TS Mocktest 2021-5

1. If the synchronous generator operating at zero power factor lagging the effect of armature reaction is

A. Demagnetizing
B. magnetizing
C. cross-magnetizing
D. partially cross magnetizing

Answer : Demagnetizing
Explanation:-

2. If in a three phase Alternator, a field current of 50A produces a full load armature current of 200 A on short circuit and a generated emf of 1000 V/phase on open circuit, then the synchronous impedance is

A. 5 Ω
B. 20 Ω
C. 10 Ω
D. 8.5 Ω

Answer : 5 Ω
Explanation:- Synchronous Impedance=(Voc)/Isc (for same field current)
Zs=1000/200=5 Ω

3. The rotor of an Induction motor cannot run with synchronous speed because

A. rotor torque would then become zero
B. Lenz's law would be violated
C. Induction motor would then become synchronous motor
D. air friction prevents it from doing so

Answer : rotor torque would then become zero
Explanation:-

4. Squirrel cage bars placed in the rotor pole faces of an alternator help to reduce hunting

A. Above synchronous speed
B. below synchronous speed
C. Above and below synchronous speed
D. does not depend on speed

Answer : does not depend on speed
Explanation:-

5. Earthing is necessary to give protection for

A. Over Voltages
B. Danger of electric shock
C. Over currents
D. Lightning

Answer : Danger of electric shock
Explanation:-

6. The stator winding of a single phase induction motor is splitted into two parts in order to

A. improve efficiency
B. improve power factor
C. develop starting torque
D. increase speed

Answer : develop starting torque
Explanation:-

7. 'V' curves of a Synchronous motor give relation between which quantities

A. Power factor and field current
B. Armature current and field current
C. Armature current and Power factor
D. Applied voltage and field current

Answer : Armature current and field current
Explanation:-
V-curves---->Armature current and field current
Inverted V-curves---->Power factor and field current

8. A 6 pole 50 HZ, three phase Induction motor is running at 950 rpm and has a rotor Cu loss of 5 KW it's rotor input is

A. 100 KW
B. 10 KW
C. 95 KW
D. 5.3 KW

Answer : 100 KW
Explanation:- Given poles=6,Frequency-50Hz N=950rpm
Synchronous speed (Ns)=120f/P=(120*50)/6=1000rpm Slip=(Ns-N)/Ns=(1000-950)/1000==0.05
Relation between Rotor input and Rotor Cu loss
P2:Pcu=1:s
Rotor input=Pcu/s==5/0.05=100 KW

9. Which one of the following methods of speed control cannot be applied for controlling the speed of a three phase squirrel cage induction motor

A. By changing the applied voltage
B. By changing the supply frequency
C. By changing the number of stator poles
D. By rotor Rheostat control

Answer : By rotor Rheostat control
Explanation:- We cannot control speed through rotor due to it is short circuited

10. Which of the following types of motors is not self starting motor

A. DC Shunt motor
B. DC Series motor
C. Three phase induction motor
D. Three phase synchronous motor

Answer : Three phase synchronous motor
Explanation:-