Electrical Machines Questions and Answers_pag161

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b) Xd = 6, Xq = 3.86 
c) Xd = 3, Xq = 3.86 
d) Xd = 6, Xq = 2.5 
View Answer 
Answer: d 
Explanation: Xd=V(max)/I(min) = 30/5 = 6 ohms; 
Xq=V(min)/I(max) = 25/10=2.5 ohms. 
13. A synchronous motor is operated at a bus voltage of 1.5 pu and drawing 1.0 pu at zpf 
leading current. Its synchronous reactance is 0.4 pu. What is the excitation emf of the motor? 
a) 2.0 pu 
b) 1.5 pu 
c) 1.0 pu 
d) 1.1 pu 
View Answer 
Answer: d 
Explanation: E = V-I*X = 1-0.5=0.5 pu. 
14. Consider a 3-phase alternator with negligible armature resistance and high rotational inertia 
delivering power to an isolated load when the armature terminals got short circuited. After 
sometime, the mechanical 
power input to the shaft of the machine is terminated at time t0. The short-circuit current 
circulating in the armature will change till the machine comes to halt at ‘ts’. Which one of the 
curves depicts this phenomena? 
a)