21. To obtain sinusoidal flux distribution, chamfering of poles is done in
- salient-pole alternators only
- smooth cylindrical pole alternators only
- both of the above
- none of the above
22. The salient pole rotors are not suitable for high speed turbo alternators due to
- excessive bearing friction
- undesirable mechanical oscillations
- high centrifugal force and windage
- loss large eddy current losses
23. In turbo-alternators, smooth cylindrical rotors have very long axial length because it
- reduces friction and windage loss
- reduces centrifugal force
- number of conductors being less, they have to be necessarily long for generating the required voltage
- none of the above
24. Cylindrical rotor alternators use _______ ducts for efficient cooling.
- only radial
- only axial
- both radial and axial
- none of the above
25. In cylindrical rotor _______ portion of rotor is wound.
- one-third
- two-third
- one half
- whole
26. The maximum possible speed at which an alternator can be driven to generate 50 Hz and 4000 V is
- 4000 rpm
- 3600 rpm
- 3000 rpm
- 1500 rpm
27. Two mechanically coupled alternators deliver power at 50 Hz and 60 Hz respectively. The highest speed of the alternators is
- 3600 rpm
- 3000 rpm
- 600 rpm
- 500 rpm
28. The most appropriate operating speeds in rpm of generators used in thermal, nuclear and hydropower plants would respectively be
- 3000, 300 and 1500
- 3000, 3000 and 300
- 1500, 1500 and 3000
- 1000, 900 and 750
29. The aircraft alternators are designed to generate emf of high frequency of 400 Hz in order to
- reduce the bulk
- compensate for high speeds
- make the systems free from external disturbances
- all the above
30. Armature winding is one in which
- emf is induced by the leakage flux
- emf is induced by the main or working flux
- working or main flux is created by the field current
- working or main flux is created by the armature current