Electrical Machines Questions and Answers_pag38

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b) ω-ω​r 
c) (ω​r​-ω)/ω 
d) ω​r 
View Answer 
Answer: b 
Explanation: The relative speed between rotor and rotating flux wave, i.e, (ω-ω​r​) is referred to as 
the slip speed in rad/sec 
slip speed = (ω-ω​r​) rad/sec 
12. If the rotor of an induction motor is made to revolve in a direction opposite to the rotating flux 
wave, then RMS value of EMF induced in one phase of rotor E is proportional to ____________ 
a) 2-s 
b) s 
c) 1-s 
d) 2+s 
View Answer 
Answer: a 
Explanation: Relative velocity between rotor winding and rotating flux wave becomes 
(ω+ω​r​) = ω+ω(1-s) = ω(2-s) and 
E = 2πf(2-s)N​phr​k​w​Φ 
13. In an alternator, frequency per revolution is equal to 
a) number of poles 
b) twice the number of poles 
c) speed in rps 
d) number of pole-pairs 
View Answer 
Answer: d 
Explanation: For a P-pole machine, in n rev/sec, P/2n cycles/second are generated and cycles 
per second is referred to as frequency f of the EMF wave. Here in this question n=1 
⇒ frequency per revolution = P/2 
⇒ number of pole pairs. 
14. The EMF generated in an alternator depends upon 
(i) speed 
(ii) maximum flux per pole 
(iii) series turns per phase 
(iv) phase spread 
(v) coil-span 
(vi) type of alternator 
Which of the above statements are correct? 
a)(i),(ii),(iii),(iv),(v),(vi) 
b) (i),(iii),(iv),(v) 
c) (i),(ii),(iii),(v),(vi) 
d) (i),(iii),(iv),(v),(vi) 
View Answer