Question

If a resistor ZR is connected between R and N, ZBR between R and B, ZRY between R and Y and ZBY between B and Y form a delta connection, then after transformation to star, the impedance at Y is?

(a) (ZRY)/(ZRY+ZBY+ZBR)

(b) (ZBY)/(ZRY+ZBY+ZBR)

(c) (ZRYZBY)/(ZRY+ZBY+ZBR)

(d) (ZRYZBR)/(ZRY+ZBY+ZBR)

I had been asked this question in an internship interview.

Question is taken from Star to Delta and Delta to Star Transformation in portion Polyphase Circuits of Network Theory

Answer 1

Right choice is (c) (ZRYZBY)/(ZRY+ZBY+ZBR)

To explain I would say: After transformation to star, the impedance at Y is

(ZRYZBY)/(ZRY+ZBY+ZBR).