## electrical

### Three Phase Induction Motor MCQ Questions and Answers Page - 6

1.
 The 3-phase induction motor provided with open slots has A Reduced leakage reactance B Increased starting current, starting torque and breakdown torque C Better pf D Both (a) and (b) Answer : D Explanation :
2.
 Semi closed or totally closed slots are used in induction motors essentially to A Improve pull out torque B Increase pull out torque C Increase efficiency D Reduce magnetizing current and improve power factor Answer : D Explanation :
3.
 The stator of a small (up to 5hp) induction motor is provided with A Open slots with parallel teeth B Open slots with tapered teeth C Semi closed slots with parallel teeth D Totally closed slots with parallel teeth Answer : C Explanation :
4.
 “Cogging” in induction motor occurs when A Number of stator teeth-number of rotor teeth=odd number B Number of stator teeth-number of rotor teeth=even number C Number of stator teeth-number of rotor teeth =0 D Number of stator teeth-number of rotor teeth=negative Answer : C Explanation :
5.
 In a 3-phase induction motor, the stator slots are A Equal to rotor slots B Exact multiple of rotor slots C Not exact multiple of rotor slots D None of the above Answer : C Explanation :
6.
 In a 3-phase induction motor, the number of slots on stator is not kept an exact multiple of the number of rotor slots because it A Facilitates cooling B Avoids magnetic locking between stator field and rotor. Improves efficiency C Improves efficiency D Improves pf Answer : B Explanation :
7.
 The difference between the number of stator slots and that of rotor slots in an induction motor should not be equal to P, 2P or 5P. It is essential in order to avoid A Synchronous cusps B Crawling C Magnetic locking D noise and vibrations Answer : A Explanation :
8.
 The rotor slots in a 3-phase induction motor are kept inclined this phenomenon is known as A Skewing B Crawling C Cogging D None of these Answer : A Explanation :
9.
 In an induction motor, skewing of motor bars reduces A Eddy currents and copper requirement B Noise, vibrations and synchronous cusps C Both (a) and (b) D None of these Answer : B Explanation :
10.
 Skew of rotor bar eliminates A The effect of space harmonics B The entire effects of crawling C Magnetic noise D Vibration due to unequal force developed on rotor Answer : A Explanation :
11.
 Jogging of an induction motor is A Energizing a motor once or repeatedly to have small movement for mechanisms B A process of synchronization C A method of braking D None of the above Answer : A Explanation :
12.
 In 3-phase induction motors, sometimes copper bars are placed deep in the rotor. It is done in order to improve A Efficiency B Starting torque C Power factor D None of these Answer : B Explanation :
13.
 In a squirrel cage induction motor high starting torque is achieved by using A High resistance in series with the rotor circuit B Low resistance across the rotor circuit C Double cage rotor D None of these Answer : C Explanation :
14.
 The outer cage of a double squirrel rotor consists of A Manganese brass B Red copper C Bronze D Steel Answer : A Explanation :
15.
 In double cage induction motor the inner cage has A High resistance and high leakage reactance B High resistance and low leakage reactance C Low resistance and high leakage reactance D Low resistance and low leakage reactance Answer : C Explanation :
16.
 In the equivalent circuit of a double cage induction motor, the two rotor cages can be considered A To be in parallel B To be in series parallel C To be in series D To be parallel with stator Answer : A Explanation :
17.
 The advantage of the double squirrel cage induction motor over single cage rotor is that its A Efficiency is higher B Power factor is higher C Slip is larger D Starting current is lower Answer : D Explanation :
18.
 In a double squirrel cage induction motor, the resistance of upper cage is ____that of inner cage. A Equal to B 4 to 5 time of C One fourth of D None of these Answer : B Explanation :
19.
 The deep bar rotors of double cage rotors are employed to A Increase starting torque B Improve efficiency C Increase pull out torque D Reduce rotor core loss Answer : A Explanation :
20.
 For high starting torque, the most suitable 3-phsae induction motor is ___induction motor A Slip ring B Squirrel cage C Double cage D Deep bar squirrel cage Answer : A Explanation :
21.
 Which of the following ac motor is widely used? A Squirrel cage induction motor B Slip ring induction motor C Double cage induction motor D Synchronous motor Answer : A Explanation :
22.
 A squirrel cage induction motor is not favored when ______ in the main consideration A Initial cost B Maintenance cost C High starting torque D Higher efficiency and higher power factor Answer : C Explanation :
23.
 A slip ring induction motor is recommended for application requiring A High starting torque B Variable speed operation C Frequent starting, stopping and reversing operation D All of the above features Answer : D Explanation :
24.
 Unbalanced 3-phase voltage supply to an induction motor causes excessive heating of A Rotor B Stator C Rotor shaft D None of these Answer : A Explanation :
25.
 Unbalance supply voltage given to a 3- phase delta connected induction motor will cause A Zero sequence currents B Less heating of motor C Negative sequence component current D All the above Answer : C Explanation :
26.
 Single phase preventer A Suppresses negative sequence currents B Compensates for voltage drops C Provides protection in the event of non-availability of one of the phases D None of the above Answer : C Explanation :
27.
 If one of the 3-phase of supply to an induction motor fails, may be due to any reason, when the motor is running normally, the motor will A Continue running at the same speed if it was running on light load B Continue running but will draw more current if it was operating at rated load. C Stop and carry heavy current causing permanent damage to the winding if it was operating overloaded D All of the above Answer : D Explanation :
28.
 Increase in supply voltage to a 3-phase induction motor will not cause A Decrease in input current and so reduction in copper losses B Increase in torque C Decrease in speed as well as in core loss D Operating temperature abnormally higher owing to increase in core loss Answer : C Explanation :
29.
 Decrease in supply voltage to a 3-phase induction motor will not cause A Any difficulty in accelerating the load B Increase in torque C Decrease in speed as well as in core loss D Increase in stator as well as rotor currents Answer : B Explanation :
30.
 If a 3-phase 350 V, 50Hz, 1440 rpm induction motor is operated on 420V, 60Hz supply, then the torque will A Increase B Decrease C Remain unchanged D None of these Answer : C Explanation :
31.
 With the increase in supply frequency ___decreases (s) A Starting current B Starting torque C Full load current D Starting and full load currents and starting and maximum running torque Answer : D Explanation :
32.
 This size of a high-speed motor is ____in comparison to that of a low speed motor for the same kW output A Smaller B Larger C Almost the same D Unpredictable Answer : A Explanation :
33.
 Which of the following induction motors will operate on the lowest value of power factor? A 7.5 kW, 950 rpm B 7.5 kW, 1440 rpm C 15 kW, 950 rpm D 15 kW, 1440 rpm Answer : A Explanation :
34.
 For starting a 3-phase induction motor ____winding is to be connected to 3 phase ac supply A Rotor B Stator C Both stator and rotor D None Answer : B Explanation :
35.
 The starting current of a 3-phase induction motor is about of its full load rated current A Half B Twice C 5 to 7 times D 15 to 20 times Answer : C Explanation :
36.
 The purpose of the starter is to A Limit the starting current B Limit the speed C Protect against over voltage D Produce back emf Answer : A Explanation :
37.
 Direct on line starter can be used for small capacity ____induction motors. A Squirrel cage B Wound rotor C Squirrel cage and wound rotor both types of D All of thses Answer : C Explanation :
38.
 For starting a small induction motor DOL starter is used in place or iron clad triple pole (ICTP) switch as it A Provides protection against over load and under volt B Reduces the starting current C Increase the starting torque D Improves the power factor and efficiency Answer : A Explanation :
39.
 Whenever any polyphase induction motor is loaded A Induced e.m.f decreases and frequency increases B Induced e.m.f in the rotor remains constant C Induced e.m.f in the rotor increases and its frequency also increases D Induced e.m.f in a rotor increase and frequency falls Answer : D Explanation :
40.
 A 3-phase induction motor is running at a load of the rated torque. What happens when one of the outer mains is interrupted while the motor is running? A The motor keeps on running B The motor keeps on running but draws more current C The motor stops immediately D The motor stops after a few seconds Answer : B Explanation :
41.
 What is the function of putting resistance in parallel to one phase of 3-phase induction motor? A To attain a higher starting torque B To achieve a smooth starting C To reduce the starting current to be very low value D To attain a higher maximum torque Answer : B Explanation :
42.
 What is the disadvantage of the speed control of slip-ring induction motor with the help of resistances in the rotor circuit? A By using this method, the speed can be easily controlled B This method is associated with high losses C With reduction in torque the speed decreases considerably D None of the above Answer : B Explanation :
43.
 The reactance per phase as compared to the resistance per phase of an induction motor is A Very small B Quite high C Almost same D Slightly large Answer : B Explanation :
44.
 What is the disadvantage of starting an induction motor with star-delta starter? A The starting torque is one third of the torque in case of the delta connection B During starting high losses are produced C The starting torque increases and the motor runs with jerks D None of the above Answer : A Explanation :
45.
 An induction motor has a rated speed of 720 r.p.m. how many poles has it rotating magnetic field? A 8 poles B 6 poles C 4 poles D 2 poles Answer : A Explanation :
46.
 Which of the following methods of speed control is not affected through the stator side? A Change of number of poles B Change of rotor resistance C Change of supply voltage frequency D Change of supply voltage Answer : B Explanation :
47.
 During starting of an induction motor hums, the probable 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 :
48.
 The possible number of different speeds that can be obtained by connecting two motors in cascade is A 2 B 4 C 6 D 12 Answer : B Explanation :
49.
 The induction motor which is provided with open slots have A Motor break down torque B Less break down torque C Leading power factor D Higher efficiency Answer : A Explanation :
50.
 For a 50 H.P motor, which type of rotor will be preferred? A Die chat aluminium rotor B Wound rotor C Squirrel cage rotor using round copper bars D Squirrel cage rotor using rectangular copper bars Answer : B Explanation :
51.
 If a three-phase squirrel cage induction motor run slow, which of the following could not be the reason for the same? A Shorted stator coils B Low voltage C Overload D High frequency Answer : D Explanation :
52.
 The probable reason for an induction motor running too hot could be A Low voltage B Uneven air gap C Clogged ventilating ducts D Any of the above Answer : D Explanation :
53.
 Which of the following motors has the highest power to weight ratio? A Synchronous motor B Capacitor motor C Induction motor D Universal motor Answer : D Explanation :
54.
 In a 3-phase induction motor, iron loss A In rotor is negligible as compared to that in stator B In stator does not occur C In rotor is equal to iron loss is stator D In rotor is much more than the iron loss in stator Answer : A Explanation :
55.
 Which of the following parameters for an induction motor varies as square of the supply voltage? A Slip B Synchronous speed C Starting current D Maximum running torque Answer : D Explanation :
56.
 Is an induction motor is to be run on unbalanced supply, then it should be run at A Lower loads B Higher loads C Low speed D Higher speeds Answer : A Explanation :
57.
 In case single phasing occurs in delta connected motor A One phase will be seriously over-loaded and two others will be slightly overloaded B Two phases will be seriously over-loaded and there will be no current in the third phase C There will be no current in one phase D  There will be no current in two phases Answer : A Explanation :