electrical

Three Phase Induction Motor MCQ Questions and Answers Page - 4

1.
 In a 3-phase induction motor the maximum torque A Is independent of rotor circuit resistance B Varies as rotor resistance C Varies as the square of rotor resistance D Varies inversely as rotor circuit resistance Answer : A Explanation :
2.
 For a slip ring induction motor, if the rotor resistance is increased then A Starting torque and efficiency increase B Starting torque decreases but efficiency increases C Starting torque increases but efficiency decreases D Starting torque and efficiency decrease Answer : C Explanation :
3.
 An induction motor has a rotor resistance of 0.002 ohm/ phase. If the resistance is increased to 0.004 ohm/phase then the maximum torque will A Reduce to half B Increase by 100w C Increase by 200% D Remain unaltered Answer : D Explanation :
4.
 Breakdown torque of a 3-phase induction motor of negligible stator impedance is A Directly proportional to the rotor resistance B Inversely proportional to the rotor resistance C Directly proportional to the reactance D Inversely proportional to the rotor leakage reactance Answer : D Explanation :
5.
 A 4 – pole, 50 Hz, 3-phase induction motor has blocked rotor reactance per phase which is four times the rotor resistance per phase. The speed at which maximum torque develops is A 1,125 rpm. B 1,500 rpm C 1,050 rpm D 1,210 rpm Answer : A Explanation :
6.
 If the rotor circuit resistance is increased in an induction motor, the maximum torque will occur at A Lower speed B High speed C The same speed D None of these Answer : A Explanation :
7.
 Beyond the point of maximum torque any further increase in loads will cause A Increase in torque developed and so the motor will run at a high speed B No change in the torque developed and so the motor will run at the same speed as before. C Decease in torque developed and the motor will slow down D Increase in current drawn from the supply mains resulting in damage to it Answer : C Explanation :
8.
 The torque-slip characteristic of a polyphase induction motor becomes almost linear at small values of slip, because in this range of slips A The effective rotor circuit resistance is very large compared to the rotor reactance B The rotor resistance is equal to the stator resistance C The rotor resistance is equal to the rotor reactance D The rotor resistance is equal to the stator reactance Answer : A Explanation :
9.
 The torque –slip characteristic of an induction motor is approximately a/an A Straight line B Rectangular parabola C Parabola D Hyperbola Answer : B Explanation :
10.
 Stable operation of an induction motor is A Between zero slip and unity slip B Between 0.5 slip and 0.95 slip C Between zero slip and slip corresponding to maximum torque D Between slip corresponding to maximum torque and unity slip Answer : C Explanation :
11.
 with the increase in load on a squirrel cage induction motor A Stator current increase B Power factor improves and Slip increase C The torque developed by the motor increases till it becomes equal to the load torque D All of the above Answer : D Explanation :
12.
 Whenever polyphase induction motor is loaded A Motor speed decreases B Stator flux cuts the rotor bars more rapidly C Emf induced in the rotor as well as its frequency increases and also Current in rotor bars increases D All of the above Answer : D Explanation :
13.
 In a 3-phase induction motor iron loss mainly occurs in A Stator and rotor B Rotor core and rotor teeth C Stator core and stator teeth D Stator winding Answer : C Explanation :
14.
 Iron loss in the rotor of a 3-kphase induction motor is negligible This is due to A Very low frequency of emf induced in the rotor B Very low emf induced in the rotor C Very low flue density in rotor parts D The fact that the rotor core is laminated Answer : A Explanation :
15.
 Rotor input of a three-phase induction motor is equal to A Input to motor stator copper and iron losses B Mechanical power developed –windage and friction losses C Input to motor –windage and friction losses D Input to motor - stator iron losses-windage and friction losses Answer : A Explanation :
16.
 In an induction motor, what is the ratio of rotor copper loss and rotor input? A 1/s B S C (1-s) D s/(1-s) Answer : B Explanation :
17.
 The rotor power output of a 3-phase induction motor is 15 kW The rotor copper losses at a slip of 4% will be A 600W B 625W C 650W D 700W Answer : B Explanation :
18.
 The synchronous watt is A The torque which under synchronous speed would develop a powr of 1 watt or power of 1 watt or power input to rotor in watts B Shaft output in watts C Input power to stator D The unit of ratings of synchronous machines Answer : A Explanation :
19.
 The no load current in an induction motor is ____in comparison to that of a power transformer. A Much lower B Much higher C Slightly less D Slightly more Answer : B Explanation :
20.
 In a 3-phase induction motor, the variable mechanical load is represented electrically by a variable ____only A Inductance B Capacitance C Resistance D Any of these Answer : C Explanation :
21.
 If the load on an induction motor is increased from no load to full load, its slip and the power factor will, respectively A Decrease, decrease B Decrease, increase C Increase, decrease D Increase, increase Answer : D Explanation :
22.
 The output power of a 3-phase induction motor will be maximum when the equivalent load resistant is equal to the standstill leakage _____of the motor A Impedance Z1 B Reactance X1 C Resistance D Capacitance Answer : A Explanation :
23.
 The power factor of an induction motor operating at no load will have a value around A 0.9 lag B 0.2 lead C 0.2 lag D 0.9 lead Answer : C Explanation :
24.
 The power factor of an induction motor at full load is likely to be A Unity B 0.85 lead C 0.85 lag D 0.5 lag Answer : C Explanation :
25.
 Out of the following methods of staring a 3-phase induction motor, which one requires six stator terminals? A Direct-on-line B Star-delta C Auto-transformer D Rotor rheostat Answer : B Explanation :
26.
 If a single-phasing occurs on the running position in an induction motor, the motor will A Fail to carry load B Produce peculiar noise C Draw unbalanced and excessive currents D Not start Answer : D Explanation :
27.
 The fractional slip of an induction motor is the ratio A Rotor copper loss/rotor output B Rotor copper loss/rotor input C Rotor copper loss/stator copper loss D Stator copper loss/stator input Answer : B Explanation :
28.
 Each of the following statements regarding stator flux of an induction motor is true expect A It induces e.m.f in the rotor bars B Its magnitude depends on the motor load C It is constant in magnitude D It revolves around the stator at synchronous Answer : B Explanation :
29.
 The rotor of an induction motor cannot run with synchronous speed because A Lenz’s law would be violated B Induction motor would then become synchronous motor C Rotor torque would be then become zero D Air friction prevents it to do so Answer : C Explanation :
30.
 When the load on a squirrel-cage induction motor is increased from no-load to full-load, which of the following statement is incorrect? A Slip increases B The magnetizing component of the stator exiting current is increased C Stator power factor is improved D Torque developed by the motor increases till it equals the applied torque Answer : B Explanation :
31.
 The power factor of an induction motor under no-load conditions is about A 0.2 lag B 0.4 lag C Unity D 0.4 lead Answer : A Explanation :
32.
 A pump induction motor is switched on to a supply 25 percent lower than its rated voltage. The pump runs eventually A The pump will get heated and consequently get damaged B The pump will start after sometimes C The pump will continue to run at the lower speed without damage D None of the above Answer : A Explanation :
33.
 If there is an open circuit in the rotor of a squirrel cage induction motor A Rotor will overheat B Line fuses will blow C Motor will be noisy D Motor will not start Answer : D Explanation :
34.
 The principle of operation of 3-phase induction motor is most similar to that of A Transformer with a shorted secondary B Synchronous motor C Capacitor-start induction run motor D Repulsion start induction motor Answer : A Explanation :
35.
 The speed of a 3-phase wound motor can be increased by A Short-circuiting slip-rings B Increasing applied voltage C Decreasing applied voltage D Decreasing external resistance in the rotor circuit Answer : D Explanation :
36.
 The A.C motor which would be best suited to drive a centrifugal pump for discharging a variable quantity of water against a fixed head is the A Repulsive motor B Synchronous motor C Squirrel cage D Slip ring induction motor Answer : D Explanation :
37.
 For staring a Schrage motor, 3-phase supply is connected to A Rotor via slip rings B Regulation windings C Secondary winding via brushes D Stator as in an ordinary induction motor Answer : A Explanation :
38.
 The primary winding of a Schrage motor is located A In stator B In lower part of rotor C In upper part of rotor D Partly in stator and partly in rotor Answer : B Explanation :
39.
 When a stationary and partly in rotor motor is switched on with one phase disconnected A It will start but run very slowly B It will make jerk start with loud growling noise C Remaining intact fuses will be blown out due to heavy rush of current D It is likely to burn out quickly unless immediately disconnected Answer : D Explanation :
40.
 The upper bars in a double squirrel cage induction motor have A Low resistance and high reactance B High resistance and high reactance C High resistance and low reactance D Low resistance and low reactance Answer : C Explanation :
41.
 The pull-out torque for a normal squirrel cage induction motor usually occurs at a percentage slip of about A 10 to 15 percent B 20 to 25 percent C 35 to 45 percent D 50 to 60 percent Answer : B Explanation :
42.
 Speed variations of a squirrel cage induction motor are essentially similar to those of A D.C shunt motor B D.C series motor C Synchronous motor D Differential compound motor Answer : A Explanation :
43.
 If air gap of an induction motor is increased, its A Power factor will decrease B Magnetizing current will decrease C Magnetizing current will increase D Power factor will increase Answer : C Explanation :
44.
 Two of the power supply terminals to a 3-phase induction motor get interchanged during reconnection after maintenance of the motor. When put back into service, the motor will be A get heated up and damaged B rotate in the same direction as it was prior to maintenance C fail to rotates D rotate in the reverse direction to that prior to maintenance Answer : D Explanation :
45.
 In three-phase induction motor, the relative speed of stator flux with respect to ……………. is zero A space B rotor C stator winding D rotor flux Answer : D Explanation :
46.
 In a 3-phase induction motor reactance under running conditions is less than its standstill value because of decrease in A stator magnetic flux B mutual flux linking the stator and rotor conductor C rotor inductance D frequency of rotor e.m.f Answer : D Explanation :
47.
 In case of a 3-phase wound-rotor induction motor, an increase in rotor resistance affects the motor performance in the following ways A the motor efficiency decreases B the motor efficiency increases C staring current decreases D staring current increases Answer : C Explanation :
48.
 A squirrel cage induction motor running on no-load is loaded, which of the following statements is incorrect? A Current in the rotor bars decreases B Motor speed decreases C Stator flux cuts the rator bars more rapidly D Stator flux keeps rotating synchronously Answer : A Explanation :
49.
 Which of the following statement about 3-phase induction motor is incorrect? A It is capable of operation under a wide range of power factors both lagging and leading B It starts up from rest and has not be synchronised C Its no-load line current may be as large as 40% of the full load current D It has no commutator and no slip rings Answer : A Explanation :
50.
 Which of the following statements about a double squirrel cage induction motor is incorrect? A It has better speed regulation than squirrel cage motor B Its upper cage is of low resistance and lower cage is high resistance C It is particularly useful where frequent starting under heavy loads is required D It maintains high efficiency during normal operating conditions Answer : B Explanation :
51.
 When a double squirrel cage motor is started, the current induced in the rotor A Flows most through the upper winding B Flows mostly through the lower winding C Is directly proportional to the impedance offered by each cage D Is equally divided between the two windings Answer : A Explanation :
52.
 Regarding single phase of a 3-phase induction motor under running conditions which of the following statements is incorrect? A It will stop and blow the remaining fuses B It will continue to run without damage if it is carrying half load or less C It will stop and promptly burnt out if it is heavily overloaded D It will try to keep running though over-heating of part of its winding if it is carrying full –load or slight over-load Answer : A Explanation :
53.
 The difference between the synchronous speed and actual speed of an induction motor is known as A Lag B Regulation C Slip D Back lash Answer : C Explanation :
54.
 In a induction motor if P is the power delivered to a rotor and s is the slip, then the power lost in the rotor as copper loss will be A P/s B sP C s²P D P/s² Answer : B Explanation :