Electrical Machines Questions and Answers_pag34

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View Answer 
Answer: d 
Explanation: For synchronous machines of large sizes, adequate cooling arrangements must be 
incorporated to dissipate the heat produced by various losses. 
5. In a synchronous generator, the frequency of the generated EMF depends on 
a) rotational speed 
b) poles 
c) both rotational speed and poles 
d) none of the mentioned 
View Answer 
Answer: c 
Explanation: f = PN/120 Hz 
6. A synchronous machine can be connected to an energy system only when ___________ 
a) frequency/rotational speed matches 
b) voltage matches 
c) current matches 
d) power matches 
View Answer 
Answer: a 
Explanation: For successful operation, the rotational speed/frequency of the synchronous 
machine must synchronize with the frequency of the energy system, and this is the reason for 
calling it a synchronous machine. 
7. In larger DC machines, the reason behind using fabricated steel instead of cast iron in the 
manufacture of yoke is ? 
a) economical considerations 
b) permeability considerations 
c) mechanical considerations 
d) insulation considerations 
View Answer 
Answer: b 
Explanation: Fabricated steel has higher permeability than cast iron. 
8. In a DC machine, the pole core is usually of smaller cross section than the pole shoe 
because ___________ 
(i) smaller cross section requires less copper 
(ii) larger pole shoe increases the air gap reluctance 
(iii) larger pole shoe increases the flux per pole entering the armature 
(iv) larger pole shoe reduces the mechanical strength 
(v) larger pole shoe supports the field winding 
Which of the above statements are correct? 
a) (i),(ii),(v) 
b) (ii),(iii),(v) 
c) (i),(iii),(v) 
d) (ii),(iv),(v)