electrician

MCQ ON 3 - PHASE INDUCTION MOTOR PAGE - 1

1.
 The speed of rotating magnetic flux of a 3-phase induction motor is given by the expression A 120P/F B PF/120 C 120F/P D F/120P Answer : C Explanation :
2.
 The synchronous speed of a 2,4 and 8 poles motor at 50 Hz can not be A 3000 RPM B 1000 RPM C 1500 RPM D 750 RPM Answer : B Explanation :
3.
 The no of poles of a 3-phase induction motor running at 500 RPM will be : (supply frequency =50 Hz) A 12 B 76 C 4 D 2 Answer : A Explanation :
4.
 The relation of the torque developed in an induction motor and the supply voltage is given by the expression A $T=&space;\sqrt{V}$ B T =V C T = V2 D T = 1/V2 Answer : C Explanation :
5.
 The starting torque of a 3 phase induction motor can be increased A By increasing the rotor resistance B By increasing the stator resistance C By increasing the air gap between the stator and the rotor D None of these Answer : A Explanation :
6.
 At the time of starting, the slip of an induction motor is equal to A Infinite B Unit C Zero D None of these Answer : B Explanation :
7.
 The reason of drawing high current at the time of starting by an induction motor is A High voltage B Low power factor C Zero back e.m.f D None of these Answer : C Explanation :
8.
 For the speed control purpose pole changing method can be adopted for A slip ring motor B Synchronous motor C Squirrel cage induction motor D Single phase motor Answer : C Explanation :
9.
 The principle of operation of an induction motor is based on A Faradya’s law B Rotating magnetic field C Fleming’s right hand rule D Ohm’s law Answer : B Explanation :
10.
 The speed of rotor of an induction motor is A The same as the synchronous speed B Higher than the synchronous speed C Lesser than the  synchronous speed D Determined by the frequency and the number of poles Answer : C Explanation :
11.
 To reverse the direction of rotation of a 3 phases induction motor, it is necessary to A Interchange all the three-line connections B Reverse the polarity of the rotor’s circuit C Increase the resistance of the rotor’s circuit D Interchange any two out of the three-line connections Answer : D Explanation :
12.
 Squirrel cage induction motor differs from a slip ring induction motor as if has no A Slip rings B Rotor windings C Stator windings D Rotating part Answer : A Explanation :
13.
 A squirrel-cage induction motor has A Zero starting torque B Very small starting torque C Very high starting torque D Highest starting torque amongst all types of induction motors Answer : B Explanation :
14.
 The frequency of the rotor current (fr) in an induction motor is given by the expression A f = s.fr B fr =s/f C fr = s.f D S = f . fr Answer : C Explanation :
15.
 The torque of a 3-phase induction motor A Increases with an increase in supply voltage B Decreases with an increase in supply voltage C Has no relation with supply voltage D T = 1/ V2 Answer : A Explanation :
16.
 The purpose of connecting  resistance in the rotor circuit of a slip ring induction motor is to A Limit the starting current B Increase the starting current C Limit the starting current and to increase the starting torque D Decrease the starting torque Answer : C Explanation :
17.
 If the induction motor of a pump is operated at 30% less supply voltage then what will happen? A It will start after some time B It will continue to run at a lower speed without any damage C It will get heated up and consequently will get damaged D Nothing adverse will happen Answer : C Explanation :
18.
 In an induction –motor, the fractional slip (s) is given by the expression A S = Nr - Ns / Ns B C D Answer : Select Correct Ans Explanation :
19.
 It is advisable to avoid the use of (d) O.L starter for starting an induction motor having 5 H.P capacity because A The starting torque will be very low B The motor will  take five to seven times more starting current than its full load current C It will pick-up very high speed and may go out of step D It will run in the reverse direction Answer : B Explanation :
20.
 A thermal overload relay is provided in a starter to protect the motor against A Short circuit B Open circuit C Excess current D Low voltage Answer : C Explanation :
21.
 In a 3 phase, induction motor stared by a (d)O.L starter, protection against short circuit is given by the A Overload relay B No volt coil C Back up fuses D Protective earth contractor Answer : C Explanation :
22.
 When a squirrel cage induction motor is started directly by a (d) O.L starter, the starting current will be A Equal to the full load current B Twice the full load current C 3 times the full load current D 5 to 7 times the full load current Answer : D Explanation :
23.
 When we connect an induction  motor in star through a starter, the applied voltage across each winding becomes A VL volts B VL  * 3 Volts C c D Answer : C Explanation :
24.
 Total electrical degrees for a 4 pole (a) (c) machine is equal to A 1800 B 3600 C 7200 D 10800 Answer : C Explanation :
25.
 A 3-phase, 400 V, 50 Hz induction motor has a rated speed of 715 r.p.m. How many poles are there in the machine? A 2 B 4 C 6 D 8 Answer : B Explanation :
26.
 With a fractional slip of 0.07 , the rotor speed of a 4 pole squirrel cage induction motor fed from a 50 Hz supply is A 25.65 rev/s B 24.48 rev/s C 23.25 rev/s D 22.11 rev/s Answer : C Explanation :
27.
 The starting torque of a squirrel cage induction motor is low because of A Low starting current, low starting P.F B Low starting current , high starting P.F C High starting current, high starting P.F D High starting current, low starting P.F Answer : C Explanation :
28.
 If a 3-phase motor runs with two phases only it is called single phasing it will ultimately A Carry no load B Burn out C Run with triple speed D  not run efficiently Answer : B Explanation :
29.
 On switching ‘ON’ the starter, the magnetic contractor vibrates heavily. The possible cause is A Low voltage and single phasing B High voltage C Dirt between the contacts D Low voltage and dirt between the contacts Answer : D Explanation :
30.
 A squirrel-cage motor of above 3 H.P should not be started by a (d) O.L starter because A The motor takes 5 to 7 times of its full load current which till spoil the starter B The motor will pick up a tremendous speed C The motor will have a low starting torque D The motor will run in the reverse direction Answer : A Explanation :