Electrical Machines Questions and Answers_pag166

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Explanation: Thermal plants use cylindrical rotor synchronous machines. 
9. The practical speed ranges for the thermal, nuclear and hydel power plants is (in rpm) 
a) 3000,3000,300 
b) 3000,300,300 
c) 1500,1500,3000 
d) 1000,500,3000 
View Answer 
Answer: a 
Explanation: Thermal and nuclear power plants have high speed requirements and hydel power 
plants have salient pole generators so the speed is also less. 
10. Synchronous motor has been preferred for driving the loads requiring high power at low 
speeds. 
a) True 
b) False 
View Answer 
Answer: a 
Explanation: Synchronous motor acts a capacitor when operated at leading p.f. with no load. 
11. The zero power characteristics used in the Potier plot can be obtained by loading the 
alternator using 
a) synchronous motor 
b) lamp load 
c) dc motor 
d) induction motor 
View Answer 
Answer: a 
Explanation: To obtain mmf and emf drops the loading is done as a synchronous motor. 
12. Below are the statements about cylindrical rotor alternator. 
A. E.m.f. generated by armature reaction lags armature current by 90°. 
B. Air gap voltage leads the field flux by 90° 
a) Only A is true 
b) Only B is true 
c) A is true, B is false 
d) A and B both are true 
View Answer 
Answer: d 
Explanation: Both the statements are right. 
13. Out of the given statements about the cylindrical rotor alternator, mark the correct 
A. Air gap voltage lags the field flux by 90° 
B. Armature reaction mmf lags the field flux by (90°+ internal p.f. angle) 
a) A is false, B is true 
b) Only A is true 
c) Only B is true 
d) Both A and B are true