Electrical Machines Questions and Answers_pag407

Text Material Preview

II. Losses : (1-k) 
III. Impedance drop = 1/(1-k) 
Which of the above are correct for an auto transformer when compared to the identical rating 
two winding transformer? 
a) I,II 
b) II,III 
c) 1,III 
d) I,II,III,IV 
View Answer 
Answer: a 
Explanation: KVA(auto)/KVA(2-W) = V2*I2/V2*(I2-I1) 
= 1/(1-I1/I2) 
= 1/(1-k) 
Losses(auto)= (1-k) *Losses(2-W) 
Impedance drop(auto)= (1-k)*Impedance drop(2-W). 
6. The voltage regulation of a transformer at full-load 0.8 p.f leading is -2%. Its voltage 
regulation at full load 0.8 p.f lagging 
a) will be positive 
b) will be negative 
c) may be positive 
d) may be negative 
View Answer 
Answer: a 
Explanation: The leading p.f. has negative v.r. and lagging p.f. has major portion of positive 
voltage regulation. 
7. The voltage regulation of a transformer depends on its 
(A)equivalent reactance (B) equivalent resistance 
(C)load power factor (D)transformer size 
(E) load current 
a) A,B,C,E 
b) A,B,C,D,E 
c) A,B,D,E 
d) A,B,C,D 
View Answer 
Answer: a 
Explanation: Voltage regulation is independent of the size of the transformer. 
8. Three transformers having identical dimensions but with core of iron, aluminium and wood 
are wound with same number of turns and have same supply. 
Then choose the order for hysteresis losses. 
a) wood > aluminium > iron 
b) aluminium > iron > wood 
c) iron > wood > aluminium 
d) iron > aluminium > wood