Question

A 3-phase transformer has 420 and 36 turns on the primary and secondary windings respectively. The supply voltage is 3300 V. The secondary line voltage on no-load when the windings are connected in star-delta is nearly
1. 22260 V
2. 1908 V
3. 164 V
4. 490 V

Answer 1

Correct Answer - Option 3 : 164 V

In a 3-ϕ transformer,

Primary turns, N₁ = 420 turns

V₁ = 3300 V

Secondary turns, N₂ = 30 turns

For star delta transformer,

\({V_L} = \sqrt 3 \;{V_p}\)  (star)

V_L = V_P (Delta)

Primary phase voltage, \({V_{p1}} = \frac{{{V_L}}}{{\sqrt 3 }} = \frac{{3300}}{{\sqrt 3 }}\)

V_p1 = 1905.255 V

Now, \(\frac{{{V_{p1}}}}{{{V_{p2}}}} = \frac{{{N_1}}}{{{N_2}}}\)

\( \Rightarrow {V_{p2}} = {V_{p1}}\frac{{{N_2}}}{{{N_1}}}\)

\(= 1905.255 \times \frac{{36}}{{420}}\)

⇒ V₂ = 164 V