Electrical Machines Multiple Choice Questions on “Physical Concepts of Synchronous Machine Operation”.
1. Floating of synchronous machine on infinite bus means ___________
A. Fa = 0 and machine neither generator nor motor
B. Fa is not zero and machine neither generator nor motor
C. Fa = 0 and machine is not excited
D. frequency is zero
Answer: A
Clarification: Floating means the armature flux is zero and it neither motoring nor generating.
2. If the field poles are ahead of resultant mmf, then the 3-phase synchronous machine is operating as ___________
A. generator
B. motor
C. reluctance motor
D. any of the mentioned
Answer: A
Clarification: Field mmf leads the resultant mmf so it is generator.
3. At steady state operation in a 3-phase synchronous generator. (where Tem is developed electromagnetic torque developed in the machine and Tpm is the prime mover torque.)
A. Tem = Tpm
B. Tem > Tpm
C. Tem < Tpm
D. can not be coupled
Answer: A
Clarification: At steady state the net torque is zero in the machine.
4. Active power in synchronous machine is proportional to __________
A. torque
B. excitation
C. sinϕ
D. all of the mentioned
Answer: A
Clarification: Active power is proportional to the torque of the synchronous machine.
5. The reactive power in synchronous machine is proportional to __________
A. torque
B. excitation
C. sinϕ
D. all of the mentioned
Answer: B
Clarification: Reactive power depends on the excitation of the synchronous machine.
6. If the synchronous machine is loaded from floating conditions, the alternator begins to operate at __________
A. leading p.f
B. lagging p.f
C. unity p.f
D. zero p.f
Answer: A
Clarification: The resultant flux will be leading in nature, so after loading it from loading it from floating, alternator will operate at leading p.f.
7. If the synchronous machine is loaded from floating conditions, the synchronous motor begins to operate at __________
A. leading p.f
B. lagging p.f
C. unity p.f
D. zero p.f
Answer: B
Clarification: The resultant flux will be lagging in nature, so after loading it from loading it from floating, alternator will operate at lagging p.f.
8. Reactive power flow in synchronous machine can be controlled by __________
A. varying field excitation
B. varying field current
C. varying field flux
D. any of the mentioned
Answer: D
Clarification: All the alternatives are same, varying the field flux matters at the end to change the reactive power in a synchronous machine.
9. If the 3-phase alternator has 4 poles and has synchronous speed of 120 rad/s. Then the mechanical speed in rad/sec is?
A. 60
B. 240
C. 120
D. 30
Answer: A
Clarification: Mechanical speed = electrical speed*2/Number of Poles.
10. The power load angle characteristic as maximum for a load angle of __________
A. 90°
B. 45°
C. more than 90°
D. 180°
Answer: A
Clarification: As per the parabolic graph, the maxima is obtained at 90°.
11. For running a 750 kW compressor, a synchronous motor is preferred because of __________
(i) lower noise (ii) faster build up of pressure (iii) better power factor (iv) low starting torque
A. (ii), (iii)
B. (i), (ii), (iv)
C. (i), (ii), (iii), (iv)
D. (iii), (iv)