1. what are the needed elements for producing emf in alternators ?
Ans: The essential elements for generating emf in alternators are prime mover, stator, rotor and exciter.
2. Why is it possible to design alternators to generate much higher voltages than DC generators ?
Ans: In case of an alternator field system is moving and armature is stationary and it is easier to insulate stationary armature winding for very high voltage because insulation of stationary armature coil is not related to mechanical stresses due to centrifugal action and large space is available on the armature winding for providing more insulation as the stator is outside the rotor. In case of dc generators field is stationary and armature is moving.
3. Which type of rotor is adopted for high speed alternators ?
Ans: Cylindrical or non salient pole rotor construction is used for high speed alternators.
4. What type of alternator will be used (a) for hydraulic turbine (b) for steam turbines as prime movers ?
Ans: For hydraulic turbine - salient pole and for steam turbine - smooth cylindrical type rotor type
5. Names the two types of alternators depending on the rotor construction.
Ans: Salient pole alternator and cylindrical rotor alternators
6. Name the types of alternator depending on the type of alternators.
Ans: 1. Turbo generators 2. Hydro generator ( water wheel generators) and) 3. diesel engine drives generators
7. What will be the number of poles of 50Hz alternator if it runs at its greatest speed ?
Ans: TWO.
8. What is the maximum speed of a 50Hz alternators ?
Ans: 3000 RPM
9. Why are salient pole alternators more suitable for low speed and non salient pole high speed operation ?
Ans: The salient pole type field structure is used almost entirely for low speed alternators, since it is least expensive and provides ample space for the field ampere turns but if can not be used for high speed alternators on account of high peripheral speed and the difficulty of obtaining sufficient mechanical strength. So non salient pole field structure is used for high speed operation.
10. Which one of the cylindrical and salient pole alternators is more stable ?
Ans: Salient pole alternator is more stable.
11. Do we use laminations for all the iron parts of an electrical machine ?
Ans: All the iron parts of an electrical machine are laminated except the end covers and the outer stator frame. the laminations are used to reduce the eddy current loss.
12. Why are the poles and pole shoes laminated ?
Ans: For reducing eddy current loss.
13. Why machines having long core length are provided with radial ventilating ducts while machines having deep cores are provided with axial cooling.
Ans: Radial ventilating ducts provided with long core machines take away heat at faster rate. similarly in case of deep cores axial ventilating ducts do. Otherwise the central portion of long core and the inner portion of deep core may attain a temperature higher than the permissible value.
14. What is the role of damper winding in 1. Synchronous generator and 2. synchronous motor ?
Ans: 1. In a synchronous generator damper winding is provided to suppress the negative sequence field and to damp the oscillations when the generator starts hunting.
2. In a synchronous motor damper winding is provided to develop starting torque so as to make the motor self starting and to develop damping torque when the machine starts hunting.
15. Why rotating machine with aluminium armature coils have increased leakage reactance ?
Ans: Rotating machine with aluminium armature coils need more space to accommodate conductors and so large size of stator slots resulting into increase leakage reactance.
16. At What voltage is the field of an alternator usually excited ?
Ans: 125 to 250 V DC.
17. What is an exciter ?
Ans: An exciter is a small dc generator (usually flat compound wound type ) to supply dc power to the filed magnet system or rotor of the alternator.
18. Why are distributed windings are preferred over concentrated windings ?
Ans: Distributed Winding are preferred over concentrated winding as they 1. Improve waveform of induced emf 2. provide better utilization of core 3. diminishes armature reaction and 4. facilitate cooling.
19. What is the necessity of chording in the alternator winding of a synchronous machine ?
Ans : The armature winding is chorded in order to 1. Improve waveform of induced emf 2. save copper in coil ends due to less span and 3. reduce inductance of the winding
20. A full pitch coil in alternator has a span of 18 slots. what should be the span if the third harmonic of the emf is to be eliminated ?
Ans: 12 slots