Electrical Machines Questions and Answers_pag366

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Explanation: The changes in the frequency is not reflected due to the flux density variation 
correspondingly. 
9. There are two identical transformers A and B such that flux density applied to B is doubled. 
Then magnetizing current of B is 
a) larger than A 
b) double of A 
c) half of A 
d) same as that of A. 
View Answer 
Answer: a 
Explanation: When we double the flux density, as per the magnetization curve of the core, the 
magnetizing current is much larger than double of the machine A. 
10. A 20KVA, 2200/220 V, 50 Hz single phase transformer has the below parameters: 
HV : r = 2.4 ​Ω, X=6 ​Ω 
LV: r = 0.03 ​Ω, X = 0.07 ​Ω 
The primary reactance referred to secondary is 
a) 0.024 ​Ω 
b) 0.06 ​Ω 
c) 3 ​Ω 
d) 7 ​Ω 
View Answer 
Answer: b 
Explanation: Primary reactance referred to secondary = x1*(N2/N1)^2 = 6*(220/2200)^2 
= 0.06 ​Ω. 
11. A 20KVA, 2200/220 V, 50 Hz single phase transformer has the below parameters: 
HV : r = 2.4 ​Ω, X=6 ​Ω, Primary 
LV: r = 0.03 ​Ω, X = 0.07 ​Ω, Secondary 
The secondary reactance referred to primary is 
a) 0.024 ​Ω 
b) 0.06 ​Ω 
c) 3 ​Ω 
d) 7 ​Ω 
View Answer 
Answer: d 
Explanation: Secondary reactance referred to primary = x2*(N1/N2)^2 = 0.07*(2200/220)^2 = 
7.0 Ω 
12. A 20KVA, 2200/220 V, 50 Hz single phase transformer has the below parameters: 
HV : r = 2.4 ​Ω, X=6 ​Ω 
LV: r = 0.03 ​Ω, X = 0.07 ​Ω 
The primary resistance referred to secondary is 
a) 0.024 ​Ω 
b) 0.06 ​Ω 
c) 3 ​Ω