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 R is?

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

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

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

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

This question was posed to me during an interview.

The origin of the question is Star to Delta and Delta to Star Transformation topic in chapter Polyphase Circuits of Network Theory

Answer 1

Right choice is (b) (ZRYZBR)/(ZRY+ZBY+ZBR)

To explain: After transformation to star, the impedance at R is

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