11. The relative speed between the magnetic fields of stator and rotor under steady state operation is zero for
- a dc machine
- an induction machine
- a synchronous machine
- all the above machines
12. A 3-phase synchronous motor has
- high starting torque
- no starting torque
- low starting current
- low starting torque
13. A 10 pole, 25 Hz alternator is directly coupled to and is driven by 60 Hz synchronous motor. What is the number of poles for the synchronous motor?
- 48
- 12
- 24
- 16
14. A 3-phase synchronous motor needs dc supply for excitation
- continuously
- at the starting instant only
- of stator
- none of these
15. The coupling magnetic field reaction with ________ system(s) is essential for extracting energy from an electrical system.
- mechanical
- electrical
- both electrical and mechanical
- none of these
16. The armature current of a synchronous motor on no load
- leads the applied voltage by 90°
- lags behind the applied voltage by 90°
- is in phase with the applied voltage
- zero
17. As the load in increased on a synchronous motor, its speed
- decreases
- increases
- remains constant and additional load is supplied by shift in relative position of the rotor with respect to stator rotating magnetic field
- remains constant for some time and then falls abruptly
18. The synchronous motor meets increase in load by taking more armature current as
- the rotor pole falls back relative to the stator pole causing an increase in motor current
- the back emf decreases causing an increase in motor current
- rotating field is strengthened causing an increase in motor current
- none of the above
19. A synchronous motor is operating with normal excitation. With the increase in load, the armature current drawn from the supply main increase due to
- increase in back emf
- fall in motor speed
- increase in resultant voltage across the armature
- increase in power factor
20. The resultant voltage acting across the armature circuit of a synchronous motor (Er) is the ________ of induced emf in the armature circuit (Eb) and supply voltage V.
- arithmetic sum
- arithmetic difference
- phasor difference
- phasor sum