JNTU ONLINE EXAMINATIONS [Mid 1 - em3]

1. The frequency of voltage generated by an alternator having 4-poles and rotating at 1800 r.p.m is ———hertz 
a. 60
b. 7200
c. 120
d. 450

2. The frequency generated in a 8-pole alternator that rotates at 750 r.p.m is 
a. 30 Hz
b. 40 Hz
c. 50 Hz
d. 60Hz

3. If P is the number of poles , f=frequency in Hz , N=speed in r.p.m, the relationship between the three for a synchronous alternator is given by 
a. f = (P / 120 N)
b. f = (P / 60 N)
c. P = (120 f / N)
d. P = (60 f / N)

4. Turbo-alternators are generally used to run to 
a. 1500 r.p.m
b. 3000 r.p.m
c. 5000 r.p.m
d. 150000 r.p.m

5. Alternator operates on the principle of 
a. Electro-magnetic induction
b. Self induction
c. Mutual induction
d. Self or mutual induction

6. Synchronous machine can operate
a. only as a generator
b. only as a motor
c. both as a generator and as a motor
d. neither generator not motor

7. In a synchronous machine, if the field flux axis is ahead of the armature field axis in the direction of rotation, the machine operating is 
a. Synchronous motor
b. Synchronous generator
c. Asynchronous motor
d. Asynchronous generator

8. An alternator generates [01S04]
a. dc
b. ac
c. pulsating dc
d. dc as well as ac

9. In modern alternators, the rotating part is 
a. Field system
b. Armature
c. Armature as well as field system
d. Neither armature nor field

10. The frequency of emf generated by an alternator depends upon the alternator speed (N in rpm) and number of poles ( P )on the alternator field and is given as
a. PN/60
b. (60N) / P
c. PN/120
d. (120N) / P

11. Squirrel cage bars placed in the rotor pole faces of an alternator help reducing hunting 
a. Above synchronous speed only
b. Below synchronous speed only
c. Above and below synchronous speeds both
d. Equal to synchronous speed 

12. Salient pole field structure, the pole shoes cover about —of pole pitch 
a. one-third
b. one half
c. two-third
d. whole

13. The high-speed turbo-alternator has rotor of —-diameter and —- axial length in comparison to those of rotor for a low speed hydro-generator.
a. smaller, smaller
b. larger, larger
c. smaller, larger
d. larger, smaller

14. Salient pole field structure has the advantages of 
a. Adaptability to low and moderate speed operations
b. Reduced noise
c. Reduced friction and windage losses
d. Reduced bearing loads

15. Larger diameter salient pole rotors have shorter axial length because 
a. It occupies much less space
b. It saves lot of copper in stator winding
c. It reduces weight of the rotor
d. Number of conductors held in large circumference of rotor is very large and so they need not be long

16. The stator core of a synchronous machine is built up of——laminations 
a. stainless steel
b. silicon steel
c. cast iron
d. cast steel

17. The stator core of a synchronous machine is laminated so as to reduce 
a. Eddy current loss
b. Hysteresis loss
c. Both eddy current and hysteresis loss
d. The size and weight of the machine