## electrician

### MCQ ON 3 - PHASE INDUCTION MOTOR PAGE - 3

1.
 The torque developed  by a 3-phase induction  motor depends on the following three factors A Speeds , frequency , number of  poles B Voltage, current and stator impendence C Synchronous speed, rotor speed and frequency D Rotor e.m.f. , rotor current and rotor power factor Answer : D Explanation :
2.
 The efficiency and power factor of a SCIM increase in proportion to its A Speed B Mechanical load C Voltage D Rotor torque Answer : B Explanation :
3.
 If the stator voltage and frequency of an induction motor are reduced proportionality, its A Locked rotor current is reduced B Torque developed is increased C Magnetizing current is decreased D Both A and B Answer : D Explanation :
4.
 An induction motor has  a shaft circuit current 7 times the fullload current and a full load slip of 4 percent. It s line starting is ____times the full load torque. A 7 B 1.96 C 4 D 49 Answer : B Explanation :
5.
 If stator voltage of a SCIM is reduced to 50 percent of its rated value, torque developed is reduced by ____percent of its full load value. A 50 B 25 C 75 D 57.7 Answer : C Explanation :
6.
 A SCIM with 36- slot stator has two separate windings one with 3 coil roups per phase per pole and the other with 2 coil grops per phase per pole. The obtainable two motor speeds would be in the ra A 3:2 B 2:3 C 2:1 D 1:2 Answer : A Explanation :
7.
 A 6 pole, 3-phase induction motor taking 25 KW from a 50 Hz supply is cumulatively cascaded to a 4 pole motor, Neglecting all losses, speed of the 4 pole motor would be ____R.P.M A 1500 B 1000 C 600 D 3000 Answer : C Explanation :
8.
 Mark the WRONG statement Schrage motor is capable of behaving as A Inverted induction motor B Slip ring induction motor C Shunt motor D Series motor Answer : D Explanation :
9.
 When  a stationary 3 phase induction motor is switched on with one phase disconnected A It is likely to burn out quickly unless immediately disconnected B It will start, but very slowly C It will make jerky start with loud growing noise D Remaining intact fuses will be blown out due to heavy inrush of current Answer : A Explanation :
10.
 If single phasing of a 3 phase induction motor occurs under the running condition, it A Will stall immediatlry B Will keep running though  with slightly increased C May either stall or keep running depending on the load carried by it D Will become noisy while it still keeps runnings Answer : C Explanation :
11.
 If single phasing occurs during the running of a 3 phase motor, what is the effect on motor performance? A Motor will run as 2 phase motor and performs normally B Will continue to run on 2 phases but fails to take load C Continue to run  on 2 phases, draws excess current , winding likely to burn D Run at reduced speed and can take only 2/3 of the load Answer : C Explanation :
12.
 You have to change the defective induction motor of a machine. The available motor is of a higher capacity than the existing one. What is the effect of using a higher capacity motor? A The new motor will be overheated B The new motor is better suitable for this work C The efficiency and the power factor will become low D The new motor has a longer useful working life and efficiency Answer : C Explanation :
13.
 The synchronous speed of an induction motor refers to the speed of the …. A Rotor B Armature C Rotating magnetic field D Value marked on the name plate Answer : C Explanation :
14.
 The synchronous speed formula is : A Ns = 120f / p B Ns  = 120 / fp C Ns  = 120p / f D Ns = pf / 120 Answer : A Explanation :
15.
 Calculate slip of an induction motor if it has 4 poles, 50 Hz, rotor speed si 1400 rpm A 6.6% B 6.0% C 5.5% D 5.0% Answer : A Explanation :
16.
 In a 3-phase motor the rotating magnetic filed is displaced at an angle of … A 300 B 600 C 900 D 1200 Answer : D Explanation :
17.
 Which one of the following 3-phase motor is NOT self-starting? A Synchronous motor B Squirrel cage induction motor C Wound rotor motor D Double cage motor Answer : A Explanation :
18.
 The motor suitable for loads where constant speed is required is A Induction motor B Synchronous motor C Capacitor  motor D Universal motor Answer : A Explanation :
19.
 The magnetic field produced in the stator of a three-phase induction motor travel at A Rotating speed B Asynchronous seed C Synchronous speed D Slip speed Answer : C Explanation :
20.
 For a small capacity induction motor, the type of starter used is…. A Star delta starter B DOL starter C Auto transformer starter D Rotor resistance starter Answer : B Explanation :
21.
 In a 3-phase induction motor started by a DOL starter, the short circuit current is protected by… A Overload relay B No volt coil C Back up fuses D Protective earth conductor Answer : C Explanation :
22.
 It is advisable to avoid direct on line starting of induction motor having a capacity of more than 3HP because A The string torque is very low B The motor takes five to seven times of its full load current C It will pick up very high speed D It will run in the reverse direction Answer : B Explanation :
23.
 On switching ON the starter the magnetic contactor vibrates heavityly , the causes may be… A High voltage B Dirt in the armature contact surface C Low voltage  and single phasing D Low voltage and dirt on the armature contact surface Answer : D Explanation :
24.
 In the manual star delta starter, the stop button connection is in series with the A No volt coil B Over laod relay contacts C No volt coil and load relay contacts D No-volt coil and start button Answer : B Explanation :
25.
 You are working with a fully automatic star delta starter. You notice that when start switch is pressed the motor starts in star, but does not change to delta. In the above case, the trouble will b A Delta contactor B Timer and its circuit C Star contactor D Line contactor Answer : B Explanation :
26.
 A 3-phase 15HP, 415 V, 50Hz squirrel cage motor’s star delta starter cables are found to be defective copper cables are replaced by same current capacity aluminium cables through the same met A Aluminum cable insulation is weaker when compared to copper cable B Aluminium cables may not have the same voltage grading as copper cables C Overall size of the aluminium cables are more than copper cables and hence insulation damaged whi D Overheat provided by aluminium cables is the cause of failure of insulation and short. Answer : C Explanation :
27.
 Rotor resistance starters are used with wound rotor induction mtor. When the motor is running the resistance in starter are A Shorted B Open circuit C Connected in delta D Connected in star Answer : A Explanation :