer : A Explanation :
15.
 In an induction motor if the flux density is reduced to one half of its normal value then the torque will A Reduce to one half B Reduce to one fourth C Remain unchanged D Increase for times Answer : A Explanation :
16.
 The torque developed in an induction motor is nearly proportional to A 1/V B V C V2 D None of these Answer : C Explanation :
17.
 The starting torque of a 3-phase induction motor varies as A V2 B V C √V D 1/V Answer : A Explanation :
18.
 The starting torque of a squirrel cage induction motor is A Very large B Very low C Slightly more than full load torque D Zero Answer : C Explanation :
19.
 For achieving high starting torque and high operation efficiency an induction motor should have _____rotor circuit resistance at starting and ____circuit resistance under operating conditions A high, low B low, high C high, high D low, low Answer : A Explanation :
20.
 In a 3-phase slip ring induction motor high stating torque is achieved by A Increasing supply voltage V B Increasing supply frequency f C Connecting a capacitor across the motor terminals D Connecting a star connected resistance across the slip ring terminals of the motor Answer : D Explanation :
21.
 When the applied voltage per phase is reduced to one half, the starting torque of a three-phase squirrel cage induction motor becomes A ½ of the initial value B ¼ of the initial value C Twice of the initial value D 4 times of the initial value Answer : B Explanation :
22.
 The torque developed by a 3-phase induction motor least depends on A Rotor current B Rotor power factor C Rotor e.m.f D Shaft diameter Answer : D Explanation :
23.
 In an induction motor if air-gap is increased A The power factor will be low B Windage losses will be more C Bearing friction will reduce D Copper loss will reduce Answer : A Explanation :
24.
 When R2 is the rotor resistance, X2 the rotor reactance at supply frequency and the slip, then the condition for maximum torque under running conditions will be A sR2X2 = 1 B sR2 = X2 C R2 = sX2 D R2 = s²X2 Answer : C Explanation :
25.
 In case of a double cage induction motor, the inner cage has A High inductance and low reactance B Low inductance and high resistance C Low inductance and low resistance D High inductance and high resistance Answer : A Explanation :
26.
 The low power factor of induction motor is due to A Rotor leakage reactance B Stator reactance C The reactive lagging magnetizing current necessary to generate the magnetic flux D All of the above Answer : D Explanation :
27.
 Insertion of reactance in the rotor circuit A Reduces starting torque as well as maximum torque B Increases starting torque as well as maximum torque C Increases starting torque but maximum torque remains unchanged D Increases starting torque but maximum torque decreases Answer : A Explanation :
28.
 Insertion of resistance in the rotor of an induction motor to develop a given torque A Decreases the rotor current B Increases the rotor current C Rotor current becomes zero D Rotor current remains same Answer : D Explanation :
29.
 For driving high inertia loads best type of induction motor suggested is A Slip ring type B Squirrel cage type C Any of the above D None of the above Answer : A Explanation :
30.
 Temperature of the stator winding of a three-phase induction motor is obtained by A Resistance rise method B Thermometer method C Embedded temperature method D All of the above Answer : D Explanation :
31.
 The purpose of using short-circuit gear is A To short circuit the rotor at slip ring B To short circuit the staring resistances in the starter C To short circuit the stator phase of motor to from star D None of the above Answer : A Explanation :
32.
 In a squirrel cage motor, the induced e.m.f is A Dependent on the shaft loading B Dependant on the number of slots C Slip times the stand still e.m.f induced in the rotor D None of the above Answer : C Explanation :
33.
 Less maintenance troubles are experienced in case of A Slip ring induction motor B Squirrel cage induction motor C Both (a) and (b) D None of the above Answer : B Explanation :
34.
 A squirrel cage induction motor is not selected when A Initial cost is the main considerable B Maintenance cost is to be kept low C Higher staring torque is the main consideration D All above consideration are involved Answer : C Explanation :
35.
 Reduced voltage starter can be used with A Slip ring motor only but not with squirrel cage induction motor B Squirrel cage induction motor only but not with slip ring motor C Squirrel cage as well as slip ring induction motor D None of the above Answer : C Explanation :
36.
 Slip ring motor is preferred over squirrel cage induction motor where A High staring torque is required B Load torque is heavy C Heavy pull out torque is required D All of the above Answer : A Explanation :
37.
 In as tar-delta starter of an induction motor A Resistance is inserted in the stator B Reduced voltage is applied to the stator C Resistance is inserted in the rotor circuit D Applied voltage per stator phase is 57.7% of the line Answer : D Explanation :
38.
 The torque of an induction motor is A Directly proportional to slip B Inversely proportional to slip C Proportional to the square of the slip D none of the above Answer : A Explanation :
39.
 The rotor of an induction motor runs at A synchronous speed B below synchronous speed C above synchronous speed D any of the above Answer : B Explanation :
40.
 The starting torque of a three-phase induction motor can be increased by A increasing slip B increasing current C both (a) and (b) D none of the above Answer : C Explanation :
41.
 Insertion of resistance in the stator of an induction motor A increases the load torque B decreases the starting torque C increases the staring torque D none of the above Answer : B Explanation :
42.
 An induction motor is identical to A D.C compounded motor B D.C series motor C Synchronous motor D Asynchronous motor Answer : D Explanation :
43.
 The starting torque of a 3-phase squirrel cage induction motor is A Twice the full load torque B 1.5 times the full load torque C Equal to full load torque D None of above Answer : B Explanation :
44.
 In induction motor, percentage slip depends on A Supply frequency B Supply voltage C Copper losses in motor D None of the above Answer : C Explanation :
45.
 The efficiency of an induction motor is ………… than that of a transformer A Lower B Higher C Either of above D None of the above Answer : A Explanation :
46.
 If the rotor circuit of a squirrel cage induction motor is open, the rotor will A Run at very high speed B Run at very low speed C Make noise D Not run Answer : D Explanation :
47.
 The advantages of a slip-ring induction motor over a squirrel cage induction motor is that A It has higher efficiency B It has higher power factor C It can be started with the help of rotor resistance starter D None of the above Answer : C Explanation :
48.
 The magnetising current drawn by the induction motors and transformers is the cause of their ……. Power factor A Leading B Lagging C Unity D Zero Answer : B Explanation :
49.
 A 3-phase slip-ring induction motor is always started with A A starting winding B Squirrel cage winding C No external resistance in rotor circuit D Full external resistance in rotor circuit Answer : D Explanation :