Match List I (Equivalent circuit parameter) with List II (values) for a 50 MVA three phase alternator and select the correct answer.

List I

List II

A.

Armature resistance

1.

1 pu

B.

Synchronous reactance

2.

0.1pu

C.

Leakage reactance

3.

0.01 pu

Question

Question

This question was previously asked in

BHEL Engineer Trainee Electrical 2019 Paper

Answer 1

Causes for voltage drop:

(i) drop due to armature resistance i.e. I_aR_a drop

(ii) drop due to armature leakage resistance i.e. I_a X_ℓ drop

(iii) drop due to armature reaction i.e. I_aX_a drop.

Now, ‘I_aX_ℓ’ will be the phase with ‘I_aX_a’

X_ℓ + X_a = X_s

R_a + jX_s = Z_s

In synchronous machine irrespective of its rating always, X_s > X_a > X_ℓ > R_a

We know, Z_s = R_a + j(X_a + X_ℓ)

Since X_s = X_a + X_ℓ

⇒ Z_s = R_s + j(X_s)