## electrician

### MCQ ON 3 - PHASE INDUCTION MOTOR PAGE - 2

1.
 Thermal overload relays are _____than magnetic overload relays. A Faster B Slower C Much bigger D Very smaller Answer : B Explanation :
2.
 Which of the following reduces vibration, chattering, noise, wear and heat in the armature of a starter? A Electromagnet B Shading coil C Centrifugal switch D Holding lock Answer : B Explanation :
3.
 Minimum requirement of contacts in a 3 phase (d) O.L starter is : A 4 NO B 3 NO and one NC C 3 NO D 2 NO and 2 NC Answer : A Explanation :
4.
 NVC used in a starter is meant to protect the motor against A Short circuit B Open circuit C Excess current D Low voltage Answer : D Explanation :
5.
 Full load line current of a 3 phase , 3H.P, 415 V, 50 Hz , squirrel cage inductionmotor is approximately: A 3A B 4.5A C 6A D 9A Answer : B Explanation :
6.
 No-load current of an induction motor is approximately ______percent of full load current A 10 B 20 C 40 D 75 Answer : C Explanation :
7.
 In a (d) O.L starter a NVC is connected across a phase and neutral of a 3-phase, 415 V, 50 Hz supply and it works normally. What is the rating of NVC? A 220V B 230V C 400V D 415V Answer : B Explanation :
8.
 A (d) O.L starter can be used satisfactorily for a double cage squirrel cage induction motor upto A 1 kW B 1.5kW C 3 kW D 5kW Answer : B Explanation :
9.
 The purpose of a backup fuse is to provide protection against A Overload B Over voltage C Excess current D Short circuit Answer : D Explanation :
10.
 A 4-pole, 3-phase, 50 Hz motor’s speed is 95 % of its synchronous speed then the actual speed of the motor is A 500 r.p.m B 1300 r.p.m C 1400 r.p.m D 1425 r.p.m Answer : D Explanation :
11.
 The induction motor of a pump is run at a supply voltage 30% lesser thanthe normal voltage (415V). The pump starts to work. After some time what possibly can happen? A The motor will become hot and  will be damaged B The motor will start after some time C The motor will function normally D None of the stated above will happen Answer : A Explanation :
12.
 The purpose of adding a rheostat to slip ring induction motor is to A Limit the starting increase B Increase the starting current C Decrease the starting current so that the starting torque may increase D None of the stated above Answer : C Explanation :
13.
 The rotor current frequency of a slip-ring induction motor depends on A Rotor conductor B Rotor inductance C Amount of slip D Inductive reactance Answer : C Explanation :
14.
 When the rotor of a slip ring induction motor is standstill, the rotor frequency is A Zero B Higher than the stator frequency C Lower than the stator frequency D Equal to the stator frequency Answer : D Explanation :
15.
 If the load on a motor is increased beyond its full load point the greatest effect of the increase in slip is in A Increasing the power factor B Decreasing the power factor C Increasing the current D Decreasing the current Answer : C Explanation :
16.
 Regarding skewing of motor bars in a squirrel cage induction motor (SCIM), which statement is false? A It prevents cogging B It increases starting torque C It produces more uniform torque D It reduces motor “hum”  during  its operation Answer : B Explanation :
17.
 The principle of operation of  a 3-phase induction motor is similar to that of a A Synchronous motor B Repulsion start induction motor C Transformer with a shorted secondary D Capacitor start, induction run motor. Answer : C Explanation :
18.
 The effect of increasing the length of air gap in an induction motor will be to  increase the A Power factor B Speed C Magnetisign current D Air gap flux Answer : C Explanation :
19.
 In a 3 phase induction motor, the relative speed of stator flux with  respect to ___ is zero A Stator winding B Rotor C Rotor Flux D Space Answer : C Explanation :
20.
 A 3-phase, 4-pole, 50 Hz induction motor runs at a speed of 1440 R.P.M The rotating field produced by the rotor rotates at a speed of ____ R.P.M with respect to the rotor A 1500 B 1440 C 60 D 0 Answer : C Explanation :
21.
 In a 3 phase induction motor, the rotor field rotates at synchronous speed with respect to A Stator B Rotor C Stator flux D None of the above Answer : A Explanation :
22.
 Irrespective of the supply frequency, the torque developed by a SCIM is the same when ever_____is the same. A Supply voltage B External load C Rotor resistance D Slip speed Answer : D Explanation :
23.
 The number of stator poles produced in the rotating  magnetic field of a 3-phase induction  motor  having three slots per pole per phase is A 3 B 6 C 2 D 12 Answer : B Explanation :
24.
 The power factor of a squirrel cage induction motor is A Low at light loads only B Low at heavy loads only C Low at light and heavy loads both D Low at rated load only Answer : A Explanation :
25.
 A 6 pole, 50 Hz, 3 phase induction motor is running at 950 R.P.M and has rotor copper loss of 5 KW. Its rotor input is _____KW A 100 B 95 C 10 D 5.5 Answer : A Explanation :
26.
 The efficiency of a 3-phase induction motor is approximately proportional to A (1-S) B S C N D Ns Answer : A Explanation :
27.
 A 6 pole, 50 Hz, 3 phase induction motor has a full load speed of 950 R.P.M. At half load, its speed would be ___R.P.M A 475 B 500 C 975 D 1000 Answer : C Explanation :
28.
 If rotor input of a SCIM running with a slip of 10% is 100 KW, gross power developed by its rotor is ______KW A 10 B 90 C 99 D 80 Answer : B Explanation :
29.
 When applied rated voltage per phase is reduced by one half, the starting torque of a SCIM becomes ____of the starting torque with full voltage A 1/2 B 1/4 C $\frac{1}{\sqrt{2}}$< D $\frac{\sqrt{3}}{2}$< Answer : B Explanation :
30.
 The fractional slip of an induction motor is the ratio of A Rotor copper loss to rotor input B Stator copper loss to stator input C Rotor copper loss to rotor output D Rotor copper loss to stator copper loss Answer : A Explanation :